UV Solid-State Light Emitters and Detectors

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I

-~­~

I

Series II: Mathematics, Physics and Chemistry- Vol. 144

UV Solid-State Light Emitters and Detectors

edited by

Michael S. Shur Rensselaer Polytechnic Institute, Troy, NY, U.S.A.

and

Arturas Zukauskas Vilnius University, Vilnius, Lithuania

Springer-Science+Business Media, B.V.

Proceedings of the NATO Advanced Research Workshop on UV Solid-State Light Emitters and Detectors Vilnius, Lithuania 17-21 June 2003

A C.I.P. Catalogue record for this book is available from the Library of Congress.

ISBN 978-1-4020-2035-3 ISBN 978-1-4020-2103-9 (eBook) DOI 10.1007/978-1-4020-2103-9

Printed an acid-free paper

AII Rights Reserved © Springer Science+Business Media Dordrecht 2004 Originally published by Kluwer Academic Publishers 2004 Softcover reprint of the hardcover 1 st edition 2004 No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work.

Contents

Contributing Authors

Preface

Basic Device Issues in UV Solid-State Emitters and Detectors M. S. SHUR AND A. ZUKAUSKAS

IX

X Ill

HYPE-Grown AlN-GaN Based Structures for UV Spectral Region 15 A. S. USIKOV, Yu. MELNIK, A. I. PECHNIKOV, V. A. SOUKHOVEEV, 0. V. KOVALENKOV, E. SHAPOV ALOVA, S. Yu. KARPOV, AND V. A. DMITRIEV

GaN-Based Laser Diodes 31 S. EINFELDT, S. FIGGE, T. BOTTCHER, AND D. HOMMEL

Quaternary AllnGaN Materials System for UV Optoelectronics 41 E. KUOKSTIS, G. TAMULAITIS, AND M. ASIF KHAN

III-Nitride Based UV Light Emiting Diodes 59 R. GASKA, M. ASIF KHAN, AND M. S. SHUR

UV Metal Semiconductor Metal Detectors 77 J- L. REVERCHON, M. MOSCA, N. GRANDJEAN, F. OMNES, F. SEMOND, J- Y. DUBOZ, AND L. HIRSCH

vi

Characterization of Advanced Materials for Optoelectronics by Using UV Lasers and Four-Wave Mixing Techniques 93 K. JARASIONAS

Quantum Phospors 111 A. P. VINK, E. VANDERKOLK, P. DORENBOS, AND C.W.E. VAN EIJK

Optical Measurements Using Light-Emitting Diodes 127 A. ZUKAUSKAS, M. S. SHUR, AND R. GASKA

Novel AlGaN Heterostructures for UV Sensors and LEDs 143 M. STUTZMANN

Nitride Photodetectors in UV Biological Effects Studies 161 E. MuNOZ, J. L. PAU, AND C. RIVERA

Promising Results of Plasma Assisted MBE for Optoelectronic Applications 179 A. GEORGAKlLAS, E. DIMAKlS, K. TSAGARAKI, AND M. ANDROULIDAKI

Low Dislocations Density GaN/Sapphire for Optoelectronic Devices 189 B. BEAUMONT, J.-P. FAURIE, E. FRAYSSINET, E. AUJOL, AND P. GIBART

Stimulated Emission and Gain in GaN Epilayers Grown on Si 199 A. L. GURSKli, E. V. LUTSENKO, V. Z. ZUBIALEVICH, V. N. PAVLOVSKli, G. P. YABLONSKII, K. KAZLAUSKAS, G. TAMULAITIS, S. ruRSENAS, A. ZUKAUSKAS, Y. DIKME, H. KAUSCH, A. SZYMAKOWSKl, R. H. JANSEN, B. SCHINELLER, AND M. HEUKEN

Materials Characterization of Group-III Nitrides under High-Power Photoexcitation 207 S. JURSENAS, G. KURILCIK, S. MIASOJEDOV AS, AND A. ZUKAUSKAS

Small Internal Electric Fields in Quaternary InAlGaN Heterostructures 215 S. ANCEAU, S. P. LEPKOWSKI, H. TEISSEYRE, T. SUSKI, P. PERLIN, P. LEFEBVRE, L. KONCZEWICZ, H. HIRAYAMA, ANDY. AOY AGI

Vll

MOCVD Growth of AlGaN Epilayers and AlGaN/GaN SLs in a Wide Composition Range 223 W. V. LUNDIN, A. V. SAKHAROV, A. F. TSATSUL'NIKOV, E. E. ZA VARIN, A. I. BESULKIN, A. V. FOMIN, AND D. S. SIZOV

Gallium Nitride Schottky Barriers and MSM UV Detectors 233 B. BORATYNSKI AND M. TLACZALA

III-Nitride Based Ultraviolet Surface Acoustic Wave Sensors 239 D. CIPL YS, A. SEREIKA, R. RIMEIKA, R. GASKA, M. SHUR, J. YANG, AND M. ASIF KHAN

Optically Pumped InGaN/GaN/AlGaN MQW Laser Structures 247 V. YU. IVANOV, M. GODLEWSKI, H. TEISSEYRE, P. PERLIN, R. CZERNECKI,P. PRYSTAWKO, M. LESZCZYNSKI, I. GRZEGORY, T. SUSKI, AND S. POROWSKI

High Power LED and Thermal Management 253 A.MAHLKOW

Detection of Blue Light by Self-Assembled Monolayer ofDipolar Molecules 261 0. NEILANDS, N. KIRICHENKO, I. MUZIKANTE, E. FONA VS, L. GERCA, S. JURSENAS, R. V ALIOKAS, R. KARPICZ, AND L. V ALKUNAS

Atomic and Molecular Spectroscopy with UV and Visible Superbright LEDs 271 G. PICHLER, T. BAN, H. SKENDEROVIC, AND D. AUMILER

Semi-Insulating GaN and its First Tests for Radiation Hardness as an Ionizing Radiation Detector 279 J. V. VAITKUS, W. CUNNINGHAM, M. RAHMAN, K. M. SMITH, AND S. SAKAI

Towards the Hybrid Biosensors Based on Biocompatible Conducting Polymers 287 A. RAMANA VICIENE AND A. RAMAN A VICIUS

viii

Optically Pumped UV-Blue Lasers Based on InGaN/GaN/Ab03 and InGaN/GaN/Si Heterostructures 297 G. P. YABLONSKII, A. L. GURSKII, E. V. LUTSENKO, V. Z. ZUBIALEVICH, V. N. PAVLOVSKII, A. S. ANUFRYK, Y. DIKME, H. KAUSCH, R. H. JANSEN, B. SCHINELLER, AND M. HEUKEN

Key Word Index 305

Author Index 307

Contributing Authors

Key Lectures

EINFELDT Sven

GASKA Remis

JARASrONAS K~stutis

KUOKSTIS Edmundas

Institute of Solid State Physics, University of Bremen, P.O. Box 330440, 28334 Bremen, Germany einfeldt@physik.uni-bremen.de

Sensor Electronic Technology, Inc., 1195 Atlas Road, Columbia, SC 29209, USA gaska@s-et. com

Institute of Materials Science and Applied Research, Vilnius University, Sauletekio 9-III, 2040 Vilnius, Lithuania kestutisjarasiunas@ff. vu.lt

Department of Electrical Engineering, University of South Carolina, Columbia, SC 29208, USA koukstis@engr. sc. edu

lX

X

MuNOZ Elias

REVERCHON Jean-Luc

SHUR Michael

STUTZMANN Martin

TAMULAITIS Gintautas

USIKOV Alexander

VINKArjan

ZUKAUSKAS Artilras

Institute for Systems Optoelectronics and Microtechnology and DIE ETSI Telecomunicaci6n, Universidad Politecnica de Madrid, 28040 Madrid, Spain elias@die. upm. es

Thales Research & Technology, 91404 Orsay Cedex, France jean-luc. reverchon@thalesgroup. com

Center for Broadband Data Transport, Rensselaer Polytechnic Institute, CII 9017, 110 8th street, Troy, NY 12180, USA shurm@rpi. edu

Walter Schottky Institut, Technische Universitat Mtinchen, 85748 Garching, Germany stutz@wsi. tum. de

Institute of Materials Science and Applied Research, Vilnius University, Sauletekio 9-III, 2040 Vilnius, Lithuania gintautas. tamulaitis@ff. vu.lt

TDI, Inc., 12214 Plum Orchard Dr., Silver Spring, MD 20904, USA usikov@tdii. com

Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands vink@now.nl

Institute of Materials Science and Applied Research, Vilnius University, Sauletekio 9-III, 2040 Vilnius, Lithuania arturas.zukauskas@ff. vu.lt

Poster Presentations

BORATYNSKI Boguslaw Faculty ofMicrosystem Electronics and Photonics, Wroclaw University of Technology, Janiszewskiego 11117, 50-372 Wroclaw, Poland bborat@wemif.pwr. wroc.pl

CIPL YS Daumantas Department of Radiophysics, Vilnius University, Sauletekio 9-III, 2040 Vilnius, Lithuania daumantas. ciplys@ff. vu.lt

GEORGAKILAS Alexandros Institute of Electronic Structure and Laser, Foundation for Research and Technology­Hellas, P.O. Box 1527, 71110 Heraklion, Crete, Greece

GIBART Pierre

GURSKII Alexander

IVANOV Vitalii

JURSENAS Saulius

alexandr@physics. uoc.gr

Lumilog, 2720, Chemin de Saint Bernard, Les Moulins I, 06220 Vallauris, France pierre.gibart@lumilog. com

Stepanov Institute of Physics ofNAS Belarus, F. Skaryna Ave. 68, 220072 Minsk, Belarus gurskii@dragon. bas-net. by

Institute of Physics, Polish Academy of Sciences, AI. Lotnik6w 32/46, 02-668 Warsaw, Poland ivanov@ifpan. edu.pl

Institute of Materials Science and Applied Research, Vilnius University, Sauletekio 9-III, 2040 Vilnius, Lithuania sauliusjursenas@ff. vu.lt

xi

Xll

LEPKOWSKI Slawomir

LUNDIN Wsevolod

MAHLKOW Adrian

MUZIKANTE Inta

PICHLER Goran

RAMANA VICIUS Ariinas

V AITKUS Juozas

Y ABLONSKII Genadii

UNIPRESS, Polish Academy of Sciences, Sokolowska 29/37, Warszawa, Poland slawek@unipress. waw.pl

A.F. Ioffe Physico-Technical Institute of the Russian Academy of Science, 194021 St.Petersburg, Russia Iundin. vpegroup@pop. ioffe. rss i. ru

Optotransmitter Umweltschutz Technologie e.V., Berlin, Germany adrian@mahlkow. com

Institute of Physical Energetics, Aizkraukles Str. 21, LV-1006 Riga, Latvia intam@edi.lv

Institute of Physics, Bijenicka cesta 46, P. 0. Box 304, HR-10001 Zagreb, Croatia pichler@ift. hr

Laboratory ofBioanalysis, Institute of Biochemistry, Mokslininkq 12, 2600 Vilnius, Lithuania arunas@imi.lt

Institute of Materials Science and Applied Research, Vilnius University, Sauletekio 9-III, 2040 Vilnius, Lithuania juozas. vaitkus@ff. vu.lt

Stepanov Institute of Physics ofNAS Belarus, F. Skaryna Ave. 68, 220072 Minsk, Belarus yablon@dragon. bas-net. by

Preface

Infrared and visible light LEDs and photodetectors have found numerous applications and have become a truly enabling technology. The promise of solid state lighting has invigorated interest in white light LEDs. Ultraviolet LEDs and solar blind photodetectors represent the next frontier in solid state emitters and hold promise for many important applications in biology, medi­cine, dentistry, solid state lighting, displays, dense data storage, and semi­conductor manufacturing. One of the most important applications is in sys­tems for the identification of hazardous biological agents.

Compared to UV lamps, UV LEDs have lower power consumption, a longer life, compactness, and sharper spectral lines. UV LEDs can provide a variety of UV spectra and have shape and form factor flexibility and rugged­ness. Using conventional phosphors, UV LEDs can generate white light with high CRI and high efficiency. If quantum cutter phosphors are developed, white light generation by UV LEDs might become even more efficient.

Advances in semiconductor materials and in improved light extraction techniques led to the development of a new generation of efficient and pow­erful visible high-brightness LEDs and we expect that similar improvements will be achieved in solid-state UV technology.

NATO Advanced Research Workshop UV Solid-State Light Emitters and Detectors took place on June 17-21, 2003 in Vilnius, Lithuania (see http://www.natoarw-uv.ff.vu.lt). It brought together leading researchers in semiconductor UV technology and systems applications. The topics covered at the workshop ranged from basic device issues to substrates, epitaxial growth, materials characterization, nitride quaternary alloys, doping, strain energy band engineering, quantum phosphors, ohmic contacts and Schottky

Xlll

XIV

barriers, UV LED and solar blind photodetector device design and perform­ance, thermal management, and applications for biological hazardous agent sensing, solid state lighting, environmental control, and optical measure­ments. All these issues are presented in these Proceedings, and we hope that this book will be useful for students, engineers, scientists, and researchers interested in solid state light emitters and detectors and in wide band gap semiconductor technology.

We gratefully acknowledge the support of the workshop by NATO, the US Defense Advanced Research Projects Agency, Ministry of National De­fense of the Republic of Lithuania, Ministry of Education and Science of the Republic of Lithuania, Lithuanian State Foundation of Science and Studies, Vilnius University, Center of Broadband Data Transport Science and Tech­nology at Rensselaer Polytechnic Institute, Sensor Electronic Technology, Inc., and EKSPLA Ltd.

MichaelS. Shur and Artiiras Zukauskas

Troy, NY, USA- Vilnius, Lithuania

BASIC DEVICE ISSUES IN UV SOLID-STATE EMITTERS AND DETECTORS

M. S. SHUR 1 and A. ZUKAUSKAS 2

1 Center for Broadband Data Transport, Rensselaer Polytechnic Institute, C/J 9017, 110 8th street, Troy, New York 12180, USA 2 Institute of Materials Science and Applied Research, Vilnius University, Sauletekio 9-III, LT-2040 Vilnius, Lithuania

Abstract: UV light emitting diodes (LEDs) and lasers are expected to find numerous applications in biotechnology, medicine, dentistry, home security, food and air safety technology, short-range covert communications, industry, and solid­state lighting. 340-400-nm LEDs are already available commercially and milliwatt power 285-nm LEDs have been demonstrated in a laboratory. In par­allel, AIGaN alloys with large molar fractions of AI for UV solar blind Schot­tky barrier, p~n junction and MSM detectors have been demonstrated. Recent work on surface-acoustic-wave (SAW) UV detectors revealed their potential for remote solar blind detection applications. However, with decreasing wave­lengths, UV LEDs power is dropping and challenges in growing high quality nitride heterostructures with a high aluminum molar fraction are becoming more formidable. The solutions to device problems lie in using better sub­strates (with bulk AIN substrates in non-polar orientations being especially promising), using better epitaxial growth techniques, improving device design and using better contact technology and design.

Key words: ultraviolet LEDs, solar blind photodetectors, aluminum gallium nitride

1. INTRODUCTION

When Monsanto introduced the first commercial visible LEDs in 1968, they produced only red light with the intensity of approximately 1 o-3 lumen, barely visible under ambient light. Much brighter LEDs with colors ranging from red to yellow and green have been developed between 70's and mid 90's. As a result, infrared and visible LEDs as well as their counterparts,

M.S. Shur and A. tukauskas (eds.), UV Solid-State Light Emitters and Detectors, 1-13. © 2004 Kluwer Academic Publishers.

2 M S. Shur and A. Zukauskas

photodetectors, have found numerous applications and have become a truly enabling technology.

Pioneering work of Pankove, Akasaki, and Nakamura has led to the de­velopment of bright green and blue LEDs based on nitride semiconductors in the recent decade. Further development of the AllnGaN materials system resulted in an appearance of ultraviolet (UV) LEDs and solar blind photodetectors, which represent the next frontier in solid-state optoelectron­ics with a huge potential in biological, medical and environmental instru­mentation, dense data storage, disinfection, deodorization, communications, and solid-state lighting.

UV spectral region spans from 100 to 400 nm and is usually divided into three subregions based on the absorption in the atmosphere and biological action of radiation. UV radiation with wavelengths from approximately 315 to 400 nm is referred to as UV A; UV radiation with wavelengths from 280 to 315 nm is referred to as UVB; and UV radiation with wavelengths from 100 to 280 nm is referred to as UVC (see Fig. 1 ). UV A pe-netrates the at­mosphere without substantial absorption and causes minor biological action, mainly premature aging of the skin. UVB is partially absorbed in the atmos­phere; it results in sunburn and may cause skin cancer. UVC doesn't reach the earth's surface due to absorption in ozone contained in the upper atmos­phere and is highly dangerous for live organisms because of strong absorp­tion in proteins.

luvcl 1 00-280 280-315 315-400

nm

UV radiation Visible radiation (light) Infrared radiation (IR)

nm nm

100 -400 nm 400 - 760 nm

> 760 nm

Figure f. Classification of spectral ranges.

Figure 2 (from [1]) shows the energy gaps of semiconductor materials and the corresponding wavelengths. As seen, the group-III nitride materials family spanning the direct energy gaps from 0.8 eV to 6.2 eV is ideal for applications in UV emitter and detector technology for UV A, UVB, and, partially, UVC regions. In particular, the band gap of InGaN alloy covers a part of the UV A region and AlGaN alloy can be tailored to wavelengths ranging form 360 to 200 nm depending on the AI molar fraction.

Basic Device Issues in UV Solid-State Emitters and Detectors

AIN C ZnS

OGaN Zn S ic%CH(fHl

ZnSe CdS CdOAIP

SiC(6H GaP

Zn Te AlAs

InN CdSeAISb CdTe lnP GaAs

GaSbSi Ge

In As lnSb

Wave length (nm )

2000 800 600 500 400 300

uv

Ba nd Gap Energy (eV)

3

200

Figure 2. Semiconductor bandgaps and corresponding radiation wavelengths. Human eye sensitivity curve (arbitrary logarithmic scale) is also shown [I].

2. UV LIGHT-EMITTING DIODES

Compared to UV lamps, UV LEDs have lower power consumption, a longer lifetime, compactness, and sharper spectral lines. UV LEDs can provide a variety of UV spectra and have shape and form factor flexibility and rugged­ness. In particular, UV LEDs are expected to be used for the disruptive tech­nology of solid-state lighting [2]. Using conventional phosphors, UV LEDs can generate white light with high color rendering properties and high effi­ciency. If quantum cutter phosphors are developed, white light generation by UV LEDs might become even more efficient. Another important application ofUV LEDs is fluorescence excitation [2,3]. Based on this technique, novel and cost-efficient instruments for detection and characterization of bio­chemical compounds and biological agents, including hazardous agents, can be developed.

Soon after the invention of the p-n junction GaN UV LED by Akasaki et a/. in 1992 [ 4], a tremendous progress in solid-state sources of UV light was achieved. Present UV LEDs are based on heterostructures developed using nitride materials systems GaN/AlGaN [5), InGaN/AlGaN [6], AlGaN/AlGaN [7], and quaternary AllnGaN (for a review, see Ref. 8). For mature UV LEDs, the main device issues to be addressed are almost the

4 M S. Shur and A. Zukauskas

same as those for advanced visible LEDs [2]: the chips must feature elec­tronic structure that facilitates high efficiency of carrier injection into the active layer, the internal quantum efficiency should be maximized by en­hancing radiative recombination and suppressing the nonradiative recombi­nation, and light generated within the chip must be efficiently extracted.

Reduction of the dislocation density and preventing cracking of epitaxial layers mismatched to the substrate is one of the most important issues. Most of fabrication approaches employ growth on sapphire substrate, which has a 16% lattice mismatch with GaN. This drawback is being bypassed through dislocation filtering by epitaxial lateral overgrowth and by using superlat­tices, strain-compensating layers and quaternary AllnGaN alloys (strain en­gineering approach). To substitute sapphire, novel substrates are being searched for UV LEDs. An example of such a substrate is bulk AlN, which offers identical crystal structure, close lattice and thermal expansion match to high Al-content nitride alloys, and refraction index favorable for UV light extraction. A UV LED grown over bulk AlN was recently reported by Xerox PARC and Crystal IS (see Fig. 3).

Figure 3. UV LED on bulk AIN substrate (courtesy Crystal IS, Inc.).

To increase the internal quantum efficiency, optimization of quantum­well structures is required through selecting composition and doping profiles of the well and barrier layers, shaping of the interfaces, and engineering of the built-in electric field to avoid the quantum-confined Stark effect. In addi­tion, basic research for unveiling the routes of nonradiative recombination in AIGaN alloys with high molar fraction of aluminum is needed.

Wide band gap of semiconductors used in UV LEDs have high ionization energies for shallow impurities, especially for acceptors. This results in dif­ficulties with p-doping and increased resistivity of the layers and contacts.

Basic Device Issues in UV Solid-State Emitters and Detectors 5

To overcome these difficulties, novel doping approaches including piezo­electric and superlattice doping, as well as co-doping are being developed.

Finally, specific issues related to UV light extraction must be addressed. Conventional plastics used in visible-LED domes absorb UV radiation and should be substituted by new materials. New plastic materials, optical cou­plers to silica windows, as well as novel transparent contacts are required for further promotion ofUV LED technology.

First commercial 375-nm LEDs were introduced by Nichia [6]. Typically, these devices feature 1.5-2 m W optical power and are available with the outcoupling optics for narrow-angle (20°) and wide-angle (110°) radiative pattern.

Cree introduced the first near-UV LED for use in the illumination market in 2001 (12 mW, 405-nm and 395-nm UV InGaN on SiC substrate devices). These LEDs have a geometrically enhanced vertical chip structure to maxi­mize light extraction efficiency and require only a single wire bond connec­tion.

Recently, considerable progress in penetration into the UVC spectral re­gion was achieved. SET /USCIRPI team has already reported on UV LEDs with the wavelength as short as 265 nm [9,10]. Deep UV LEDs with the peak optical powers of3 mW (1 A) at 280 nm and 10 mW (1 A) at 325 nm were fabricated and characterized (see Fig. 4). Sandia National Laboratories have demonstrated UV LEDs with 290 nm wavelength with 1.3 m W of out­put power and with 2 7 5 nm wavelength with 0.4 m W of output power [ 11].

~ s ..... <!)

~ 0 c.. ~

~ 0

10 pulse - .... o-- -0 l_A_--

o-

I de -·-- -· IOOmA .... ....

....

..... 0.1

270 280 290 300 310 320 330 340 350

Wavelength, run

Figure 4. Output power ofUV LEDs versus wavelength [9].

A high-power 365-nm UV LED is being developed by Nichia and is ex­pected to enter the market in 2004. The chip containing an active InGaN/ AlGaN multiple quantum well structure is separated from its sapphire substrate and mounted on a Cu W heat sink. The device has an output power

6 MS. Shur and A. Zukauskas

of 210 mW when driven at de 500 rnA and 4.3 V and has an external quan­tum efficiency of 12.4% [12].

3. SOLAR BLIND PHOTODETECTORS

Equally impressive progress has been achieved in solar blind nitride based UV detectors. Figure 5 shows different types of nitride based UV photodetectors.

- ohmic = n-GaN - i-GaN

= transparent Schot tky D n-AIGaN

= Schottky - p-GaN

[ J photoconductive

Cl3N

transparent optoelectronic HFET

Schottky contact

[ 1 photoconductive Meta i-GaN-Meta l

p-i-n june! ion

AIGaN

E J Met ai-AIGaN-Met al

E J photoconductive

AIGaN

Figure 5. Schematic representation of photodetectors realized using the GaN materials system [13].

Direct band-gap AlxGa1_xN layers with x;:::: 0.4 exhibit a sharp transmis­sion cut-off at A < 280 nm. Devices using such layers found applications in jet engines, furnaces, environmental monitors, and missile detection systems. Both AlGaN based photoconductive and photovoltaic detectors have been explored [ 14-16]. Photoconductive devices have large gains but slow re­sponse times: Also, photoconductive devices cannot operate at zero bias and, therefore, have an extra noise coming from the dark current. AlGaN p-i- n photodiodes have several drawbacks [17-20] related to the difficulties of p­type doping of AlGaN layers with a high Al content [18] and to a high resis­tance of ohmic contacts to p-type AlGaN layers. This resistance can be de­creased by using p-GaN as the contact layer [ 17, 19,20] with i-AlxGal-xN (x > 0.4) as the active layer. However, the contact GaN layer absorbs a sig­nificant fraction of the optical beam reducing the device responsivity and

Basic Device Issues in UV Solid-State Emitters and Detectors 7

deteriorating UV/visible selectivity. Also, to avoid cracking, i-AlGaN active layer thickness has to be well below 2000 A.

A metal-semiconductor-metal (MSM) design [21] does not require oh­mic contacts. However, the MSM devices cannot operate at zero bias, which increases the noise. Also, at moderate bias values, the photo response of MSM diodes has a significant slow photoconductive component, since the space charge width in the AlGaN layer is smaller than the electrode spacing. Lateral geometry transparent Schottky barrier photodetectors avoid most of the above problems [22]. However, this design requires an n-doped Al0.4Gao.6N layer, and such doping attempts using Si had resulted in insulat­ing material.

Carrano et a!. [23] studied the current transport mechanisms in GaN­based MSM photodetectors. They concluded that thermionic and thermionic­field emissions were the dominant transport mechanisms. The traps affecting the current transport seemed to be surface defects (such as threading disloca­tions) and deep defect states, which are within the tunneling distance from the heterointerface.

Adivarahan et a!. [24] reported on a new In-Si co-doping approach to obtain n-Al0.4Ga 0.6N active layers with resistivity as low as 0.16 ohm·cm. In addition to a significantly increased doping efficiency, the introduction of a small concentration of In also allows for the direct deposition of a crack-free 0.5 J..l.m thick Si-doped Alo.4Ga0.6N layer over a 200 A thick AIN buffer layer on basal plane sapphire substrates. They also demonstrated the potential of using these In-Si co-doped layers for a lateral geometry, true solar-blind Schottky barrier detector (..1 cut-off at 278 nm).

The increased n-type doping due to the addition of In can result from the introduction of a shallow impurity level. Indium incorporation might also reduce the defect formation as indicated by the improved structural quality and morphology of the grown films [25]. Indium might counteract the incor­poration of defects responsible for the self-compensation of high AI mole fraction AlGaN layers, such as DX centers and cation vacancies [26].

Figure 6 (from [24]) shows optical transmission and photoluminescence spectra ofln co-doped AlGaN layer at room temperature.

Figure 7(a) shows the current-voltage characteristics measured between two 50 J..l.m x 150 J..l.m transmission line model (TLM) pads separated by a 2 J..l.m gap [24]. The characteristics are linear due to the low sheet resistivity of the AlGaN layer. The TLM measurements yielded the specific contact resistivity to be 2.5x10-3 ohm·cm. This was the first ever-reported data on ohmic contact resistivity to thick AlGaN epilayers with AI fraction of about 40%.

Figure 7(b) shows the dark I-V characteristics for the transparent Schottky barriers fabricated by Adivarahan eta!. [24]. As seen, the tum-on voltage and the forward differential resistance were approximately 1.2 V and 500

8 M S. Shur and A. Zukauskas

ohm, respectively, and the reverse leakage current at a bias of -3 V was as low as 6 nA. The effective Schottky barrier height extracted from tempera­ture measurements was 0.64 V.

260 290

Wavelength (nm)

60 ~ ~

~

40 ~ E "' c: co

20 t=

Figure 6. Spectra of optical transmission and photoluminescence of the In co-doped AlGaN epilayer with approximately 40% of AI [24].

3.0 (a)

2.5 (b)

2.5 2.0

<( E ~ 1.5 ~ ~ 2.5 ! 1.0 :; -

(.) 6.3nA 0.5

0.0

-3 -2 -1 0 1 2 3 Voltage,V Voltage, V

Figure 7. Current-voltage (I- V) characteristics of the ohmic contacts to In co-doped AlGaN (a) and of the AlGaN Schottky photodiode (b). Dashed line in the left figure shows the I-V curve for the AlGaN layer with no In co-doping [24].

Figure 8 shows the measured responsivity of this In co-doped Schottky photodiode.

Basic Device Issues in UV Solid-State Emitters and Detectors

-~ ->-+-' ·::;: ·c;; c: 0 0... C/) Q)

0::

0.01

1E-3

1 E-4

260 280 300 320 340 360 380 400

Wavelength (nm) Figure 8. Photoresponsivity spectrum of the A!GaN photodiode [24].

9

Rumyantsev et al. [27] studied low-frequency noise in Schottky barrier Alo.4Gao.6N diodes. At forward bias, the low-frequency noise is a superposi­tion of the 1/f and generation-recombination noise. The spectral noise den­sity, S1, of current fluctuations increases as S, ~ II. 5 at low currents I and as S, ~ p.s at high currents (see Figure 9). The measured dependencies of noise on forward current show that the noise is a superposition of the noise from Schottky barrier and from the series resistance of the contacts and/or the base. At high current densities, when the noise from the base or contacts is dominant, the upper bound of the Hooge parameter in AlGaN was estimated as a< 10. However, this high value of a does not present an obstacle for practical applications of these photodetectors, since their detectivity is pri­marily limited by the thermal noise of the load resistance.

Osinsky et al. [28] demonstrated visible-blind GaN Schottky barrier de­tectors grown on Si (Ill). The spectral response of the lateral Schottky bar­rier detectors had a cutoff at 365 nm with peak responsivities of 0.05 AIW at zero gate bias and 0.1 A/W at --4 V bias. This work demonstrated a possibil­ity of integrating GaN-based photodetectors with Si electronics.

Van Hove et al. [29] fabricated GaN and AlGaN 1x10 photodetector ar­rays for high temperature sensing applications. The device epitaxial layers were grown on sapphire substrates by RF atomic nitrogen plasma molecular beam epitaxy (MBE). At room temperature, GaN p-i-n photodetectors had a peak sensitivity of 0.198 A/W at 360 nm (which corresponded to the internal quantum efficiency of85%). The devices operated up to 400 °C.

10 M S. Shur and A. Zukauskas

Current density (A/cm2)

1 o-5 1 o-3 1 o-1 1 o 1

~t:: 1o·"0 4 8 12 16 8 Voltage (V) .. , .... -reverse bias ,. .. · n.-Tl-~ 11.s . ··-· ---- ----"": s-~kriR?------ --~--- ~ ~~-~--~-- .... ~-:· -

IT cq .---·- · S 2 I _.- ·- lc= q

1 o -26 E........... ........ ~.........L~.......~. ............ .....L ............ ......._ ............. "'""--~......_~ ....

1 o-10 1 o-8 1 o-6 1 o-4 1 o-2

1 o -22

Current (A)

Figure 9. Dependence of spectral noise density of current fluctuations S1 on current. T= 300 K,f = 1 Hz. Open and closed symbols show experimental data for forward and reverse biases, respectively. Dashed line shows the level of thermal noise S1r = 4kTIReq (Req =50 Q). Dot­dashed line shows the level of the shot noise S1c = 2ql (q is the electron charge). Dotted line shows the slope of current dependence of noise for the reverse bias. Inset shows current­voltage characteristic of Schottky barrier AI0.4Gao.6N diode (device area 9x I o-4 cm2) [27].

Khan et al. [30,31] reported on a photodetector based on a 0.2-J.lm gate AlGaN/GaN heterostructure field effect transistor (HFET). The epilayer structure and processing details for the gated photodetectors are similar to those for short gate AlGaN/GaN HFETs.

Ciplys et al. [32-34] reported on a SAW-based UV GaN sensor by plac­ing a SAW element into an oscillator feedback loop. The output of such sen­sor was a radio signal with UV radiation-dependent frequency, which made this sensor attractive for remote sensing applications.

Several review papers and book chapters discuss nitride based UV photodetectors, see, for example, Refs. 13,35-38.

4. CONCLUSIONS

Further progress in nitride based UV emitters and photodetectors will de­pend on several materials and device issues ranging from using large area substrates, improved homo- and heteroepitaxy of nitrides, improved p-type and n-type doping of high AI molar fraction epilayers, better light extraction designs for UV LEDs, thermal management, and UV resistant packaging.

Basic Device Issues in UV Solid-State Emitters and Detectors 11

Quantum phosphors (quantum cutters) might be essential for applications of UV LEDs in solid-state lighting. And, of course, reaching higher yields and reducing costs will be important. The development of UV semiconductor lasers and moving toward shorter wavelengths, especially into the UVC re­gion, which is important for protein excitation, will stimulate biophotonics applications such as clinical screening, point-of-care medical instrumenta­tion, environmental control, and biological hazardous agent detection.

REFERENCES

I. M. S. Shur, Introduction to Electronic Devices (Wiley, New York, 1996). 2. A. Zukauskas, M. S. Shur, and R. Gaska, Introduction to Solid State Lighting

(Wiley, New York, 2002). 3. A. Zukauskas, M.S. Shur, and R. Gaska, "Optical measurements using

light-emitting diodes," this volume. 4. I. Akasaki, H. Amano, K. ltoh, N. Koide, and K. Manabe, "GaN-based UV/blue

light emitting devices," !nsf. Phys. Conf Ser. 129, pp. 851-856 ( 1992). 5. J. Han, M. H. Crawford, R. J. Shu!, J. J. Figiel, M. Banas, L. Zhang, Y. K. Song,

H. Zhou, and A. V. Nurmikko, "AlGaN/GaN quantum well ultraviolet light emit­ting diodes," Appl. Phys. Lett. 73, pp. 1688-1690 ( 1998).

6. T. Mukai, M. Yamada, and S. Nakamura, "Current and temperature dependences of electroluminescence of lnGaN-based UV /blue/green light-emitting diodes," Jpn. J Appl. Phys. 37, pp. Ll358-LI361 (1998).

7. T. Nishida, and N. Kobayashi, "346 nm emission from AlGaN multi-quantum-well light emitting diode," Phys. Stat. Sol. A 176, pp. 45-48 ( 1999).

8. E. Kuokstis, G. Tamulaitis, and M. AsifKhan, "Quaternary AllnGaN materials sys­tem for UV optoelectronics," this volume.

9. A. Chitnis, V. Adivarahan, J.P. Zhang, M. Shatalov, S. Wu, J. Yang, G. Simin, M. As if Khan, X. Hu, Q. Fareed, R. Gaska, and M. S. Shur, "Milliwatt power AI­GaN quantum well deep ultraviolet light emitting diodes," Phys. Stat. Sol. A 200, pp. 99-101 (2003)

10. R. Gaska, A. Khan, and M.S. Shur, "Ill-nitride based UV light emitting diodes," this volume.

11. M. Hatcher, "Sandia UV LEDs emit record power," Compound Semiconductor 20, November 2003.

12. D. Morita, M. Sano, M. Yamamoto, M. Nonaka, K. Yasutomo, K. Akaishi, S. Nagahama, and T. Mukai, "Over 200 mW on 365 nm ultraviolet light emitting diode of GaN-free structure," Phys. Stats. Sol. A 200, pp. 114-117 (2003).

13. M.S. Shur and M.A. Khan, "GaN/AlGaN heterostructure devices: Photodetectors and field effect transistors," MRS Bull. 22, pp. 44-50 (1997).

14. M.A. Khan, J. Kuznia, D. T. Olson, M. Blasingame, and A. R. Bhattarai, "Schottky barrier photodetector based on Mg-doped p-type GaN films:· Appl. Phys. Lett. 63, pp. 2455-2456 (1993).

15. M. A. Khan, Q. Chen, C. J. Sun, M.S. Shur, M. F. Macmillan, R. P. Devaty, and J. Choyke, "Optoelectronic devices based on GaN, AIGaN, lnGaN homo­heterojunctions and superlattices," Proc. SPIE 2397, pp. 283-293 (1995).

12 M S. Shur and A. Zukauskas

16. A. Osinsky, S. Gangopadhyay, B. W. Lim, M. Z. Anwar, M.A. Khan, D. V. Kuksenkov, and H. Temkin, "Schottky barrier photodetectors based on AlGaN," Appl. Phys. Lett. 12, pp. 742-744 (1998).

17. D. Walker, V. Kumar, K. Mi, P. Sandvik, P. Kung, X. H. Zhang, and M. Razeghi, "Solar-blind AlGaN photodiodes with very low cutoff wavelength," Appl. Phys. Lett. 16, pp. 403-405 (2000).

18. S.C. Jain, M. Willander, J. Narayan, and R. Van Overstraeten, "III-nitrides: Growth, characterization, and properties," J Appl. Phys. 87, pp. 965-1006 (2000).

19. G. B. Parish, S. Keller, P. Kozodoy, J.P. lbbetson, H. Marchand, P. T. Fini, S. B. Fleischer, S. P. DenBaars, U.K. Mishra, and E. J. Tarsa, "High-performance (Al,Ga)N-based solar-blind ultraviolet p-i-n detectors on laterally epitaxially over­grown GaN," Appl. Phys. Lett. 15, pp. 247-249 (1999).

20. E. J. Tarsa, P. Kozodoy, J. lbbetson, B. P. Keller, G. Parish, and U. Mishra, "Solar­blind AlGaN-based inverted heterostructure photodiodes," Appl. Phys. Lett. 77, pp. 316-318 (2000).

21. T. Li, D. J. H. Lambert, A. L. Beck, C. J. Collins, B. Yang, M. H. Wong, U. Chowdhury, R. D. Dupuis, and J. C. Campbell, "Solar-blind AlxGa1_xN-based metal-semiconductor-metal ultraviolet photodetectors," Electron. Lett. 36, pp. 1581-1583 (2000).

22. V. Adivarahan, G. Simin, J. W. Yang, A. Lunev, M. AsifKhan, N. Pala, M. Shur, and R. Gaska, "Si02-passivated lateral-geometry GaN transparent Schottky-barrier detectors," Appl. Phys. Lett. 77, pp. 863-865 (2000).

23. J. C. Carrano, T. Li, P. A. Grudowski, C. J. Eiting, R. D. Dupuis, and J. C. Campbell, "Current transport mechanisms in GaN-based metal­semiconductor-metal photodetectors," Appl. Phys. Lett. 12, pp. 542-544 (1998).

24. V. Adivarahan, G. Simin, G. Tamulaitis, R. Srinivasan, J. Yang, M. AsifKhan, M.S. Shur, R. Gaska, S. L. Rumyantsev, and N. Pala, "Indium-silicon co-doping of high aluminum content AlGaN for solar blind photodetectors," Appl. Phys. Lett. 19, pp. 1903-1905 (200 1 ).

25. G. Tamulaitis, K. Kazlauskas, S. Jursenas, A. Zukauskas, M.A. Khan, J. W. Yang, J. Zhang, G. Simin, R. Gaska, and M. S. Shur, "Optical bandgap formation in Al­InGaN alloys," Appl. Phys. Lett. 77, pp. 2136-2138 (2000).

26. Stampfl and C. G. Van de Walle, "Doping of Al,Ga1_xN," Appl. Phys. Lett. 12, pp. 459-461 ( 1998).

27. S. L. Rumyantsev, N. Pala, M.S. Shur, R. Gaska, M. E. Levinshtein, M. AsifKhan, G. Simin, X. Hu, and J. Yang, "Low frequency noise in Al04Ga060N based Schottky barrier photodetectors," Appl. Phys. Lett. 19, pp. 866-868 (2001).

28. A. Osinsky, S. Gangopadhyay J. W. Yang, R. Gaska, D. Kuksenkov, H. Temkin, I. K. Shmagin, Y. C. Chang, J. F. Muth, and R. M. Kolbas, "Visible-blind GaN Schottky barrier detectors grown on Si(lll)," Appl. Phys. Lett. 12, pp. 551-553 (1998).

29. J. M. Van Hove, P. P. Chow, R. Hickman, II, J. J. Klaassen, A.M. Wowchak, and C. J. Polley, "GaN and AlGaN photodetectors for high temperature sensing applica­tions," in Abstracts of Materials Research Society Conference (December 1997, Boston, MA), 019.5.

30. M. A. Khan, M. Shur, and Q. Chen, "High transconductance AlGaN/GaN optoelec­tronic heterostructure field effect transistor," Electron. Lett. 31, pp. 2130-2131 (1995).

Basic Device Issues in UV Solid-State Emitters and Detectors 13

31. M. A. Khan, M.S. Shur, Q. Chen, J. N. Kuznia, and C. J. Sun, "Gated photodetec­tor based on GaN/AlGaN heterostructure field effect transistor," Electron. Lett. 31, pp. 398-400 (1995).

32. D. Ciplys, R. Rimeika, M.S. Shur, R. Gaska, A. Sereika, J. Yang, and M. AsifKhan, "Radio frequency response of GaN-based SAW oscillator to UV il­lumination by the Sun and man-made source," Electron. Lett. 38, pp. 134-135 (2002).

33. D. Ciplys, R. Rimeika, M.S. Shur, S. Rumyantsev, R. Gaska, A. Sereika, J. Yang, and M. Asif Khan, "Visible-blind photoresponse of GaN-based surface acoustic wave oscillator," Appl. Phys. Lett. 80, pp. 2020-2022 (2002).

34. D. Ciplys, A. Sereika, R. Rimeika, R. Gaska, M.S. Shur, J. Yang, M.A. Khan, "III­nitride based ultraviolet surface acoustic wave sensors," this volume.

35. M. Razeghi and A. Rogalski, "Semiconductor ultraviolet detectors," J Appl. Phys. 79, pp. 7433-7473 (1996).

36. M.S. Shur and M.A. Khan, "GaN and AlGaN devices: Field effect transistors and photodetectors," in GaN and Related materials II, ed. by S. J. Pearton, Optoelec­tronic Properties of Semiconductors and Super lattices, Vol. 7 (Gordon and Breach Science Publishers, Amsterdam, 2000), pp. 47-92.

37. H. Morkoij:, A. DiCarlo, and R. Cingolani, "GaN-based modulation doped FETs and UV detectors," in Condensed Matter News, ed. by Patrick Bernier, Vol. 8, is­sue 2, pp. 4-46 (200 1 ).

38. H. Morkoij:, "Wurtzite GaN based modulation doped FETs and UV detectors," in Handbook of Thin Film Devices: Hetero-Structures for High Performance Devices, ed. by M. H. Francombe, Chapter 5 (Academic Press, San Diego, 2000), pp. 193-216.

HVPE-GROWN AIN-GaN BASED STRUCTURES FOR UV SPECTRAL REGION

A. S. USIKOV 1, Yu. MELNIK 1, A. I. PECHNIKOV 1,

V. A. SOUKHOVEEV \ 0. V. KOVALENKOV \E. SHAPOVALOVA ., S.Yu. KARPOV 2, and V. A. DMITRIEV 1

1 TDI, Inc., 12214 Plum Orchard Dr., Silver Spring, MD 20904 2 Soft-Impact, Ltd., P.O. Box 83, 27 Engels av., St. Petersburg, 194156 Russia E-mail: usikov@tdii.com, Phone: + 1 (301) 572 7834, Fax: + 1 (301) 572 6438

Abstract: In this paper we describe ultraviolet light emitting diodes (LEOs) emitting in the spectral range from 305 to 340 nm based on AIGaN/AIGaN multi-layer submicron heterostructures grown by hydride vapor phase epitaxy (HVPE). The developed HVPE process possesses unique features such as ability (i) to combine deposition of thick low-defect layers and thin device multi-layer structures in the same growth run and (ii) to easily grow high-quality AlGaN layers in the whole composition range. HVPE is carbon-free growth technique producing GaN materials with very low background impurity concentrations. For a packaged LED with the peak wavelength of 340 nm, an optical output power of 2 m W was achieved at pulsed injection currents of 110 rnA. The ob­tained results prove the developed HVPE technique to have a significant po­tential for production of device epitaxial wafers, particularly for fabrication AIGaN-based light emitters.

Key words: HVPE, AIGaN/GaN heterostructures, UV LED

1. INTRODUCTION

Group-III nitride semiconductors (AlGaN) with high aluminum content could be developed as light emitters operating in the ultra-violet (UV) spec­tral range (350-220 nm). High performance AlGaN-based UV light emitters can find a lot of applications in UV optoelectronics for military, industrial and medicine needs. These devices would allow the implementation of miniaturized and inexpensive system for biological agent detection. Being

15

M.S. Shur and A. Zukauskas (eds.), UV Solid-State Light Emitters and Detectors, 15-29. © 2004 Kluwer Academic Publishers.

16 A. Usikov et al

realized through fluorescence excitation of 1-5 !!ill diameter sampled parti­cles by UV light (emission wavelength should be shorter than 340 nm), bio­detection of vanishing concentration of dangerous biological agents will be effective and reliable with high optical power of the UV light emitters. In addition, these devices can operate in the solar blind region of the spectrum (240-280 nm) where the earth's atmosphere is opaque. A particular interest is in high efficiency light emitters for 280-360 nm wavelength range. These devices would enable satellite communication secure from the ground and non-line-of-sight covert communication. Underwater submarine communica­tion would also be possible. Civilian applications include medicine and solid-state lighting. UV emission could be converted into white light utiliz­ing luminescent polymers and the resulting white light sources can be much more efficient, compact, and rugged than conventional fluorescent lamps.

To date, the main technological method to fabricate UV light emitters based on AlGaN materials is metal organic chemical vapor deposition (MOCVD). This method is proven to be a reliable fabrication tool for blue/green GalnN/GaN-based light-emitting diodes (LEDs) [1,2] and violet laser diodes (LDs) [3,4]. The LDs with GaN active region emitting at 366 nm under pulsed current injection were also fabricated by MOCVD [3]. To shift to the shorter wavelength range ( <360 nm), AlGaN-based light emitters with high AlN composition need to be grown. However, epitaxial growth of device structures based on AlGaN epitaxial layers with a high AlN concen­tration is a challenging technological task. First, MOCVD technology faces significant difficulties in producing highly doped p-type AlGaN layers as AlN concentration increases. Second, the radiative efficiency of light emit­ting AlGaN layers reduces drastically for shorter wavelength emitters possi­bly due to high concentration of deep centers in light emitting materials. A potential reason for the poor emission efficiency is carbon contaminations unavoidable for MOCVD technology. Another issue is related to high stress. As a result, the output power and external efficiency in the AlGaN-based UV LEDs have a tendency to decrease as the wavelength becomes shorter as seen in Fig. 1.

In this paper, an alternative epitaxial method to produce UV emitters based on AlGaN materials, hydride vapor phase epitaxy (HVPE) is de­scribed. Generally, the vapor phase epitaxy is well known as a reliable manufacturing tool for devising of structures based on compound semicon­ductor materials [5]. For group-III nitride semiconductors, HVPE was the first technique to produce high quality thick GaN epitaxial layers [6,7,8]. However, device applications of this epitaxial technique were not considered for a long time.

HVPE-Grown AIN-GaN Based Structures 17

AlGaN-based AllnGaN-based AlGaN-based AllnGaN-based active region active region active region active region

5 • 1A {pulsed) _,~;.. l I 10 ,_,A , i

~4 / • I ! f

~ 150mA i • 20mA >- \ 20mA E ~

<.>

! . c 3 i I -~ .. I <.> ~ i = II 0

.Jc ... 20mA e c. • Q)

-:; 100mA....- c;; 2

' l c. c -:; JC Q; 0 ;;:

w 1 i 1 A (pulsed) i

A ! -0.1 0

240 280 320 360 400 240 280 320 360 400

Wavelength, nm Wavelength, nm

Figure 1. Optical output power and efficiency of UV LEOs as a function of emission peak wavelength. ( .& ,.&) MOCVD grown structure, University of South Carolina; (. ) MOCVD grown structure, NTT Corporation, Japan; ( + ) MOCVD grown structure, University of Tokushima, Japan; info source: Technical Digest of ICNS-5, May 25-30, 2003, Nara, Japan. ( ) HYPE grown structure, this work

Recently, substantial progress has been achieved in the development of advanced HYPE technology for device applications [9,10,11]. The devel­oped HYPE process possesses unique aspects such as ability (i) to combine deposition of thick low-defect layers and thin device multi-layer structures in the same growth run and (ii) to easily grow high-quality AlGaN layers in the whole composition range. Moreover, HYPE technology can provide low­impurity materials with a record-high carrier mobility [12]. Additional posi­tive features of the HYPE method are a high growth rate forGaN, AlN, and AlGaN materials (from 50 to 100 times of MOCYD technology), low raw source material cost, and low capital equipment cost.

The key HYPE elements recently demonstrated for the fabrication of group-III nitride semiconductor devices are the following:

• Growth of n-type AlGaN alloy layers with AlN concentration ranging from 0 to 100 mol.% [9];

• Growth of p-type GaN and AlGaN materials; the hole mobility inp-GaN exceeds 10 cm2N·s for the hole concentration of ~1018

cm-3 (300 K) [13] ; • Growth of multi-layer submicron AlGaN/GaN and

AlGaN/AlGaN heterostructures. An AlGaN/GaN double het­erostructure displaying low-threshold stimulated emission under

18 A. Usikov eta/

optical pump at room temperature has been grown by HYPE (10,14];

• First HYPE grown structures with the EL peak wavelengths of 430-470 nm (blue spectral range) and 320 nm were demon­strated [ 10,13].

Growth of the multi-layer submicron AlGaN/GaN-based heterostructures resulted in the world first HEMT devices grown by HYPE [ 11].

Specific features of the GaN and AlGaN single layers growth on sapphire substrates and characterization are discussed in this paper. Optimization of the LED structure design is also considered. We describe device characteris­tics for the violet [15] and UV LEDs fabricated on HYPE grown AlGaN­based submicron multiplayer structures. The obtained results indicate that the HYPE technique has a significant potential for mass production of Al­GaN-based device epitaxial wafers.

2. HVPE TECHNOLOGY FOR GROWTH OF GaN AND AlGaN SINGLE LAYERS

The GaN, AlN, and AlGaN layers and multi-layers structures were grown by HYPE on c-plane 2-inch sapphire substrates in a horizontal-flow reactor. The growth was performed in a temperature range from 1000 to 1 050°C and at atmospheric pressure. Ammonia and hydrogen chloride (HCl) were used as precursors and argon served as a carrier gas. Ga and AI metals were used as material sources and were located in a source zone of HYPE growth ma­chine. ForGaN growth, HCI gas was passed over the Ga source forming gal­lium chloride that was transported into a growth zone of the machine. Reac­tion between ammonia and gallium chloride resulted into GaN growth on sapphire substrate. To grow the AlGaN alloy, HCl was passed separately over the Ga and Al sources. Variation of the HCl flow through the sources allows growing AlGaN layers with different composition. Depending on the source zone conditions and the source design, the reaction of aluminum chloride gas with quartz parts of the reactor, which potentially may result in oxygen contamination of grown layers, can be suppressed. Magnesium metal and silane gas were used for doping. The gas flow rates were controlled to obtain GaN and AlGaN growth rates in the range from 0.2 to 3.0 J.lrnlmin.

The grown structures were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), photo- and electroluminescence (PL, EL), capacitance-voltage (C-V) mercury probe, and Hall effect method. Doping profiles in the grown structures were measured by secondary ion mass spectrometry (SIMS). This method also provided us precise growth rate calibration for thin epitaxial layers.

HVPE-Grown AlN-GaN Based Structures 19

The SIMS depth profiles through a 6 J..tm-thick Mg doped GaN layer is shown in Fig. 2. Low background oxygen and carbon concentration ((2-

16 -3 16 -3 4)x10 em and less than 2x10 em for oxygen and carbon, respectively) are clearly seen. These data are also typical for both undoped and Si-doped GaN layers grown by HVPE. Background concentration of other elements such as Mn, Co, Cr, Fe, and Ni that promote deep non-radiative centers in group III-nitride materials were les than 1015 cm-3, i.e. beyond the detectabil­ity limit of the SIMS method.

1E+21 Ga->

1E+09 u .!:!. 1 E+20 1E+08 1/)

1/) c; ~ 1E+19 1E+07 ::I

0

1ii 1E+06 u c:::u

~ 1E+18 0 Gl 1E+05 ·- 1/)

0 ~!i ~ 1E+17 1E+04 Ill u ... AI-> -o-

~ 1E+16 c::: 1E+03 0

u u S 1E+15

Gl 1E+02 (/)

(.)

1E+14 1E+01

0 2 4 6

Depth ( 1-1m) 1E+21 1E+08

u 1E+07 J2 1E+20 Ga-> .!!l 1/) c::: ~ 1E+19

1E+06 ::I 0

1ii 1E+05 u c:::u

~ 1E+18 0 Gl 1E+04 ··- Cl)

0 ~(); ~ 1E+17

1E+03 Ill t; ... "0 -

~ 1E+16 c:::

1E+02 0 u u

S 1E+15 Gl

1E+01 (/) (.)

1E+14 1E+OO

0 2 4 6

Depth ( 1-1111)

Figure 2. Concentrations of the dopant (top) and unintentional impurities (bottom) obtained by SIMS in a typical HYPE-grown GaN:Mg layer. The sample surface is on the left-hand side.

The atomic Mg concentration of 2x10 19 cm-3 (Fig. 2) corresponds to the NrNo concentration up to 8x1018 cm-3 as revealed by C-V measurements. Typically, the as-grown Mg-doped material has a reasonable p-type conduc­tivity after the HYPE epitaxial run. In some samples, the annealing proce­dure resulted in an increase of the Nr N 0 concentration. The Hall effect measurement results for GaN layers with various doping are shown in Ta­ble I. Controllable Zn doping of GaN layers grown by HYPE led to semi-

20 A. Usikov et al

insulating or p-type material [16,17]. Thin (<1 J.tm) semi-insulating GaN layers were grown on silicon carbide. To evaluate the specific resistivity of GaN :Zn material, the value of the de current through GaN mesa structure was measured at a fixed bias of 10 V in the temperature range from 200 to 500 K. The value of the specific resistivity was found to be 1012 O·cm at 300 K and 109 O·cm at 500 K [16] (see Table 1). These values are the highest resistivity values published forGaN so far. For p-type GaN doped with Zn, the concentration NrND as high as 1018 cm-3 was measured.

Table I. Electrical properties ofGaN layers grown by HYPE.

Type of the material Carrier concentra- Mobility (300 K), Specific tion (300 K), cm-3 cm2 y-1 s-1 resistivity, n em

n-type undoped GaN 4x 1016 760 n-type GaN:Si 3x1018 360 p-type GaN:Mg lx10 18 10 Semi-insulating N/A N!A 1012 (300 K) GaN:Zn 109 (500 K)

Figure 3 shows the full width at half maximum (FWHM) of the X-ray rocking curve (ffi-scan, (00.2) GaN reflection) as a function of thickness and Si-doping level of the GaN layers. The structural quality improves with in­creasing GaN layer thickness and decreasing Si doping.

Figure 4 shows a relation between the c and a lattice parameters for both undoped and doped GaN layers. Experimental data agrees well with a linear approximation. It was found that the cia ratio deviates little from the average value of 1.63. This is an indication that these layers have a volume conserv­ing distortion of the unit cell and are mainly under biaxial compressive stress. Some scattering, however, is observed in the experimental data in Fig. 4. It is more pronounced for undoped GaN layers with larger c- and lower a-parameters. The data variation can be attributed to the presence of a hydrostatic stress component originated by point defects for undoped GaN layers and dopant for Si-doped GaN layers. The dashed-line in Figure 4 de­notes the c lattice constant for bulk GaN (c = 5.1864 A and a= 3.189 A) [18]. It is seen that biaxial stress can reverse its sign in heavily Si-doped GaN layers (NrND> 6x1017 cm-3). This fact can explain smaller critical thickness for crack formation in Si-doped layers.

HVPE-Grown A/N-GaN Based Structures 21

700

600 \~ u

~~ .. +

., ~ 500 ~

~ 400

~ ' + - -300

200

0 10 20 30 Layer thickness (~m)

Figure 3. Variation of the FWHM of the x-ray (00.2) reflection with GaN layer thickness for undoped (+),and Si-doped layers for two silane flows of350 seem (0) and 150 seem(+).

5.192

5.19

~ - 5.188 I.>

5.186

~ 0 ........... . . ........,..

~~ ! · -·-·-·-·- · -·- ·- ·-·- ·-·~---·-·-·-·-·r·-·

3.182 3.184 3.186 a(A)

0~ 0 ! ~ !

I

3.188 3.19

Figure 4. Relation between a and c lattice parameters measured for different GaN samples. Dotted line is a linear fit. Chain lines intersection point corresponds to the c and a parameters for bulk GaN [14]. Silane mixture flow: •- 0 seem, 0- 50 seem,+- 75sccm, • - 100 seem, o - 160 seem, !>. - 300 seem.

Optical properties of HYPE grown GaN layers were studied by photolu­minescence and showed exciton related PL peaks as narrow as 1.7 meY at 6 K. Results of optical characterization of AlGaN alloy layers grown by HYPE on sapphire and SiC substrates are presented in Fig.5. The thickness of Al­GaN layers ranged from 0.3 to 0.5 J.lm. Composition of AlGaN layers was measured by electron probe microanalysis and XRD under assumption of the Yegrd's law validity. These results proved that AlGaN alloys could be

22 A. Usikov et al

grown by HYPE in a wide composition range. As example, for UV emitter operating at 31 0 nm, AlN concentration in AlGaN light emitting region must be approximately 22 mol.% as seen in Fig. 5. In this case, AlN concentration inn-type and p-type AlGaN cladding layers should be around 30 mol.% or higher. Auger electron spectroscopy (AES) revealed uniform material com­position through the AlGaN layers depth. As measured by the mercury probe, undoped AlGaN layers had n-type conductivity with Na-NA concen­tration from 3xl017 to 1018 cm-3 for the composition range from 20 to 50 mol.% of AlN. C-V measurements of Al0.15Gao.85N layer doped with Mg and grown on p-GaN-on-sapphire template revealed p-type conductivity with the uniform NA- No concentration of2-3x1018 cm-3.

400

AlxGai -x on sapphire • ,.....

350 AlxGa 1-xN on iC • El • ..=. • i: • (.!) • • z 300 c..J

'"" •• c..J • > < • ~ 250

200 ,...."'1""',_""'1""....,........,........,......,_ ....... .,......+ 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0. 7 0.8 0.9 1.0

Ga X AIN

Figure 5. The PL and CL edge peak wavelength as a function of AIGaN alloys composition [9].

Results described above showed an ability of HYPE method to produce GaN and AlGaN materials for light emitting devices.

3. DESIGN OF AlGaN/AlGaN LIGHT EMITTERS

Modeling of the LED operation was found to be helpful in optimization of the LED heterostructure design. The simulations were carried out with the SiLENSe code implementing one-dimensional drift-diffusion model and ac­counting for the specifics of group-III nitrides, huge polarization charges at the structure interfaces and carrier recombination on threading dislocation cores as the principal nonradiative recombination channel [19]. Accounting

HVPE-Grown AlN-GaN Based Structures 23

for the latter mechanism was important for adequate prediction of the inter­nallight emission efficiency and carrier diffusion lengths.

A number of structures with a GaN or AlGaN active region sandwiched between two wide-bandgap AlGaN cladding layers has been considered to find correlation between the structure parameters and LED characteristics. The modeling results show that the concentration of holes injected into the active (light emitting) region from the p-AlGaN emitter is much lower than that of electrons for the forward biases of interest. As that is the factor even­tually controlling the LED brightness, the properties of the p-AlGaN emitter play the most important role for the LED performance. To enhance the hole injection, increase in both acceptor concentration and AlN fraction in the p­AlGaN emitter is mandatory. Modeling has also indicated that the local po­tential barrier at the n-GaN/n-AlGaN interface negatively affects the distri­bution of electron concentration in the LED structure. This barrier originates from the negative polarization charge localized on the interface and does not disappear even at high injection levels. High doping of the n-AlGaN emitter results in the lowering of the barrier due to screening of the polarization charge.

Due to both radiative and nonradiative carrier recombination, the hole concentration decays rapidly towards the n-AlGaN emitter in the active n­GaN or n-AlGaN region where the electron concentration is nearly constant. The decay results in a non-uniform distribution of the electroluminescence intensity, which is localized primarily next to the Jrn junction.

Simulations have also demonstrated that reduction of the dislocation den­sity, which controls the nonradiative carrier recombination in the Al­GaN/GaN heterostructures, is extremely important to enhance the UV LED brightness. In InGaN blue/green LEDs, the negative effect of dislocations is partly compensated by the InGaN composition fluctuations preventing the carriers from the nonradiative losses. Contrary, deep-UV light emitter does not contain InGaN alloys in the structure making it much more sensitive to dislocations. On the other hand, an active region of a much higher thickness can be grown in the GaN/ AlGaN heterostructures what results in a large emission volume. In the case of InGaN/GaN LEDs, the latter approach can­not work because it is impossible to growth high-quality thick InGaN active layers. It should be noted that the concept of a large emission volume is be­ing successfully used in AlGaAs and AllnGaP red LEDs where the active region thickness may reach a few micrometers.

The ideas discussed above were used to design first UV LED structures to be fabricated by HYPE. These structures are described in the next para­graph.

24 A. Usikov et a/

4. FABRICATION OF AIGaN-BASED LED WAFERS

Recently, we have demonstrates violet LED epistructures grown by HVPE technology [15]. The structure contained 0.2-0.4 ~m-thick GaN active re­gion sandwiched between p- and n-AlxGa1_xN wide bandgap emitters (x ~ 0.21-0.24), all grown on from 4 to 6 ~m-thick n-GaN:Si base layer (n ~ 6-8x10 18 cm-3). A 0.2 ~m-thick p-GaN contact layer doped with Mg covers the structure. The Mg doping produces hole concentration up to 2xl018 cm-3 in the contact layer. After the structures have been grown, die processing was performed and LED lamps were assembled. Figure 6a pre­sents on-wafer measured EL spectrum for AlGaN/GaN/AlGaN LED struc­ture after die processing. The peak wavelength is 415 nm. The forward

Energy (eV) 3.4 3.2 3 2.8 2.6 2.4

a

350 375 400 425 450 475 500 525

Wavelength (nm)

3.0 2.1

~ 2.5 1.8

~ /() ~ >- 1.5

0 2.0 /() 5 c /() Q) 1.2 Q; '13

i:E 1.5 /() ~ ()-Q) ()/ 0.9 a.

E ()/ 'S ::J 1.0 a. 'E ()/ 0.6 'S ro

()/ 0 ::J a 0.5

()/ b 0.3 / .

0.0 ()

0.0 0 5 10 15 20 25

Electric current (rnA)

Figure 6. Emission spectrum (a), external quantum efficiency, and output power (b) of A!GaN/GaN-based violet LED [15].

HVPE-Grown A/N-GaN Based Structures 25

voltage of the packaged diodes at 20 rnA ranged from 4.1 to 6.0 V. For the best devices, the output power at 20 rnA exceeds 1.4 mW (Fig. 6b). The ex­ternal quantum efficiency reached a value of ~2.5%. For the best of our knowledge, this is the highest external efficiency reported for an In-free LED structure emitting in the violet spectral range. The brightness of the LED lamps was up to 500 mcd.

In this work, we applied HYPE growth technology to develop UV emit­ting structures. The LED structures consisted of 4-6 f.liD-thick n-GaN:Si base layer (n ~ 6-8xl018 cm-3) grown on sapphire substrate, a 100-400 nm thick n-AlzGa1_2N:Si (z = 21-27 mol.% AlN) cladding layer, a 50-100 nm thick n-type AlYGa1_yN (y = I 0- 21 mol.% AlN) active region, and a p­AlxGa1_xN injection layer similar in thickness and composition to the n­AlzGa1_2N cladding layer. Composition of the AIGaN layers was determined

1 E+21 1E+05

S 1 E+20 Ga->

Vl (); 1E+04 ~ g1E+19 :1

0 - (,.)~

~1E+18 1E+03 c (,.) 0 Q) c · - Vl 0 ~]! ~ 1 E+17 1E+02 Ill 0 ... "0-

~ 1 E+16 c 0

(,.) 1E+01 (,.)

c Q)

o 1E+15 1/'J (.)

1E+14 1E+OO

0 2 3 4 5 Depth (l!m)

1 E+21 1E+08

S 1 E+20 Ga->

1E+07 !! (); 1E+06 c

g 1E+19 :1 <-Si 0 - 1E+05 (,.)~

.!!.1E+18 c 0 0 Q) c

1E+04 ·~ ~ 0

~ 1E+17 1E+03 {! ~ -~ 1 E+16 c

1E+02 0 (,.) (,.)

c Q)

o 1E+15 1E+01 1/'J (.)

1E+14 1E+OO

0 2 3 4 5 Depth (l!m)

Figure 7. SIMS concentration profiles for dopants (a) and unintentional impurities (b) in a typical HYPE-grown UV LED structure. The AI profile s hows location of the A IGaN layers. The sample surface is o n the left-hand side.

26 A. Usikov et al

by XRD 2 theta-omega scan. For ease of making an electrical contact to the p-side of the devices, an 80-nm thick p-GaN layer was grown to cap the structure.

The SIMS depth profiles shown in Fig. 7 demonstrate Si and Mg doping profiles in the UV LED structure. The AGaN active region sandwiched be­tween the AlGaN:Mg injection layer and AlGaN:Si cladding layer is clearly seen. The typical feature of the LED structures observed by SIMS is a low background oxygen and carbon concentration in thick GaN:Si base layers (2-4xl0 16 cm-3 and less than 2xl016 cm-3 for oxygen and carbon, respec­tively). The other features of the LED structures are abrupt impurity concen­tration profiles, and a relatively high oxygen concentration in the AlGaN layers (up to lxl019 cm-3). The carbon concentration in the AlGaN layers increases slightly up to (6-7)x1016 cm-3 .

We suggest that it is high reactivity of Al that promotes oxygen incorpo­ration in the AlGaN layers. A high oxygen content in the AlGaN layers pos­sibly contributes to electron concentration and, thus, lowers the n-AlGaN cladding layer resistivity. For p-type AlGaN layers, concentration NrND determined by C-V measurements was ~(3-6)x 1017 cm-3, i.e. compensation of acceptors by oxygen donors was not observed. This may be due to co­doping effects of Mg and 0 extensively discussed in literature [20]. Thus, oxygen incorporation as a background impurity in HYPE process could be in favor of AlGaN layers doping. We suppose this effect to promote high effi­ciency of fabricated UV LEDs.

Different compositions of the AlGaN active regions were used to control the peak emission wavelength from 300 to 350 nm, for different LED

; 6.0x10-5

$ ~ 4.0x10.5

c Q)

c - 2.0x1o·5

Wavelength (nm)

Figure 8. EL spectra of a UV LED lamp (right) and view of a chip (left). The current flow

was 5, 10, 20, 30, 50, 70, and 100 rnA.

HVPE-Grown AlN-GaN Based Structures 27

structures. Device structures emitting at the peak wavelength of 340 nm were selected for device processing and packaging. Figure 8 demonstrates UV LED chip view and the EL spectra. The chip dimension was 350 IJ.ffi x 350 !J.m. Circle pads in the center and in the left comer are p- and n-contact pads, respectively. The Ni/Au transparent contact was deposited on the top. The single sharp peak at 340 nm dominates the spectrum over the whole cur­rent range (20-100 rnA). The peak position did not shift under current injec­tion. The FWHM of this peak was about 17 nm. Additional impurity related broad peak at 370 nm is also present. In Fig. 8, it looks like a shoulder. He intensity of this peak did not exceed 5-7% of the main peak intensity. Possi­bly, it can be attributed to recombination in p-AlGaN layer.

Packaged UV lamps were characterized at the Air Force Research Lab, Wright-Patterson AFB. Detailed results of this study will be published else­where [21]. Figure 9 shows the LED's optical output power and external quantum efficiency versus the forward current. To eliminate the heating ef­fect, EL spectra were detected under pulsed de current with the pulse dura­tion of 500 ns and duty cycle of 0.5%. The packaged UV LED had a voltage drop less than 5.0 V at the forward current of 20 rnA. The optical output power of approximately 2.2 mW was obtained for the wavelength of340 nm at the pulsed injection current of 110 rnA. The output power is almost line­arly proportional to input current over the entire test range. This linearity is a reasonable indicator that heating was not a problem for the injection current levels used during testing. Over the entire current test range the external quantum efficiency was changed between 0.57 and 0.62%. A lowering emis­sion efficiency at higher input currents can be due to (i) electron leakage in p-AlGaN where nonradiative camer recombination is especially

2.

:; 200)

~ ... L ~

~ 0 c.. '"I .!:! • Q. • 0

500 • 0

0 :ll 40

• • • • • • •

60

urrent (nt \)

120

- 0.62 . ~

::;· 0.61 " .. ·;:;

5 0.6

§ = 0.59 "' "' 0 0.58

0 57 ~.--_.. _ _._ _ _,____.___.. _ __,

0 20 40 60 80 100 120

Current (mr\)

Figure 9. LED's optical output power (left) and external quantum efficiency (right) versus the forward current [21].

28 A. Usikov et al

intensive and/or (ii) laterally non-uniform carrier injection in the LED structure due to the distributed n-contact resistance. Increasing of the p­AlGaN composition or introducing special electron confinement layer can suppress electron leakage.

The obtained data are also depicted in Fig. 1. It is seen that parameters of the HYPE grown UY LEDs are comparable with the record parameters of MOCYD grown structures. It is worth mentioning that HYPE results were obtained on non-optimized structure having a UY non-transparent top p-type GaN contact layer and a n-GaN base layer substantially reducing light out­put. If GaN layers covering AlGaN layers in the UY LED structure were replaced, the output power and external efficiency of the UY LED lamp would increase by a factor of 4 to 6 [22].

5. CONCLUSIONS

UY LED structures based on group III nitride semiconductor materials were fabricated by HYPE technology. UY light emitters contain an AlGaN light emitting layer sandwiched between n-AlGaN and p-AlGaN cladding layers. Epitaxial structures were grown on 2-inch sapphire substrates. For different samples, AlN concentrations in light emitting layer and cladding layers ranged from 10 to 21 mol.% and from 21 to 27 mol.%, respectively. The peak emission wavelength was varied from 305 to 350 nm by changing Al­GaN composition in light emitting layer. The FWHM of emission peak was about 17 nm. Packaged UY LED lamps had voltage drop less than 5.0 Y at forward current of 20 rnA. At the peak wavelength of 340 nm, 2 m W optical output power was achieved for packaged lamps at pulsed injection currents of 110 rnA. The obtained results show a high potential of HYPE technology for fabrication of high power UY light emitters based on group-III nitride semiconductors.

ACKNOWLEDGEMENTS

The authors are thankful to G. Smith, T. Dang, and T.R. Nelson, Sensors Directorate, Air Force Research Laboratory, Wright-Patterson AFB for UY LED lamp characterization and D. Look, Wright-Patterson University for Hall measurements of HYPE grown materials. The authors also thank J.A. Frietas, Naval Research Laboratory and N. Shmidt, W. Lundin, and N. Kuznetsov, Ioffe Physico-Technickal Institute for fruitful discussions and material characterization. The work at TDI was partly supported by MDA.

HVPE-Grown AIN-GaN Based Structures 29

REFERENCES

1. Nakamura S., Senoh M., Nagahama S., Iwasa N., Yamada T., Matsushita T., Ki­yoku H., Sugimoto Y., Kozaki T., Umemoto H., Sano M., Chocho K. Jpn. J. Appl. Phys. 1997; 36: Ll568.

2. Akasaki 1., Amano H., Itoh K., Koide N., Manabe K. Inst. Phys. Conf. Ser. 1922; 129: 851.

3. Nagahama S., Yamamoto T., Sano M., Mukai T. Jpn. J. Appl. Phys. 2001; 40: L785-7.

4. Ikeda M., Uchida S. Phys. Stat. Sol. (a) 2002; 194: 407. 5. Moon R. L. J. Cryst. Growth 1997; 170: 1. 6. Maruska H. P., Tietjen J. J. Appl. Phys. Lett. 1969; 15: 327. 7. Kelly M. K., Vaudo R. P., Phanse V. M., Gorgens L., Ambacher 0., Stutzmann M.

Jpn. J. Appl. Phys. 1999; 38: L217. 8. Oshnima T., Eri T., Shibata M., Sunakawa H., Kobayashi K., lchihashi T., Usui A.

Jpn. J. Appl. Phys. 2003; 42: Ll. 9. Melnik Yu.V., Nikolaev A. N., Stepanov S. 1., Zubrilov A. S., Nikitina I. P., Vas­

silevski K. V., Tsvetkov D. V., Babanin A. 1., Musikhin Yu. G., Tretyakov V. V., Dmitriev V. A. Mater. Res. Soc. Symp. Proc. 1998; 482: 245.

10. Tsvetkov D., Melnik Yu., Davydov A., Shapiro A., Kovalenkov 0., Lam J. B., Song J. J., Dmitriev V. Phys. Stat. Sol. (a) 2001; 188:429.

11. Mastro M.A., Tsvetkov D., Soukhoveev V., Usikov A., Dmitriev V., Luo B., Ren F., Baik K.H., Pearton S.J. Solid State Electronics 2003; 47: I 075.

12. Look D.C., Jones R.L., Sun X.L., Brillson L.J., Ager III J.W., Park S.S., Han J.H., Molnar R.J., Maslar J.E. J.Phys.: Condens. Matter 2002; 14: 13337.

13. Niko1aev A.E., Melnik Yu.V., Kuznetsov N.I., Strelchuk A.M., Kovarsky A.P., Vassilevski K.V., Dmitriev V.A. Mater. Res. Soc. Symp. Proc. 1998; 482:251.

14. Lam B., Gainer G.H., Bidnyk S., Elgawadi A., Park G.H., Krasinski J., Song J.J., Tsvetkov D.V., Dmitriev V.A. Mater. Res. Soc. Symp. Proc. 1998: 639: G6.4.1.

15. Usikov A.S., Tsvetkov D.V., Mastro M.A., Pechnikov A.I., Soukhoveev V.A., Shapovalova Y.V., Kovalenkov O.V., Gainer G.H., Karpov S.Yu., Dmitriev V.A., O'Meara B., Gurevich S.A., Arakcheeva E.M., Zakhgeim A.L., Helava H. Phys. Stat. Sol. (a) 2003; 195: accepted for publication.

16. Kuznetsov N. et al. Appl. Phys. Lett. 1999; 75: 3138. 17. Polyakov A.Y., Govorkov A.V., Smimov N.B., Nikolaev A.E., Nikitina I.P. Solid

State Electronics 2001; 45: 249. 18. Leszczynski M., Teisseyre H., Suski T., Gregory 1., Bockowski M., Jun J.,

Porowski S., Pakula K., Baranowski J.M., Foxon C.T., Cheng T.S. Appl.Phys.Lett. 1996; 69: 73.

19. Karpov S.Yu., Makarov Yu.N. Appl. Phys. Lett. 2002: 81; 4721. 20. Korotkov R.Y., Gregie J.M., Wese1s B.W. Appl. Phys. Lett., 2001; 78: 222. 21. Smith G., Dang T., Nelson T.R., Brown J., Tsvetkov D., Usikov A., Dmitriev V.

Appl. Phys.Lett., submitted for publication. 22. Nishida T., Ban T., Kobayashi N. Technical Digest 5-th International Conference

on Nitride Semiconductors; 2003 May 25-30; Nara, Japan: p.159.

GaN-BASED LASER DIODES Device design and performance

S. EINFELDT, S. FIGGE, T. BOTTCHER, and D. HOMMEL University of Bremen, Institute of Solid State Physics, P. 0. Box 330440, 28334 Bremen, Germany

Abstract: Laser diodes have been fabricated from group-III nitride layer structures grown by metalorganic vapor phase epitaxy on c-plane sapphire substrates. The gain-guided devices emitted at a wavelength of around 400 nm. The threshold current density decreased with increasing the width of the injection stripe, which was attributed to lateral current spreading below the p-metal con­tact. Device operation was limited to pulsed current injection due to the heat­ing of the material. By measuring the light emission in dependence of the de­vice temperature, the drop in intensity during a single pulse could be converted to a rise in temperature of the active region. This experimental data was in good agreement with simulations of the heat dissipation based on solving the two-dimensional heat-conduction equation. In view of reducing the threshold current density of the device, the confinement of the optical modes guided in the structure was simulated. Coupling of modes mainly guided either in the la­ser waveguide or in the GaN buffer layer is predicted. The use of an AlxGa1_xN buffer layer or an InxGa1_xN waveguide is suggested for complete suppression of mode coupling.

Key words: group-III nitrides, laser diodes, heat dissipation, optical waveguiding

1. INTRODUCTION

Laser diodes based on group-III nitrides have recently attracted a lot of in­terest particularly due to their use in high-density optical storage systems. The performance requirements for these devices are fairly stringent. Among other things, cw operation with high output powers for thousands of hours along with a proper beam profile are needed. In order to achieve this, one has to minimize, e.g. device degradation due to heat accumulation. This can

31

M.S. Shur and A. Zukauskas (eds.), UV Solid-State Light Emitters and Detectors, 31-39. © 2004 Kluwer Academic Publishers.

32 S. Einfeldt eta!

only be achieved by carefully optimizing both the device design and the quality of the used materials.

In this paper we summarize results on the experimental characterization and theoretical simulation of laser diodes that have recently been fabricated in our lab. Although the device performance is still far from what is needed for commercial applications, the conducted investigations are helpful with regard to understand the physics behind the performance limitations. The goal is to provide roadmaps for a further optimization of the device. Here, we focus on the lateral spreading of the current injected through the contact stripes, the heating of the device due to limited heat dissipation and the cou­pling of optical modes guided in the multilayer structure.

2. EXPERIMENTAL AND THEORETICAL DETAILS

Laser diodes were grown by metalorganic vapor phase epitaxy on c-plane sapphire substrates using a 3 x 2" close-coupled showerhead reactor from Thomas Swan Inc. Standard precursors and carrier gases were employed. The heterostructure consisted of a thick GaN:Si buffer layer (2.7 f.!m) grown on a thin GaN nucleation layer, an Al0mGa0.93N:Si cladding layer (500 nm), a GaN:Si waveguide layer (100 nm), an active region with three ln0.08Gao.92N quantum wells (4 nm) separated by GaN:Si barriers (7 nm), an Alo.2Gao.sN:Mg electron blocking layer (15 nm), a GaN:Mg waveguide layer (85 nm), an Alo.o?Gao.93N:Mg cladding layer (500 nm) and a GaN:Mg contact layer (90 nm). Gain-guided laser diodes with a cavity length and an injection stripe width of 500 f.!m and 5-20 f.!m, respectively, were fabricated using standard photolithography, metallization and reactive ion etching. The smoothness of the laser facets was enhanced by wet chemical etching as de­scribed in Refs. [1,2]. One facet was coated with a quarter-wavelength di­electric mirror consisting of four pairs ofTi02 and Si02.

Heat dissipation in the devices was modeled by solving the heat­conduction equation numerically. An implicit integration scheme on a two­dimensional grid of variable spacing was applied to a cross-section area of 300 f.!m x 600 f.!m. The numbers for the thermal conductivity and the spe­cific heat were taken from the literature. Whereas the specific heat of the alloys was derived from a linear interpolation between the corresponding values for the binary constituents, a strongly nonlinear interpolation was chosen for the thermal conductivity as described in Ref. [3] to consider the effect of mass disorder.

Optical waveguiding in the laser diodes was simulated using the software WAVEGUIDE [ 4]. The complex refractive indices for a wavelength of 400

GaN-Based Laser diodes 33

run were taken from the literature. The confinement of the optical wave was determined by the fraction of the mode overlapping the quantum wells [5].

3. RESULTS AND DISCUSSION

3.1 Device Performance

A typical emission spectrum of a laser diode operated at a current 10% above threshold is shown in Fig. 1(a). The wavelength was typically around 400 run. The threshold current density decreased with increasing injection stripe width as shown in Fig. 1 (b). This effect is believed to result from lat­eral current spreading between the injection stripe and the active region. If the spreading is in the order of or even larger than the stripe width, the cur­rent density at the active region can be significantly smaller than the current density at the stripe. Assuming the threshold current density in the active region to be identical for all devices under investigation, the current spread­ing can be estimated assuming the simple model illustrated as insert of Fig. 1(b). A corresponding fit to the experimental data provided a spreading width of about 25 Jlm. It is noted that the device resistance increases with the reduction of the injection stripe width. Therefore, the device temperature at the threshold level increases for lower stripe widths. Hence, the threshold

100 duty cycle 0.06 % I

........ I l.,-·· 1.,0 ( t+2x / w) pulse wldlh 60 ns E I

I I

1- I 1 x 11 80 I

~ I X w X I E .?;- I

::J ·u; I

.e c: 60

1. w stripewKith Ql

~ "t:l x · current spread1ng .?;- E iii Ql c: 1::: 40 Ql ::J ' .S 0 ' :!2 'i-,, ..J 0 w .s=

(/) 20 X• 25 :!: 101J111 '''--+-------e -- - ~ 1.,0 · 5:t21<A i cm 2 -

398 399 400 401 402 403 00 5 10 15 20 25 30 Wavelength (nm] Stripe width (~m}

Figure 1. Performance of gain-guided laser diodes operated under pulsed current injection: (a) Emission spectrum and (b) Threshold current density in dependence of the injection stripe width. The dashed line in (b) corresponds to a fit of a model on the lateral current spreading. (Images taken from Refs. [1,2].)

34 S. Einfeldt et al

current density in the active region might be higher for smaller injection stripe widths and the calculated spreading width has to be taken as an upper limit. The spreading might be enhanced due to both the AlxGa1_xN: Mg clad­ding layer and the various interfaces where highly mobile two-dimensional carrier gases may have formed.

Device operation was limited to pulsed injection. When the duty cycle was increased above 2%, the device operation failed. This can be attributed to a significant rise in temperature as heat is accumulated in the structure rather than dissipated between two pulses.

3.2 Heat Dissipation

To quantify the heating of the device, one can make use of the drop in elec­troluminescence intensity with increasing temperature. Figure 2(a) shows this dependence measured with a laser diode operated as light emitting diode (LED). An activation energy of 50 meV was derived from the Arrhenius plot. Using this value, the temperature change during a single pulse could be determined. A low duty cycle of 0.1% was chosen for this experiment to al­low complete heat dissipation between the pulses. Figure 2(b) shows the drop in intensity during a 100 )lS pulse along with the derived increase in temperature. Heating by about 70 K was determined for the current density of5 kA/cm2.

(a)

2S

WW---~--~3~~~~~~~4~5~0~~~ 1cmpcm1un: (K)

(b)

?w~~~~w~~~~-ro~~w~~~oo~ 1irnc (I' )

Figure 2. Performance of laser diodes operated in LED mode under pulsed current injection. (a) Arrhenius plot of the average light output power. (b) Light output power during a single 100 J.IS pulse (circles) and estimated temperature evolution in the active region (squares). The dashed line corresponds to simulation results based on the solution of the heat-conduction equation. (Images taken from Ref. [3].)

GaN-Based Laser diodes 35

The temperature evolution of the active region during device operation was also extracted from simulations. Pure Joule heat was assumed to be gen­erated at the p-contact and within the p-type doped layers. Current spreading was neglected for simplicity reasons. The dashed line in Fig. 2(b) corre­sponds to the theoretical results, which are in excellent agreement with the experimental data. Figure 3 illustrates the impact of the duty cycle on the heating of the device calculated for 100 ns injection pulses. The solid and dotted lines correspond to the temperature of the active region under pulsed and cw operation, respectively, assuming the same average thermal load. When the duty cycle is low, the heating occurs mainly during the current pulse followed by a cooling due to heat dissipation. By increasing the duty cycle, the heat gets accumulated as typical heat dissipation times become larger than the period between two pulses. Consequently, the average tem­perature rises and the variation of the temperature due to pulsed operation is reduced. Note that for pulse lengths of 1 ms, the temperature raise can be several hundred Kelvins.

duty cycle (%):

100

50

·o.s ~

ci~o----~s~oo~--,~oo~o---,~~~0~~2~~~

time (ns)

Figure 3. Temperature evolution of the active region in the laser diodes for various duty cy­cles as predicted by theory. The solid and dotted lines correspond to pulsed and cw operation, respectively, assuming the same average thermal load. (Image taken from Ref. [3].)

3.3 Optical Waveguiding

The threshold current density of a laser diode is related to the confinement of the optical mode in the waveguide structure. Therefore, the optical modes were simulated for various laser designs in search of layer structures with improved device performance. It has been found that the several microns

36 S. Einfeldt eta!

thick GaN buffer layer can act as a parasitic waveguide as it is sandwiched between materials of lower refractive index, namely sapphire and AlxGaJ_xN [5]. This so-called "buffer waveguide" can couple to the waveguide embed­ded between the two cladding layers such that a significant part of the mode is guided in the buffer layer [6,7]. The coupling is strongest when the effec­tive refractive indices of the two waveguides coincide. These resonance points can be observed as periodic dips in plots of the optical confinement factor versus the buffer layer thickness. Besides a lack in confinement, the coupling results in a far-field pattern with multiple peaks, which are obvi­ously undesired for laser applications.

(a) 4

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(b) 20 25 30 GaN buffer layer th•ckness [Jlm)

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.·.: · .. · . .......... ./ \. ... ~~.~ ~

0 Angle [deg)

Figure 4. (a,b) Confinement factor of the most strongly confined mode in dependence of the GaN buffer layer thickness. The aluminum mole fraction and the thickness of the AlxGa1_xN cladding layers was varied from the parameters mentioned in section 2 as specified. (c,d) Far field distribution of the laser diodes for buffer layer thicknesses with maximum confinement factors in (a) and (b), respectively. (Images taken from Ref. [5].)

Figure 4 summarizes results of simulations of laser diodes containing dif­ferent AlxGa1_xN cladding layers. The confinement factor increases when the aluminum mole fraction of the cladding layers increases, as shown in Fig. 4(a). However, the resonances remain. They become only narrower when the thickness of the cladding layers is increased, as shown in Fig. 4(b ). In prac­tice, the range of possible variations of the cladding layer composition and thickness is limited by the risk of cracking of the tensile stressed AlxGa1_xN. Therefore, Fig. 4 suggests that merely varying the layout of the cladding lay­ers is not sufficient for suppressing mode coupling between the buffer and laser waveguides. The far field pattern of all structures under consideration is not gaussian-like as illustrated by Figs. 4( c) and (d).

GaN-Based Laser diodes

(a) tzv:X1=== 3 .· ·•······ · ··········· ... ; '\./··· ·····•· .• / ....

c ~ "' .5 "E 8

(b)

2

~0 25 30 Buffer layer thickness [pm]

" .. .... --· 005 •..... 003 - o

x .. • -- 003 ...... 002 - o

/\ FWHM·

/ \

(c)

__./:\.!;~

. ....... .... ../ vv \ ....... ~.~~ ~

·50 0 Angle [deg]

37

Figure 5. (a,b) Confinement factor in dependence of the buffer layer thickness. The aluminum mole fraction of the AlxGa1_xN buffer layer and the indium mole fraction of the lnxGa1_xN waveguide was varied as specified. (c,d) Far field distribution of the laser diodes for buffer layers thicknesses with maximum confinement factors in (a) and (b), respectively. (Images taken from Ref. [5].)

The use of AlxGat-xN and lnxGa1-xN instead of GaN for the buffer layer and the waveguide, respectively, are promising concepts to suppress mode coupling completely. Whereas in the first case the confinement in the buffer layer is reduced, the confinement in the laser waveguide is enhanced in the second case. Figures 5(a) and (b) suggest that minimum mole fractions of 0.05 and 0.03 are required for the aluminum in the AlxGa1_xN buffer layer and the indium in the lnxGa1_xN waveguide, respectively. The corresponding far-field patterns, which are shown in Fig. 5(c) and (d), become gaussian­like under these conditions. It is noted that, recently, the successful fabrica­tion and improved performance of laser diodes containing an AlxGai-xN buffer layer [8] or an lnxGa1_xN waveguide [9,10] have been reported.

4. SUMMARY

The performance of gain-guided laser diodes based on group-III nitride lay­ers grown by metalorganic vapor phase epitaxy on c-plane sapphire sub­strates was investigated. Lateral current spreading is proposed to reduce the threshold current density with increasing the width of the injection stripe. A fit of a simple model to the experimental data provided a spreading width of about 25 J.lm. All devices could only be operated in pulsed mode with duty cycles of up to 2%. This can be attributed to heat accumulation in the mate-

38 S. Einfeldt et al

rial due to insufficient heat dissipation between the pulses. The light emis­sion of laser diodes operated in LED mode was measured in dependence of the device temperature. This data was used to relate the drop in intensity dur­ing a single pulse to a rise in temperature of the active region. The tempera­ture distribution in the device was calculated by solving the two-dimensional heat-conduction equation. Excellent agreement with the experimental data was obtained. To optimize the device design with regard to the confinement of the guided optical modes, simulations of various structure layouts were performed. The results could be interpreted by the coupling of two waveguides which are formed by the GaN waveguide between the cladding layers on one hand and the GaN buffer layer on the sapphire substrate on the other hand. Complete suppression of mode coupling can be obtained by the use of an AlxGa1_xN buffer layer or an lnxGa1_xN waveguide.

ACKNOWLEDGEMENT

The authors gratefully acknowledge Wiley VCH for their permission to re­produce material from publications in Physica Status Solidi. This work was supported by the Deutsche F orschungsgemeinschaft.

REFERENCES

1. T. Bottcher, Ch. Zellweger, S. Figge, R. Kroger, Ch. Petter, H.-J. Biihlmann, M. Ilegems, P. L. Ryder, and D. Hommel, phys. stat. sol. (a) 191, R3 (2002).

2. T. Bottcher, S. Figge, S. Einfeldt, R. Chierchia, R. Kroger, Ch. Petter, Ch. Zell­weger, H.-J. Biihlmann, M. Diel3elberg, D. Rudloff, J. Christen, H. Heinke, P. L. Ryder, M. Ilegems, and D. Hommel, accepted for publication in phys. stat. sol. (c).

3. S. Figge, T. Bottcher, D. Hommel, Ch. Zellweger, and M. Ilegems, accepted for publication in phys. stat. sol. (a)

4. http://www.edunotes.org/notes/eng/electricaVoptics/lasers/waveguide/software.html 5. S. Einfeldt, S. Figge, T. Bottcher, and D. Hommel, accepted for publication in phys.

stat. sol. (a). 6. D. Hofstetter, D.P. Bour, R. L. Thronton, and N. M. Johnson, Appl. Phys. Lett. 70,

1650 (1997). 7. G. Hatakoshi, M. Onomura, S. Saito, K. Sasanuma, and K. Itaya, Jpn. J. Appl. Phys.

38, 1780 (1999). 8. P. G. Eliseev, G. A. Smolyakov, and M. Osinski, IEEE J. Select. Topics Quantum

Electron. 5, 771 (1999). 9. T. Takeuchi, T. Detchprohm, M. Iwaya, N. Hayashi, K. Isomura, K. Kimura, M.

Yamaguchi, H. Amano, I. Akasaki, Yw. Kaneko, amd N. Yamada, Appl. Phys. Lett. 75, 2960 (1999).

10. M. Koike, S. Yamasaki, S. Nagai, Y. Tezen, S. Iwayama, A. Kojima, T. Hiramatsu, T. Umezaki, M. Itoh, H. Yamashita, M. Ohashi, A. Kimura, M. Sato, and K. Ohgu­chi, Proc. Int. Workshop on Nitride Semiconductors, lAP Conf. Ser. 1, 886 (2000).

GaN-Based Laser diodes 39

11. T. Asano, M. Takeya, T. Tojyo, T. Mizuno, S. Ikeda, K. Shibuya, T. Hino, S. Uchida, and M. Ikeda, Appl. Phys. Lett. 80, 3497 (2002).

QUATERNARY AllnGaN MATERIALS SYSTEM FOR UV OPTOELECTRONICS.

E. KUOKSTIS 1, G. TAMULAITIS 2, and M. ASIF KHAN 1

1 University of South Carolina, USA. 2 Vilnius University, Lithuania

Abstract: Quaternary AllnGaN materials represent a novel class of semiconductor com­pounds, which can be used in UV optoelectronics demonstrating a number of advantages. After brief evaluation of peculiarities of structure and growth spe­cifics of AllnGaN layers and multilayered structures, we review the results on investigation and analysis of the radiative recombination dynamics of MOCVD-grown AllnGaN-based layers and multiple quantum wells (MQWs) in a wide temperature range and under different excitation conditions. The dis­cussion is focused on carrier localization and the influence of indium on mo­bility and recombination of photoexcited carriers. Recent results on study of indium spatial distribution and simulation of exciton hopping in AllnGaN are summarized. Additional influence of built-in electrostatic fields due to sponta­neous and piezoelectric effects in MQWs on the photoluminescence dynamics is also addressed. Different mechanisms of optical transitions in AllnGaN sys­tems are analyzed. The prospects for practical application of these quaternary materials for UV emitters are discussed.

Key words: III-nitrides, AllnGaN, wide-band-gap semiconductors, quaternary compounds, exciton localization, built-in electric field, heterostructures

1. INTRODUCTION

Within the decade after the first report on growth of AllnGaN epitaxial lay­ers [1], the fabrication technology of this quaternary nitride compound was considerably improved, usually using metalorganic chemical vapor deposi­tion (MOCVD) technique [2] and especially its modifications [3,4]. Side by side with ternary AlGaN, the quaternary AllnGaN is considered as a promis­ing wide-band-gap material for light emitters in the UV region, as well as in other electronic and optoelectronic applications. In particular, ultraviolet

41

M.S. Shur and A. Zukauskas ( eds.), UV Solid-State Light Emitters and Detectors, 41-58. © 2004 Kluwer Academic Publishers.

42 E. Kuokstis, G. Tamulaitis and M AsifKhan

LEDs emitting at 305-340 nm [5-7] have been fabricated by using quater­nary AllnGaN multiple quantum wells, and a laser diode with AllnGaN ac­tive layer emitting at the wavelength as short as 366.4 nm was demonstrated [8].

1.1 Band Gap Engineering

First of all, the quaternary AllnGaN compounds have been introduced to improve lattice matching of the layers with substrate and between the adja­cent layers in quantum wells and so to inhibit formation of the strain-induced defects in multilayered wide-band-gap epitaxial structures [9-11]. The pos-

~ 0.1

-0.4

>2 Cl)

- 1 a; ~ 0

"0 ·1 c ~ ·2

-3

loGaN (a)

Molar fraction -

AIGaN

Figure 1. Lattice mismatch (a) and band offset (b) between GaN and ternary AIGaN and InGaN com­pounds as a function of AI and In molar fraction, respectively.

sibility of lattice matching of AllnGaN with GaN is illustrated in Fig. 1. The lat­tice in quaternary AllnGaN with Alto In content ratio approximately equal to 1 :5 is matched with the GaN lattice. Note that the band offset for this pare of materials turned out to be lower after the recent up­date of the band gap value of InN [12-14 ], however, it is still high enough to form a quantum well structure, as it is il­lustrated m a linear approximation m Fig. lb.

Often, the lattice engineering in AllnGaN is aimed not at exact lattice matching of the two adjacent materials, but rather at elimination of the built-in electric field, what is possible when the field due to spontaneous polarization is compensated by the piezoelectric field of the opposite direction engineered by an appropriate lattice mismatch.

1.2 New Approaches in Growing of AllnGaN Compounds

Until recently, the vast majority of nitride-based devices were fabricated us­ing MOCVD technique since it ensures optimum temperatures and growth rates for nitrides in comparison with other methods. However, for conven­tional MOCVD (using continuous flows of precursor gases) the optimal

Quaternary AllnGaN Materials Systems 43

growth temperatures for In- and At-containing materials are quite different. On the other hand, it is challenging to grow high-quality and high-At-content compounds using this conventional approach. Figure 2 illustrates typical PL spectra dependence on AI content in conventional MOCVD AlGaN com­pound layers using ArF excimer laser as excitation source (..1. == 193 nm). It can be seen that the PL rapidly degrades with an increase of AI mole frac­tion. It was noticed that introduction of In for quaternary alloys improves the situation.

....J Cl..

280 300 320 340

wavelength (nm) 360 380

Figure 2. Typical PL spectra of conventional MOCVD AIGaN epitaxial 1-~Jm-thick layers with different AI content.

One of the alternative novel approaches using a pulsed regime of gas flow in MOCVD scheme was developed in the USC [3,4]. This technique enables reduction of the growth temperature and thereby significantly enhances the In incorporation in high At-content quaternary AllnGaN alloy with improved quality of the layers. This method was successfully ap­plied for growing Alln-GaN/AllnGaN MQWs (see be­low). For example, the sequential introduction of precursor pulses of AI-, In-, Ga-, and NH3 was applied in the PALE technique [3] along

with continuous ammonia flow in PMOCVD approach [4]. The time regime for unit growth-cell in PALE case is illustrated in Fig. 3. Actually, the pulse

__I1______jl___ TMA

LilfLJUU NH,

_ll_IL_TMI

n_TMG

10 20 30 40 50 60 70

Time (seconds)

Figure 3. An example of PALE time­pulse-diagram for unit growth-cell. The number of repeats of AI, In, and Ga pulses in the unit growth-cell are 2, 2, and 1, respectively, and this is marked as (2,2, 1 ).

260 280 300 320 ~40 360 wavelength {nm)

380

Figure 4. PL spectra of PALE AllnGaN 0.1 IJm-thick layers with the marked unit growth-cells. "+" indicates doubling of the flux for In precursor pulse.

44 E. Kuokstis, G. Tamulaitis and M As if Khan

regime ensures deposition of GaN/AlN/InN short-period superlattices with composition control at atomic/molecular level. The X-ray diffraction, atomic force microscopy, and PL results revealed essential improvement of struc­tural and optical quality of the quaternary compounds and quantum struc­tures. The PL results for AllnGaN PALE layers are illustrated in Fig. 4 indi­cating much stronger PL signals for PALE layers up to 290 nm.

1.3 Improvement of Structural Quality of AIGaN by In-doping

The incorporation of In into AlGaN requires reduced growth temperature in comparison with the growth of high crystalline quality AlGaN. The growth at lower temperatures may lead to disordering of the AlGaN sub lattice and, thus, to the degradation of the materials quality. On the other hand, indium is known to improve the crystal structure of III-N layers via isoelectronic

3.500 3.600 3.700 3.800

Photon Energy (eV)

Figure 5. Room-temperature luminescence spectra of 200 nm thick epitaxial layers of AlGaN (a) and Al0.09InooJG<lo.9oN (b). Ar­rows indicate the incident photon energy for off-resonant excitation (open points) and resonant excitation (solid points). Solid lines, Gaussian approximation.

Excitation Energy (eV)

Figure 6. Incident photon energy depend­ence of photoluminescence peak position in AlGaN (squares) and AllnGaN with 1% (tri­angles) and 2% (dots) of In. Arrows indicate the exciton positions.

doping [15,16). The recent study of the optical properties of AlGaN indicates such improvement [17]. We investigated reflectivity, site-selectively excited photoluminescence (SSEPL ), photoluminescence excitation (PLE), and time-resolved luminescence (TRL) of 200-nm thick Al0•091nyGa0,9 1-yN epi-

Quaternary AllnGaN Materials Systems 45

taxial layers with different In content. The samples were grown over a basal plane sapphire substrate by using a low pressure metalorganic chemical va­por deposition. Typical spectra measured at resonant excitation to exciton energy and with excess energy of 100 meV are presented in Fig. 5. These SSEPL spectra were measured by using a tunable dye laser pumped by the fourth harmonic of a Q-switched Y AG:Nd laser radiation (10 ns pulse dura­tion). The dependence of the peak position of the PL spectra on the photon energy of excitation is presented in Fig. 6. In quaternary AllnGaN the PL band blueshifts until the exciton energy, and the shift saturates with further increase of the excitation photon energy. In contrary, the PL band in AlGaN continues to shift to higher energies even above the exciton energy. Such a behavior is peculiar to a disordered system. Thus, the incorporation of In decreases the disorder.

Improvement of crystal quality is also demonstrated by PLE spectra. Since the PL line shifts with changing excitation photon energy, spectrally integrated PL intensity was estimated from the measured spectra and was plotted against the incident photon energy (see Fig. 7). The PLE spectrum of the buffer GaN layer is also shown in Fig. 7 for comparison.

-~ c: :::s

-e ro .._.,

3.400 3.600 3.800

Excitation Energy (eV)

Figure 7. Dependence of spectrally integrated luminescence intensity on incident photon energy in AlGaN (squares) and AllnGaN with 1% (triangles) and 2% (dots) ofln.and GaN buffer layer (diamonds). Arrows indicate the exciton energies.

Two features progressively appear in the PLE spectra with increasing In content. First, the Urbach tail below the exciton energy becomes steeper, what indicates reduction of the band-tail states. Second, the range between the exciton energy and the saturation energy in the PLE spectrum becomes smaller, what is an indication of increasing diffusion length of the photoex-

46 E. Kuokstis, G. Tamulaitis and M As if Khan

cited carriers. When the absorption length for small photon energies of the incident light is larger than the carrier diffusion length, an increase in the photon energy leads to a stronger light absorption, which results in a higher carrier density. This, in tum, yields higher photoluminescence intensity. The saturation of PL intensity occurs when the absorption length becomes com­parable to or less than the diffusion length. The saturation in AllnGaN oc­curs with increasing In content closer to the exciton energy, i.e. at longer absorption lengths. Thus, the diffusion length is enhanced with increasing In content. The increase in the carrier diffusivity is probably associated with smoothing of the energy band potential profile. The improvement in the crystal quality of the quaternary AllnGaN with increase of In content was supported by our results of time-resolved experiments. The results are also consistent with X -ray diffraction study [ 18] revealing that In-doping im­proves the crystalline properties of AlGaN quantum wells, and with PL mapping for linewidth and peak wavelength [19], which showed that incor­poration of In into AlGaN improves uniformity of the PL.

Indium-silicon co-doping is shown to increase n-doping of high­aluminum-content AlGaN. This doping technique was used to fabricate so­lar-blind transparent Schottky barrier photodetectors [20].

2. LOCALIZATION EFFECTS IN AllnGaN

2.1 Photoluminescence Features Reflecting Exciton Localization in AllnGaN

It is well known that composition of InGaN is inhomogeneous. Accumula­tion of nonequilibrium carriers in the In-rich regions with lower band gap is probably favorable in light emitting devices based on InGaN, though size, In content and other characteristics of the In-rich regions and relationship of these properties to growth conditions are not completely understood up to now. The epitaxy of quaternary AllnGaN is even more complicated. Typical growth temperature for At-containing layers is above 1000 °C. However, indium has a relatively high vapor pressure, and In-N bond energy (1.93 eV in InN) is considerably smaller than Al-N bond energy (2.88 eV in AIN). Thus the temperature for incorporation of indium must be lower (typically <800 °C). It is highly probable that indium distribution is inhomogeneous also in AllnGaN. Existence of In-rich regions in AllnGaN was revealed by spatially resolved cathodoluminescence, X-ray energy dispersive micro­analysis, and high-resolution transmission electron microscopy [21-23]. Lo­calization of carriers in these regions as well as in smaller potential fluctua­tions due to inhomogeneous In distribution may be partially characterized by

Quaternary AllnGaN Materials Systems 47

using photoluminescence properties. In particular, an S-shaped temperature dependence of the luminescence band peak was previously observed in In­containing alloys InGaN [24,25]. This feature is a manifestation of phonon­assisted motion of localized excitons (hopping) [26-31]. The initial redshift of the luminescence band can be attributed to redistribution of nonthermal-

AIGaN T, K

10' (a) 8 ~ 70 90

103 110 130 160

~ 190

'2 10' 220 :> 250

.e 3()0

~ 10' z. 'iii T, K <: 2 6 E 60 ...J 70 a.. 90

110 13() 1&1 190 220 2~

300

10'.......,"'"""""""-.......J.~........._.........,..., -0.20·0.15 -0.10·0.05 0.00 0.05 0.10

Energy E-E0 (eV)

Figure 8. Photoluminescence spectra of AIGaN (a) and AllnGaN with 2% of In (b) at various temeratures (indicated) shown in photon energy relative to the exciton energy. Solid points indicate the peaks of the PL spectra as a guide for the eye. Vertical dashed line indicates the exciton energy £ 0 .

ized excitons into lower-energy states via hopping. Increased tem­peratures enable excitons to achieve thermal balance with the lattice and to occupy higher energy states. As a result, the peak position of the lumi­nescence band is blueshifted. The S­shaped peak shift was also observed in AllnGaN [22,32]. We studied how the feature evolves with incorpora­tion of In [33,34]. The evolution of the luminescence spectra of AlGaN and AIInGaN (2% In content) with increasing temperature is illustrated in Fig. 8. The spectra were measured at relatively low excitation power density (7 kW/cm2) under quasi­steady state excitation by 10 ns long pulses of the fourth harmonic of the Y AG:Nd laser radiation. The energy scale for the spectra is plotted rela­tive to the exciton position to reveal the red shift (the Stokes shift) of the luminescence band.

In AlGaN, the Stokes shift gradu­ally decreases with temperature, whereas in AllnGaN the Stokes shift

has an S-shaped temperature dependence, and this feature is enhanced by increasing In content as illustrated by solid points in Fig. 9 (a-c) for three samples (with 0%, I%, and 2% of In, respectively). Meanwhile, a monoto­nous increase of the bandwidth with temperature in ternary AlGaN gradually transforms with incorporation of indium to a W -shaped dependence (de­crease-increase-decrease-increase) in quaternary AllnGaN [see Fig. 9 (d-f)].

48 E. Kuokstis, G. Tamulaitis and M As if Khan

~ 0.00

c: -0.01 0 ~ -002 0 a.. -0.03 .:£

m -o o4 a.. ~ 0.10 > ~ 0.08 .c '6 "§: 0.06

" ~ 0.04 m ~~~~~~~~~~~~~~~~~~

0 100 200 0 100 200 0 100 200 300

Temperature (K)

Figure 9. Stokes shift of the peak position (a)- (c) and full width at half maximum (d)- (f) of photoluminescence band as a function of temperature for AIGaN (a and d), and AllnGaN epilayers with I% (b and e), and 2% ( c and f) of In at the excitation power density of 7 kW/cm2 (solid points) and 0.91 MW/cm2 (open points), respectively.

0.00

~ -001

¢::

155 -0 02

"' Q)

-"' a -0 03

-0.04

0.01 0.1

Excitation Power Density (MW/cm2)

Figure 10. Stokes shift of the photolumi­nescence peak in respect to the exciton energy as a function of the excitation power density at different temperatures (indicated) for AIGaN (open points) and AllnGaN with 2% (solid points).

With increased excitation power density, the non-monotonous tem­perature behavior of the photolumi­nescence characteristics disappears (open points in Fig.9). Moreover, as presented in Fig. 10, the character of the band shift with increasing excitation power density strongly depends on temperature and is different in AlGaN and AllnGaN. The band shift is probably governed mainly by two effects of an oppo­site sign: i) band filling effect shift­ing the band to high energy side, and ii) band gap renormalization due to many-body effects, which redshifts the PL band. At low tem­peratures, the band filling effect has the prevailing role for both AlGaN and AllnGaN, while the importance

of the band gap renormalization becomes more evident with increasing tem­perature. The renormalization-caused redshift of the PL band is more pro­nounced in AllnGaN. This is an indication of the higher carrier density than that in AlGaN at the same pump intensity. The difference might be caused

Quaternary AllnGaN Materials Systems 49

by different contribution of nonradiative recombination as well as by spatial redistribution of carriers to form regions with higher density. To reveal pecu­liarities of the carrier movement we performed simulation of the process by using the Monte Carlo technique.

2.2 Simulation of Exciton Hopping: Double-Scaled Potential Profile Model

The Monte Carlo simulation procedure of the phonon-assisted exciton hop­ping over the localized states distributed randomly in space was similar to that described in [30,35]. The hopping probability from a localized state ito any other state j separated by the distance ru was defined by Miller­Abrahams expression

(1)

where E; and E1 denote energies of initial and final states, respectively, v0 is the attempt-to-escape frequency, and a is the characteristic decay length of the exciton wave function. Taking into account all the calculated hopping probabilities as well as the probability of radiative recombination, either hopping to another state or radiative annihilation from a current state was randomly selected. The energies of the localized states associated with the event of radiative annihilation were used to compose the emission spectra. Dashed lines in Fig. 11 depict the best fit of the experimental data with the results of a straightforward Monte Carlo simulation. Although the simulated temperature dependence of the Stokes shift quantitatively agrees with the experimental one (Fig. lla), the simulated FWHM appears to be considera­bly narrower than that obtained by measurements (Fig. 11 b).

To achieve a quantitative description of the temperature dependence of the Stokes shift and FWHM simultaneously, we propose a model of a dou­ble-scaled potential profile, which is schematically illustrated in Fig. 12. The model takes into account formation of In-rich clusters due to composition fluctuations in the AllnGaN alloy. At low temperatures, the excitons move via hopping over chaotic potential fluctuations with the roughness described by dispersion (J mainly within the In-rich cluster. Thus, the PL band is a re­sult of a convolution of the contributions from different In-rich regions, which have different average energies for excitons due to possibly different

50 E. Kuokstis, G. Tamulaitis and M As if Khan

0.08 (b)

0.07

> 0.06 D D

D .!£. 0.05 ~ I 0.04 ---------------:s: LL 0.03

0.02 --r=42meV

·- r=O 0.01

0 50 100 150 200 250 300

Temperature (K)

Figure II. Stokes shift and line width as a function of temperature in AllnGaN. Points, experimental data. Lines, simulated de­pendencies. Solid curves were obtained by using Gaussian convolution with r = 42 meV. Dashed curve, line width for a straightforward simulation. A fit of the broadened simulated spectrum (solid line) to the measured PL spectrum (points) at 8 K is illustrated in the inset.

Figure 12. Double-scaled potential profile in AllnGaN compound due to inhomoge­neous In distribution

In content. The inhomogeneous broadening of the PL band was taken into account by convoluting the spectra, Sa(v'), obtained di­rectly from the Monte Carlo simu­lation with a Gaussian-like curve, G( 0, with dispersion parameter r reflecting the distribution of the average exciton energies in differ­ent clusters:

S(v)= fs0 (v')G(v-v')dv'. (2)

This model gives a quantitative fit of the Monte Carlo simulations with the experiment for tempera­ture dependence of both peak po­sition and bandwidth. The second redshift in the W -shaped tem­perature dependence of the band­width can be interpreted in the frames of this model as caused by ability of excitons to redistribute between different In-rich clusters This shift takes place at elevated temperatures and results in pre­dominant occupation of clusters with the lowest potential. This re­distribution causes narrowing of the PL band and increases carrier density in the preferred clusters. Probably, the increased carrier density enhances the band gap re­normalization in certain regions of the InAlGaN compound, what is reflected in pronounced redshift of the PL band with increasing the power density of the photoexcita­tion. Thus, our results confirm the formation of In-rich clusters in AllnGaN. In AllnGaN with 1% of In, the best fit was obtained for the dispersion of the potential fluctua­tions within the clusters

Quaternary AllnGaN Materials Systems 51

a= 0.016 eV and the distribution m average localization energy r = 0.042 eV, respectively.

3. POLARIZATION-INDUCED UV EMISSION PECULIARITIES IN HIGHLY EXCITED AllnGaN MULTIPLE QUANTUM WELLS

Nitride-based epilayers and quantum structures are basic elements for opto­electronic applications and usually are grown on the basal plane of either sapphire or 6H-SiC resulting in deposition of wurtzite material with (0001 )­plane (c-plane) orientation [36,37]. In this case, due to the singular polar axis parallel to the growth direction and noncentrosymmetric nature of the c­plane oriented films huge built-in electrostatic fields appear as a result of spontaneous and piezoelectric polarization [38]. Those fields are responsible for quantum-confined Stark effect (QCSE) [39--41], which usually is unde­sirable for light-emitting devices. The QCSE in MQWs is responsible for spatial separation of electrons and holes that leads to a reduced oscillation strength [ 42] and lower quantum efficiency of radiative transitions. Besides,

....J Q_

j AllnGaN MOWs l(\1 0 K

p~ ~!\L

~ ~~0.0035 0.0030

.:::;:://\ ~ 0. 0 16

3.25 3.50 3.75 4.00

Photon Energy (eV)

Figure 13. PL spectra of Al022Ino.o2Gao 76N/

Al038In0.otGao.61N MQWs at low temperature (10 K) under different excimer laser excita­tion (A= 193 nm) power densities.

QCSE causes an undesirable red shift of the emission spectra in MQW-structures designed for UV region [42,43]. Due to an inde­pendent tunability of band gap and lattice constant the quaternary AllnGaN alloys provide an excel­lent vehicle for band engineering and the investigation of strain and built-in electrostatic field effects in quantum structures. From this point of view, the PL dynamics analysis can provide simply ac­cessible and reliable information about phenomena and parameters, which are important in device fab­rication. Figure 13 illustrates the excitation intensity-induced trans­formation of the PL spectra at 10 Kin AllnGaN MQWs, which were grown by PALE technique. The long-wavelength band, which dominates at low excitation ( <3

52 E. Kuokstis, G. Tamulaitis and M As if Khan

kW/cm2) and low temperature, can be ascribed to complexes of bound exci­tons [44]. The most important PL feature is an excitation-induced blue shift of the PL band under higher pumping levels. This peculiarity of the MQWs PL spectra was observed in a wide temperature range. Meanwhile, the bulk epilayers with the same AllnGaN composition didn't exhibit any significant shift with excitation. This is illustrated in Fig. 14, which shows the PL maximum position as a function of excitation power density at low (a) and room (b) temperatures for MQWs and bulk layers.

3.90

5:'3.85 .!.

.. AAoGoNbu~ I 3 .86t;=:=:!:===:::::::;:~....._~'"":-....... l

I AllnGaN MOWs I ~

5 3.80 ., ~ 3.75 "--§ 3.70 E -~ 3.65 ::;: ...J 3.60 "--

.

2

3

10K

o AJJnGaN MOWs

.......... ... • • • •• ..

JCliXl

D

0 tl .......... ......

(a)

3.55 -l..,--~...------.......---~..-----........---~.,......-........,.. ........ 10... 1x104 b:10""' 10..3 10-: 10·' 10°

Excitation Power Density (MW/cm')

> 3.84 .. ~ 3.82 .g -~ 3.80 2 "-- ~ E E 3.78 I'. 1 0 MV/cm I ·;;: 2 - 1 2 MVIcm ~ 3.76 3 - 1.5 MV/cm ...J "-- 3.74

(b) 3

300K

10_. 1x1o·• 1x10" 10'' 10-2 10·• 10°

Excitation Power Density (MW/cm2)

Figure 14. Position of the experimental PL spectra maximum as a function of excimer laser power density in bulk Alo_22In0_o2Gao_76 (triangles) and Al0 _22In0_02Ga0_76N/Al038In0_otGao 6 1N MQWs (other types of points) at (a) 10 K and (b) 300 K. Black squares in (a) correspond to the band of bound exciton complexes. The curves 1- 3 are theoretical dependencies for different built-in electrostatic field strength F0•

The observed PL dynamics can be explained taking into account QCSE and field screening by injected carriers. Assuming that radiative recombina­tion corresponds to interband optical transitions between the first quantum well electron levels, the PL peak position can be evaluated as [45,46]

h = E _ d'F( ) ( -J / 3 - t/3 !__ 9trF(n) { 2 Jl/3 [ ]2/3 v !( e n + me + mh 2 8 (3)

Here Eg is the band gap, dis the well width, and F(n) is the resulting carrier­density-dependant (due to screening) electrostatic field in a well. The de­pendence can be approximately expressed as F(n) zF0 - nedlcc:o with the field strength in unexcited sample F0.

The calculated dependencies of position of optical transitions with re­spect to Eg are demonstrated in Fig. 15. Interestingly, only slim spectral shift can be observed in wells with thickness less than ~4 nm even for very strong

Quaternary AllnGaN Materials Systems 53

built-in fields, whereas field screening starts at carrier densities exceeding ~1018 cm-3•

200

100

~ 0 s LLJ':100 :.

-<: -200

~ m

'

·-.-. ---. n-/5 nm

(a) ·-v·-... _ 6nm

0.5 1.0 1.5 2.0

Electrostatic field strength F0 (MV/cm)

50 F0 = 0.3 MV/cm

0 0.6 ~ s -50

~rn -100 :.

0.8 1.0

1.4 -<: -150 1.6 (b)

-200

1~ 1~ 1~ 1~

Carrier density (cm-3)

Figure 15. Calculated difference between energy of optical transitions in quantum wells h v and bulk material band gapE~ as a function of (a) built-in electrostatic field strength and (b) injected carrier density for typical high-barrier AllnGaN MQWs. In (a), solid and dashed lines correspond to MQWs with carrier density of 10 16 and 8xl018 cm-3, respec­tively, and different well thicknesses. In (b), curves correspond to 5-nm-thick wells and different built-in electrostatic fields.

The same model can be applied in order to explain experimentally ob­served excitation-induced spectra shifts. The theoretical curves 1-3 in Fig. 14 are obtained assuming the bimolecular recombination as a dominant channel for band-to-band transitions and using corresponding material pa­rameters [ 4 7]. As can be seen, quite satisfactory agreement is achieved with experimental data for F0 = 1.2 MV/cm.

4. AllnGaN-BASED UV EMITTERS

The advantages of In-incorporation in the ternary AlGaN layers for UV light emitting devices have been demonstrated in [4,6,48-54]. Khan et al. [6] and Hirayama et al. [50] have reported UV LEDs with AllnGaN/AllnGaN MQWs as an active medium in ~300 nm region. Nagahama et al. [5] have also reported lasing at 366 nm with AllnGaN MQW active layer. We illustrate application of quaternary AllnGaN compounds in emitters us­ing an example of UV LED. The active medium of the LED was PALE­grown AllnGaN/AllnGaN MQWs with peak emission wavelengths tuned in the region from 340 nm to 305 nm by varying the alloy composition. Fig­ure 4 shows the room temperature emission spectra for the LEDs with three different MQW configurations, which included 10 x 2-nm-thick wells and 11 x 2.5-nm thick barriers. Those MQWs correspond to three PALE regimes

54 E. Kuokstis, G. Tamulaitis and M As if Khan

in an unit growth-cell: (1) (2,2,1)/(3,3,1), (2) (2,2+,1)/(3,3\1), and (3) (1,3,1)/(1+,T,1). Other technical details of the devices can be found else­where [2]. As seen in Fig. 16, LED electroluminescence (EL) is peaked at 305 nm, 320 nm, and 340 nm with good agreement between the PL and EL spectra. The output power of LEDs could reach up to 1 m W at ~ 1000 rnA in pulsed regime. These data demonstrate the viability of quaternary AllnGaN compounds and the PALE approach for tuning the LED emission wave­lengths by simply adjusting the number of precursor pulses in the unit growth-cell.

::J ro >; .... ·u; c Q) .... c

"'C Q)

.~ ro E I.... 0 z

10 •· o: <( E

,. E 8 ,.

E c o: c 0 ,. 0

6 "i """ ,. N

C') ,: C')

~ •: , . :::::1 ,:

2 () o: ,. ,. 5-:J<:'

0 1.·

PL "J~

-10 -5 0 5 10 15 Voltage, V

280 300 320 340 360 380 400 420

Wavelength, nm

Figure 16. Room temperature normalized electroluminescence (EL) spectra of LEDs with

three different MQW configurations (solid lines). PL spectra of the corresponding MQWs are

demonstrated as dotted lines. The 1-V curves for LEDs are depicted in the insert.

5. SUMMARY

We have reviewed the basic properties of quaternary AllnGaN alloys, which represent a novel class of semiconductors for UV optoelectronic applica­tions. We have analyzed the dynamics of radiative recombination of those materials and their quantum structures under wide range of temperatures and excitations. The PL properties can be explained by taking into account car­rier/exciton localization associated with In incorporation. Influence of strong built-in electrostatic field due to spontaneous and piezoelectric polarization on PL peculiarities of AllnGaN-based MQWs has been also analyzed. At­tractive radiative properties of the first AllnGaN-based UV optoelectronic

Quaternary AllnGaN Materials Systems 55

devices confirm a prospective potential for practical application of those quaternary alloys.

ACKNOWLEDGEMENTS

The authors would like to thank J. Zhang, C. Chen, G. Simin, J. W. Yang, M. Shatalov, M. Gaevski, H. Wang, and V. Adivarahan at University of South Carolina, R. Gaska and Q. Fareed at SET, Inc., M. Shur at Rensselaer Polytechnic Institute, A. Zukauskas, S. Jursenas, and K. Kazlauskas at Vil­nius University for collaboration and fruitful discussions.

REFERRENCES

1. Masumoto T., Yoshimoto N., Sasaki T., Katsui A. Wide-gap semiconductor InGaN and InGaAlN grown by MOVPE. J. Electron. Mater. 1992; 21:157-63

2. Mcintosh F. G., Boutros K. S., Roberts J. C., Bedair S. M., Piner E. L., and El­Masry N. A. Growth and characterization of AllnGaN quaternary alloys. Appl. Phys. Lett. 1996; 68:40-42

3. Zhang J., Kuokstis E., Fareed Q., Wang H., Yang J., Simin G., Khan M. A., Gaska R., Shur M. Pulsed atomic layer epitaxy of quaternary AllnGaN layers. Appl. Phys. Lett. 2001; 79:925-7

4. Chen C., Yang J., Ryu M. Y., Zhang J., Kuokstis E., Simin G., Khan M. A. Pulsed metal organic chemical vapor deposition of quaternary AllnGaN layers and multiple quantum wells for ultraviolet light emission. Jpn. J. Appl. Phys. 2002; 41:1924-8

5. Adivarahan V., Chitnis A., Zhang J. P., Shatalov M., Yang J. W., Simin G., Khan M. A., Gaska R., Shur M. S. Ultraviolet light-emitting diodes at 340 nm using quaternary AllnGaN multiple quantum wells. Appl. Phys. Lett. 2001; 79:4240-2

6. Chitnis A., Adivarahan V., Shatalov M., Zhang J., Gaevski M., Shuai W., Pachipu­lusu R., Sun J., Simin K., Simin G., Yang J., Khan M.A. Submilliwatt operation of AllnGaN based multifinger-design 315 nm light emitting diode (LED) over sap­phire substrate. Jpn. J. Appl. Phys. 2002; 41:L320-L322

7. Khan M. A., Adivarahan V., Zhang J. P., Chen C., Kuokstis E., Chitnis A., Shata­lov M., Yang J. W., Simin G. Stripe geometry ultraviolet light emitting diodes at 305 nanometers using quaternary AllnGaN. Jpn. J. Appl. Phys. 2001; 40:L1308-Ll310

8. Nagahama S, Yanamoto T., Sano M., Mukai T. Characteristics of ultraviolet laser diodes composed of quaternary AlxlnyGa1+yN. Jpn. J. Appl. Phys. 2001; 40:L788-L791

9. KhanM. A., YangJ. W., SiminG., GaskaR., ShurM. S., zurLoyeH. C, Tamu­laitis G., Zukauskas A., Smith D. J., Chandrasekhar D., Bicknell-Tassius R. Lattice and energy band engineering in AllnGaN/GaN heterostructures. Appl. Phys. Lett. 2000; 76:1161-3

10. Li J., Nam K. B., Kim K. H., Lin J. Y., Jiang H. X. Growth and optical properties of In,AlyGa1_x-yN quaternary alloys. Appl. Phys. Lett. 2001; 78:61-3

56 E. Kuokstis, G. Tamulaitis and M As if Khan

II. Aumer M. E., LeBoeuf S. F., Moody B. F., Bedair S. M. Strain-induced piezoelec­tric field effects on light emission energy and intensity from AllnGaN/InGaN quan­tum wells. Appl. Phys. Lett. 2001; 79:3803--6

12. Davydov V. Yu., Klochikhin A. A., Seisyan R. P., Emtsev V. V., Ivanov S. V., Bechstedt F., Furthmtiller J., Harima H., Mudryi A. V., Aderhold J., Semchi­nova 0., Graul J. Absorption and emission of hexagonal InN. Evidence of narrow fundamental band gap. Physica Status Sol. B 2002; 229:Rl

13. Wu J., Walukiewicz W., Yu K. M., Anger III J. W., Haller E. E., Lu H., SchaffW. J., Saito Y., Nanishi Y. Unusual properties of the fundamental band gap of InN. Appl. Phys. Lett. 2002; 80:3967-9

14. Matsuoka T., Okamoto H., Nakao M., Harima H., Kurimoto E. Optical bandgap en­ergy ofwurtzite InN. Appl. Phys. Lett. 2002; 81:1246-8

15. Yamaguchi S., Kariya M., Nitta S., Amano H., Akasaki I. Strain relief and its effect on the properties of GaN using isoelectronic In doping grown by metalorganic va­por phase epitaxy. Appl. Phys. Lett. 1999; 75:4106-8

16. Chung H. M., Chuang W. C., Pan Y. C., Tsai C. C., Lee M. C., Chen W. H., Chen W. K., Chiang C. 1., Lin C. H., Chang H. Electrical characterization of isoelectronic In-doping effects in GaN films grown by metalorganic vapor phase epitaxy. , Appl. Phys. Lett. 2000; 76:897-9

17. Tamulaitis G., Kazlauskas K., Jursenas S., Zukauskas A., Khan M. A., Yang J. W., Zhang J., Simin G., Shur M. S., Gaska R. Optical bandgap formation in AllnGaN alloys. Appl. Phys. Lett. 2000; 77:2136-8

18. Kariya M., Nitta S., Kosaki M., Yukawa Y., Yamaguchi S., Amano H., Akasaki I. Effect on GaN/ Al0 17Gao.s3N and Al0.05Gao.95/ Al0.17Gao.83N quantum wells by isoelectronic In-doping during metalorganic vapor phase epitaxy. Jpn. J. Appl. Phys.2000;39:Ll43-Ll45

19. Yamaguchi S., Kariya M., Nitta S., Amano H., Akasaki I. The effect of isoelec­tronic In-doping on the structural and optical properties of (AI)GaN grown by meta­l organic vapor phase epitaxy. Jpn. J. Appl. Phys. 2000; 39:2385-8

20. Adivarahan V., Simin G., Tamulaitis G., Srinivasan R., Yang J., Khan M. A., Shur M. S., Gaska R. Indium-silicon co-doping of high-aluminum-content AIGaN for solar blind photodetectors. Appl. Phys. Lett. 2001; 79:1903-5

21. Cremades A., Navarro V., Piqueras J., Lima A. P., Ambacher 0., Stutzmann M. In­homogeneous incorporation of In and Al in molecular beam epitaxial AllnGaN films. J. Appl. Phys. 2001; 90:4868-70

22. Hirayama H, Kinoshita A., Yamabi T., Enomoto Y., Hirata A., Araki T., Nan­ishi Y., Aoyagi Y. Marked enhancement of320-360 nm ultraviolet emission in qua­ternary In,AlvGa1_x-yN with In-segregation effect. Appl. Phys. Lett. 2002; 80:207-9

23. LiD., Dong X., Huang J., Liu X., Xu Z., Wang X., Zhang Z., Wang Z. Structural and optical prpperties of InAlGaN films grown directly on low-temperature buffer layer with (000 I) sapphire substrate. J. Cryst. Growth 2003; 249:72-7

24. Eliseev P. G., Perlin P., Lee J., Osinski M. "Blue" temperature-induced shift and band-tail emission in InGaN-based light sources. Appl. Phys. Lett. 1997; 71 :569-71

25. Cho Y. H., Gainer G. H., Fischer A. J., Song J. J., Keller S., Mishra U. K., Den­Baars S.P. "S-shaped" temperature-dependent emission shift and carrier dynamics in InGaN/GaN multiple quantum wells. Appl. Phys. Lett. 1998; 73:1370-2

26. Momoe D. Hopping in exponential band tails. Phys. Rev. Lett. 1985; 54:146-9 27. lvchenko E. L., Reznitsky A. N. Hopping and low-temperature photoluminescence

in solid solutions and amorphous semiconductors. Phil. Mag. B 1992; 65:733-7

Quaternary AllnGaN Materials Systems 57

28. Zimmermann R., Runge E., Excitons in narrow quantum wells: disorder localization and luminescence kinetics. Phys. Status Solidi A 1997; 164:511-16

29. Baranovskii S. D., Eichmann R., Thomas P. Temperature-dependent exciton lumi­nescence in coupled quantum wells. 1998; 205:R19-20

30. Baranovskii S. D., Eichmann R., Thomas P. Temperature-dependent exciton lumi­nescence in quantum wells by computer simulation. Phys. Rev. B 1998; 58:13081-7

31. Tarasenko S. A., Kiselev A. A., Ivchenko E. L., Dinger A., BaldaufM., Kling­shirn C., Kalt H., Baranovskii S. D., Eichmann R., Thomas P. Energy relaxation of localized excitons at finite temperature. Semicond Sci. Techno!. 2001; 16:485-92

32. Wang T., Liu Y. H., Lee Y. B., Ao J.P., Bai J., Sakai S. 1 mW AllnGaN-based ul­traviolet light-emitting diode with an emission wavelength of 348 nm grown on sap­phire substrate. Appl. Phys. Lett. 2002; 81 :2508-10

33. Kazlauskas K., Tamulaitis G., Zukauskas A., Khan M. A., Yang J. W., Zhang J., Simin G., Shur M. S., Gaska R. Localization and hopping of excitons in quaternary AllnGaN. Phys. Status Solidi C 2002; 0:512-15

34. Kazlauskas K., Tamulaitis G., Zukauskas A., Khan M. A., Yang J. W., Zhang J., Kuokstis E., Simin G., Shur M. S., Gaska R. Exciton and carrier motion in quater­nary AllnGaN. Appl. Phys. Lett. 2003; 82:4501-3

35. Silver M., Schoenherr G., Baessler H., Dispersive hopping transport from an expo­nential energy distribution of sites. Phys. Rev. Lett.l982; 48:352-5

36. Abermathy C. A., in GaN and Related Materials, edited by S. J. Pearton (Gordon and Breach, New York, 1997), pp. 11-51.

37. Jain S.C., Willander M., Narayan J.,Van Overstraeten R. III-nitrides: growth, char­acterization, and properties. J. Appl. Phys. 2000; 87: 965-1006.

38. Bernardini F., Fiorentini Y, Vanderbilt D. Phys. Rev. B. Spontaneous polarization and piezoelectric constants ofiii-V nitrides. 1997; 56:R10024-Rl0027; Bernardini F., Fiorentini V., ibid. Macroscopic polarization and band offsets at nitride hetero­junctions. 1998; 57:R9427-30; Fiorentini V., Bernardini F., Della Sala F., DiCarlo A., Lugli P., ibid. Effects of macroscopic polarization in III-V nitride multiple quan­tum wells. 1999; 60:8849-58; Bernardini F., Fiorentini V., D. Vanderbilt, ibid. Ac­curate calculation of polarization-related quantities in semiconductors. 200 I; 63:193201-4.

39. Miller D. A. B., Chemla D. C., Damen T. C., Grossard A. C., Wiegmann W., T. Wood T. H., Burrus C. A., Phys. Rev. Lett. Band-edge electroabsorption in quan­tum well structures: the quantum-confined Stark effect. 1984; 53: 2173-6.

40. Miller D. A. B., Chemla D. C., Damen T. C., Grossard A. C., Wiegmann W., Wood T. H., Burrus C. A., Phys. Rev. B. Electric field dependence of optical absorption near the band gap of quantum-well structures. 1985; 32: I 043-60.

41. Takeuchi T., Takeuchi H., Sota S., Sakai H., Amano H., Akasaki 1., Jpn. J. Appl. Phys. Optical properties of strained AlGaN and GalnN on GaN. 1997; 36:L177-9.

42. Im J. S., Kollmer H., Off J., Sohmer A., Scholz F., Hangleiter A., Phys. Rev. B. Re­duction of oscillator strength due to piezoelectric fields in GaN/AlxGa 1 ~xN quantum wells. 1998; 57:R9435-8.

43. Takeuchi T., Sota S., Katsuragawa M., Komori M., Takeuchi H., Amano H., Aka­saki 1., Jpn. J. Appl. Phys. Quantum-confined Stark effect due to piezoelectric fields in GalnN strained quantum wells. 1997; 36:L382-5.

44. Klochikin A., Reznitsky A., Permogorov S., Breitkoph T., Grii M., Hetterich M., Klingshirn C., Lysenko V., Langbein W., Hvam J. M., Phys. Rev. B. Luminescence spectra and kinetics of disordered solid solutions. 1999; 59:1294 7-72.

58

45.

46.

47. 48.

49.

50.

51.

52.

53.

54.

E. Kuokstis, G. Tamulaitis and M AsifKhan

Kuokstis E., Yang J. W., Simin G., Khan M.A., Gaska R., Shur M. S. Appl. Phys. Lett. Two mechanisms of blueshift of edge emission in InGaN-based epilayers and multiple quantum wells. 2002; 80:977-9. Weisbuch C., Vinter B. Quantum Semiconductor Structures (Academic, London, 1991), 252 p. Palmer D. W. www.semiconductors.co.uk, 2001.04. Shatalov M., Chitnis A., Adivarahan V., Lunev A., Zhang J., Yang J. W., Fareed Q., Simin G., Zakheim A., Khan M.A., Gaska R., Shur M.S. Band-edge luminescence in quaternary AllnGaN light-emitting diodes. Appl. Phys. Lett. 2001; 78:817-9. Adivarahan V., Chitnis A., Zhang J.P., Shatalov M., Yang J. W., Simin G., Khan M.A., Gaska R., Shur M. S. Ultraviolet light-emitting diodes at 340 nm using qua­ternary AllnGaN multiple quantum wells. Appl. Phys. Lett. 200 I; 79:4240-2. Hirayama H., Kinoshita A., Aoyagi Y. Growth and optical properties of InAIGaN for 300 nm-band UV emitting devices. The Fourth Int. Conf. Nitride Semiconduc­tors (ICNS-4), Denver, B.5.1. 200 I. Nagahama S, Yanamoto T., Sano M., Mukai T. Characteristics of ultraviolet laser diodes composed of quaternary AUnvGa1_x-rN. Jpn. J. Appl. Phys. 200 I; 40:L 788-91. Kipshidze G., Kuryatkov V., Borisov B., Holtz M., Nikishin S., Temkin H. Al­GalnN-based ultraviolrt light-emitting diodes grown on Si(lll). Appl. Phys. Lett. 2002; 80:3682--4. Asano T., Takeya M., Tojyo T., Mizuno T., Ikeda S., Shibuya K., Hino T., Uchida S., Ikeda M. High-power 400-nm-band AIGainN-based diodes with low aspect ra­tio. Appl. Phys. Lett. 2002; 80:3497-9. Wang T., Liu Y. H., Lee Y. B., Ao J.P., Bai J., Sakai S. I mW AllnGaN-based ul­traviolet light-emitting diode with an emission wavelength of 348 nm grown on sapphire substrate. Appl. Phys. Lett. 2002; 81 :2508-10.

III-NITRIDE BASED UV LIGHT EMITING DIODES

R. GASKA 1, M. ASIF KHAN 2, and M.S. SHUR 3

1 Sensor Electronic Technology, Inc., Columbia, SC, USA 2 University of South Carolina, Dept. of Electrical Engineering, Columbia, SC, USA 3 Rensselaer Polytechnic Institute, Dept. of Electrical, Computer, and Systems Engineering, Troy, NY, USA

Abstract: The development of deep UV LEDs requires growing high quality AIGaN epitaxial layers with high molar fractions of AI. Our Strain Energy Band Engi­neering approach has considerably improved the quality of such layers grown on sapphire substrates. Recent improvements in our epitaxial Pulsed Atomic Layer Epitaxy (PALE) technology have allowed us to grow very uniform high quality AIGaN heterostructures over four inch sapphire wafers. However, for deep UV emitters, a more promising approach might be to use bulk AIN sub­strates that improve lattice matching to high molar fraction AIGaN com­pounds, have a much better match for a thermal expansion coefficient, and have a superior thermal conductivity. We recently reported on high Al-content AIGaN-based deep UV emitter structures grown over single crystal, slightly off c-axis (5.8 degrees) bulk AIN substrates. AIN/AIGaN multiple quantum well (MQW) structures with up to 50% of AI in the well material were grown by using low-pressure MOCVD and characterized by using X-ray, AFM, SEM and photoluminescence techniques. We observed stimulated emission at wave­length as short as 258 nm in Al0.5Gao.5N/AIN MQWs grown on bulk AIN sin­gle crystals. These results confirm a high potential of epitaxial structures grown on bulk AIN for scaling our 285 nm UV LEDs on sapphire technology for developing UV LEDs and laser diodes at 260 nm.

Key words: aluminum-gallium nitride, epitaxial growth, PALE, strain relief, UV emitter

1. INTRODUCTION

Wide energy band gap of AlN, GaN, and their alloys makes them well suited for the fabrication of ultraviolet (UV) LEDs and visible-blind and solar-blind

59

M.S. Shur and A. Zukauskas (eds.), UV Solid-State Light Emitters and Detectors, 59-75. © 2004 Kluwer Academic Publishers.

60 R. Gaska, M As if Khan and M S. Shur

photodetectors. Figure 1 shows two basic designs of nitride based UV LEOs relying on the emission through the substrate and the edge emission, respec­tively. The LEOs emitting through the substrate can provide a better light extraction. However, SiC used in the edge emitting LEOs while absorbing the UV radiation has an advantage of having a larger thermal conductivity. Both advantages-a high thermal conductivity and emission through a trans­parent substrate-are combined in AlN bulk substrate that have an additional advantage of being a "native" substrate for AlGaN alloys with a high molar fraction of Al.

Tran. parent Suhstrate Surface emitting LED

Ahsorhing Sub ·trate Edge emitting LED

Figure 1. Basic UV LED structures.

The wavelength of these LEOs is determined by a molar fraction of Al as seen from Fig. 2 that shows the photoluminescence spectra of AIGaN films with different AI molar fraction.

Major challenges in realizing efficient UV LEOs with sufficient power include the growth of crack free high quality AIGaN layers and doping these layers p-type and n-type with high enough carrier densities.

Another major challenge is to control strain and polarization induced electric fields. Nitride semiconductors are pyroelectric materials that possess strong piezoelectric properties. Since the first papers on strain induced po­larization in GaN-based heterostructures [1] that pointed out to the existence of the polarization induced two-dimensional electron gas at the AlGaN/GaN heterointerfaces [1,2], it has become clear that strain control is the key issue in nitride based electronic and photonic devices. Figure 3 (from [1]) clearly shows how a 20 electron or hole gas can form at the AlN/GaN heterointer­face. The strain polarization depends on the crystal orientation [3] and on the thickness and composition of the epitaxial layers [4).

Ill-Nitride Based UV Light Emitting Diodes

Wavelength (nm)

340 320 300 280 260

1 -AI0 ,.Ga070N

2 -AI032Ga068N

3-A1042Ga0 .. N

4.0

3

T = 300K

4.5 5.0

Photon Energy (eV)

Figure 2. Photoluminescence spectra of AlGaN films with different Al molar fraction.

[0001] .....

2.... GaN AlN GaN GaN p- u P Buff

A BA BA BA En (I) (2) (3)

f4l-E\. E =r:::Ff

-L 0 z

[0001] ..... F

+-GaN AlN GaN GaN n + u n Buff

8 A 8A 8 A 8 (I) (2) (3)

Efn::lEc EF

¥ -L 0 z

61

Figure 3. GaN- AIN- GaN p-type and n-type SIS structures with undoped regions (marked u). A denotes cation face; B denotes anion face. 8Ec I 8z and 8Ev I 8z are not continuous at the interfaces because of polarization (after [ 1 ]).

An importance of lattice mismatching and strain-relating issues for UV light emission and photodetector devices is illustrated in Fig. 4.

62 R. Gaska, M AsifKhan and MS. Shur

6.::>.,--------------,

6.

5.

~ 5.

g: bJl 4.

~ &l 4.

3.

0.35

s 'r---...,...----1 0.30 ::1.

~ 5

" 0.25 ~ c

·~ "§

AIOJ G<~o7N

3 ·"t--.---,---F/;...-.,-~-,------,----,----.------1 0.20 w..l

0.0 0.2 0.4 0.6 0.8 1.0 AI mole fraction

Figure 4. The emission wavelength dependence on the Al fraction in AlGaN alloy [6].

Strain Energy Band Engineering approach [5,6] controls the strain using several different techniques or their combination:

• Superlattice buffers for strain relief [2, 7]. • PALE and MEMOCVD epitaxial growth [5,6,8,9]. • Homoepitaxial substrates [10-13]. • Non-polar substrates [14-21].

In this paper, we briefly review these approaches and their applications for the development ofUV LEDs.

2. SUPERLATTICE BUFFERS FOR STRAIN RELIEF

Figure 5 compares the critical thickness for the development of the strain relieving dislocations for a single hetero layer with that for a superlattice [2,6,7]. As seen, the superlattice provides a noticeable advantage that might be further enhanced if the heterointerfaces are not atomically sharp.

Using AlN/AlGaN superlattice (SL) defect filtering [7], Zhang et al. grew thick (>2 !lm) n+-Al0.2Ga0.8N layers with low dislocation densities. Us­ing them as buffer layers, they have also reported on deep UV LEDs on sap­phire with emission at 324 nm and powers as high as 4 mW. Combining a new epilayer design with the low defect density n + -Al0.4Ga0.6N buffer layers, they fabricated sub-milliwatt power AlGaN p-n junction deep UV LEDs with emission at 285 nm.

III-Nitride Based UV Light Emitting Diodes 63

70

60 I I

'E 50 .s "' 40 "' Gl

30 I: -"' 0 :<: 20 1--

10

\I \ ' ~' ...

.......... .... .......... ......_

--:- ..:::' --0 0.0 0.2 0.4 0.6 0.8 1.0

AI molar fraction

Figure 5. Critical thickness as function of AI molar fraction: superlattice (solid line), a single layer (SIS) approximation (dashed line) (after [2]).

The epilayer structure of the 285 run LED is shown in inset to Fig. 6 [6]. It consists of a 0.2 j.lm thick Al0.4Ga0.6N layer that is deposited over base plane sapphire using conventional low-pressure metalorganic chemical vapor deposition (LPMOCVD). This is followed by a 10 period AlN (20 A) I Al0.4Gao.6N (300 A) SL for strain relief and dislocation filtering and a 1.8 llm thick Si-doped n+-Al0.4Ga0.6N buffer layer. As reported earlier, the use of the strain relief AlNI AlGaN superlattices results in a reduction of threading dislocations by a factor of 5 and thus enables the deposition of 1.8 j.lm thick n+-A0.4Ga0.6N layers without cracking. It also significantly im­proves the emission characteristics of the active layers by reducing the non­radiative recombination. The device active region consisted of an AlxGa1_xN (x = 0.36, 100 A) I AlxGa,_xN (x = 0.32, 30 A) I AlxGa,_xN (x = 0.36, 100 A) single quantum well (SQW) which was capped with a Mg-doped p-

25

~ 20

~ 15 i:: QJ

~ 10 u

5

2 4 6 8

Voltage, V

Figure 6. 1-V characteristics of285 nm deep UV LED (after [6]).

64 R. Gaska, M As if Khan and M S. Shur

Al0.4Gao.8N (200 A) and a p+-GaN (500 A) layer. All layers of the structure were deposited at 1050 oc and 76 Torr.

Figure 6 also shows the current-voltage (I-V) characteristics of a 200 J.liD x 200 J.lm device. The device turns on at 6 V with a series resistance of about 74 n. A major contribution to this value comes from the lateral spreading resistance of the n-Al0.4Gao.6N layer. The device emission spectra as a function of pulsed pump current are shown in Fig. 7. For these meas­urements, the emitted light was collected through the sapphire substrate. As seen, at low pump currents, the emission spectra consist of two peaks, one at 285 nm and the other at 340 nm. As the pump currents are increased, the peak at 340 nm rapidly saturates while that at 285 nm shows a near linear increase. We believe the peak at 340 nm to arise from deep levels in the bot­tom n-Alo.4Ga0.6N layer. This is validated by photoluminescence and ca­thodoluminescence (PL and CL) measurements [6].

The peak at 285 nm is the band edge emission from the quantum well. This conclusion is based on observing identical peaks in excimer laser (193 nm) excited PL from test structures without the p-Al0.4Ga0.6N/p+-GaN layers. For these PL measurements the pump is incident from the top (SQW side).

RT 300mA

pulse 500 ns, 0.5%

200mA

!OOmA

260 280 300 320 340 360 380 400 420

Wavelength, nm

Figure 7. Spectral characteristics of285 run deep UV LED (after [7]).

The 285 nm LEDs were then bonded to pads on ceramic carriers and mounted on T0-39 headers. The emitted powers were then measured under CW and pulsed-pumping. A calibrated integrating sphere and an UV­enhanced Si detector with a 285 nm bandpass filter were used for these measurements. Figure 8 shows that a peak CW-power of 10 J.!W rapidly saturated at pump currents in excess of 60 rnA. Current crowding is primar­ily responsible for this saturation. Under pulse pumping no power saturation

III-Nitride Based UV Light Emitting Diodes 65

was observed even up to pump currents as high as 400 rnA. At 400 rnA, a power of 0.15 mW for the 285 nm emission was obtained. A better device geometry and proper heat sinking might significantly improve these power levels.

0.20

RT 0.15

~ 0.10 !-<~ il)

~ 0

P-; 0.05

Current, rnA

Figure 8. CW and pulsed output optical power of the 285 nm deep UV LED (after [7]).

3. PALE AND MEMOCVD EPITAXIAL GROWTH

Pulsed atomic layer epitaxy (PALE) is a novel approach for depositing the AllnGaN layers for the quaternary active region. PALE allows for an accu­rate control of the active layer composition and thickness [5,6,8] and reduces the single crystal epitaxy temperatures thereby increasing the In incorpora­tion in the quaternary AllnGaN layers with high Al-mole fractions. This is an outcome of the increased precursor mobility in the PALE process. Using PALE deposited high Al-mole fraction AllnGaN layers, Khan et al. reported on a p-n junction LEDs with peak emission wavelength tuned from 305 nm to 340 nm by adjusting the AllnGaN alloy composition (different number of AI-, In- and Ga- pulses in the unit growth cell) [6].

For all the AlxinyGa1_x-yN depositions, they used trimethyl aluminum (TMA), trimethylgallium (TMG), trimethyl indium (TMI) and NH3 as the precursors and the basal plane sapphire substrates. Prior to the quaternary layer, a 250 A thick AlN buffer layer and a 1.5 J.lm thick intrinsic i-GaN layer were grown using conventional low-pressure MOCVD. The growth

66 R. Gaska, M As if Khan and M S. Shur

pressures for the two layers were kept at 40 Torr whereas their growth tem­peratures were 450 oc and 1000 °C respectively. Quaternary AlxinyGa1+yN layers were then grown at 760 °C by 150 repeats of a unit cell. The unit cell had a growth sequence illustrated by Fig. 9.

'----TMI

-------___Jil__ TMG

10 20 30 40 50 60 70

Time, sec.

Figure 9. A representative growth unit cell of PALE. We denote this cell as (3AJ,l 1mloa), or simply as (3,1,1) (after [6])

6-sec. long pulses of the precursors TMA, TMI, TMG and NH3 were al­ternately introduced in the low pressure MOCVD reactor. An ammonia pulse always followed the metal organic pulses. As an example, the unit cell in Fig. 9 consisted of 3 repeats of Al and N pulses, followed by one In and N and oneGa and N pulse. Khan et al. referred to the resulting AlxlnyGa1_x-yN layer as the (3A~,l,n,loa)Jso or simply as a (3,1,1),so layer. The subscript de­notes the number of unit cell repeats. The precursor fluxes in individual pulses were adjusted in such a way that the deposited thickness in each unit cell as determined from the total layer thickness divided by the number of unit cell repeats, was around 6 A.

Recently, Fareed et al. reported on a new epitaxial growth technique called Migration Enhanced Metalorganic chemical vapor deposition (MEMOCVD) method [9]. MEMOCVD is an improved version of PALE, which deposits quaternary AlxlnyGa1_x-yN layers by repeats of a unit cell grown by sequential metal organic precursor pulses of AI-, In-, Ga- and NH3. In PALE, the duration of each pulse in the unit cell is fixed, and the NH3 pulse always followed each metalorganic pulse. In MEMOCVD, the dura­tion and waveforms of precursor pulses are optimized, and the pulses might overlap allowing for a continuum of growth techniques ranging from PALE to conventional MOCVD. MEMOCVD combines a fairly high growth rate for buffer layers with reduced growth temperature (by more than 150 °C) and improved quality for active layers. Using this new technique, we achieved a better mobility of pre-cursor species on the surface and thus, bet­ter atomic incorporation and improved surface coverage. In [9], Fareed et al.

Ill-Nitride Based UV Light Emitting Diodes 67

reported on the growth of InN films of the quality comparable to MBE grown films using the MEMOCVD approach.

MEMO-CVD combines a fairly high growth rate for buffer layers with re­duced growth temperature (by more than 150 °C) and improved quality for active layers. Using this new technique, we achieved a better mobility of pre-cursor species on the surface and, thus, better atomic incorporation and improved surface coverage. Figure 10 shows typical results for sheet resis­tance for a two-inch wafer. The electron sheet density for the wafer shown in Fig. 10 was 1.5x1013 cm-2 with the electron mobility over 1,500 cm2N·s at room temperature.

Figure 10. Sheet resistance mapping for an AIGaN/GaN heterostructure grown by MEMO­CVD

MEMOCVD technique achieved impressive results in growing crack-free undoped AlGaN layers with the following thicknesses: 1.6 /-liD for Alo.49Gao.s1N; 2.4 !-!ill for Alo.42Gao.ssN, and 3.9 flill forAlo.3oGao.7oN. MEMOCVD has also enabled scale up of our epitaxial technology to four inch and allowed for precise growth of Strain and Energy Band Engineered structures resulting in elimination of aging effects in AlGaN/GaN het­erostructures grown on large area substrates. This technology should find applications in growing heterostructures for nitride based photonic devices, such as deep UV LEDs, UV lasers, and solar blind UV photodetectors.

An interesting feature of the MOCVD technology is an ability to control the thickness variation over the wafer (see Fig. 11). The thickness could vary in the opposite way to that obtained using a conventional PALE technique. As a result, large area wafer can be grown with extremely uniform thickness distribution by adjusting the MEMOCVD growth regime (see Fig. 12.)

68 R. Gaska, M As if Khan and M S. Shur

MEMO-CVD Growth PALE Growth 2000 Edge

iL 2000 1600 ~ ~. I ~ Center Q. 1600 -Q) 1200 \ u

\ 1: 1200 .s Center ·c:; 800

\ Edge I'll 800 Q. 400 \ I'll 0 · 400 I

0 1 2 3 4 5 0 1 2 3 4 5

Voltage (V) Voltage (V)

Figure II. Capacitance voltage characteristics in the centers and at the edges of wafers grown by MEMO-CVD and PALE.

Thickness

ln·Spec: 100.0% Below: 0.0%

11m 4.200

3.950

3.700

3.450

3.200

2.950

2.700

2.450

2.200

Avge : 3.221 Median : 3.204 Std Dev: 3.816 '-

(0.123)

Above: 0.0% --------------------------------------

Figure 12. Thickness mapping for MEMO-CVD wafer.

Ill-Nitride Based UV Light Emitting Diodes 69

4. HOMOEPITAXIAL SUBSTRATES

Heteroepitaxial growth of nitride epilayer structures on sapphire or SiC re­sults in a large dislocation density (varying from I 08 to I 010 cm-2), which reduces device reliability and life-time, causes premature breakdown and degrades noise performance. One way to reduce the dislocation density is to use Lateral Epitaxial Overgrowth (LEO) or related technologies [23,24]. However, a large background doping of GaN with silicon penetrating from the masking layers degrades device performance of the LEO-grown devices.

The use of bulk GaN or AlN substrate allows one to reduce the dislocation density in the epitaxial layers by more than four orders of magnitude down to 104-105 cm2 in case of AIN substrates [25]. At the same time, bulk AlN substrates have superior thermal conductivity (3 W/cm-K or higher) [26], comparable to that of semi-insulating 4H-SiC (3.9 W/cm-K).

A new approach for growing high-quality AlGaN/ AlN multiple quantum wells (MQW) emitting in deep UV region is to use structures deposited on bulk AlN substrates. Structural analysis by using X-ray diffraction con­firmed high crystalline quality of these structures. Alo.sGao.sN/ AlN MQW grown on bulk AlN demonstrated emission at 260 nm with high emission intensity. Stimulated emission of these structures at 258 nm was observed (see Fig. 13.)].

"' ~ =>

4 ~ i!::' ·;;; c

"' .5 "' u c

"' u

"' "' c "§ => -'

240

. . .. .. IlL l'i"om edg.e a L 400 pm

-- - PLfromcdge 'a l .. 4ol0pm

f ~ - Sp0il1:1Jleous l)t

300

Wavelength (nm)

Figure 13. Photoluminescence spectra of Al0.5Gao.5N/AlN MQWs on bulk AlN. Solid curve represents the spectrum of spontaneous emission detected in the direction perpendicular to the sample surface; dotted and dashed curves correspond to the spectra measured from the sample edge along the direction of a 30 f.lm wide stripe, which length was 400 f.lm and 440 f.lm, re­spectively. Intensity of the spontaneous emission is deliberately normalized. The excitation wavelength was 213 nm. (After [13]).

70 R. Gaska, M As if Khan and M S. Shur

The results prove great potential of growing structures with high­aluminum-content layers on bulk AIN substrates [ 13].

5. NON-POLAR SUBSTRATES

As discussed above, epitaxy over the (0001) crystal orientation leads to strong polarization effects. The resulting strong built-in electrostatic field leads to the quantum confined Stark effect [14-16], which reduces the opti­cal emission intensity from quantum wells grown on polar substrate orienta­tions. The deposition of planar m-, and a-plane GaN films [17-19] on non­polar substrates should enable light emitting devices with polarization-free active regions and increased efficiency. Recently, Sun et al. investigated the growth of GaN/Alo.2oG~4l.8oN multiple quantum wells (MQWs) on selective-area-grown a-plane GaN pillars over r­plane sapphire [20- 21]. In contrast to the MQWs grown on planar a-plane GaN templates, these GaN/Al0.2Ga0.8N MQWs on the pillars exhibited pit­free and atomically smooth surface morphology. Their structural quality and their UV emission at 357 nm increased with the underlying pillar height. The photoluminescence data shown in Fig. 14 clear illustrate the different proper ties of layers grown on polar and non-polar substrates. The epitaxy of GaN/AlGaN MQWs on the selective-area-grown pillars is a promising ap­proach for fabricating high-efficiency, non-polar UV emitters.

100

~

~ ~ 10 "iii c

r-Piane Sapphire -C:::r-- c-Piane Sapphire

~ (b) . ./ 0.1 +-~~~~~~~~~~~~~~-+

1E-5 1E-4 1E-3 0.01 0.1 1

Excitation Power Density (MW/cm2)

Figure 14. Integral PL intensity dependence o n excitation power density [2 1].

III-Nitride Based UV Light Emitting Diodes 71

Tamulaitis et al. reported on the influence of substrate surface polarity on optical properties ofGaN grown on single crystal bulk AlN [22].

6. DEVICE PERFORMANCE

Figure 15 shows the comparison of I-V curves for devices with peak emis­sions at 278 nm, 325 nm and 338 nm [23]. The turn-on voltages range from 4 V to 5.2 V with the series resistances ranging from 15 n to 30 n. This in­crease in resistance with a decrease in the wavelength is due to lower doping (especially p-type) efficiency of the AlGaN layers with high Al content.

40

325 nmLED

280nmLED

2 4 6 8 10

Voltage, V

Figure 15. I-V curves of 278 nm, 325 nm and 340 nm emission LEDs [23].

Figure 16 shows normalized room temperature electroluminescence spec­tra of these deep UV LEDs measured under 100 rnA pulsed current. To avoid self-heating these LEDs were pumped by 500 ns long current pulses with 0.5% duty cycle. The suppression of the long wavelength emission band associated with deep level transitions is about 1:10 for 278 nm emis­sion LED and better than 1:40 for 325 nm and 340 nm LED.

Chitnis et al. obtained room temperature de powers as high as 0.47 mW (at 260 rnA), 1 mW (at 100 rnA) and 1.2 mW (at 100 rnA) for the 278 nm, 325 nm and 338 nm devices, respectively. In output power measurements only the emission around the main peak was measured and the long wave­length band was filtered out. As shown in Fig. 17, these devices exhibited pulsed powers as high as 3 mW, 10 mW and 13 mW for 1 A of pumping current. Inset to Fig. 17 shows the external quantum efficiency of these

72 R. Gaska, M AsifKhan and MS. Shur

LEOs as a function of current. Maximum quantum efficiencies of about 0.1% (278 nm), 0.45% (325 nm) and 0.55% (338 nm) were obtained. For 325 nm and 340 nm emitting LEOs, the external quantum efficiency reaches maximum values at 20-50 rnA indicating relatively low non-radiative re­combination rate in the active region. For 280-nm emitting devices, the non­radiative recombination in the active layer dominates at low currents thereby additionally reducing the output power and external quantum efficiency.

~ 278 nm 325 nm 338 nm c.j

.~ 00. ;:::: Q) ...... . s

"0

.~ ~ § 0 z

9~ ~ 8.5~ 10.2nm r-

RT 500 ns 10kHz

240 260 280 300 320 340 360 380 400 420 440

Wavelength, nm

Figure 16. Normalized electro luminescence spectra of 278 nrn, 325 nrn and 340 nrn emission LEDs [23].

20 10~~~~~~~~~~~ -D-340nm

15 u.f C/ 0.2 w

Current, rnA

--o-- 325 nm ------i::r- 2 8 0 nm

Figure 17. LED output power of UV LEDs vs. current under pulse pumping by 500 ns long current pulses with 0.5% duty cycle. rnset shows the external quantum efficiency [23].

III-Nitride Based UV Light Emitting Diodes 73

Figure 18 shows the optical power vs. wavelength for these deep UV LEOs. As seen from the figure, when the heating is eliminated (pulsed cur­rent) and non-radiative recombination is suppressed (high pulsed current), the output power at shorter wavelength drops by a factor of 4-5. The similar behavior was observed for external quantum efficiency. This is a result of lower material quality and reduced doping of high Al-content material re­quired to achieve deep UV emission. Under de operation, both the non­radiative recombination and the device self-heating due to relatively high operating voltage strongly limit the output power. This leads to about 10 times power reduction at 280 nm compared to 340 nm.

~ 8 1-<~ Q)

~ 0 0.. :; ~ 0

10 pulse -o- --0

1_A_--

o-

de .... -- -· ... 100 rnA..- ... ... ... ... ... ... ....

0.1

270 280 290 300 310 320 330 340 350

Wavelength, nm

Figure 18. LED output power as a function of wavelength [23].

7. CONCLUSION

In conclusion, we reported on the development of deep UV AlGaN MQW LEOs on sapphire with peak emission wavelength from 278 nm to 340 nm. Innovative growth techniques and structure design resulted in the pulsed op­tical powers as high as 3 mW, 10 mW and 13 mW for 1 A of pumping cur­rent. A further improvement in material quality, device design and packag­ing techniques should allow for additional power and efficiency enhancement leading to the high power deep UV LEOs.

74 R. Gaska, M As if Khan and M S. Shur

REFERENCES

1. Bykhovski, B. Gelmont, and M. S. Shur, "The influence of the strain-induced elec­tric field on the charge distribution in GaN-AlN-GaN SIS structure," J Appl. Phys. 74, pp. 6734-6739 (1993).

2. D. Bykhovski, B. L. Gelmont, and M. S. Shur, "Elastic Strain Relaxation in GaN­AlN Superlattices," in Proceedings of International Semiconductor Device Re­search Symposium (December 1995, Charlottesville, VA), Vol. II, pp. 541-544.

3. A. Bykhovski, B. Gelmont, and M. S. Shur, "Strain and charge distribution in GaN­AlN-GaN SIS structure for arbitrary growth orientation," Appl. Phys. Lett. 63, p. 2243 (1993).

4. M. S. Shur, A. D. Bykhovski, R. Gaska, and A. Khan, "GaN-based pyroelectronics and piezoelectronics," in Handbook of Thin Film Devices, ed. by C. E. C. Wood, Hetero-structures for High Performance Devices, Vol. 1 (Academic Press, San Diego, 2000), pp. 299-339.

5. M.A. Khan, J. W. Yang, G. Simin, H. zur Loye, R. Bicknell-Tassius, R. Gaska, M. S. Shur, G. Tamulaitis, A. Zukauskas, "Energy band/lattice mismatch engineering in quaternary AllnGaN/GaN heterostructure," Phys. Stat. Sol. A 176, pp. 227-230 (1999).

6. M. AsifKhan, J. Yang, G. Simin, R. Gaska, and M.S. Shur, "Strain energy band engineering approach to AlN/GaN/InN heterojunction devices," in Frontiers in Electronics: Future Chips, ed. by Y. S. Park, M. S. Shur, and W. Tang (2003), pp. 195-214.

7. J. P. Zhang, H. M. Wang, M. E. Gaevski, C. Q. Chen, Q. Fareed, J. W. Yang, G. Simin, M. Asif Khan, "Crack-free thick AlGaN grown on sapphire using high tem­perature AlN/AlGaN superlattice for strain management," Appl. Phys. Lett. 80, pp. 3542-3544 (2002).

8. J.P. Zhang, E. Kuokstis, Q. Fareed, H. M. Wang, J. W. Yang, G. Simin, M. Asif Khan, G. Tamu1aitis, G. Kurilcik, S. Jursenas, A. Zukauskas, R. Gaska, and M. Shur, "Pulsed atomic layer epitaxy of quaternary AllnGaN layers for ultraviolet light emitters," Phys. Stat. Sol. A 188, pp. 95 -99 (200 1 ).

9. Q. Fareed, R. Gaska, M. S. Shur, J. Wu, W. Walukiewicz, and M. A. Khan, High Quality InN/GaN Heterostructures Grown by Migration Enhanced Metalorganic Chemical Vapor Deposition, submitted to Appl. Phys. Lett.

10. E. Frayssinet, W. Knap, P. Lorenzini, N. Grandjean and J. Massies, C. Skier­biszewski, T. Suski, I. Grzegory, S. Porowski, G. Simin, X. Hu, M. Asif Khan, M. Shur, R. Gaska, and D. Maude, "High electron mobility in AlGaN/GaN heterostruc­tures grown on bulk GaN substrates," Appl. Phys. Lett. 77, pp. 2551-2553 (2000).

11. M. AsifKhan, J. W. Yang, W. Knap, E. Frayssinet, X. Hu and G. Simin, P. Prys­tawko, M. Leszczynski, I. Grzegory, S. Porowski, R. Gaska, M. S. Shur, B. Beau­mont, M. Teisseire, and G. Neu, "GaN-AlGaN heterostructure field effect transis­tors over bulk GaN substrates," Appl. Phys. Lett. 76, pp. 3807-3809 (2000).

12. X. Hu, J. Deng, N. Pala, R. Gaska, M. S. Shur, C. Q. Chen, J. Yang, S. Simin, A. Khan, C. Rojo, and L. Schowalter, "AlGaN/GaN hetero-structure field effect tran­sistor on single crystal bulk AlN," Appl. Phys. Lett. 82, pp. 1299-1302 (2003).

13. R. Gaska, C. Chen, J. Yang, E. Kuokstis, A. Khan, G. Tamulaitis, I. Yilmaz, M. S. Shur, J. C. Rojo, and L. Schowalter, "Deep-ultraviolet emission of AlGaN/ AlN quantum wells on bulk AlN," Appl. Phys. Lett. 81, pp. 4658-4660 (2002).

14. S. Ghosh, P. Waltereit, 0. Brandt, H. T. Grahn, and K. H. Ploog, Phys. Rev. B 65, 075202 (2002).

III-Nitride Based UV Light Emitting Diodes 75

15. T. Deguchi, K. Sekiguchi, A. Nakamura, T. Sota, R. Matsuo, S. Chichibu, and S. Nakamura, J Acoust. Soc. Jpn. 38, L914 (1999).

16. D. A. B. Miller, D. C. Chem1a, T. C. Damen, A. C. Gossard, W. Wiegmann, T. H. Wood, and C. A. Burrus, Phys. Rev. B 32, 1043 (1985).

17. P. Waltereit, 0. Brandt, M. Ramsteiner, R. Uecker, P. Reiche, and K. H. Ploog, J Cryst. Growth 218, 143 (2000).

18. M.D. Craven, S. H. Lim, F. Wu, J. S. Speck, and S. P. DenBaars, Appl. Phys. Lett. 81, 469 (2002).

19. C. Q. Chen, M. E. Gaevski, W. H. Sun, E. Kuokstis, J. P. Zhang, R. S. Q. Fareed, H. M. Wang, J. W. Yang, G. Simin, M.A. Khan, H. P. Maruska, D., W. Hill, M. M. C. Chou, and B. H. Chai, Appl. Phys. Lett. 81, 3194 (2002).

20. W. H. Sun, J. W. Yang, C. Q. Chen, J.P. Zhang, M. E. Gaevski, E. Kuokstis, V. Adivarahan, H. M. Wang, Z. Gong, M. Su, and M. AsifKhan, "GaN/AlGaN multi­ple quantum wells on a-plane GaN pillars for stripe-geometry nonpolar ultraviolet light-emitting devices," Appl. Phys. Lett. 83, 2599 (2003).

21. W. H. Sun, E. Kuokstis, M. Gaevski, J.P. Zhang, C. Q. Chen, H. M. Wang, J. W. Yang, G. Simin, M. AsifKhan, R. Gaska, and M.S. Shur, "Strong ultraviolet emis­sion from non-polar AlGaN/GaN quantum wells grown over r-plane sapphire sub­strates," Phys. Stat. Sol. A 200, 48- 51 (2003).

22. G. Tamulaitis, I. Yilmaz, M. S. Shur, R. Gaska, C. Chen, J. Yang, E. Kuokstis, A. Khan, J. C. Rojo, L. J. Schowalter, "The influence of substrate surface polarity on optical properties of GaN grown on single crystal bulk AlN," Mat. Res. Symp. Proc. 743, L3.34 (2003).

23. A. Chitnis, V. Adivarahan, J. Zhang, M. Shatalov, S. Wu, J. Yang, G. Simin, M. Asif Khan, X. Hu, Q. Fareed, R. Gaska, and M. S. Shur, "Milliwatt power AlGaN quantum well deep ultraviolet light emitting diodes," Phys. Stat. Sol. A 200, pp. 99-101 (2003).

UV METAL SEMICONDUCTOR METAL DETECTORS A robust choice for (Al, Ga)N based detectors

J-L. REVERCHON \ M. MOSCA 1, N. GRANDJEAN 2, F. OMNES 2,

F. SEMOND 2, J-Y. DUBOZ 2, and L. HIRSCH 3

1 Thales Research & Technology, 91404 Orsay Cedex, France 2 CRHEA-CNRS, rue Bernard Gregory, Sophia Antipolis, 06560 Valbonne, France 3 IXL-CNRS-ENSEIRB, University of Bordeaux I, 33405 Talence, France

Abstract: UV detection is interesting for combustion optimization, air contamination control, fire and solar blind rocket launching detection. Most of these applica­tions require that UV detectors have a huge dynamic response between UV and the visible, and a very low dark current in the range of the UV flux meas­ured. (Al,Ga)N alloys present a large direct bandgap in this range and there­fore can be used as an active region in such detectors. To take advantage of the large Schottky barrier, the good alloy quality, and to avoid any doping prob­lems, we have developed MSM photodetectors. High quality material has been grown with MOCVD and MBE on sapphire substrates. Stress management is employed for aluminum contents up to 65% to reduce crack density. This is correlated with non-ideal features like dark current, sub-bandgap response and non-linearity between photocurrent and optical flux. The spectral selectivity between UV and visible reaches five orders of magnitude. A geometry of in­ter-digitized fingers is optimized in regards to the peak response. The Schottky barrier and a dielectric passivation result in dark currents lower than 1 fA up to 30 V for a 100 x 100 ).1m2 pixel. Consequently, detectivity is mainly limited by shot noise and corresponds to a noise of 500 photons per second and per pixel.

Key words: UV solar blind detectors, Metal-Semiconductor-Metal detectors, stress man­agement in (Al,Ga)N, IBICC.

77

M.S. Shur and A. Zukauskas (eds.), UV Solid-State Light Emitters and Detectors, 77-92. © 2004 Kluwer Academic Publishers.

78 J- L. Reverchon et al

1. INTEREST AND SPECIFICATIONS FOR (Al,Ga)N AS AN ACTIVE LAYER FOR UV DETECTION

1.1 (Al,Ga)N for UV Detection

Due to their use for blue LEDs and lasers, GaN based materials are gaining more and more importance and a great effort has been undertaken in order to improve their quality. As a result, nitrides can now be considered for many other applications such as high power-high frequency electronics and ultra­violet (UV) detection [1,2]. As a direct band gap III-V semiconductor, (Al,Ga)N is well suited for detecting light at energies higher than its band gap energy and providing a large rejection at lower energies. The band gap energy varying from 3.43 eV (GaN) to 6.2 eV (AlN), makes it possible to adjust the wavelength of absorption from 360 nm to 195 nm. In particular, we will focus on wavelengths of about 280 nm for which sunlight is ab­sorbed by the ozone layer and never reaches the surface of the earth. Conse­quently, detectors sensitive in this range see only UV sources coming from the earth and are said to be solar blind.

Fundamentally (Al,Ga)N based devices suffer from difficulties such as a large activation energy required not only for magnesium p doping but also for n doping in high aluminum content alloys. For the same reasons, ohmic contacts are also difficult to achieve. On the contrary, this large barrier gives the opportunity to achieve high barrier Schottky contacts. This is a great ad­vantage for obtaining of low dark current in Schottky based detectors. Fi­nally the main difficulties come from the quasi absence of GaN or AlN sub­strates. Nitrides are traditionally grown on sapphire or SiC with a lattice and thermal expansion mismatch inducing strain, dislocations and cracks. In Sec­tion 2, we will discuss how to avoid cracks and to reduce the non-ideal fea­tures attributed to related electrical defects.

1.2 Specifications for UV Detection

UV is in the range of energy involved in chemical bonding. Thus, UV detec­tion presents a great interest for combustion optimization, air contamination control, UV A/UVB medical control, and fire/flame detection and in particu­lar solar blind detection. Most of these applications require stringent specifi­cations because of the low fluxes to be measured. Indeed UV radiation is diffused by the Rayleigh mechanism especially when UV sources are far away in the atmosphere. As a consequence, dark current must be as small as possible in comparison to photocurrent. Moreover, as far as noise is con-

UV Metal Semiconductor Metal Detectors 79

cerned, a low current would diminish the shot noise and the lifnoise, which are respectively proportional to current and current squared.

In this paper, we present (Al,Ga)N based detectors which are in competi­tion with photomultipliers (PM) and silicon based charge coupled devices (CCD). PMs are not available in large array configuration, are fragile, and use a high bias. Large array CCDs are available with a huge detectivity and even with photon counting mode but only when cooled to reduce dark cur­rent. One of the advantages of (Al,Ga)N based detectors versus PM and ceo would be the intrinsic spectral selectivity between uv and visible. It prevents use of interference filters whose sensitivity to non-normal incidence is a drawback. In the case of (Al,Ga)N based detectors, such interference filters may be added to the intrinsic selectivity to obtain even larger rejec­tion.

Obviously, we require from photodetectors a responsivity as large as pos­sible. It means that gain (the ratio between electron pairs created per photon absorbed) may be close to one in the case of photovoltaic detectors and as large as possible in the case of photoconductor or phototransistor structures. Moreover, the proportionality between photocurrent and incident power (linearity), must be preserved. Concerning the response time, a short one may be expected due to the low capacitance and transit time of device [3]. Capacitance can be estimated to be lower than 0.1 pF for 100 x 100 f.1m2 de­vices. Nevertheless, because of the need for large detectivity in imaging with low fluxes, a long integration time is necessary. Thus the time response is not so important and needs only be reasonably fast for imaging at several hundreds of hertz.

1.3 The Choice of Metal-Semiconductor-Metal Detectors

A first kind of semiconductor detectors is the photoconductor that may show a high internal gain defined as the ratio of lifetime of carriers to transit time between electrodes. As we will see in Section 1.5, the lifetime depends on density and occupancy of deep levels, so that non-ideal and uncontrolled behaviors may appear like, e.g. a non-linear dependency of the photocurrent on the incident power. Moreover, photoconductors present an intrinsic high dark current leading to lifnoise. Therefore, photoconductors are not suitable as flame detectors in terms of dark current, noise and detectivity.

(Al,Ga)N p-i-n photodiodes do not exhibit the above-mentioned draw­backs. The gain is limited to one, the current is low and the responsivity is linear. But p-type doping with high Al content is difficult to obtain. Some attempts to use p-GaN show that long wavelength contribution could be lim­ited by convenient band diagram design [ 4]. Another difficulty is to make good ohmic contacts on such wide bandgap semiconductors even if they are

80 J- L. Reverchon et a!

highly n doped. Generally, a high-temperature annealing cycle (700-900 °C) is necessary to make the contact ohmic. For this reason, together with the need for a good n-type conducting layer, good Schottky photodiodes are not so easy to achieve at high aluminum content.

Consequently, the situation is far more convenient for (Schottky) Metal­Semiconductor-(Schottky) Metal (MSM) detectors that don't need either doping or ohmic contacts. The only but important difficulty is a mismatch between the substrates and (Al,Ga)N layers. A MSM consists only of a photoabsorbing epitaxial layer with two interdigitated Schottky metal con­tacts deposited on the semiconductor surface. In this paper, we will focus on this planar technology whose simplicity contributes to robustness.

1.4 Electrical and Optical Characterization Tools

In most detectors, dark current is measured with a picoammeter ( 485, Keith­ley), but when necessary, dark current is measured with a source/meter (6430, Keithley) in the fA range taking care of the connections (Guarded Tri-axial Cable). For the photoresponse measurement, we use a Xenon lamp filtered by a monochromator and the light is focused on the back side of the detector for samples grown on Ab03, and on the front side for samples grown on Si(lll ). The incident power is measured by a calibrated pyrome­ter. The detectors are biased with a voltage source and connected in series with a transimpedance amplifier. The photocurrent is measured both in AC conditions with a chopper and a lock-in amplifier (7220, EG&G Instru­ments) and in DC conditions with a picoammeter (485, Keithley). There­sponsivity is calculated as the ratio of the photocurrent to the power incident on the detector. All measurements are made at room temperature.

1.5 Non Ideal Features in MSM

Due to dislocations or cracks, some layers may present defects that are elec­trically active and lead to traps or recombination centers. For MSM based on such material, the high quantity of defects and deep levels gives poor rectify­ing contacts. These levels give both channels across the junction and a bow­ing of the conduction band that diminish the depletion thickness at the Schottky barrier. Finally, this injection via trap-assisted tunneling corre­sponds to a photoconductive behavior. But, in photoconductors, responsivity depends on the lifetime of carriers. This lifetime has been linked in many ways to traps or deep levels [5,6,7,8]. It results in a strong nonlinearity of photoresponse versus absorbed optical flux. These spectra also present a sub­band-gap absorption and a reduced dynamics depending strongly on fre­quency when spectra are acquired with a chopped flux.

UV Metal Semiconductor Metal Detectors 81

2. MATERIAL GROWTH

We present here the structure used for back illuminated samples and the conditions of growth by MOVPE and MBE. The choice of nucleation layer and the efforts to eliminate cracks will be particularly stressed.

2.1 Sample Structures for UV Detection

One of the main difficulties encountered in the growth of nitride materials has been the absence of a lattice matched substrate. In the case of large array detectors, we have also to take into account that a Readout Integrated Circuit (ROIC) on the front side obliges us to use a substrate transparent to UV. Thus, even if GaN, AlN substrates or pseudo substrates have been improved during the last few years, GaN ELOG (Epitaxial Lateral OverGrowth) [9] or f.!ELOG [10] and bulk GaN [11] cannot be used. On the other hand HYPE [12] or bulk AIN [13] substrate may be adapted to UV detection if a good transparency to UV is guaranteed. Up to now, sapphire is still the substrate of choice. The choice of the nucleation layer must provide good optical and electrical qualities for (Al,Ga)N. As far as optical properties are concerned, an AlN buffer layer is the only solution to provide transparency at 280 nm. GaN buffer layer can be used only if its thickness is sufficiently low to guar­anty transparency to UV. After the buffer growth, cracking may arise from the lattice mismatch between (Al,Ga)N and sapphire and also between (Al,Ga)N layers with very different aluminum contents. Thus, one of the greatest challenges is to manage this mismatch whereas we have to use lay­ers as thick as possible to minimize dislocations. In our case, we use a thick window layer of 1 f.!m transparent to UV to improve materials quality. Then the active layer is grown with a thickness of 0.4 f.!m, sufficient to easily col­lect carriers.

Figure 1. Left: cross section of sample structures for UV detection. ROIC is on the front side and light comes from backside. Right: overview of interdigitized fingers of a MSM.

82 J- L. Reverchon et a/

2.2 Samples Grown by Low-Pressure Metalorganic Vapour-Phase Epitaxy

Some samples are grown by low-pressure metalorganic vapour-phase epi­taxy (LP-MOVPE) on c-sapphire substrates in an Aixtron growth chamber AIX200 RF. Trimethylgallium, trimethylaluminum, and ammonia are used as precursors. GaN or AIN buffer layers are 25-nm and 1 0-nm thick and are grown at 525 oc and 890 °C, respectively, in a pure nitrogen carrier gas. (Al,Ga)N alloys are grown at 1180°C with a V/III ratio between 2000 and 3100 in a pure hydrogen carrier gas. NH3 flux is 2 !/min and the total flux is 5 1/min. The growth pressure is low (20 mbar) in order to avoid parasitic re­actions between NH3 and TMAI. Finally, the growth rate is 1 f..Lm/h for the window layer (Alo.6sGao35N) and 1.8 flm/h for the active layer (Al0.5Ga0.5N). More details are given in Ref. 14. We now pay attention to layers grown with a GaN buffer layer. We notice a strong sub band gap absorption corre­sponding to deep levels (Fig. 2, left) even if no crack networks are present. All devices grown on this layer present high dark current with non-ideal fea­tures of photoconductors already mentioned. For example, we notice in Fig. 2 (right) that the dynamics can be reduced when high bias is applied and participate to trap-assisted tunneling across contacts. The frequency depend­ence also shows the long time needed to neutralize sub-band-gap absorption .

...........

104 0.8 i c OHz. 20V .Q 0.6 (/) . >. 102 , -·--80Hz. 20V (/)

, ...... E 0.4 :~ ·,'·.:·--80Hz, 2V

(/) \ '• (/) Buffer GaN c 10° ' ................. _

c 0 ' ··-ctl 0.2 I - Buffer AIN ' .. Buffer GaN .... a. 1- -Buffer AIN (/) ~ ... , ...

Q) 1 o-2 .. ·--0.0 0:::

200 500 300 400 Wavelength (nm) Wavelength (nm)

Figure 2. Left: transmission spectra for layers grown on a GaN or AlN buffer. Right: spectral response depending on bias and frequency for sample with GaN buffer layer.

On the contrary, the best samples have been obtained with an AlN buffer layer. The transmission is good down to 280 nm showing the absence of deep levels. This is confirmed by the dynamics independent of bias and AC or DC mode used for spectral acquisition (Figure 3). Then, no deep level contribute to sub-band-gap absorption or trap assisted tunneling across the Schottky barrier. Consequently, the time response is due only to the transit

UV Metal Semiconductor Metal Detectors 83

time needed for carriers to cross the spacing between electrodes and non­ideal features disappear. We must stress here that some good results have been observed with the layers grown with a GaN buffer layer in the past [14]. We don't have any clear explanation for these differences. We can only mention that materials quality has been shown to depend closely on growth parameters and that the average aluminum content is closer to AlN than to GaN in such layers.

10'2 . .. ,..

i 10-3 OHz/SV ...... 190Hz /5V

>. -- 80Hz/5V ...... ·:;;

104 - 80Hz/20V

(/)

r::

l 4(1.1 ,N UA ~un

Al "'(la :~N o H5 ~·m

0 c. 10'5 (/) Q)

0:::: 10'6

lluO<r · !liN I 01 ~ 11111

Sarrhon;

280 320 360 400 Wavelength (nm)

Figure 3. Responsivity spectra as a function of bias and frequency (AC) for the sample with AlN buffer layer.

2.3 Samples Grown by Molecular Beam Epitaxy

The Molecular Beam Epitaxy (MBE) samples were grown most often on Ah03 and exceptionally on Si(l11) substrates using NH3 as nitrogen source in a RIBER growth chamber. Samples grown on Si(lll) even if not suitable for backside illumination are used for transport studies. More information concerning the growth procedure and the properties of layers can be found in references [ 15, 16]. The best electrical properties have been achieved with a GaN buffer layer. In Fig. 4, transmission measurements show that only 25% of the optical flux is absorbed by this 25-nm thick buffer GaN layer. We also notice that absorption due to the GaN buffer layer has no consequence in spectral response. Thus, we prefer to keep this buffer layer instead of AlN buffer. Some good quality samples have been grown with 40% of aluminum in the window layer and 30% Al in the active layer. But when the aluminum content increases, some cracks appear after cooling down due to an exces­sive tensile stress concomitant with electrical leakage in the Schottky con­tacts [17]. Thus a 100-nm thick AlN layer is incorporated to put epilayers in a compressive stress, a thinner window layer was grown to reduce elastic energy, and a lower growth temperature of 800 oc is used to limit thermal

84 J- L. Reverchori et a/

expansion consequences. In this way, we obtain layers without any cracks and with exceptional electrical and optical properties.

0.8

I 10-2

0.6 -I t\1_.,(.;) ~' o . .a 1-1m ..., >- 10·3 Ill

...... ::J \ l ,..,(fil lcN I Jim "> (/)

"iii 0.4 3 i\IN IUilnm c 10"4 iii" 0 (/) (t.l ~; nm Q. 0.2 a· (/)

10-5 ::J \apphin: Q)

cr: 0.0

250300350400450500 Wavelength (nm)

Figure 4. Transmission and response spectra of the layer grown by MBE with a GaN buffer layer.

3. OPTIMIZATION OF PROCESSING

3.1 Surface Preparation and Metallization

Most MSM detectors were processed for defining interdigitated fingers by optical lithography. The spacing equals 2 or 5 11m whereas the width varies from 1 to 10 f.lm. The surface was deoxidized in HCl for one minute and rinsed in de-ionized water during four minutes just before being introduced into the Joule evaporation chamber (Plassys chamber MEB550S). The con­tact consists of 10 nm of platinum followed by 100 nm of gold. Even if we take care to limit time between cleaning and deposition, we can expect (Al,Ga)N to be oxidized. Some studies have shown that an oxide could pre­vent leakage via dislocations. For example, some enhancement of Schottky barrier height on (Al,Ga)N/GaN heterostructures by oxidation annealing has also been reported [ 18]. It may explain the exceptionally low dark current low obtained with some samples (1 fA up to 35 V). Even if oxide presence has not been investigated here, we have observed that a smooth etching just before deposition could increase leakage via induced defects and oxide elimination. After lift off, annealing at 400 °C during 10 minutes in nitrogen atmosphere is used only for mechanical requirements. Higher temperatures would induce leakage currents.

UV Metal Semiconductor Metal Detectors 85

3.2 Contact, Passivation and Connection

If we want to perform a fast evaluation of epilayers, we can use a one-step processing for which contact pads are evaporated at the same time as elec­trodes. But in the case of a backside-illuminated device, we observed that both the interdigitated electrodes and the contact pad areas contribute to the overall photocurrent even if contact pads are placed at several tens of mi­crometers from the interdigitated area. In order to avoid the parasitic current due to the contact pads, we developed another process where the Pt/ Au Schottky contacts are deposited on the (Al,Ga)N surface whereas the contact pads are sputtered on a dielectric. Several dielectric films were tested for their electrical passivation capability. Si3N4 (300 nm) and Si02 (300 nm), were deposited by plasma-enhanced chemical vapor deposition (PECVD) at 300 °C. Benzocyclobutene (BCB) (1500 nm) was deposited by spin coating and annealed under vacuum at 250 oc for 30 min. The first two PECVD ma­terials show good passivation up to fA range at several tens of volts. Passiva­tion has been particularly efficient in the case of layers having developed microcracks related to excess stress. In that case, we showed that both the dark current and the responsivity strongly depend on the crack density. By using our two-level process, we have reduced the parasitic effects of cracks on the dark current.

4. TRANSPORT PROPERTIES

As we can see in Fig. 4, responsivity is limited to 0.04 A/W. This value is .low compared to the absolute photovoltaic limit e/h v that would be 0.22A /W at 280 nm. Indeed, MSM detectors have been fabricated by many groups on GaN [19,20] or on (Al,Ga)N [3,14,21,22] and exhibit good per­formance but with the same limited collection of carriers between fingers. Collection efficiency in MSM detectors is studied here with submicronic lithography, the ion beam induced charge collection method (IBICC), and numerical 2-dimensional calculations of the electric field distribution.

4.1 Submicronic MSM byE-beam Lithography

A way to improve collection of carriers is to reduce spacing and trapping between the electrodes. Here, we will study the effects of spacing on both the spectral response and the absolute value of the photoresponse. We com­pare sub-micron devices obtained by electron beam lithography (the width equals 1 !lm, and the spacing is 0.6 !lm) to interdigitized fingers defined by optical lithography (the finger width and spacing equal to 2 !lm) in terms of

86 J- L. Reverchon eta!

responsivity and spectral selectivity. Exceptionally, the (Al,Ga)N layers are grown on a Si(lll) substrate. Pt/Au Schottky contacts are evaporated and lifted off. The dark currents are in the pA range for biases up to 10 V and 50 V for 0.6-f.lm and 2-11m spacing, respectively.

The spectral responses of two different detectors are shown in Fig. 5 (left). The cut-off wavelength is 280 nm, with a 3 decades rejection ratio between 280 and 300 nm for both spacings. For the 2-f..lm MSM, the re­sponse presents a plateau from 300 to 365 nm corresponding to the GaN layer that is grown underneath the active (Al,Ga)N region [21]. This compo­nent, not present in de measurements is due to a capacitive coupling between the 2-D electron gas at the AlN/GaN interface and the electrodes. At a posi­tive bias of 40 V, the responsivity is 0.044 A/W corresponding to a 20% quantum efficiency. As far as the 0.6-f.lm MSM is concerned, the response decreases more regularly, without any plateau, and shows an overall better rejection of near-UV light. Thus, the parasitic response in the underlying GaN layer is largely reduced for the applied de field and for the ac photo­voltage. This is due to a reduced coupling between the GaN layer and the electrodes when the finger spacing is reduced.

\ I 0.30~ 1 .i:' 0.20 I :2 • ~ 0.10 • .

' &. ' ~ 0.00 .. 0:: 0 20 40 60

Bias {V)

280 320 360 400 Lamnda (nm)

Figure 5. Left: response spectra with 2 J.tm and 0.6 J.tm spacing MSM; responsivity versus bias is given in the inset. Right: contours of equi-values of electric field found from the 2 MSM geometries.

The variation of responses with bias is shown in the inset of Fig. 5 (left). We verify that the dark current at a given bias generally varies as the inverse of the finger spacing, although deviations from this law can be seen. The responsivity increases first sub-linearly ( ~ V0·7) and then linearly with bias. The knee at about 40 V for the 2-11m MSM and 10 V for the 0.6-f.lm MSM corresponds to a transition from photovoltaic to photoconductive behavior for which the contacts start to inject current. We note that the responsivity is

UV Metal Semiconductor Metal Detectors 87

much larger in the 0.6-flm MSM than in the 2-!lm one at the same bias, or reaches a given responsivity value at a much lower bias.

4.2 IBICC Measurements

We now present experiments based on IBICC measurements on MSM fabri­cated on the same layer on Si(lll). IBICC measurements consist offocusing a 2 MeV 4He + micro beam down to a 1 11m2 spot size with a low flux of less than 400 ions per second. Ions are absorbed in the crystal and create about 105 electron-hole pairs per ion. One electrode (called anode) is grounded while a negative bias is applied on the other electrode (cathode). For each incident ion, a signal was obtained, with the pulse height proportional to the number of collected charges. More details of the experimental procedure can be found in Ref. 23. Figure 6 shows maps of collected charges at 75 V. In Fig. 6 (left), we have selected the events that give rise to a small charge per ion. We observe that these events are located at the edges of the anode. In Fig. 6 (middle), we have selected the events that give rise to a large charge per ion. These events are now located close to the cathode in the Schottky depletion region. Regions in between fingers give rise to a moderate collec­tion. The collection efficiency is given as a function of position for different voltages from 0 to 75 V in Fig. 6 (right). On the anode edges, the collection efficiency increases rapidly with bias up to 30 V, and then remains almost independent of bias. On the cathode the collection efficiency is increasing with bias, and is almost flat below the electrode. As far as the region be­tween the electrodes is concerned, the decrease of the current when moving away from the cathode presents an attenuation length of 5 f.tm. It is a typical length for minority carriers already found on EBIC measurements [24] .

20

..,rJ IS

"':.. j u 10

' a Ill IS 20 B JO

d (p .. )

Figure 6. IBICC response at anode (left) and cathode (middle). Response is plotted versus bias on the right-hand side.

88 J- L. Reverchon et al

Some precautions have to be taken before extending these IBICC results to UV MSM detectors. For instance, the detector is uniformly illuminated by photons whereas the beam is focused in 1 Jlm2 • Nevertheless, we can think that below the cathode, carriers are created in the depletion region so that holes are easily collected even at low bias. Electrons drift towards the anode where they are collected after the screening of the build-up field of the Schottky diode.

4.3 Electric Field Calculation

In order to explain these results, we performed a 2-dimensional calculation of the applied electric field in the structure using a commercial 2-D solver (Atlas-Silvaco ). Parameters used for this calculation are described elsewhere [21]. Figure 5 (right) shows the distribution ofthe electric field in the direc­tion perpendicular to fingers in the 2-Jlm and 0.6-Jlm MSMs for a bias of 15 V. The comparison clearly shows that the high field region extends through the whole spacing between fingers in the 0.6-Jlm MSM whereas it remains confined to the electrode edge in the 2-Jlm MSM. It also shows that the ver­tical extension of the high field region is reduced when the spacing between fingers is reduced. The calculated field distribution thus explains the larger response and the reduced coupling to the GaN layer when the spacing is lim­ited.

In order to calculate the response value from the field distribution, we made the assumption that electron-hole pairs are collected in high-field re­gions only. The high-field criterion was the following: (Al,Ga)N alloys show some localization with a typical energy of 50 me V on a spatial scale of 50 nm [21, 25]. Then, a field higher than 10 kV/cm is needed to collect carriers. For a front side illumination, photons above band edge are absorbed in the first 0.2 Jlm, and the volume of the high-field region is just proportional to the lateral extension of the high-field region beside fingers that are not trans­parent to UV. Because of a slight dependence on the structure parameters such as doping or finger spacing, or on the field value used to define the high field region, the response depends on bias as vr with yin the range of 0.65 to 0.72. The inset of Fig. 5 shows the calculated response as a function of bias for both MSM. It varies as V0· 7, and the absolute value is close to the meas­ured one up to biases where internal gain starts to appear. This variation is intermediate between the extension of the depletion region in a vertical Schottky diode (r= 0.5) and the linear response (y= 1) of a photoconductor with an uniform field assumed.

We describe IBICC results with the same kind of hypotheses and simula­tions as those previously used. Incident ions are absorbed in the (Al,Ga)N

UV Metal Semiconductor Metal Detectors 89

layer on a scale that is larger than the layer thickness, so that we can con­sider that the electron-hole pair generation is uniform in the vertical direc­tion. Electrons and holes are efficiently separated where the field is high enough to overcome localization [21,23]. At the cathode, the high field sepa­rates carriers, and holes are all the more easily collected since the distance to travel is small. Electrons are swept towards the anode, once the build-in field of the Schottky diode is screened. When the bias increases, the high-field region extends below the cathode and separates more and more electron­hole pairs.

4.4 Conclusion for the Geometry of UV Detectors

From IBICC studies, we have shown that it is interesting to increase the cathode area. As far as the region between the electrodes is concerned, sub­micronic studies have shown that spacing between the fingers must be as short as possible. For example, we can see in Fig. 7 (left) that the responsiv­ity increases with cathode area for a constant area and spacing between the fingers. We see also this tendency in Fig. 7 (right) with a larger responsivity for a lower spacing and larger area.

~15 >. u ~ 10 ·u !E w 5

0 10 20 30 Bias voltage (V)

~ 20

[) 15 c -~ 10 !E w 5

0 10 20 30 Bias voltage (V)

Figure 7. Responsivity versus bias for different electrode area and spacing in MOCVD sam­ple (left) and MBE sample (right).

5. PERFORMANCES AND CONCLUSION

MSM detectors benefit from the large band-gap and Schottky barrier of high quality undoped materials. The most impressive performance is the dark cur­rents that are still in the femtoampere range at 35 V. We couldn't measure noise in the best samples. Thus we estimated shot noise, Johnson noise and 1/f noise corresponding to this dark current. A conservative assumption for the ' constant p of fJP/f noise ( 5 x 1 o-5) shows that noise is dominated by shot

90 J- L. Reverchon et al

noise in the fA range. Then we obtain a detectivity of 4xl014 w-1 with a frame rate of 100 Hz. It corresponds to an equivalent power of 2.5 fW or 500 photons/second per pixel of 100 x 100 j.lm2 • In our case, capacity is not measured but may be estimated to 10 fF for 100 x 100 11m2 pixels. MSM are also well suited to work at high frequency. Furthermore, we can stress the advantage of (Al,Ga)N based devices which is the intrinsic selectivity be­tween UV and visible close to five orders of magnitude. We notice that this dynamics of UV/visible is obtained without any antireflection coating that would improve both the peak responsivity and dynamics.

So, we have shown that MSM photodiodes present all of the desirable at­tributes of a flame detector: fabrication simplicity, robustness, large UV/visible rejection, high sensitivity, high speed, low dark current, low noise, high detectivity. Theses performances approach the ones of photo­multipliers (PM) and the best cooled charge coupled devices (CCD). Now a new challenge is to design a Readout Integrated Circuits capable of reading 1 fA with an optimal collection of carriers at 10 V. Risks of breakdown in circuits designed on a small area are important. In a first time, it may be eas­ier to find circuits for large linear array.

If we compare MSM to (Al,Ga)N-based Schottky or p-i-n photodiodes, we observe that spectral selectivity of 4 orders of magnitude has been achieved between UV and visible with an excellent detectivity [26,27,28,29]. The latter devices require a low voltage which is an advantage to adapt to standard ROIC. On the contrary, it is more difficult to achieve dark currents as low as those of MSM owing to the mesa processing and remaining mate­rial difficulties (dark current in the nA or pA range are typical). Conse­quently, different detectors may be adapted to different kinds of applications: MSM for extremely low fluxes for which very low dark current is required (fA), and Schottky or p-i-n photodiodes for larger ones (pA). In all cases, the key reason for choosing (Al,Ga)N-based device would be the spectral selectivity between UV and visible light.

ACKNOWLEDGEMENTS

This work was partially supported by DGA (contract N° 00-34-068). One author (MM) wishes to acknowledge financial support from a Curie Re­search Grant (G5TR-CT-2001-00064). Thanks are due to R. Me Kinnon (NRC) for numerical simulations and ONERA for technical support.

UV Metal Semiconductor Metal Detectors 91

REFERENCES

1. B. Gil, Group III Nitride Semiconductor Compounds: Physics and applications (Clarendon Press, Oxford, 1998).

2. Y. A. Goldberg, Semicond. Sci. Teclmol. 14, R41 (1999). 3. E. Momoy, F. Calle, E. Mufioz, and F. Omnes, Appl. Phys. Lett. 74,3401 (1999). 4. C.Pemot, A. Hirano, M. Iwaya, T. Detchprohm, H. Amano, and I. Akasaki, Jpn. J.

Appl. Phys. Part 2, 39, L387 (2000). 5. F. Binet, J.Y. Duboz, E. Rosencher, F. Scholz, and V. Hlirle, Appl. Phys. Lett. 69,

1202 (1996). 6. E. Mufioz, E. Monroy, J.A. Garrido, I. Izpura, F.J Sanchez, M.A. Sanchez-Garcia,

E. Callera, B. Beaumont, and P. Gibart, Appl. Phys. Lett. 71, 870 (1997). 7. J. A. Garrido, E. Monroy, I. Izpura, and E. Mufioz, Semicond. Sci. Techno!. 13, 563

(1998). 8. J. L. Reverchon, M. P. Poisson, and J. Y. Duboz, Semicond. Sci. Teclmol. 16, 720

(2001). 9. B. Beaumont and P. Gibart, Proc. SPIE 3725, 2 (1999). 10. B. Beaumont, J.P. Faurie, E. Frayssinet, E. Aujol, and P. Gibard, this volume. 11. V. Kirchner, H. Heinke, D. Hommel, J. Z. Domagala, and M. Leszczynski, Appl.

Phys. Lett. 77, 1434 (2000). 12. Nikolaev, I. Nikitina, A. Zubrilov, M. Mynbaeva, Y. Melnik, and V. Dmitriev,

MRS Internet J. Nitride Semicond. Res. 581, W6.5 (2000). 13. L. J. Schowalter, Y. Shusterman, R. Wang, I. Bhat, G. Arunmozhi, and G. A. Slack,

Appl. Phys. Lett. 76, 985 (2000). 14. F. Omnes, N. Marenco, B. Beaumont, Ph. De Mierry, E. Momoy, F. Calle, and E.

Mufioz, J. Appl. Phys. 86, 5286 (1999). 15. F. Semond, P. Lorenzini, N. Grandjean, and J. Massies, Appl. Phys. Lett. 78, 335

(2001). 16. N. Grandjean, J. Massies, P. Vennegues, M. Leroux, F. Demongeot, M. Renucci,

and J. Frandon, J. Appl. Phys. 83, 1379 (1997). 17. M. Mosca, J.- L. Reverchon, N. Grandjean, F. Omnes, J.-Y. Duboz, I. Ribet, and M.

Tauvy, Mat. Res. Soc. Symp. Proc. Vol 764, Material Research Society (2003). 18. C. M. Jeon and J.- L. Lee, Appl. Phys. Lett. 82, 4301 (2003). 19. J. C. Carrano, T. Li, D. L. Brown, P. A. Grudowski, C. J. Eiting, R. D. Dupuis, and

J. C. Campbell, Appl. Phys. Lett. 73,2405 (1998). 20. S. W. Seo, K. K. Lee, Sangbeom Kang, S. Huang, William A. Doolittle, N. M. Jok­

erst, and A. S. Brown, Appl. Phys. Lett. 79, 1372 (2001). 21. J. Y. Duboz, J. L. Reverchon, D. Adam, B. Damilano, N. Grandjean, F. Semond,

and J. Massies, J. Appl. Phys. 92, 5602 (2002). 22. N. M. Wong, U. Chowdhury, C. L. Collins, B. Yang, J. C. Denyszyn, K. S. Kim, J.

C. Campbell, and R. D. Dupuis, Phys. Stat. Sol. (a), 188, 333 (2001). 23. L. Hirsch, P. Moretto, J.-Y. Duboz, J.-L. Reverchon, B. Damilano, N. Grandjean, F.

Semond, and J. Massies, J. Appl. Phys. 91, 6095 (2002). 24. A. Cremades, M. Albrecht, J. Krinke, R. Dimitrov, M. Stutzmann, and H. P. Strunk,

J. Appl. Phys. 87, 2357 (2000). 25. F. Semond, N. Antoine-Vincent, N. Sclmell, M. Leroux, J. Massies, P. Disseix, J.

Leymarie, and A. Vasson, Phys. Stat. Sol. (a), 183, 163 (2001). 26. E. Momoy, F. Calle, E. Mufioz, F. Omnes, B. Beaumont, and P. Gibart, J. Elect.

Mat., 28, 240 (1998).

92 J- L. Reverchon et a!

27. G. Parish, S. Keller, P. Kozodoy, J. P. Ibbetson, H. Marchand, P. T. Fini, S. B. Fleischer, S. P. DenBaars, U.K. Mishra, and E. J. Tarsa, Appl. Phys. Lett. 75, 247 (1999).

28. V. Adivarahan, G. Simin, G. Tamulaitis, R. Srinivasan, J. Yang, M. AsifKhan, M. S. Shur, and R. Gaska, Appl. Phys. Lett. 79, 1903 (2001).

29. N. Biyikli, 0. Aytur, I. Kimukin, T. Tut, and E. Ozbay, Appl. Phys. Lett. 81, 3272 (2002).

CHARACTERIZATION OF ADVANCED MATERIALS FOR OPTOELECTRONICS BY USING UV LASERS AND FOUR-WAVE MIXING TECHNIQUES

K. JARASIONAS Institute of Materials Science and Applied Research, Vilnius University, and Joint Center of Semiconductor Optoelectronics, Sauletekio ave. 9, Bldg. 3, LT-2040 Vilnius, Lithuania E-mail: kestutis.jarasiunas@ff.vu.lt; tel. (3705) 236 6036

Abstract: Development of four-wave mixing on picosecond free-carrier grating tech­nique and its recent applications to probe carrier dynamics in various het­erostructures are presented. Carrier plasma parameters, such as carrier life­times and bipolar/monopolar diffusion coefficients as well as surface recombination velocities in heterostructures of GaN/sapphire, InGaN/GaN, CdTe/GaAs, ZnTe homoepitaxial structures, and heavily doped p-GaAs dou­ble heterostructures have been determined. It has been shown that investiga­tion of electrical properties allows characterization of epilayer and interface quality as well as related changes of structural properties by optical means.

Key words: optical nonlinearities, four-wave mixing, nonequilibrium carriers, diffusion, recombination, surface recombination velocity, heterostructures, GaN, InGaN, GaAs, ZnTe, CdTe

1. INTRODUCTION

Semiconductor optoelectronics brought together the materials technology and nonlinear optics, demonstrating possibilities of bridging the gap between the optics and electronics. The field developed rapidly, exploiting a progress in semiconductor technology and the subsequent nonlinear-optical studies.

93

M.S. Shur and A. Zukauskas (eds.), UV Solid-State Light Emitters and Detectors, 93-109. © 2004 Kluwer Academic Publishers.

94 K. Jarasiunas

As a result, many practical applications of semiconductor lasers, light emit­ting and laser diodes, modulators, and high-speed detectors became a reality.

The studies of light interaction with matter furthered the deeper under­standing in optical nonlinearities and developed methods of nonlinear spec­troscopy as well as engineering applications. Recent achievements in the fields of coherent nonlinear spectroscopy, basic science of nonlinear optics, and modem technology for applied research are reviewed in monographies [1,2]. Nevertheless, further progress in development of nonlinear optical techniques and especially their applications for characterization of semicon­ductor materials still remain scattered through various publications. Nonlin­ear optical techniques, based on a strong correlation between electrical and optical processes, opened a possibility to analyze electrical processes in non­destructive "all-optical" way (i.e. without electrical contacts) and to develop relevant measurement techniques for studies of nonequilibrium carrier dy­namics. Light-induced dynamic grating technique [3,4] has been demon­strated as a simple but powerful technique for control of photoelectrical processes and transport in bulk crystals, at the surface, and in the multiple quantum well structures.

This paper presents development of four-wave mixing on picosecond free carrier grating technique and its recent applications to probe carrier dynam­ics in various materials and structures. The aspects of materials research will be underlined here instead of an analysis of optical nonlinearities via nonlin­ear susceptibility i 3 l. Experimental studies of carrier generation, diffusion and recombination processes and determination of carrier plasma parameters (as nonequilibrium carrier concentration, recombination time, diffusion coef­ficient, and surface recombination velocity) in nitride-based, III-V, and II-VI heterostructures are reviewed.

2. BASIC PRINCIPLESANDADVANTAGES OF THE FWM TECHNIQUE

Light diffraction on light-induced dynamic gratings has been an object of intensive studies a decade ago (see review papers [2-5]). A particular advan­tage of this technique is its capability to monitor carrier generation, transport and recombination, as it bridges the photoelectrical properties of semicon­ductors with related optical nonlinearities, which are monitored by time­resolved dynamic holography. The latter approach-excitation of a crystal with light interference pattern-has a number of advantages against the other "excite-probe" techniques (Fig. 1 ). Light diffraction creates new beams in a dark field, thus increasing the signal-to-noise ratio. Next, the intensity of diffracted beams nonlinearly reflects the modulation amplitude of refractive

Characterization of Advanced Materials for Optoelectronics 95

a)

INDUCED ABSORPTION INDUCED REFLECTION

INOUCEO REFRACTION INDUCED DIFFRACnON

b)

9

K - 2!!... g- A

lighl intcn~i l)' I

I = 10 (1+mcosK,x)~

N0 + ' cosK, x

Figure 1. a) Techniques of the light-induced absorption, reflection, refraction, and diffrac­tion; b) formation of a dynamic grating by interference of two coherent laser beams.

index. In addition, spatial modulation of carrier density allows simple moni­toring of diffusion processes along the grating vector by varying the grating period, A, and, thus, enables separation of diffusion from recombination. Finally, variation of excitation wavelength leads to various thickness of pho­toexcited region. The spatial in-depth resolution is defined by the thickness of a bulk crystal, if excited by a quantum with the energy h v < Eg, and may reach a submicron thickness at strong light absorption (h v> Eg). In-plane resolution is limited by the number of grating lines in the excited spot and vary from 50 Jlm to I mm.

Development of the FWM technique requires specific knowledge of dy­namic holography, nonlinear optics, and semiconductor physics. Table I lists the quantitative relationships, which bridge the light-induced modulation of optical properties and diffraction efficiency with optical and electrical parameters of a semiconductor. From the point of view of materials research, the most important part in this loop is the set of equations, which join quanti­tatively the generation, recombination, and transport processes with the measured light diffraction characteristics, i.e. grating kinetics at various pe-

Table I. Optical nonlinearity: a bridge between the optical and electrical parameters. Field What it provides

Dynamic • A configuration to create and monitor spatial complex refractive holography index modulation n*=n+ik:

n(x,t) = n0+~n(t)cos(Kx) k(x,t) = k0+~t)cos(Kx).

• A type of diffraction grating: phase, amplitude, or mixed one. • Relationships between the diffraction efficiency 1] = 11/Ir and

modulation amplitude of~ or M: 17 = ( tm.nd/ lli + ( tm.kdi'Ai Nonlinear Optics • Modulation mechanisms and their coefficients

Semiconductor Physics

~nFc = n eh M e,h ~EO= n eo Esc· • Nonequilibrium carrier density M e,h or SC field Esc and their

spatio-temporal evolution, governed by carrier generation­transport-recombination: i.e. by optical (a, /1) and electric pa­rameters (D, TR, S). Quantitative relationships follow from the so­lution of a set of continuity and current density equations.

96 K. Jarasiunas

riods and excitation intensities, dependence of diffraction efficiency on exci­tation intensity.

It goes without saying that the a priori knowledge of the dominant refrac­tive index modulation mechanism in the conditions of experiment is of the utmost importance. As a rule in the spectral region below the band gap, the free-carrier nonlinearity is dominant with the refractive index modulation !!..n1c prevailing over light-induced absorption index change !!..k1c = !!..a1<.A/4;r< !!..nfc· The refractive index modulation by free carriers !!..nfc = neh Me,h is given by the well-known Drude- Lorentz model with the refractive index modulation coefficient by one electron-hole pair, neh, [3,4,6]

It is seen that the electrons dominate in refractive index modulation, even at bipolar carrier generation, because me < mh. The value of refractive index modulation neh varies in the range of (4-7)x10-21 cm-3 (for CdTe, GaAs, GaN) at the wavelength of 1.06 ).till of a probe beam. This coefficient to­gether with the thickness of excited region d determines the sensitivity of the technique, i.e. the minimum value of the product (.1nd), which can be de­tected in diffraction. Usually, the signal/noise ratio of~ 1 is reached at the diffraction efficiency of '7 ~ 1 o-5, what corresponds to (.1nd) 2:: 0.001 em, and this limit corresponds to the lowest carrier density of 1015 cm-3 (in a few mm thick bulk crystals) or 1018 cm-3 (in a 1-)..lm thick epitaxial layer) which can be assessed by the FWM technique.

3. CONFIGURATIONS AND INSTRUMENTATION OF TIME-RESOLVED FWM

Two following figures (Figs. 2 and 3) present configurations of optical schemes and instrumentation, used for free carrier grating recording and probing in bulk crystals and in layered structures, correspondingly. These schemes present two common configurations of FWM interaction, involving two combinations of four waves. The case of degenerate FWM (Fig. 2) in­volves four waves whose wavelengths are equal: the forward s-polarized waves record a grating in the bulk and the backward p-polarized delayed probe wave monitors the grating decay. The diffracted wave counterpropa­gates to one of the recording beams at the Bragg-matched angle and is ex­tracted by a Glan prism. The scheme allows extraction of all the diffracted beam intensity, which may reach 100% of the probe beam for light diffrac-

Characterization of Advanced Materials for Optoelectronics 97

Controller PC

Figure 2. Scheme of a four-wave mixing setup. The elements shown are: I. Sample; 2. Translation stages; 3. Glan prism; 4. Delay time of the probe beam; 5. Shutter; 6. Attenu­ator; 7. Detectors; 8. Beam splitters; 9. Dielectric mirrors.

tion on a thick grating. The DFWM is used to study cases of weak interac­tion in bulk crystals, as deep impurity-related carrier generation in semi­insulating materials or two-photon interband transitions. If a sample is thin and the grating period is large enough to satisfy condition that the factor Q = 2;rM/(nA2) ~ 1, then the regime of light self-diffraction takes place, and the diffracted pump beams can be observed in the far field of diffraction (the latter case was used to monitor homogeneity of defect distribution in GaAs wafers [7]).

The second arrangement is used for investigation of highly absorbing thin samples (epitaxial layers, heterostructures, multiple quantum well struc­tures). Here the beams at wavelengths above the Eg are used to record the free-carrier grating, while the probe beam wavelength is in the transparency region of a sample. The carrier grating is created at the very surface with the excitation depth deff being close to the inverse absorption coefficient. The further bipolar diffusion of carriers to the sample depth significantly expands

Coherent ps light pulses from VAG or pa rarnelrk la:;er ~ •

-:.~ AIR

Pump 101 AI lcllf

Figure 3. Configuration for a surface grating recording.

98 K. Jarasiunas

the excited area in bulk crystals, while a potential barrier in heterostructures may localize carriers in the front layer. A delayed probe beam monitors the electron-hole density as a function of time via the time-varying diffraction efficiency of the grating. This configuration has been applied initially to de­termine surface recombination velocities in GaAs and InP crystals [8].

The intensities of recording (10), probing (lp), transmitted probe (lpT) and diffracted (11) beams are controlled by Si photodiodes. The data acquisition system measures the instantaneous diffraction efficiency of the grating IJ(t) = (11 - J,)llpT (the scattered background signal J, is extracted from the diffracted beam) and the transmission of the sample T(t) = lpr/lr The system measures all the signals, and the Lab View system presents the kinetics in a required intensity window.

The FWM technique requires coherence of the laser beams that record the dynamic grating. In our experiments, we used a picosecond mode-locked Nd:YAG laser model PL-2143 (Ekspla Co.), which also emits the second (532 nm) and third (355 nm) harmonics, used for recording "surface" grat­ings in III-V compounds and nitrides, respectively. A parametric generator PG-401 (Expla Co.) emitting in the spectral range from 420 to 2000 nm was also used to record gratings in II-VI heterostructures and the grating decay was monitored by the delayed beam of PL-2143 at 1064 nm.

The grating decay kinetics is measured at various grating periods to plot an "angular" dependence of the grating decay time TG versus grating period. The decay time of the grating efficiency IJ(t) ~ exp( -2t/ rG) corresponds to the time interval t = TG,in which the 17 value decreases by e2 times, while the carrier modulation decreases by e times in this time interval. The plot in a form 1/rG vs. (2tr/Ai allows separation of diffusion processes from carrier recombination, as the inverse of rG(t) is the sum of two decay process of spa­tial carrier modulation:

(2)

where TR is the recombination time and TD = A2/(4reD) is the diffusion time of the grating erasure. A slope of the plot provides D value, while the inter­section with the ordinate axis yields the 1/rR value.

An "exposure" characteristic of diffraction is the dependence of diffrac­tion efficiency on excitation beam intensity, 17(10), which is also called a "lux-diffraction" (LDCh) characteristic (like "lux-ampere" characteristic in measurements of photoconductivity). The slope of a LDCh in log-log plot, y, reveals carrier generation rate vs. excitation: for linear carrier generation r= 2, while at two-photon absorption r= 4. At deep-trap assisted carrier generation, the value of r may vary from r= 2 to r< 1, indicating an ex-

Characterization of Advanced Materials for Optoelectronics 99

hausting of the traps with excitation, or increase up to y= 4 at high excita­tions, when two-step transitions via impurity states dominate [ 6].

The one more characteristic is a mapping of a wafer, which provides pla­nar distribution of defects responsible for carrier generation or recombina­tion. For mapping, the sample is positioned on a translation stage and in­plane variation of the diffraction efficiency 'l(x,y) is measured while other parameters (the excitation energy, grating period, and delay time of probe beam) are fixed. The mapping of commercial 3-inch GaAs wafers in regime of self-diffraction revealed W -shape distribution of deep donor EL2, which correlated well with the dislocation density [7].

4. SAMPLES

We investigated various heterostructures by FWM technique with an attempt to reveal peculiarities of electrical properties in the epitaxial layers and in the vicinity of interfaces, which are very sensitive to lattice mismatch and re­lated changes in structural/optical properties. Here we present some results of recent studies carried out in MOCVD-grown GaN layers on sapphire, SiC, or Si substrates, InGaN/GaN, as well MOVPE and hydrogen-transport-VPE grown II-VI heterostructures CdTe/GaAs, and in heavily C-doped GaAs lay­ers embedded in a double-heterostructure (DHS).

In bulk crystals, the application of FWM technique for investigation of electrical activity of deep defects and modification of their charge state by co-doping as well as related carrier transport features were the objects of recent studies in semi-insulating II-VI compounds [6,7,9-11]. Defect related properties in semi-insulating bulk crystals of GaAs:EL2, CdTe and CdZnTe, doped by deep vanadium impurity and co-doped by shallow donors or accep­tors (Cl,As) as well as in proton-irradiated GaAs:Cr crystals allowed studies of impurity-related carrier generation, determination of carrier lifetimes, type of dominant carriers, photoexcited from deep traps, density of residual de­fects, and control uniformity of commercial GaAs and CdTe wafers. The studies of bulk SiC and GaN wafers grown for the electronics and optoelec­tronics industry are in progress.

5. EXPERIMENTAL RESULTS AND DISCUSSION

5.1 GaN-Based Heterostructures

The experiments were performed on a 2.6 ~m undoped GaN sample grown at 1075 °C by MOCVD on 25-nm thick GaN buffer layer, deposited at low

100

a) 1

= " >: u 0.1 c .~ u

l.: .... "' c . ~ u " 0.0 1

!i:l Q

0

S (cm ls):

--- lxl04

-- Sxl04

1:1.106

De lay tim e, ps

b)

1019

·~ E u

.£ 1018 <I)

c <> " ... 0

' C 101 7 ...

" u

0.0 0.5

K. Jarasiunas

I 0.4 mJ em-' .1 10.5 )Jnl

1.0

z, ~m 1.5

Figure 4. Grating decay kinetics in 2.6 J.Lm-thick GaN epilayer on sapphire at various grating periods, fitted by numerical calculations using Eq. 3 (a) and carrier in-depth pro­files for various probing times (b).

temperature (450 °C) on sapphire (0001) substrate [12] . The thickness ofthe buffer layer was optimized to reduce the residual strain in the GaN epilayer. The lnxGa1_xN samples used in this study were grown on a basal-plane (0001) sapphire substrate by MOCVD [13]. Prior to the InxGa1_xN alloy layer, a 1000-nm thick GaN epilayer was deposited at 980 °C under a pres­sure of 76 Torr and served as a buffer layer. All InGaN samples were nomi­nally undoped. The thickness of lnxGa1_xN epitaxial layers was 50 nm. The averaged In fraction x was estimated by X-ray diffraction measurements. In this study, we show the results of three representative samples with In con­tent varying from 8 to 15%.

The transient free carrier gratings were recorded by exciting the front layer of the structure by two coherent 25-ps duration laser pulses at 355 nm of the Nd:YAG laser directed to the sample at a certain angle, which could be varied from 2 to 7 degrees. This allowed us to change the dynamic grating period from 3.1 to 10.5 1-1m. We probed this free carrier grating by the de­layed probe laser pulse at 1064 nm through measuring the intensity of its first diffraction order. For comparison, we also measured carrier dynamics the same way in the vicinity of internal buffer-epilayer interface excited through the sapphire.

Figure 4a shows the grating decay kinetics measured in the 2.6-1-!m thick GaN epilayer at various grating periods. A typical behavior of a surface grat­ing in bulk crystals [8,14], a very fast initial decay that at later times evolves into a slower exponential process, was observed. The decay indicated contri-

Characterization of Advanced Materials for Optoelectronics 101

bution of the surface and nonlinear recombination in the initial stage fol­lowed by a linear recombination and diffusion at later times (M ~ 500 ps). Using the TG values extracted from the tail parts of kinetics, we plotted a dependence of 1/rG vs. (27r/A)2 and determined the bipolar dif­fusion coefficient Da = 1.7 cm2/s and effective carrier lifetime •~u= 670 ps (the effective time accounts for all channels of recombination; at given con­ditions we assume that both linear and surface recombination contribute to its value). The Da value allows estimation of hole diffusion coefficient Dh ::::: Dj2, hole mobility Jih = 32 cm2N·s, and hole diffusion length Lh = 0.28 J..tm.

The surface recombination velocity was evaluated by solving two­dimensional continuity equation

oN(x,z,t) G( _....:.....c____:___:_= x,z,t) + V[D(N)VN(x,z,t)]

ot

N(x,z,t) 2 ( ) BN x,z,t (3)

with the boundary conditions for a semi-infinite media, as the carrier diffu­sion length is much less than the epilayer thickness:

iN(x,z,t)l =·-8 -N(x,O,t),N(x,oo,t) = 0, & z~o D(N)

(4)

where G(x,z,t) is the carrier generation rate by light interference pattern, B is the bimolecular carrier recombination rate, and S is the surface recombina­tion velocity. Using the determined value of Da = 1.7 cm2/s and the coeffi­cient B = 4.7x10- 11 cm3/s at 300 K, we fitted the numerical calculations to the experimentally measured set of kinetics (Fig. 4a), and determined the values of S = 5x104 cm/s and linear recombination time of TR = 950 ps [15].

The numerical simulation of carrier distribution within the epilayer is also useful in determining the effective thickness of the photoexcited region and evaluation of in-depth resolution of the technique. In Fig. 4b, we present the calculated in-depth profiles of carrier density for various probing times, using the values of D, S, and TR determined above. It is seen that diffusion into the bulk and surface recombination rapidly dilute the carrier plasma generated initially in the depth deff close to the inverse absorption coefficient 11 a= 130 nm. The deffvalue increases with time up to 1 J..tm, and the carrier density simultaneously decreases to a value of (0.5-1 )x 1018 em -J, thus allow­ing results being independent on nonlinear recombination.

Comparison of the determined D and TR values of the given GaN epilayer with published ones have shown the high quality of the used layer. The stud-

102 K. Jarasiunas

Excitation: a) 3.0 b) =! lnGa (layer) I I 4n:' ~ =- D • Ga (sub tratc) r,, r • "' 2.8 <.)

"' <> -;; ·t:,; ::: <= - 2.6 <....

'" ~

<: ... 0 '.::l <.)

E <,;; ..... • r , 0 •

2 3 4 0.0 0.5 1.0

41!' (1~-tm ' ) Probe delay time (ns )

Figure 5. Carrier dynamics in lnGaN/GaN structure and in the buffer layer ofGaN (a) and the angular dependence of grating decay times (b).

ies of 7-!lm thick GaN epilayer at its homogeneous in-depth excitation by 532 nm a provided trap-assisted carrier recombination lifetime of 100 ps and 1.1 ns in a bulk [ 16]. The determined hole diffusion length of 0.28 1..1.m at a 1-!lm distance from the interface is comparable with that of Lh = 0.25 1..1.m at a distance of -8 1..1.m from the GaN/sapphire interface for thick HYPE-grown quasi-bulk n-GaN samples [17]. The carrier dynamics in MOCVD grown - 1-!lm thick GaN/sapphire layers exhibited a relaxation time of - 50 ps and bipolar diffusion coefficient of Da ~ 0.16 cm2 /s [ 18], thus confirming essen­tial contribution of dislocations as centers of nonradiative recombination in GaN [19].

The 50-nm thick lnxGa1_xN epitaxial layers present a case of very thin single layers, in which in-plane diffusion and recombination of carriers have been studied by FWM technique at 300 K. At excitation by UV beams at 355 nm, the photoexited carriers are confined in the front layer, since the InGaN/GaN interface presents potential barriers both for electrons and holes. Nevertheless, the incident light reaches the 1-!lm thick GaN buffer and cre­ates free carriers, which also contribute to light diffraction. More detailed analysis of the carrier spatial distribution and its evolution, using Eq. (3), allowed evaluation of the contribution of InGaN and GaN layers to the dif­fraction signal [20]. Their initial contribution to diffraction signal, which is proportional to the squared carrier density, equals to the ratio of about 2:1. With time, the contribution of GaN to diffraction vanished rapidly due to essentially faster carrier decay in the buffer than in the InGaN layer (Fig. 5a). This simplified the analysis of diffraction kinetics, as at delay time 11t > 100-150 ps the diffraction is solely determined by the carrier modula­tion dynamics merely in the InGaN layer. The grating decay kinetics, in the range of A from 3 to 10.5 1..1.m allowed determination of the bipolar diffusion

Characterization of Advanced Materials for Optoelectronics 103

Content of In:

I- 8%

II- 10% III- 15% IV- GaN/sapphire interface

Excitation (m J/cm2)

Figure 6. Exposure characteristics in InGaN layers with different In content and in the GaN/sapphire interface.

coefficient Da = 2.1±0.2 cm2/s and the effective recombination time TR eff= 480 ps for highly excited InGaN layer (x = 8%), see Fig.5b.

Simulation of carrier dynamics in the 50-nm thick InGaN layer [20] has shown that the nonexponential part of decay that is present on experimental curves of InGaN/GaN (see, e.g. Fig. 5a) describes nonlinear processes in the front layer and can be attributed to radiative bimolecular recombination of carriers in InGaN. This conclusion was strongly supported by the experimen­tal data that the non-exponential part in grating decay was becoming faster with excitation (in the range from 0.4 to 1 mJ/cm2) as well as with increasing In content due to carrier localization effects, which stimulate processes of radiative recombination [21 ,22].

The dependence of diffraction efficiency vs. excitation intensity sensi­tively revealed the carrier density dependent changes of the recombination rate in InGaN and GaN layers. Nearly linear carrier recombination was found at the excitation below 1 mJ/cm2, while at higher excitations the char­acteristic exhibited a tendency for strong saturation of diffraction efficiency (Fig. 6). We attribute the latter behavior to a decrease of carrier lifetime to values shorter than the laser pulse duration due to reaching the threshold of radiative recombination, which prevails over other recombination channels at high carrier densities. Figure 6 shows that this threshold is reached most easily in the layer with 10% of In content, thus indicating the optimal In con­tent to localize carriers. The excitation of internal GaN/sapphire interface shows that carrier capture at dislocations dominate with short enough carrier lifetime, thus no signature of stimulated emission is present (see curve IV in Fig. 6). The correlation between the grating decay time and the saturation efficiency was also observed in GaN/Si layers: the longer was the carrier lifetime, the more pronounced was saturation of the diffraction efficiency.

104 K. Jarasiunas

Deeper analysis of carrier recombination mechanisms vs. excitation requires studies of temporally and spectrally resolved photoluminescence.

5.2 Heavily Doped Double GaAs:C Heterostructures

Investigation of carrier dynamics at extremely high doping concentrations (>1019 cm-3) by time-resolved FWM is demonstrated below. Carrier trans­port and recombination have been investigated in heavily carbon-doped GaAs layer (p0 = (1-2)xl0 19 cm-3), embedded in a double-heterostructure. The carriers were injected into the 1-f..tm thickp-GaAs layer sandwiched be­tween 50-nm thick AlGaAs:C (p0 = 1018 cm-3) or InGaP:Si layers, using light interference pattern of two picosecond laser pulses at 532 nm. Due to the density gradient along the z-axis, carriers diffuse away from the surface and become confined in the GaAs:C layer. The measurements were carried out at 300 K at the excitation intensity which corresponds to nonequilibrium carrier density of about 1018 cm-3 in the GaAs layer.

The carrier dynamics in heavily-doped DHS (Fig. 7) was found very dif­ferent from that in the bulk GaAs crystals. The fast component nearly fol­lowed the laser pulse temporal shape in both samples but was more pro­nounced in the DHS with the AlGaAs:C barriers. After this fast decay, a slower exponential relaxation with a time constant of -1 ns was dominant in both samples. Assuming that the latter decay component resembles the diffu­sion and recombination of carries confined in the quantum wells [ 11], we measured its decay time constant ra at various values of A and determined the in-plane diffusion coefficient D as well as the carrier lifetime rR in the p­doped GaAs:C layers (Fig. 7b): D = 35 cm2/s and rR = 1.5 ns for the layer with p 0 = 2x1019 cm-3, and D = 27 cm2/s, rR = 2 ns for another one with

1 1019 - 3 Po = x em .

a)

The values of D correspond to minority electron diffusion, as the meas-

101

0 250

Grming period /1 :

-<>- 7.1 ~m -- 9.5~m

----- 19.1 flnl

1.1 ns

Probe delay (ps)

b)

0.4 0.6 4 , 21112 (~m -2)

Figure 7. Grating decay kinetics in GaAs:C heterostructure with AlGaAs barriers (a) and the determined carrier plasma parameters in the heavily doped p-GaAs layers.

Characterization of Advanced Materials for Optoelectronics 105

urement regime ensured the condition that the doping density was essentially higher than that of photoexcited electrons: p0 >> !lNe. The corresponding electron mobilities in heavily doped GaAs layers, estimated from the meas­ured D values, are of about 1100 cm2N·s and 1450 cm2N ·s. The increase of mobility in the heavier doped layer (withp0 = 2x1019 cm-3) is in quantitative agreement with carrier scattering features in heavily-doped GaAs: at the hole density of2x1019 cm-3, the Fermi energy is 60 meV in the valance band and many states for holes are occupied. This effect of degeneracy effectively re­duces the electron-hole scattering according to Pauli exclusion principle, and Monte Carlo calculations predict an increase of minority electron mobil­ity in highly dopedp-GaAs atp > 1019 cm-3 [23,24].

The performed FWM studies complementary with time-resolved PL de­cay time measurements in heavily doped layers [25] will provide access to minority carrier lifetimes vs. doping. More detailed evaluation of carrier cap­ture in the front barrier and a role of interface is in progress [26].

5.3 CdTe and ZnTe based heterostructures

Similarly to given above studies of GaAs heterostructures, we investigated II-VI hereostructures grown by different techniques. In Fig. 8, carrier dy­namics in MOVPE-grown ~1-)lm thick CdTe on ZnTe/GaAs buffer and in 36-)lm thick HYPE-grown CdTe epilayer on GaAs are compared. FC grating kinetics in the thin epilayer revealed 870-ps decay times [14], while the 36-Jlm thick H2T-grown layers exhibited very fast decay followed by a weak relaxation tail of 300 ps time [27]. The latter decay was found very similar to

"' c -~ "' c 0

I

~ II

III

Excitation energy (a.u .) 10-3 L....-...1-.--~~-----'----~----=-'---'----'

0 300 600 Delay time (ps)

Figure 8. Free carrier grating kinetics in as-grown (I) and aged (Ill) CdTe/ZnTe/GaAs heterostructure and in HVPE-grown thick CdTe layer (II). The inset shows an exposure characteristic of diffraction in the thick layer.

106 K. Jarasiunas

dynamics in the aged 1.6-f..lm thick CdTe/ZnTe/GaAs layer, where migration of defects from the strained interface towards the epilayer surface caused the epilayer degradation: the carrier lifetime in the aged layer decreased to 60 ps. High number of nonradiative recombination centers in as-grown HYPE lay­ers leads to very fast carrier recombination at dislocations, which attract both carr!ers and also cause band-gap modulation by dislocation stress and strain fields [28]. A slope r= 1.6 of exposure characteristic corresponds to sublin­ear carrier density dependence on excitation (N ~ 1112, see inset of Fig. 8) and confirms the bimolecular carrier recombination at dislocations in the HYPE­grown ZnTe epilayer.

Recent studies of homoepitaxial ZnTe layers, grown on ZnTe:P sub­strates, have been carried out using FWM with wavelength-tunable picosec­ond pulses and revealed different recombination features [29]: the decrease of carrier lifetime with excitation in the epilayers, while the increase of TR at the surface of the substrate was caused by saturation of residual traps.

This tendency, together with ns-duration lifetimes showed by the epilay­ers, can be clearly ascribed to a dominant bimolecular recombination rate and a low density of trapping defects in the epilayers. The high value of D = 11 cm2/s in ZnTe epilayer was very close to that in the substrate, point­ing out towards an occurrence of similar density ( ~ 1018 em -3) of ionized im­purities in both epitaxial and bulk crystals.

6. HOLO-DEVICES FOR NONDESTRUCTIVE CONTROL OF SEMICONDUCTORS

There is a constant need of novel techniques able to control the material electrical parameters, in spite that commercially available devices (e.g. TEM, AFM, XRD, SIMS, EPR, FTIR) allow control of a semiconductor ma­terial morphology, defect density, strain, density of impurities, their charge state, etc. Studies of photoelectrical properties under laser excitation in Vil­nius University resulted in development of light-induced transient grating technique and its applications for nondestructive studies of technologically important materials, as Si, GaAs, CdTe, etc. We foresee useful applications of the time-resolved FWM technique for control of wide bang gap materials, both bulk wafers and heterostructures.

Recently, we implemented the picosecond FWM technique into devices and assembled Gointly with Ekspla Co, www.ekspla.com) HOLO modules, which can be attached to a pulsed laser source. The novel devices (Fig. 9) are able to control bulk materials or heterostructures, using one or two fixed wavelengths and advanced data acquisition system.

Characterization of Advanced Materials for Optoelectronics 107

Figure 9. A pilot device HOL0-1 (Ekspla Co.) for nondestructive control of bulk crystals,

based on DFWM at 1.06 !liD.

In conclusion, the feasibility of picosecond FWM technique on free­carrier gratings for studies of carrier dynamics in heterostructures has been demonstrated. Carrier diffusion and recombination in differently grown GaN, InGaN, heavily doped GaAs:C and CdTe-based heterostructures or ZnTe homoepitaxial structures have been investigated and a number of car­rier parameters were determined. The studies allowed implementation of the FWM technique into novel HOLO devices (Ekspla Co.) for nondestructive control and metrology of nonequilibrium carrier parameters in bulk crystals and layered structures.

ACKNOWLEDGEMENTS

The research was sponsored by NATO's Scientific Affairs Division in the framework of the Science for Peace Programme (Project SfP-974476), European Commission (Contract No. GSMA-CT-2002-04047), and Lithua­nian State Science and Studies Foundation. The author would like to thank co-workers Dr. M. Sudzius, Dr. V. Gudelis, R. Aleksiejunas, T. Malinauskas for substantial contribution, and acknowledges the cooperation of colleagues from Rensselaer Polytechnic Institute, Troy, USA (Prof. M. S. Shur), Sensor Electronic Technology, Inc. (Dr. R. Gaska), Leece University, Italy (Prof. N. Lovergine), University of Tokushima, Japan (Prof. S. Sakai), Ferdinand­Braun-Insitut fUr Hochstfrequentztechnik, Germany (Dr. M. Weyers), and Institute of Physics, Belorus (Prof. G. Yablonskii).

108 K. Jarasiunas

REFERENCES

1. J. Shah, Ultrafast Spectroscopy of Semiconductors and Semiconductor Nanostruc­tures (Springer, Berlin, 1999).

2. Nonlinear Optics in Semiconductors, I & II, ed. by E. Garrnire and A. Kost, Semi­conductors and Semimetals Vol. 59 (Acad. Press, New York, 1999).

3. H. J. Eichler, P. Gunter, and D. Pohl, Light-Induced Dynamic Gratings, Springer Series in Optical Sciences Vol. 50 (Springer, Berlin, 1986).

4. R. K. Jain and M. B. Klein, in Optical Phase Conjugation, ed. by R. A. Fisher (Acad. Press, New York, 1983), chap. 10.

5. Special Issue on Dynamic Gratings and Four-Wave Mixing, IEEE J. Quant. Electr. QE-22, No.8 (1986).

6. M. Sudzius, R. Aleksiejilnas, K. Jarasiilnas, D. Verstraeten, and J. Launay. Semi­cond. Sci. Techno!. 18, 367 (2003).

7. J. Vaitkus, K. Jarasiilnas, E. Gaubas, and M. Petrauskas, Semicond. Sci. Techno!. 7, Al31 (1992).

8. C. A. Hoffman, K. Jarasiilnas, H. J. Gerritsen, and A. Nurmikko, Appl. Phys. Lett. 33, 536 (1978).

9. K. Jarasiilnas, L. Bastiene, J. C. Launay, P. Delaye, G. Roosen, Semicond. Sci. Techno!. 14,48 (1999).

10. K. Jarasiilnas and N. Lovergine, Mater. Sciece & Eng. B91-92, I 00 (2002). II. K. Jarasiilnas, V. Mizeikis, S. Iwamoto, M. Nishioka, T. Someya, K. Fukutani, Y.

Arakawa, T. Shimura, and K. Kuroda, Jpn. J. Appl. Phys. 39, 5781 (2000). 12. T. Wang, T. Shirahama, H. B. Sun. H.X. Wang, J. Bai, S. Sakai, and H. Misawa,

Appl. Phys. Lett. 76, 2220 (2000). 13. E. Kuokstis, J. W. Yang, G. Simin, M. AsifKhan, R. Gaska, and M.S. Shur, Appl.

Phys. Lett. 80, 977 (2002). 14. V. Mizeikis, K. Jarasiilnas, N. Lovergine, and P. Prete, J. Cryst. Growth 214/215,

234 (2000). 15. R. Aleksiejilnas, M. Sildzius, T. Malinauskas, J. Vaitkus, K. Jarasiilnas, and S. Sa-

kai, Appl. Phys. Lett. 83, 1557 (2003). 16. B. Taheri, J. Hays, J. J. Song, and B. Goldenberg, Appl. Phys. Lett. 68, 587 (1996). 17. L. Chemyak, A. Osinsky, A. Schulte, Solid State Electron. 45, 1687 (200 1 ). 18. H. Haag, B. Honerlage, 0. Briot, and R. L. Aulombard, Phys. Rev. B 60, 11624

(1999). 19. T. Sugahara, H. Sato, M. Hao, Y. Naoi, S. Kurai, S. Tottori, K. Yamashita, K. Ni­

shino, L.T. Romano, and S. Sakai, Jpn. J. Appl. Phys. 37, L398 (1998), Part 2. 20. R. Aleksiejunas, V. Gudelis, M. Sudzius, K. Jarasiunas, Q. Fareed, R. Gaska, M. S.

Shur, J. J. Zhang, J. Yang, E. Kuokstis, and M.A. Khan, Phys. Stat. Sol. (c) 0 (2003) (in press).

21. E. Kuokstis, J. W. Yang, G. Simin, M. Asif Khan, R. Gaska, and M. S. Shur, Appl. Phys. Lett. 80, 977 (2002).

22. L. H. Robins, A. J. Paul, C. A. Parker, J. C. Roberts, S.M. Bedair, E. L. Piner, N.A. El-Masry, MRS IntemetJ. Nitride Semicond. Res. 4S1, G3.22 (1999).

23. T. Furuta and M. Tomizava, Appl. Phys. Lett. 56,824 (1990). 24. J.R. Lowney and H.S. Bennett, J. Appl. Phys. 69,7102 (1991). 25. A. Maasdorf, S. Gramlich, E. Richter, F. Brunner, M. Weyers, G. Trankle, J.W.

Tomm, Y. I. Mazur, D. Nickel, V. Malyarchuk, T. Gunther, Ch. Lienau, A. Bar­wolff, and T. Elsaesser. J. Appl. Phys. 91, 5072 (2002).

Characterization of Advanced Materials for Optoelectronics 109

26. K. Jarasiunas, R. Aleksiejunas, V. Gudelis, M. Sudzius, A. Maa~dorf, F. Brunner, and M. Weyers, Abstracts of lOth Int. Con[. on Defects DRIP-X (September 2003, Batz-sur Mer, France).

27. K. Jarasiunas, E. Gaubas, R. Aleksiejunas, M. Sudzius, V. Gudelis, T. Malinauskas, P. Prete, A.M. Mancini, and N. Lovergine, Phys. Stat. Sol. (a) 195, 238 (2003).

28. V. Kafukauskas, J. Storasta, and J. Vaitkus, J. Appl. Phys. 80, 2269 (1996). 29. R. Aleksiejunas, T. Malinauskas, M. Sudzius, K. Jarasiunas, N. Lovergine, M.

Traversa, P. Prete, A.M. Mancini, and T. Asahi, Proc. lOth European Workshop on MOVPE (June 2003, Leece, Italy), paper PS.II.06.

QUANTUM PHOSPORS Observation of the photon cascade emission process for Pr3+­

doped phosphors under vacuum ultraviolet (VUV) and X-ray excitation

A.P. VINK 1'2, E. VANDERKOLK 1, P. DORENBOS 1,

and C.W.E. VAN EIJK 1

1 Radiation Technology Group, Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands 2 Chemical Sciences, Netherlands Organisation for Scientific Research, P.O. Box 93470, 2509 AL The Hague, The Netherlands

Abstract: In luminescent-tube lighting (TL), mercury is used to excite (Amax = 254 nm) three phosphors, resulting in white light. The use of mercury however gives environmental problems and causes an undesired delay in lamp startup. If mercury is replaced by xenon, which is already gaseous at room temperature and harmless to the environment, both problems are solved. Xenon however emits at higher energy (.A.max = 172 nm) and the phosphors used in mercury­based tube lighting show a less efficient absorption to this vacuum ultra violet (VUV) radiation. Therefore, much effort is put in' developing new phosphors. The Pr3+ ( 4/) ion shows strong absorption in the VUV range, which can be as­signed to the 4/---+4/5d1 transition. Another interesting effect is that in some hosts the Pr3+ ion can, after excitation into the 4/ 5d1 bands, show a two-step relaxation to the ground state. This process is called photon cascade emission (PCE) or quantum cutting and could result in a quantum efficiency larger than 100%. The emitted photons are typically in the violet and green spectral re­gion. This contribution describes the principles of Pr3+ quantum cutting and presents methods to select host materials in which Pr3+ shows quantum cutting. Furthermore, several Pr3+ -doped hosts are presented, which show the PCE process both under VUV and host excitation.

Key words: TL lighting, quantum cutting, photon cascade emission process, Pr3+, host materials

Ill

M.S. Shur and A. Zukauskas (eds.), UV Solid-State Light Emitters and Detectors, 111-126. © 2004 Kluwer Academic Publishers.

112 A. P. Vink et a!

1. NEW GENERATION LIGHTING

For ages, mankind has sought for ways to make light when there is only darkness. These efforts resulted in the discovery of fire many thousands of years ago. More recently (in 1879) Thomas Alva Edison (1847-1931) in­vented the light bulb, which makes use of the blackbody radiation of hot carbon.

Although the light bulb still is commonly used for lighting applications, the disadvantage is that it produces mainly heat instead of light.

Just before the second world war, a new kind of lighting was invented with the main purpose to get more light than heat. In this type of lighting, emission originating from a mercury discharge is used to excite phosphor material, which is located on the inside (C in Fig. 1) of the glass tube (A). The emission from the mercury discharge is mainly at 254 nm. This type of lighting is called TL after the French name "tube luminescent".

A

B Hg~Hg*-+ Hg

c

Figure 1. Schematic representation ofTL lighting.

Phosphors of the early period (1938-1948) were based on mixtures of two phosphors (MgW04 and (Zn,Be)2Si04:Mn2+) to make white light. MgW04 shows broad-band emission over the whole spectral region, whereas the Mn2+ -doped silicate covers the green to red spectral region [ 1].

As the silicate material is not very stable under discharge conditions and the used beryllium is highly toxic, alternatives for these phosphors were in­vestigated. The mix of two phosphors was replaced by a single phosphor, of which emission covers almost the whole visible spectral region. This phos­phor was based on the halophosphate Ca(P04) 3F /Cl and was doubly doped with Sb3+ and Mn2+. The trivalent antimony emits around 500 nm. Part of the energy is however transferred to the Mn2+ ion, which emits around 600 nm. The manganese ion itself does not absorb the 254-nm light from the mercury

Quantum Phosphors 113

discharge. These halophosphate phosphors show high brightness and high color rendering (CRI of about 60) [1].

At the start of the seventies, a set of three phosphors was developed which generally not show broad-band emission, but narrow line emission. The commonly used phosphors are BaMgAI100 17:Eu2+ (blue emitting), (Ce,Gd)MgB50 10:Tb3+ (green emitting) and Y20 3:Eu3+ (red emitting). A mix of these three phosphors, emitting around 450, 550 and 610 nm respectively, results in white light with an even higher color rendering (CRI of 80-85) than the halophosphate phosphors [ 1].

All three commonly used phosphors are based on the lanthanides. The lanthanide group is located at high-atomic number end of Mendeleev's peri­odic table and is characterized by a partially filled 4fshell. The 4fshell is an inner shell, which is shielded from its surrounding by the already-filled 5i and 5l shell. Therefore, the position of the energy levels of the lanthanides is mainly determined by the electrostatic interaction between the electrons and not by the static and dynamic crystal field.

More recently, effort came to replace the commonly used mercury by a noble gas. The advantages for getting rid of mercury are based on environ­mental considerations and the need to extend the application for TL lighting. The applications for mercury-based TL lighting are limited as a certain startup time was found. The startup time is present because the mercury first has to evaporate before it can emit UV light. The delay in startup is un­wanted for possible applications of TL lighting in photocopiers and brake lights. The three phosphors present in the commonly used TL lighting

1,0 8.3eV

0.9 7 2eV

0,8

0,7

;- 0,6

'" z: 0,5 ·c;; ~ 0,4

\__ 'E

I - 0,3

0,2

0,1 I ) '-....

0,0 120 130 140 150 160 170 180 190 200

Wavelength (nm)

Figure 2 . Emission spectrum of a X e discharge. The emission i s situated in the vacuum ultra­violet (VUV) spectral r egion.

114 A. P. Vink et al

are optimized for the ultraviolet discharge, but not for the xenon discharge, which shows only emission below 200 nm (see Fig. 2).

2. QUANTUM CUTTING

The major disadvantage of using xenon in the new-generation TL lighting is the low energy efficiency. At 254 nm, about 50% ofthe excitation energy is lost, whereas at 172 nm this loss is more than 70%. A solution to this prob­lem could be the introduction of phosphors, which can emit two photons per absorbed photon. The dopant (e.g. a lanthanide) however needs specific en­ergy levels to make a two-step emission to the ground state possible. The two-step emission process is called Photon Cascade Emission (PCE) or quantum cutting. In the past, several different quantum cutting systems where found, which are described below.

In 1974 Piper [2] and Sommerdijk [3] independently found that if Pr3+­

doped YF 3 is irradiated with a zinc or mercury lamp, emission from the high­energy 1 S0 level occurs. This results in the emission of two photons at 404 nm and 480 nm.

More recently, research led to a quantum cutting system based on energy transfer. It was found that excitation of the LiGdF 4:Eu3+ phosphor led to the emission of two red photons. At around 50,000 cm-1 (200 nm), the 6GJ levels of Gd3+ can be populated. After a Gd-Eu cross-relaxation step, the 5D0 level of Eu3+ can be populated and emission of the first red photon occurs. The remaining energy (Gd3+ in the 6PJ level) can be transferred to the Eu3+, where the 5D0 level can be populated. From this level, emission of a second photon can occur. A schematic representation of the Gd-Eu quantum cutting system is presented in Fig. 3 [ 4,5].

Although the theoretical quantum efficiency can be as high as 200%, the actual quantum efficiency was found to be much lower. The low value of 32% (including quantum cutting) for the measured quantum efficiency can be ascribed to weak absorption in the Gd3+ ions. The 8 S712~ 6P J transition is parity forbidden and therefore competes with the host absorption in the same region. The addition of co-dopants, with a strong absorption in the 50,000 cm-1 region, could increase the quantum efficiency [5].

Another concept of quantum cutting without activators is exciting of a semiconductor (or insulator) at twice the band gap energy. Via Auger­interaction, an electron high up in the conduction band and a hole at the bot­tom of the valence band generate two electron-hole pairs located at the top of the valence band (holes) and the bottom of the conduction band ( elec­trons) [5].

Quantum Phosphors 115

Although this process sounds very promising, major limitations are set to the use of these materials. It was discovered that the Auger effect is efficient at energies of about 2.5 times the band gap. As the xenon discharge emission is located around 7.2 eV (see Fig. 2), materials with a band gap smaller than 2.9 e V are needed. Such materials however are not transparent to visible light. Using such higher excitation energy would again imply a low energy efficiency.

30

I ,, po-1 ..., •• ,

20

10

So

-- 1f Gd •

Figure 3. Quantum cutting using the Gd-Eu system.

v'·

In this contribution, the focus will be on the Pr3+ -doped materials. Al­though the Pr3+ quantum cutting system show major disadvantages, like emitted photons with different energies and photons in the violet spectral region, it offers the advantages that it shows a strong absorption of the Xe discharge emission region and quantum cutting without the use of co­dopants.

3. PHOTON CASCADE EMISSION WITH PR3+

3.1 Introduction

In the PCE process, a high-energy photon is absorbed, what results in two­step emission to the ground state. This process was already predicted in 1957

116 A. P. Vink et a!

by Dexter. He called this process "photon splitting" [6]. The first actual ex­ample for this process was discovered in 1974 independently by two differ­ent research groups [2,3].

The group of Sommerdijk from Philips Research Laboratories found the PCE process in YF3:Pr3+ and a-NaYF4:Pr3+ under excitation with a zinc lamp [3], whereas Piper and co-workers from General Electric found the PCE process in a large number ofPr3+-doped hosts, also including YF3:Pr3+ and a­NaYF4:Pr3+, by excitation with a mercury lamp [2]. Piper already stated that, to obtain the PCE process, the 4/ 1 So level must be located below the 4/ 5d1

bands. Piper also measured Pr3+-doped materials, like LiYF4 where only 4/5i~4/ emission was found [2].

In Fig. 4, the energy level scheme of the Pr3+ ion is shown. The energy level scheme consists of 13 Pr3+ ( 4/) levels, which are mainly situated in the energy region up to about 22,000 cm-1• Around 47,500 cm- 1, the isolated 1S0

level can be found. PCE can occur when this level is populated. This level can be populated by excitation with ultraviolet (UV) light of about 215 nm, but the 3H4~ 1 S0 transition is inefficient as it is parity forbidden. A more ef­ficient population of the 1 S0 level can be achieved by pumping into the 4/ 5i bands. If the bands are situated above the 1S0 level, efficient nonradiative relaxation leads to population of the 1 S0 level. From this level, two-step emission to the ground state can occur.

60

E

I "~ >- 50 e> Ill c:

4115<1 1

-+--.----- -- 's. w

40

30

20

10

0

-+----t---'G, ~~~~~~~:: - 'F,.'F, = 3H6.'F,

' H :::t::::::::t::::= 'H:

Figure 4. Energy level scheme of the Pr3+ ion showing the PCE process.

Quantum Phosphors 117

The most efficient two-step emission route is via 1 S0~ 1 I6 and 3Po~3H4 transitions. This results in photons with a wavelength around 404 nm Cso~1 I6 transition) and 480 nm CP0~3H4 transition). Loss processes are other 1 S0~ 2s+ 1 L; transitions, which are visible as emissions in the UV re­gion. Nonradiative relaxation from the 3P0 to the 10 2 leads to red emission. The process of nonradiative relaxation is more efficient for hosts with higher phonon energies. In principle, fluoride quantum cutters show strong green emission, whereas the Pr3+ emission for oxide-based hosts is situated in the red spectral region. This red emission is however weak as the emission from the 10 2 level is quenched by cross-relaxation at low Pr3+ concentrations.

In Fig. 5, an excitation- and emission spectrum of SrAlF5:Pr3+, a typical quantum cutter, is shown. The different emissions are assigned [7].

1 .o----------~,~--------, 3H. -> 4f'5d ' •s -> •1 A. em =404 nm

0 8

A.exc=189 nm 0,8

=! ~ 0,6 ?;-·u; c ~ c 0,4

0.2 host

0, 0 -}r."T"TT,....,...,TT"",..,.,.-MT~;:;:;:T~rn=;':.-rn;:;:;;::;::;.,.,c-r=r::;:.....r;:r;:;:,........,...;:;=;~

50 100 150 200 250 300 350 400 450 500 550 600 650 700

Wavelength (nm}

Figure 5. Excitation- (Aem = 404 nm) and emission (J.exc = 189 nm) spectrum of SrAIF5:Pr3+

measured at T = I 0 K.

Many Pr3+ -doped hosts show quantum cutting. It was found for Pr3+­doped SrAl12019 [8], LaB306 [9], LaMgB50 10 [10], KMgF3 [11], NaMgF3 [12], LiCaAlF6, LiSrAlF6 [13], LuF3, BaMgF4 [14], LaZrF7, a-LaZr3F1s [15], BaSiF6 [16], Sro.7Lao.3Al,uMgo.3019, SrB407 [17], SrS04, and BaS04 [18].

Not all Pr3+-doped hosts show PCE. The occurrence of the PCE process is determined by the position of the high-energy levels. As the 4f orbitals are shielded form their surrounding by the filled 5i and 5p6 orbitals, it can be expected that the location of the 4/ energy levels, like the 1 S0 level, is almost independent of the host in which the lanthanide is doped. The interaction between the host and 5d1 orbitals is much stronger, however. Therefore it is

118 A. P. Vink et a!

expected that the position of the 4/5d1 energy levels show a strong variation with respect to the host in which the Pr3+ ion is doped. This is clearly illus­trated in Figs. 6 and 7 where the excitation- and emission spectra of BaS04:Pr3+ and CaS04:Pr3+ are shown [19].

1 ·0 ~------~-------------.-s.-->-.-,. ~--~E~x~ci-ta~ti-on~(~~.-m-=4-0~3~nm-,

0,8

::J .i 0,6

~ 04 (/) ' c Q)

c 0,2

-- Emission (2,,,=187 nm

's.->'o,

250 300 350 400 450 500

Wavelength (nm)

550

'o,->' H, +

3P0->3F2

I

600 650

Figure 6. Excitation- (Aem = 403 run) and emission (Jcm. = 187 run) spectrum of BaS04:Pr3+

measured at T = I 0 K.

1 ·0 ~a-:_4_t'5_d_'_->,-H-.~.~a-.-4f715-d~1-~>,~F------------~E~xc~it~a~ti-on~(-~.-m-=2~3~0~n-m~)

b·. 4f'5d'->3H, M 4f'5d'->>'F, E · · (;. 190 m) 0,8

::J cti 0,6 -i!' 'iii 0,4 c $ c 0,2

<

100 150

, -- miSSIOn "oxc= n

:

~ 'D,->3H,

4f15d1->1D 1 , 1 2 +

4f 5d -> G, j 4f15d1->3P 11 (J·O 1 2) ' p ->'F I J' • .•• 1 2

~_).__ 200 250 300 350 400 450 500 550 600 650

Wavelength (nm)

Figure 7. Excitation- (Jc,, = 230 run) and emission (Aexc = 190 run) spectrum of CaS04:Pr3+

measured at T = I 0 K.

The emission spectrum of BaS04:Pr3+ is that typical of an oxide-based quantum cutter. It shows a strong 1S0- 116 emission but almost no green and red emission. The absence of these emissions can be ascribed to the high

Quantum Phosphors 119

phonon energy of the sulphate host, favoring nonradiative relaxation from the 3P0 to the 1D2 level and quenching of the 1D2 level by cross-relaxation (20].

The emission spectrum of CaS04:Pr3+, however, shows no 4/ line emis­sion but the broad-band emission, which is mainly located far in the UV spectral region. For CaS04:Pr3+ the lowest-energy 4/5d1 band is located be­low the I So level. Therefore, excitation into the 4/ si bands yields a parity­allowed 4/ 5i ----.4/ emission and no population of the 1 S0 level.

3.2 Selecting Hosts

To predict which Pr3+-doped hosts show the PCE process, data from the op­tical properties of Ce3+ can be used. The Ce3+ ion only has one electron in the 4fshell. This gives rise to only two 4/ levels: 7F512 ground state and the 7F712

excited state. The 4/5d1 bands are located at higher energy. Like for Pr3+, the energy of these bands is strongly dependent on the host in which the lantha­nide is doped. The Ce3+ 4/Si bands are however located at much lower en­ergy that the Pr3+ bands. In Fig. 8A and 8B, the excitation spectra of CaS04:CeH,Na+ and CaS04:Pr3+ are shown.

B ~ (1 II I \ I I \

J \ ;\ eno 575 ss.o 52.5 ~.o «17.5 450 42.5 .so.o 37.5 35.0 32.5 ll.O 72.5 10.0 67.5 sso 52.5 eno 575 55.0 52.5 so.o •7.5 450 ·~s

Eivgy (to' an'> Eivgy (1o' an'>

Figure 8. Excitation spectra of CaS04:Ce3+,Na+ (A, l em = 326.5 nm ) and CaS04:Pr3+ (8, Aem = 230 nm). Note the difference in the energy scale.

From Fig. 8, it can be clearly observed that the structure of the 5d bands is roughly the same. An analysis of the position of the 4f-15d1 bands in many different lanthanide host shows that the energy difference between the lowest-energy 4}15i band of Ce3+ and Pr3+ is around 12,240 cm-1 (21,22]. This fixed energy difference can be used to predict Pr3+ -based quantum cut­ters from Ce3+ data. The position of the Ce3+ 4/5d1 bands is known in many hosts as these bands are generally located in the UV spectral region and are therefore relatively easy to measure.

120 A. P. Vink eta!

The position of the 4f~1 5i bands in general is roughly determined by two independent factors: (i) by the centroid energy Ec, which is mainly de­termined by type of ligands in the host and (ii) by the crystal field splitting E:cf' which is determined by the symmetry and coordination number (CN) of the dopant in the host [21,22].

To get quantum cutting in a Pr3+ -doped host, a host material is needed in which the lanthanide has a high centroid energy and a small crystal field splitting. Most of the Pr3+ -doped materials are based on fluorides, which have a large centroid energy. The BaS04:Pr3+ material, on the other hand, shows the PCE process (see Fig. 6), because c:,1, is rather small.

3.3 Two Types of Emissions

Some Pr3+ -doped materials show both broad-band 4/5d1 emission and 4l [ 1S0) emission (resulting in quantum cutting) under excitation into the 4/si bands. An explanation for this behavior can be the presence of two different cation sites, like in CaF2:Pr3+ [23]. This behavior can however not explain the emission behavior of the BaS04:Pr3+ material, where only one Pr-site site is expected.

0.8

--:- 0.6 :::l

~ -'=' ·u; c 0.4 2 c

0.2

T= 10K -T=292K

200 250 300 350 400 450 500

Wavelength (nm)

Figure 9. Emission spectrum (A.exc = 188 nm) of BaS04:Pr3+ at T= 10 K (dashed line) and 292 K (solid line). The 4/ and 4/Si emissions are assigned.

The intensity ratio between all 4/5d1 and 4l emissions was found to be strongly temperature dependent as is shown in Fig. 9. Also, a decrease of more than three times was found for the decay time of the 1 S0 emissions (from 190 to 56 ns) [24).

Quantum Phosphors 121

This behavior can be explained by assuming thermal population of the lowest energy 4/ 5i band from the I So level. This process is temperature dependent as at higher temperatures more electrons can cross the energy bar­rier between the 1 S0 and the 4/ 5d1 band. By fitting both the intensity ratio and the decay time for the different temperatures, a value for the energy bar­rier M of about 0.04±0.006 eV (325±50 cm-1) was found [25].

The process of thermal population is present for all Pr3+ -doped materials, which show the PCE process, but thermal population can only be observed at room temperature for materials where the 4/ 5i bands are relatively close to the 1 S0 level. This thermal population process is a serious concern for the application of oxide-based Pr3+ -doped quantum cutters.

3.4 Quantum Cutting with X Rays

The observation of the PCE process is not only limited for excitation into the 4/5i bands using high-energy vacuum ultraviolet (VUV) light. Recent lit­erature also shows quantum cutting for Pr3+ -doped hosts under X ray excita­tion [25,26].

In Fig. 10 the emission spectrum of SrAlF5:Pr3+ at T= 100 K and T= 350 K under X-ray excitation is shown. From this figure, it can be ob­served that emission from 1 S0 level is absent at 100 K, whereas it is present at 350 K.

0.5 T=100K

-T=350K

0.4

:::i ~ 0.3

~ ·c;; c $ 0.2 c

0.1

200 300 400 500 600 700 800

Wavelength (nm)

Figure 10. X-ray excited emission spectra of SrAlF 5 :Pr3+ measured at T = I 00 K (dotted line) and T= 350 K (solid line). The emission spectra are corrected for the response of the measur­ing system and photo multiplier.

122 A. P. Vink et a/

Other emission originating from the lower-lying Pr3+ levels, like 3P0 and 1D2,

is however clearly visible at both temperatures. This must mean that a route other than the 1S0- 116 emission populates the 3P0 level. Figure 10 also shows that at 100 K, a broadband emission around 450 nm is visible. This emission can be assigned to emission from a localized electron-hole pair (Self­Trapped Exciton, STE). At higher temperature, energy transfer from the STE to the Pr3+ ion can occur. Energy transfer from the STE to the Pr3+ can ex­plain the presence of Pr3+ emission from the lower-lying 3P0 and 1D2 levels, which is present at both temperatures in the region from 480 to 720 nm (see Fig. 10).

It is however crucial for resonant energy-transfer to the 1 So level that the STE band extends up to the energy corresponding to the 3H4- 1S0 transition at a 215 nm. As can be observed from Fig. 10 the STE starts to emit from 240nm.

The process, which is responsible for the typical quantum cutting behav­ior under host excitation at different temperatures, is the direct recombina­tion (without the formation of a STE) of the electron and the hole with the Pr3+ ion. It is however expected that the process of direct recombination is temperature independent.

Two possible explanations for the temperature dependence of the recom­bination process can be proposed: (1) The electron can recombine with Pr3+

forming Pr+ and the hole can be trapped in a VK center and (2) the hole re­combines with Pr3+ to form Pr4+ and the electron is trapped for example at an anion vacancy, forming an F center. The hole (for process 1) or the electron (for process 2) can be de-trapped at elevated temperature and recombine at Pr4+ and Pr2+ resulting in population of the 4/5d1 bands.

Using a temperature study of the 1S0 emission intensity, assuming Ar­rhenius behavior for the intensity, the activation energy 11£ could be deter­mined. The value of M (450 cm- 1 or 0.06 eV) is so small that it can be in­terpreted as an electron trap. The activation energy for migration of holes is much higher, typically in the order of tenths of electronvolts [27]. Therefore it seems that the observation of the PCE process originates from process (2) [7].

The different excitation, energy transfer and emission processes de­scribed above are visualized in Fig. 11. The band gap of SrAlF 5 is about 90,000 cm-1, corresponding to 11 eV. The different processes resulting in Pr3+ emission are visualized in Fig. 11. Excitation of electrons into the con­duction band results in holes in the valence band (1 ). The first process is the formation of an STE, which is shown as (2a) and (2b). The STE can either emit radiatively (3a) or can transfer its energy to Pr3+ (3b ). This energy trans­fer is more efficient at higher temperatures as the STE becomes mobile. Mi-

Quantum Phosphors 123

gration to Pr3+ is followed by energy transfer, populating the lower-lying CPJ, 116 and 10 2) Pr3+ levels. Emission from the 3P0 is shown as (4) [8].

The other process, which leads to PrJ+ emission, is population of the lan­thanide without an intermediate exciton state. Here, the hole is trapped on PrJ+ (5a) and the electron in an electron trap (5b). This situation does not result in any emission from praseodymium at temperatures lower than 150 K. Above this temperature, the electrons in the shallow traps are re­leased and populate the higher 4/5d1 bands of Pr3+ (6). This population of the 4/5d1 bands results in two-photon emission, which is shown as 1 S0-~ 1 I6 (7a) and 3P0~3H4 emission (7b) [8].

": ......... 90 E 4-----~----------~ CB

C) .., 0

5b ...--":__.___,~'­electron trap

0 75 >-e' 2a Q)

tD 60

45

30

15

2b 5a

0~------~---------

~--------------------------~----~~~~VB

Figure 11. Schematic description of the different excitation, emission, and energy transfer processes. The processes, described in the text, are numbered accordingly.

3.5 Energy Transfer of the 180 Emission

For lighting applications, the emission of the first PCE step (' S0~ 1 16) is too much on the short-wavelength side of the visible spectral region. For appli­cation of quantum cutting phosphors in lighting, it is highly preferable to convert the violet photon to a visible photon. A possibility for energy con-

124 A. P. Vink et al

version is to add a co-dopant, which can convert the energy from the 1 Sa-1 16 emission to the visible spectral region. A possible candidate could be the Mn2+ ion as the 6A1 - 4A1/E absorption bands show strong overlap with the 1 S0- 1 16 emission (see Fig. 12).

60

E u ~0 ~

>- 50 e> <I> c: w

40

30

20

10

0

4f15d1

• 'so ------4T

4 2

====== T, 4A,_

--t--+--- 'G• 3 3

==II==!=== F 3, F4

==II==!=== 3Ha,3F2 --11--1--- 3H -~~~--3H'-:-~-~--aA,

Pr3+ Mn2+

Figure 12. Energy level schemes of both Pr3+ and Mn2+ showing the possibility of Pr-Mn en­ergy transfer.

Measurements on SrAlF5:PrJ+,Mn2+ however showed no typical green Mn2+ emission under excitation into the PrJ+ 4/5i bands. X-ray excited emission measurements however show the presence of built-in Mn2+ ions. Up till now it is not understood why the energy transfer does not occur. It was suggested that selection rules for exchange interaction apply, making the energy transfer from the Pr3+ 1S0 state to Mn2+ a forbidden transition [5]. Up till now no theoretical background was found for this claim.

4. CONCLUSIONS

It was shown that quantum cutting is possible for PrJ+ in many, mainly fluo­ride-based, hosts. Furthermore, a possibility to predict whether Pr3+, in a cer-

Quantum Phosphors 125

tain host, shows PCE was explained. This method makes use of the large amount of data, which is available on the optical properties of the Ce3+ ion.

Measurements on an oxide-based quantum cutter (BaS04:Pr3+) showed that the quantum efficiency in the visible spectral region is severely lowered at higher temperatures by an increase of UV emission. This makes the appli­cation of oxide-based quantum cutting phosphors more difficult.

Using X-ray excitation, quantum cutters were studied under band gap ex­citation, creating electrons and holes. It was shown that at low temperatures, broadband STE emission is visible. Only at higher temperatures, PCE be­comes visible. It is expected that for fluoride-based quantum cutters, no band-to-band transitions occur at typical xenon discharge excitation ener­gies. For oxide-based quantum cutters, host lattice excitation is efficient and gives rise to efficiency losses.

The fluoride-based quantum cutters cannot be applied in new-generation TL lighting, as the energy of one of the emitted photons is too far into the UV side of the visible spectral region. Efforts to convert this photon more to the visible region were not successful up till now.

It is highly unlikely that the lanthanide-based quantum cutting phosphor will be applied for (TL) lighting in the near future. Both systems (Gd-Eu and Pr) still have major difficulties, which have to be overcome first. It must also be noted that the application of these phosphors in Light Emitting Di­odes (LEDs) is not possible as the emission of LEDs is too much on long wavelength side to excite the high-energy levels of the quantum cutting phosphors.

ACKNOWLEDGEMENTS

The authors thank Dr. M. Kirm (HASYLAB, DESY Hamburg) for his assis­tance during the experiments performed on the SUPERLUMI set-up and Dr. M. Weil (Institute for Chemical Technology and Analytics, Vienna Univer­sity of Technology) for sample preparation.

The investigations were supported by the Dutch Technology Foundation (STW) and by the IHP-Contract HPRI-CT-1999-00040 of the European Commission.

REFERENCES

1. G. Blasse and B. C. Grabmaier, Luminescent materials (Springer-Verlag, Berlin, Heidelberg, New York, 1994).

2. W.W. Piper, J. A. deLuca, and F. S. Ham, J. Lumin. 8, 344-348 (1974). 3. J. L. Sommerdijk, A. Bril, and A. W. de Jager, J. Lumin. 8, 341-343 (1974).

126

4.

5. 6. 7.

8. 9. 10. 11. 12.

13. 14. 15.

16.

17.

18.

19.

20.

21. 22. 23. 24. 25. 26.

27.

A. P. Vink et al

R. T. Wegh, H. Donker, K. D. Oskam, and A. Meijerink, Science 283, 663-666 (1999). C. Ronda, J Lumin. 100, 301-305 (2002). D. L. Dexter, Phys. Rev. 108, 630-633 (1957). A. P. Vink, P. Dorenbos, J. T. M. de Haas, H. Donker, P. A. Rodnyi, A. G. Avane­sov, and C. W. E. van Eijk, J Phys.: Condens. Matter 14, 8889-8899 (2002). A.M. Srivastava and W. W. Beers, J Lumin. 71,285-290 (1997). A.M. Srivastava and D. A. Doughty,J Electrochem. Soc. 144, Ll90-192 (1997). A.M. Srivastava and D. A. Doughty, J Electrochem. Soc. 143,4113-4115 (1996). I. Sok61ska and S. Ktick, Chern. Phys. 270, 355-362 (2001). N.J. M. Le Masson, A. P. Vink, P. Dorenbos, A. J. J. Bos, J.P. Chaminade, and C. W. E. van Eijk, J Lumin. 101, 17 5-183 (2003 ). S. Ktick and I. Sok61ska, J Electrochem. Soc. 149, 127-130 (2002). S. Klick and I. Sok61ska, Chern. Phys. Lett. 364, 273-278 (2002). E. van der Ko1k, P. Dorenbos, and C. W. E. van Eijk, Opt. Commun. 197, 317-326 (2001). E. van der Kolk, P. Dorenbos, C. W. E. van Eijk, A. P. Vink, C. Fouassier, and F. Guillen, J Lumin. 97, 212-223 (2002). E. van der Ko1k, P. Dorenbos, and C. W. E. van Eijk, J Phys.: Condens. Matter 13, 5471-5486 (2001). E. van der Ko1k, P. Dorenbos, A. P. Vink, R. C. Perego, C. W. E. van Eijk, and A. R. Lakshrnanan, Phys. Rev. B 64, 195129 (200 1 ). A. P. Vink, E. van der Kolk, P. Dorenbos, and C. W. E. van Eijk, J Alloys Camps. 341, 338-341 (2002). A. P. Vink, P. Dorenbos, and C. W. E. van Eijk, J Solid State Chern. 171, 308-312 (2002). P. Dorenbos, J Lumin. 91,91-106 (2000). P. Dorenbos, J Lumin. 91, 155-176 (2000). K. D. Oskam, A. J. Houtepen, and A. Meijerink, J Lumin. 97, 107-114 (2002). A. P. Vink, P. Dorenbos, and C. W. E. van Eijk, Phys. Rev. B 66, 075118 (2002). A.M. Srivastava and S. J. Duclos, Chern. Phys. Lett. 275,453-456 (1997). P. A. Rodnyi, S. B. Mikhrin, P. Dorenbos, E. van der Kolk, C. W. E. van Eijk, A. P. Vink, and A. G. Avanesov, Opt. Commun. 204,237-245 (2002). K. S. Song, Solid State Commun. 9, 1263-1266 (1971).

OPTICAL MEASUREMENTS USING LIGHT-EMITTING DIODES

v I 2 3 A. ZUKAUSKAS , M. S. SHUR , and R. GASKA 1 Institute of Materials Science and Applied Research, Vilnius University, Sauletekio 9-l/L LT-2040 Vilnius, Lithuania 2 Center for Broadband Data Transport, Rensselaer Polytechnic Institute, CII 9017, 110 8th street, Troy, New York 12180, USA 3 Sensor Electronic Technology, Inc., 1195 Atlas Road, Columbia, South Carolina 29209, USA

Abstract: Recent advances in optical measurements using light-emitting diodes (LEDs) are reviewed. The review covers applications of LEDs as stable and compact sources of light, fluorometry including fluorescence lifetime measurements, and spectroscopic applications (photoluminescence line shape, absorption and absorption correlation, surface-plasmon resonance, photoreflection, and Ra­man measurements).

Key words: light-emitting diodes, optical measurements, fluorescence sensing, spectroscopy

1. INTRODUCTION

The advances in semiconductor materials and in improved light extraction techniques led to the development of a new generation of efficient and pow­erful high-brightness LEDs [1]. Red AlGalnP LEDs and violet InGaN LEDs demonstrated efficiencies approaching 60% [2] and in excess of 40% [3], respectively. Visible colored and white phosphor-conversion LEDs are al­ready available in the electrical power range of 1 to 5 W with the optical power output of hundreds of milliwatts [2,4,5]. Further progress in the de­velopment of AllnGaN materials system has resulted in an appearance of ultraviolet (UV) LEDs with the wavelengths as short as 265 nm [6]. High­power near-UV LEDs with the output power of 200m W have been reported recently [7].

127

M.S. Shur and A. tukauskas (eds.), UV Solid-State Light Emitters and Detectors, 127-142. © 2004 Kluwer Academic Publishers.

128 A. Zukauskas, M S. Shur and R. Gaska

In addition to wide spread use of LEDs in signals, full-color video dis­plays and lighting, advanced LEDs find many new applications in optical measurement technology, where they substitute for conventionally used sources of light, such as incandescent or discharge lamps and even lasers. Optical measurement technology benefits from advantages of LED technol­ogy, such as broad range of available emission wavelengths, high efficiency and power, stable output, long lifetimes (1 00,000 h and more), durability, low driving voltage, small dimensions, low self heating, reliability, and low cost. In combination with extremely low noise of the radiant flux [8] and with possibility of high-frequency modulation [9] and subnanosecond pulse generation [10], these advantages resulted in the novel instrumentation based on a variety of optical techniques ranging from simple transmission meas­urements to a more sophisticated spectroscopy.

This paper reviews instrumental applications of LEDs. Section 2 de­scribes applications requiring compact, reliable, and stable sources of light with a narrow-continuum spectrum. Section 3 deals with LED-based fluorometry, including fluorescence lifetime and fluorescence anisotropy measurements. Section 4 briefly describes spectroscopic applications.

2. LEDS AS COMPACT AND STABLE SOURCES OF LIGHT

LEDs are widely used as compact and stable narrow-continuum light sources for optical transmittance and reflectivity measurements, photodetector cali­bration, and as well as for generation of light pulses with stable parameters.

One of the most famous transmittance applications is in pulse oxymetry, a noninvasive method for monitoring arterial oxygen saturation. The method is based on different absorption spectra of hemoglobin saturated with oxygen and desaturated hemoglobin. Typically, the sensor probe consisting of two LEDs (660 nm and 910 nm, respectively) and a photodetector is mounted on a finger of the patent. An ac component of the photodetector signal, which is due to blood pulsation, is used to estimate the amount of oxygen saturation.

Another example of simple application Of LEDs is optical thermometer (Fig. 1). Using a glass fiber, light emitted by the LED is guided to a conven­tional low-pass color filter that is placed in the environment under testing. Another fiber is used to bring the light passed through the filter to a photodetector. The wavelength of the LED is matched with the absorption edge of the filter, which contains semiconductor particles dispersed in a glass matrix. A temperature change shifts the absorption edge of the filter due to semiconductor band gap variation and, as a result, the amount of the transmitted light is altered. The all-glass thermometer probe can be used in

Optical Measurements Using Light-Emitting Diodes 129

strong electric and magnetic fields, under nuclear radiation, and in corrosive chemical environments.

LED

0 D Fiber

Photodetector Color glass filter

Figure 1. Schematic of the LED-based optical thermometer.

Direct switching of the LED driving current can yield light flashes of precise duration and intensity. Such flashes are widely used in machine vi­sion to obtain images of moving objects. A remarkable application of LED­generated stable light pulses is in optical dating of sediments [11]. The method relies on freeing the electrons produced by radiation of natural radio­isotopes and trapped at defect sites of the sediment. Some of these electrons do not relax on a geological time scale unless excited by light. The number of trapped electrons depends on the radiation dose accumulated since the last exposure to light. Under illumination with a precise portion of light, the sediment might exhibit characteristic anti-Stokes luminescence with the amount of photons emitted being proportional to the time since the sediment deposition. Dating of quartz and feldspars was demonstrated using high­brightness 525-nm LEDs. An array ofLEDs was shown to successfully sub­stitute for much more expensive argon-ion- or organic-dye laser systems.

LEDs are narrow-continuum sources with the linewidth of the emission spectrum in the range of 1 0 nm. The relevant coherent length is of the order of 10 J.lm. This implies that LEDs can be used in "white-light" interferome­ters for measurement of small absolute displacements. In comparison with interferometers based on monochromatic or long-coherence-length sources, LED-based interferometers offer simple central-fringe identification. A LED-based fiber-optic interferometer was described in Ref. 12. The interfer­ometer featured a two-arm Michelson design and a three-LED broad­spectrum light source for improved central-fringe identification. The device was successfully tested as a strain sensing system.

130 A. Zukauskas, M S. Shur and R. Gaska

3. FLUOROMETRY

Using advanced LEDs, detectable levels of fluorescence can be excited in a variety of objects. The development of blue and UV chips resulted in a dra­matic increase of LED-based fluorometry applications, especially in the fields of biochemistry, life sciences, and environmental control. This section reviews LED-based fluorescence sensing as well as measurements in time­and frequency domain.

3.1 Fluorescence Sensing

Simple high-precision fluorometers consist of an LED operating in a con­tinuous regime, an optical filter (filtering the fluorescence), and a photodetector [ 13]. An additional optical filter can be used to spectrally iso­late the detector from the excitation emission. Optical fibers that deliver both the excitation light and the fluorescence signal are used in remote configura­tions. To separate the fluorescence signal from the ambient light, the LED is pulsed and amplitude-modulation [14] or phase-sensitive [15] detection technique is used. Owing to low-noise output of LEDs, such inexpensive fluorometers can exhibit high sensitivity and precision, comparable with that of state-of-the-art systems that employ bulky xenon lamps. Compact and simple LED-based fluorometers can be useful for detection of various or­ganic and inorganic compounds in biotechnology, chromatography, water purity control, and hazardous biological agent detection.

LED-based chemical sensors with fluorophores that are either quenched or activated by the substance detect gases, proteins, nucleic acids, etc. [1].

Advanced fluorometric systems employ arrays of different LEDs for multi wavelength excitation that results in partially selective fluorescence measurement [ 16]. Under appropriate signal processing, such systems were demonstrated to resolve the individual contributions in multicomponent mix­tures. By using an array of 370 to 640 nm LEDs, correct identification and quantification of six fluorescent dyes in two to six component mixtures has been achieved [ 16].

3.2 Time-Domain Fluorescence Measurements

LEDs operating in a short-pulse generation regime offer an attractive alter­native to costly and bulky lasers or pulsed arc lamps. In particular, time­resolved fluorescence measurements can be implemented using repetitive short pulses of LED light. The time-domain measurements trace the decay of fluorescence intensity and polarization after excitation with a short pulse, with the obtained fluorescence transient being used to extract the decay time.

Optical Measurements Using Light-Emitting Diodes 131

A simple LED-based system for fluorescence lifetime measurements on the nanosecond time scale [10] contained an avalanche-transistor-based cur­rent driver that pumped a high-brightness blue AllnGaN LED by pulses with a peak current of 2 A at a repetition rate of 10 kHz. In this regime, the LED generated 4-ns UV light pulses with the output of 40 m W. Fluorescence de­cay in quinine sulfate solution was recorded using a conventional time­correlated single-photon counting system and the fluorescence lifetime of 19.5 ns was extracted from the decay kinetics.

Another example of LEDs replacing pulsed lasers and arc flash lamps is time-resolved polarization anisotropy measurements [17]. The fluorophore molecules are excited with their optical transition vectors being parallel to the polarization plane of the exciting light. The initial biased population of molecules becomes increasingly randomized with time due to Brownian ro­tational diffusion. Since the polarization plane of a fluorescence photon is determined by the molecule orientation, the fluorescence polarization meas­urement provides information on molecular interaction. An LED-based tech­nique was applied for time-resolved fluorescence anisotropy measurements in dilute solution of Coumarin 6 in ethylene glycol. The arrangement con­sisted of a blue LED producing highly reproducible 680-ps pulses at a repeti­tion rate of 10 MHz, a low-pas optical filter, sheet polarizers, and a time­correlated single-photon counting system. By measuring the fluorescence intensity decay in two polarizations with high statistical precision, the tran­sient of the anisotropy was extracted and the anisotropy decay time of 2.1 ns was determined.

The simplicity of driving LEDs in a pulsed regime and availability of LEDs over a broad range of wavelengths allow one to compose experimental protocols for even more sophisticated investigation of fluorescence tran­sients using the series of excitation and sampling pulses of different wave­lengths and polarizations with an independent control of intensity and of temporal profile [18].

3.3 Frequency-Domain Fluorescence Lifetime Measurements

Fluorescence lifetime measurements provide useful information on structure, environment, and transient evolution of molecular compounds. Frequency­domain measurements allow one to measure the lifetime without a detailed analysis of the fluorescence decay kinetics, in contrast to time-domain meas­urements, which require numerous data points to be processed for extraction of the lifetime values. Figure 2 illustrates the principle of the frequency­domain measurement [19]. The excitation source is modulated by a sinusoi­dal waveform at an angular frequency w. This results in the modulation of

132 A. Zukauskas, M S. Shur and R. Gaska

the fluorescence signal with the same frequency. However because of a fi­nite fluorescence lifetime r , the fluorescence signal has a phase shift ¢ and the modulation depth is decreased by & factor m. For a single-exponential decay, the phase shift and the relative modulation depth are given by

tan¢ = m r,

---~-~---··--···------~

Fluorescence

-- -- I - ' - '

B/A m=--b/a

' '

a

---·------------'L_

Time

Excitation

I

A

Figure 2. Time variation of the excitation and fluorescence intensity in frequency-domain lifetime measurements. (After Ref. 19.)

These relations imply that by measuring the phase shift and modulation depth, the lifetime can be extracted in two independent ways. Typically, a dependence of the phase shift and/or modulation depth is measured as a function of frequency and the lifetime is extracted by fitting the data points to a simulated dependence. The frequency-domain method uses narrow­frequency-band electronics (typically, standard lock-in amplifiers) and, therefore, has a higher precision than the time-domain measurements relying on wide-band electronics.

LEDs ideally fit the requirements for light sources in frequency-domain fluorescence lifetime measurements on the nanosecond time scale, since they can be directly modulated up to frequencies of hundreds of megahertz. The use of LEDs in frequency-domain measurement regime resulted in a sub­stantial decrease in complexity and in the cost reduction of the instrumenta­tion. An example of fluorescence lifetime measurement in standard fluoro­phore, fuorescein disodium salt, was described in Ref. 9. Blue and green

Optical Measurements Using Light-Emitting Diodes 133

LEDs were biased at 5 rnA current, which was modulated with a radio­frequency power of 4 m W. The measured fluorescence decay time of 3.51 ns indicated that the inexpensive LED-based instrumentation is capable to sub­stitute for much more complex phase-modulation technique with argon-ion laser modulated with a Pockets cell.

Introduction of cost-efficient frequency-domain measurement technology led to the development of numerous sensors using chemically quenched fluorophores, such as ruthenium and platinum ligand complexes. In particu­lar, LED-based fluorescent pH indicators and gas sensors have been devel­oped [1]. Use ofUV LEDs offers even more possibilities, since most organic compounds and biological agents exhibit excitation spectra in the UV range.

Compact LED-based sensors can be assembled into arrays to monitor several chemical or biological species simultaneously. An example is a mul­tichannel detection system that is able to monitor fluorescence lifetimes of many samples in real time [20]. The system detects LED-excited fluores­cence at different wavelengths by means of a multianode photomultiplier and resolves lifetime changes of different fluorophores using phase meter software.

4. LED BASED SPECTROSCOPY

Benefits offered by advanced LEDs, such as direct modulation, stability, small dimensions, low cost, and unique spectral properties were successfully utilized in a variety of spectroscopic applications. In this section, we briefly review recent applications of LEDs in photoluminescence spectroscopy, ab­sorption and absorption correlation spectroscopy, surface-plasmon resonance sensing, photoreflection, and Raman spectroscopy.

4.1 Photoluminescence spectroscopy

Owing to low noise and high stability, LEDs can be used in photolumines­cence measurements for precise characterization of the spectral features.

An example of application of LEDs in luminescence spectroscopy is the investigation of photoluminescence in an InGaN alloy, the key material for fabrication of efficient green to near-UV LEDs. Figure 3 shows temperature dependences of the luminescence band linewidth and peak position in an InGaN epitaxial layer. The InGaN layer was photoexcited using a 3 7 5-nm LED with the active layer based on the InGaN alloy with a smaller indium molar fraction. The luminescence spectra were recorded using a double monochromator and a photon-counting system.

134 A. Zukauskas, M S. Shur and R. Gaska

3.08 0.110

lnGaN I~ Peak position I • 0.108 coo 3.07 0 Linewidth •

0.106 0

> o0000oooo .. >

00~0 306

~ 0.104 ~ • c ..c 0.102 0 '5 3.05 :;::::;

'§ 0.100 • 'iii • • • 0 Q) •• •• .ao a. c

~ 0.098

-~· 3.04 ~ • 0 ctl

Q) 0.096 c.. 0 3.03 0.094

0 0.092 •• • 0 3.02 •• 0.090

0 50 100 150 200 250 300

Temperature (K)

Figure 3. Temperature dependence of the luminescence band linewidth and peak position in an InGaN epilayer excited by an InGaN LED.

Application of a low-noise light source for photoexcitation resulted in a highly precise determination of linewidth and peak position of the lumines­cence band. (Note that the experimental points for the linewidth are scattered within~ 1 meV whereas the linewidth value is of about 100 meV.) This pre­cision resulted in revealing of tiny nonmonotonous behavior of the measured temperature dependences. This nonmonotonous behavior is a signature of an intricate character of the exciton motion over the band potential fluctuations in InGaN alloys [21].

It is worth noting that the price of the LED light source is smaller by a factor of 103 than the price of a He-Cd laser commonly used in such experi­ments. A deeper penetration of LEDs into the UV region [6] may result in partial substitution of excimer lasers and with a commensurate price reduc­tion by a factor of 104•

4.2 Absorption and Absorption Correlation Spectroscopy

Absorption spectroscopy provides with a simple and powerful insight into many physical phenomena. Absorption measurements yield the energies of electronic transitions with high precision and quantitatively characterize the transition probabilities based on the absorption coefficients. Recently, short­wavelength LEDs received attention as light sources for absorption meas­urements in the near UV region. The interest in use of LEDs for near-UV absorption spectroscopy is due to several reasons. First, conventional incan­descent lamps generate a relatively weak flux in this region what diminishes

Optical Measurements Using Light-Emitting Diodes 135

the signal-to-noise ratio, and discharge lamps suffer from inherent instabili­ties. Second, a narrow-band continuum is highly desirable in spectrally re­solved absorption measurements in order to avoid stray light in other spectral regwns.

One of the first demonstrations of LEDs for absorption spectroscopy was performed in dense cesium vapor [22]. A high-brightness blue LED with the driving current increased over its standard value was used as a source of nar­row-band continuum in the spectral range around 390 nm. Owing to a low­noise output of the LED, a few new spectral features were distinguished in the absorption spectrum.

LED1

LED2 Reference cell Measurement cell

Reference detector

Measurement

detector

Figure 4. System schematic for detection of gases by correlation spectroscopy (After Ref. 23).

Based on absorption measurements, an elegant method for target-specific detection of gases, which takes advantage of the full multi-line structure of the absorption spectrum, can be implemented using LEDs [23]. The pro­posed correlation spectroscopy method is based on two complementary sources, an LED with narrow-continuum emission covering the target ab­sorption spectrum and a similar LED with the emission partially absorbed by target gas contained in a reference cell (see Fig. 4). The LEDs are modulated in anti-phase and the driving currents are balanced to produce no net inten­sity modulation at the reference photodetector, which measures the sum in­tensity of these two sources. The two beams are passed through the meas­urement cell and detected by the second photodetector. Once the target gas is present in the measurement cell, the intensity balance between the two beams is violated, since the original beam is absorbed to a larger extent than the partially-absorbed one. The resulting modulation is detected by the sec­ond photodetector. A non-specific absorption with non-matching spectral lines attenuates both beams in the same proportion and no modulation oc­curs. The method was shown to be capable of detecting various concentra­tions of 02, CO, and C02 gases.

136 A. Zukauskas, MS. Shur and R. Gaska

4.3 Surface-Plasmon Resonance

Surface-plasmon waves are guided optical modes, which can be excited in the interface between a conductor and a dielectric. The frequency of the waves is very sensitive to the refractive index of the dielectric. When the wave vector of the incident light matches with that of the surface-plasmon mode, the reflected light is strongly attenuated and a dip in the reflectivity spectrum occurs indicating the presence of a resonance. As a result, small changes in the refraction index can be monitored with the accuracy similar to that of the resonance-frequency measurement. Owing to high sensitivity, surface-plasmon resonance (SPR) is widely used in a variety of gas-, liquid-, and bio-sensors.

Typically, halogen incandescent lamps are used in SPR sensors. The use of LEDs reduces the size and cost of SPR sensors and makes them portable, much more reliable, and suitable for long-lasting real-time measurements. Both narrow-continuum color LEDs [24] and wide-spectrum white LEDs [25] have been already demonstrated as light sources in SPR measurements. An example of white-LED based SPR sensor is schematically shown in Fig. 5.

Monochromator

Lock-in amplifier

Collimator

Polarizer

Figure 5. Schematic setup for the LED-based surface-plasmon resonance measurements. (After Ref. 25.)

The sensor contains a right-angle prism with the sensing surface coated with a 50-nm gold film. The sample fluid is run over the sensing surface

Optical Measurements Using Light-Emitting Diodes 137

within a chamber attached to the prism. A parallel beam of light emitted by the LED is formed by a two-lens collimator and polarized. The reflected light is collected and focused to the entrance slit of the monochromator that disperses the spectrum. An essential part of the setup is a lock-in amplifier that both modulates the driving current of the LED and implements phase­sensitive detection. Like in other LED-based optical measurements, direct modulation of the optical source improves the measurement precision and results in a substantial advantage over halogen-lamp-based systems.

The described white-LED based SPR sensor was shown to be able tore­liably extract values of the refractive index of glycerin-water solutions with four-digit accuracy.

4.4 Photoreflectance

Photoreflectance is a kind of modulation spectroscopy that produces sharp derivative-like features in the reflectance spectra. Free carriers, which are photoexcited by a modulated source of light, modulate the reflectance by altering the refractive index. Photoreflectance is widely used for sensitive characterization of band structure, surface properties, and built-in electric fields in semiconductors. Typically, the modulated photoexcitation is pro­duced by mechanical chopping of emission from lasers or arc lamps, and the reflectance signal is processed by a lock-in amplifier synchronized by an additional photodetector.

Bright and easy-to-modulate LEDs provide a cost-efficient alternative for modulated sources of light in photoreflectance experiments. LEDs offer so­lutions with a variety of photoexcitation wavelengths, high stability, broad frequency range, and absence of mechanical parts. No additional photodetec­tor is required for synchronization, since the LED can be driven by an inter­nal oscillator contained in most lock-in amplifiers. Advantages of the LED­based photoreflectance technique were demonstrated in Ref. 26. Blue and green AllnGaN LEDs (with no de bias) were driven at 5kHz frequency and 100% modulation depth. The LEDs shielded by colored glass filters were positioned next to the samples without focusing optics. Reliable photoreflec­tance spectra of GaAs structures were measured.

4.5 Raman Measurements

Raman spectroscopy is widely used for characterization of vibrational spec­tra in molecules and crystals. Typically, laser radiation is used to produce narrow spectral lines caused via inelastic scattering of light by vibrations. Although the linewidth of LED emission is too large for resolving narrow Raman lines, the low-noise Raman signal can be used as an intrinsic inten-

138 A. Zukauskas, M S. Shur and R. Gaska

sity standard for scaling fluorescence signals. An example of such an appli­cation is measurement of Raman spectra in water for the normalization of fluorescence [27].

Water has a Raman feature with a huge shift of about 3300 cm- 1 and a large intrinsic width (::::: 400 cm-1) that is comparable with the linewidth of emission from LEDs (typically 380 cm-1 for AlGalnP red and amber LEDs and around 1000 cm-1 for green, blue, and near-UV AllnGaN LEDs, respec­tively). The feature is due to five overlapped O-R-intramolecular stretching modes of tetrahedral hydrogen bonded structure of liquid water.

A Raman spectrum was recorded using a blue AllnGaN LED with the emission line peaked at 465 nm and with the 5 W electrical and 0.5 W opti­cal powers. The LED driven by a constant current was mounted directly on the steel optical table that also served as a heat sink. To remove the long­wavelength wing of the LED emission due to localization of carriers at band­tail energy states of the semiconductor alloy and to narrow the excitation spectra in the short-wavelength region, the emission was passed through band-pass and long-pass color-glass filters. The resulting spectrum of LED emission is shown in Fig. 6 by dashed line indicating a peak at 21600 cm- 1

and a full width at half magnitude of -800 cm- 1• The emission was focused on water contained in a fused-silica cell by an optical grade acrylic collima­tor and a lens. The scattered light was collected by a condenser, polarized perpendicular to the scattering plane by a sheet polarizer, and projected on the entrance slit of a double monochromator equipped with a photomultiplier-

Wavenumber I cm·1

16500 18000 19500 21000 22500

11 LED I I

I

0 1500

Figure 6. Raman spectra o f distilled and plumbing water recorded under high-power light emitting diode excitation (points) [ 27]. Solid line, fluorescence background; dashed line, the Rayleigh spectrum.

Optical Measurements Using Light-Emitting Diodes 139

based photon counting system. Pure distilled water as well as potable water from Vilnius city water supply was investigated at room temperature. Lower points in Fig. 6 depict a spectrum recorded for the distilled water sample. The minimal value of the measured Raman shift (2000 cm-1) is limited by the transmission threshold of the band-pass filter used. The feature at 3300 cm-1 is seen to be clearly resolved with the linewidth of about 900 cm-1

due to the broad line of the excitation source. The standard deviation of the Raman signal exactly equaled ..JN: , where N s is the number of photon counts. This deviation was entirely due to the randomness of the spontane­ous photon emission within Poisson distribution, and the limiting value of the signal to noise ratio of Jii: was achieved. This suggests that LEDs can serve as the lowest-noise sources for Raman spectroscopy, similarly to their applications in other optical measurements.

Regular city water exhibits a spectrum with a pronounced fluorescence background (upper points in Fig. 2; the extracted background is shown by solid line). Since the Raman feature is much narrower than the fluorescence band and can still be clearly resolved, it can be used to scale the fluorescence signal. This technique was introduced in airborne laser fluorometry of water to correct the fluorescence signal for water transmittance· [28] and is still widely used for laser-based fluorescence analysis of wastewater. The Raman feature due to 0-H stretching modes makes the normalization convenient and reliable because of the high cross-section of the scattering, large shift, and thermal stability of the central frequency. LEDs make this kind of meas­urements much more cost efficient, since the price of the light source drops by 3 to 4 orders of magnitude as compared to lasers. In addition, LEDs offer higher stability, lower noise and a longer lifetime. The drawbacks that pre­vent the using of LEDs in conventional Raman spectroscopy (a high radia­tion divergence and a large bandwidth) are not essential in the particular case.

The sensitivity of Raman-normalized fluorescence measurements can be substantially improved by introduction of LEDs emitting UV light, which is more favorable for excitation of fluorescence in many organic compounds and biological agents. In addition, Raman-normalization technique might be. useful in LED-based fluorometers with multi wavelength excitation where fluorescence signals are excited by different-wavelength LEDs with different output power. In water purification systems based on UV LEDs, Raman and fluorescence signals excited by emission of the LEDs can be used for the control of the purification process.

140 A. Zukauskas, M S. Shur and R. Gaska

5. SUMMARY

Numerous advantages of LEDs over conventional sources of light and their unique properties have already been exploited in a variety of optical meas­urement techniques. Cumulative progress in LED-based optical measure­ments is expected with deeper penetration of solid-state technology into the UV region and utilization of multiple-wavelength LED arrays. In particular, novel cost-efficient bio-optical applications might be anticipated with devel­opment of LEDs emitting below 280 nm.

ACKNOWLEDGEMENT

The work at Vilnius University was supported by the Lithuanian State Foun­dation of Science and Studies and European Commission supported SELITEC Center (contract No.GSMA-CT-2002-04047). A. Zukauskas ac­knowledges the Lithuanian Ministry of Education and Science for his Fel­lowship.

REFERENCES

I. A. Zukauskas, M. S. Shur, and R. Gaska, Introduction to Solid-State Lighting (Wiley, New York, 2002).

2. M. R. Krames, M. Ochiai-Holcomb, G. E. Hofler, C. Carter-Coman, E. I. Chen, I.-H. Tan, P. Grillot, N. F. Gardner, H. C. Chui, J.-W. Huang, S. A. Stockman, F. A. Kish, M.G. Craford, T. S. Tan, C. P. Kocot, M. Hueschen, J. Posselt, B. Loh, G. Sasser, and D. Collins, "High-power truncated-inverted-pyramid (Al,Ga1_x)o 5

ln05P/GaP light-emitting diodes exhibiting >50% external quantum efficiency," Appl. Phys. Lett. 75, pp. 2365-2367 ( 1999).

3. H. Kudo, Y. Ohuchi, T. Jyouichi, T. Tsunekawa, H. Okagawa, K. Tadatomo, Y. Sudo, M. Kato, and T. Taguchi, "Demonstration of high-efficient InGaN-based violet light-emitting diodes with an external-quantum efficiency of more than 40%," Phys. Stat. Sol. A 200, pp. 95-98 (2003).

4. J. J. Wierer, D. A. Steigerwald, M. R. Krames, J. J. O'Shea, M. J. Ludowise, G. Christenson, Y.-C. Shen, C. Lowery, P. S. Martin, S. Subramanya, W. Gi:itz, N. F. Gardner, R. S. Kern, and S. A. Stockman, "High-power A!GalnN flip-chip light-emitting diodes," Appl. Phys. Lett. 78, pp. 3379-3381 (2001).

5. D. A. Steigerwald, J. C. Bhat, D. Collins, R. M. Fletcher, M. Ochiai Holcomb, M. J. Ludowise, P. S. Martin, and S. L. Rudaz, "Illumination with solid state light­ing technology," IEEE J. Select. Topics Quantum Electron. 8, pp. 310-320 (2002).

6. A. Chitnis, V. Adivarahan, J.P. Zhang, M. Shatalov, S. Wu, J. Yang, G. Simin, M. AsifKhan, X. Hu, Q. Fareed, R. Gaska, M.S. Shur, "Milliwatt power AlGaN quan­tum well deep ultraviolet light emitting diodes," Phys. Stat. Sol. A 200, pp. 99-101 (2003); R. Gaska, A. Khan, M. S. Shur, this volume.

Optical Measurements Using Light-Emitting Diodes 141

7. D. Morita, M. Sano, M. Yamamoto, M. Nonaka, K. Yasutomo, K. Akaishi, S. Nagahama, and T. Mukai, "Over 200 mW on 365 nm ultraviolet light emitting diode of GaN-free structure," Phys. Stat. Sol. A 200, pp. 114-117 (2003).

8. L. Callegaro and E. Puppin, "Lasers and light-emitting diodes as sources for fixed­wavelength magneto-optical phase modulated ellipsometry," Rev. Sci. Instrum. 66, 5375-5376 (1995).

9. J. Sipior, G. M. Carter, J. R. Lakowicz, and G. Rao, "Single quantum well light emitting diodes demonstrated as excitation sources for nanosecond phase­modulation fluorescence lifetime measurements," Rev. Sci. lnstrum. 67, pp. 3795-3798 (1996).

I 0. T. Araki and H. Misawa, "Light emitting diode-based nanosecond ultraviolet light source for fluorescence lifetime measurements," Rev. Sci. Instrum. 66, pp. 5469-5472 (1995).

II. R. B. Galloway, D. G. Hong, and H. J. Napier, "A substantially improved green­light-emitting diode system for luminescence stimulation," Meas. Sci. Techno!. 8, 267-271, (1997).

12. L. Yuan, "White-light interferometric fiber-optic strain sensor from three-peak­wavelength broadband LED source," Appl. Optics 36, pp. 6246-6250.

13. B. W. Smith, B. T. Jones, and J.D. Winefordner, "High-precision fluorimetry with a light-emitting diode source," Appl. Spectrosc. 42, pp. 1469-1472 (1988).

14. Karsten U, Klimant I, and G. Holst, "A new in vivo fluorimetric technique to meas­ure growth of adhering phototrophic microorganisms," Appl. Environ. Microbial. 62, pp. 237-243 (1996).

15. B. D. Mac Craith, G. O'Keeffe, C. McDonagh, and A. K. McEnvoy, "LED-based fibre optic oxygen sensor using sol-gel coating," Electron. Lett. 30, pp. 888-889 (1994).

16. S. J. Hart and R. D. JiJi, "Light emitting diode excitation emission matrix fluores­cence spectroscopy," Analyst 127, pp. 1693-1699 (2002).

17. P. Kapusta, R. Erdmann, U. Ortmann, and M. Wahl, "Time-resolved fluorescence anisotropy measurements made simple," J Fluorescence 13, pp. I 79-183 (2003 ).

18. M. Trtilek, D. M. Kramer, M. Koblizek, and L. Nedbal, "Dual-modulation LED ki­netic fluorometer," J Lumin. 72-74, pp. 597-599 (1997).

19. J. R. Lakowicz, Principles of Fluorescence Spectroscopy (Kluwer Academic/Plenum, New York, 1999).

20. E. Rabinovich, M. J. O'Brien, S. R. J. Brueck, and G. P. Lopez, "Phase­sensitive multichannel detection system for chemical and biosensor arrays and fluo­rescence lifetime-based imaging," Rev. Sci. lnstrum. 71, pp. 522-529 (2000).

21. K. Kazlauskas, G. Tamulaitis, A. Zukauskas, M.A. Khan, J. W. Yang, J. Zhang, G. Simin, M. S. Shur, and R. Gaska, "Double-scaled potential profile in a group-III nitride alloy revealed by Monte Carlo simulation of exciton hopping," Appl. Phys. Lett. 83, pp. 3722-3724 (2003).

22. T. Ban, H. Skenderovic, S. Ter-Avetisyan, and G. Pichler, "Absorption measure­ments in dense cesium vapor using UV-blue light emitting diode," Appl. Phys. B 72, pp. 337-341 (2000); G. Pichler, T. Ban, H. Skenderovic, and D. Aumiler, this vol­ume.

23. J.P. Dakin, M. J. Gunning, P. Chambers, and Z. J. Xin, "Detection of gases by correlatioin spectroscopy," Sens. Actuators B 90, pp. 124-131 (2003).

24. J. Melendez, R. Carr, D. U. Bartholomew, K. Kukanskis, J. Elkind, S. Yee, C. Furlong, and R. Woodbury, "A commercial solution for surface plasmon sens­ing," Sens. Actuators B 35/36, pp. 212-216 (1996).

142 A. Zukauskas, M S. Shur and R. Gaska

25. H. P. Ho, S. Y. Wu, M. Yang, and A. C. Cheung, "Application of white light­emitting diode to surface plasmon resonance sensors," Sens. Actuators B 80, pp. 89-94 (200 1 ).

26. K. Gaskill, R. T. Holm, and 0. J. Glembocki, "GaN light emitting diode as a photoreflectance pump source," Rev. Sci. Instrum. 71, pp. 4341-4343 (2000).

27. A. Zukauskas, A. Novickovas, P. Vitta, M.S. Shur, and R. Gaska, "Raman meas­urements in water using a high-power light-emitting diode," J. Raman Spectrosc. 34, pp. 471-473 (2003).

28. M.P. Bristow, D. Nielsen, D. Bundy, and R. Furtek "Use of water Raman emission to correct airborne laser fluorosensor data for effects of water optical attenuation," Appl Optics 20, pp. 2889-2906 (198 I).

NOVEL AlGaN HETEROSTRUCTURES FOR UV SENSORS AND LEDS

M. STUTZMANN Walter Schottky Institut, Technische Universitiit Munchen, 85748 Garching, Germany

Abstract: The use of novel AlGaN/GaN heterostructures for UV applications is re­viewed. Multiple AlGaN layers can be employed to realize spectrally selective narrow-band UV sensors. Epitaxial heterostructures of n-type AlN on p-type diamond were grown by MBE and exhibit surprisingly good electronic proper­ties, suggesting a possible application for future UV light-emitting diodes. Fi­nally, the use of AlGaN/GaN heterostructures for biosensors is briefly dis­cussed.

Key words: narrow-band UV detectors, UV light-emitting diodes, AlGaN/GaN biosensors

1. INTRODUCTION

The ternary AlGaN-alloy system is particularly suited for optoelectronic de­vices in the ultraviolet spectral region. As indicated by Fig. 1, the band gap of AlGaN layers spans the entire spectral region between 3.4 (350 nm) and 6.2 eV (200 nm), thus encompassing the historically defined UVA (380-315 nm), UVB (315-280 nm), and UVC (280-200 nm) spectral ranges of ultra­violet radiation. Because of the onset of ozone formation by UV dissociation of oxygen for wavelengths smaller than 200 nm, this so-called vacuum ul­traviolet (VUV) region is of less importance for the applications to be dis­cussed in the following.

Because of their tunable band gap and their direct band structure, AlGaN alloys are very favourable materials for UV sensors [1]. Here we concentrate on particular sensor structures which allow a narrow-band detection of spe­cific UV spectral lines, e.g. the 320 nm OR-emission line for combustion control purposes, or the 250-270 nm spectral range for ozone detection and the monitoring of UV radiation which gives rise to maximum DNA damage.

143

M.S. Shur and A. lukauskas (eds.), UV Solid-State Light Emitters and Detectors, 143-159. © 2004 Kluwer Academic Publishers.

144 M Stutzmann

As can be deduced from Fig. 1, this corresponds to AI contents in AlGaN of 30 at.% or more. Unfortunately, for such high AI concentrations, substitu­tional doping of AlGaN becomes increasingly difficult and, in the case of p­type doping, actually has not been realized so far. Therefore, UV sensors in the spectral range of interest here are commonly based on photoconductor or Schottky diode structures [2).

6.5

6.0 PIMBE AI,Ga1.,N

T = 300 K 5.5

~ Cl 5.0

w Q.

uvc .. Cl , c: 4.5 ..

ID

UVB 4.0

3.5 UVA

3.0 -'--;r-----+--+---+---t---1--' 0.0 0.2 0.4 0.6 0.8 1.0

AI Content

Figure 1. Experimental values for the room temperature optical band gap of AIGaN with dif­ferent AI contents. Differently shaded regions indicate the spectral ranges of UV A, UVB, and UVC radiation. PIMBE refers to the growth method, namely plasma-induced molecular beam epitaxy.

Because of the p-type doping problem, also bipolar optoelectronic de­vices based on AlGaN at present are limited to the range of AI concentration below about 15 at.%. This is a particular handicap for UV light emitting di­odes in the UVB and UVC region. As a potential solution of this problem, we will discuss the properties of heterostructures consisting of n-type AIGaN-layers epitaxially grown on p-type diamond substrates. This combi­nation allows to overcome the fundamental doping problems in these two wide gap semiconductor systems and, in addition, provides an attractive syn­ergy between the possibility ofUV band gap engineering in the AlGaN alloy system on one hand and the excellent thermal, chemical and mechanical properties of diamond on the other hand.

Novel AlGaN Heterostructures 145

Finally, we will have a brief look at the use of AlGaN/GaN heterostruc­tures for future biosensor applications. A very attractive feature in this con­text is the optical transparency of AlGaN in the spectral range between 360 run and 800 nm, which is commonly employed in fluorescence investiga­tions and microscopy of biological systems. Making use of the spontane­ously formed two-dimensional electron gas at AlGaN/GaN heterointerfaces, this can be combined to realize a new generation of optoelectronic sensors for the investigation of electronic and ionic processes in biology and medi­cine.

2. AlGaN ULTRAVIOLET SENSORS

UV sensors based on simple AlGaN layers have already been studied and optimized extensively. Depending on the Al content of the sensitive layer, various sensor designs have been used: photoconductors, Schottky diodes, MSM detectors, p-n diodes, avalanche detectors, and phototransistors [3-5]. The spectral sensitivity of such detectors is generally determined by the opti­cal absorption coefficient of the respective AlGaN layer and the surface re­combination of photoexcited carriers in the case of strongly absorbed light. A typical example ofthe spectral sensitivity of AlGaN Schottky diodes with a Pt-Schottky contact is shown in Fig. 2. At the direct band gap of the re­spective alloy film, the sensitivity drops by several orders of magnitude and is determined by defect related absorption in the subgap region. Above the band edge, the sensitivity remains constant within an order of magnitude, depending on the particular metal used for the Schottky contact, details of the surface preparation, etc.

In order to achieve a narrower sensitivity characteristics, a more sophisti­cated heterostructure has to be used. As shown in Fig. 3, the most simple narrow-band detector structure consists of a three-layer sequence with dif­ferent Al contents. The first layer on the transparent sapphire substrate acts as an optical filter, which only transmits light with a photon energy below the respective band gap. Thus, this layer determines the high-energy cutoff of the sensitivity curve. The filter layer is followed by an electrical isolation layer with a much higher Al content. This isolation layer prevents the spill­over of photoexcited carriers created in the filter layer into the uppermost photoconductor layer. The band gap of the photoconductor layer finally de­termines the low energy cutoff of the spectral sensitivity, similar to the curves in Fig. 2 [6].

146 M Stutzmann

Wa elength (nm) 500 400 300 250

10_,

102 ....... GaN c ::3

.D 103

a .._ >. 164

> ......

105 Vl c <:)

{/)

1o"6

107 1,0 2.0 3 0 4,0 5,0 6,0 7,0

nergy (eV)

Figure 2. Spectral sensitivity of different Pt/AIGaN Schottky diodes at 300 K.

For the specific structure shown in Figure 3, the filter layer with an AI content of 40 at.% is expected to absorb all UV radiation with a photon en­ergy above 4.15 eV. The isolation layer with an AI content of 60 at.% has a band gap of 5.1 e V and, therefore, serves as an energy barrier of many times kT at room temperature, both for the photoexcited electrons and holes. The photoconductive layer with an AI content of 33 at.% absorbs efficiently for photon energies above 3.95 eV, so that the entire three-layer-structure is ex­pected to exhibit a significant response only in the wavelength region be­tween 314 nm and 295 nm. Within experimental accuracy, this is confirmed by the linear spectral response curve in Fig. 4.

Novel A!GaN Heterostructures

Active Layer

Filter

147

i Light

Figure 3. Layer structure of a narrow-band AlGaN UV sensor. The function of the three dif­ferent layers as optical filter, electrical isolation, and photoconductor is described in the text in more detail.

Wavelength [nm)

400 360 320 2JIO 3.0 r-r-

303nm

2 .5

~ 2 .0 0 0 :c. i!:' > 1.5 ·;;; c &. "' Gl a: 1.0

0.

_/ 0 3 .0 3 .5 4.0 4.5 5.0

Photon Energy [eV)

Figure 4. Linear spectral response curve of the AIGaN-heterostructure shown in Figure 3.

The experimental sensitivity curve shown in Fig. 4 agrees very well with the expected spectral sensitivity discussed above. A significant response is only observed for wavelengths between 290 and 310 nm. On the low-energy

148 M Stutzmann

side, the UV sensor already exhibits a noticeable response at 340 nm due to structural imperfections in the photoconductive layer. On the high energy side, a sensitivity cutoff at about 290 nm is observed which, however, de­pends strongly on the thickness of the filter layer. This is shown in more de­tail in Fig. 5, where the spectral response of several UV sensors with differ­ent thicknesses of the filter layer is plotted on a semilogarithmic scale.

·-~ § 0 z

Wavelength (nm)

600 500 450 400 350 300 250 1 0 r-""""" ----.----

1

10-6

1 o-7 t 2,0 2,5

filter

.i~ouo w o .._ J-

fjj \-0 360 nm .w 0 ~~ '(.,. ·o 'i .:)· ~ ~ u-l..l \ 720 nm

y~J v Vvvv v·v·v-V l vv 1400nm 1 v

3,0 3,5 4,0 4,5

Photon Energy ( e V) 5,0 5,5

Figure 5. Normalized photocurrent (on a logarithmic scale) of UV sensor structures as the one shown in Fig. 3, with different thicknesses of the AlGaN filter layer.

As can be seen from Fig. 5, the sensor structure without a filter layer ex­hibits a spectral sensitivity comparable to that of the simple AlGaN Schottky diodes in Fig. 2. With increasing filter layer thickness, the narrow-band characteristics of the sensor become more and more pronounced. For the highest filter layer thickness of 1400 nm investigated here, the suppression

Novel A!GaN Heterostructures 149

of radiation detection outside of the desired wavelength region exceeds three orders of magnitude. In addition, the thick filter layer acts as a buffer layer for the final photosensitive AlGaN layer, leading to a considerable reduction of defect related absorption in the lower photon energy range [7].

The results presented above have been obtained without a systematic op­timization of the structural quality and the design of the AlGaN heterostruc­tures involved and, thus, should only be viewed as a proof of concept. Obvi­ously, real ·sensor devices with much better spectral selectivity and sensitivity will be possible, if such an optimization is performed for a spe­cific application.

3. AlGaN/DIAMOND HETEROSTRUCTURES

Although in the last decade a lot of progress has been made in the prepara­tion of high quality wide band gap semiconductors such as GaN, ZnO, or CVO diamond, both as homoepitaxial as well as heteroepitaxial layers, the fabrication of high efficiency light-emitting diodes (LEOs) or laser diodes (LOs) in the UVB and UVC spectral range ( cf. Fig. 1) still remains an elu­sive dream. The reason for this setback is the general problem of the bipolar dopability of wide band gap semiconductors, which is a conditio sine qua non to transform a wide gap material of sufficien.t structural quality into an exciting new material for semiconductor applications. Usually, all wide band gap semiconductors can be doped quite efficiently by at least one shallow dopant. Thus, the n-type doping of GaN or ZnO is fairly straightforward, whereas the p-type doping of these materials still remains problematic or at least a nuissance for the realization of efficient bipolar devices such as LEOs. In the case of AlGaN, both n- and p-type doping become increasingly difficult with increasing Al-content. Similarly, p-type doping of diamond by boron has been known and employed for many years [8], whereas n-type doping by, e.g. phosphorous or sulphur has been very problematic and diffi­cult to realize [9].

In particular, the application of wide band gap semiconductors for UV­LEOs or LOs requires a complete understanding and control of both, n- and p-type doping, since otherwise the achievable efficiency of UV light emis­sion will be limited by the capability to inject electrons or holes into the op­tically active region of such devices. The problems encountered so far with both, the n- and p-type doping of AlGaN with high Al contents thus explain why despite of the favourable electronic and optical properties of the AlGaN materials system (in particular the possibility to produce quantum wells, waveguides, Bragg mirrors, etc.) no deep UV light emitters have been real­ized so far.

150 M Stutzmann

According to the present state of knowledge, in particular the p-type dop­ing of AlGaN rapidly becomes inefficient with increasing Al concentration. Whereas GaN can be doped p-type by Mg acceptors up to free hole concen­trations at room temperature of the order of 1018 em -J, all acceptor atoms tested so far tum into deep defects already at AI contents of 20-30%. Fortu­nately, the situation is considerably better for n-type doping of AIGaN with Si, at least in the case of MBE grown material. As shown in Fig. 6, Si forms a shallow donor with an ionization energy increasing linearly from about 20 meV in GaN to 320 meV in AIN [10].

700

>600 ~500 -e>400 Q) c:: ~300 0

~ 200

~100

AIGaN

0,2

' undoped

0,4 0,6 0,8 1,0

Al mole fraction

Figure 6. Conductivity activation energy of different MBE-grown AlGaN films doped with silicon. The dashed line shows the corresponding results for nominally undoped films contain­ing a residual concentration of oxygen contamination.

Since the solubility of substitutional silicon in AlGaN is very high due to the similar atomic sizes and bond energies of AI and Si with nitrogen, good n-type contacts required for UVC LEOs can in principle be realized with Si­doped AIN layers. The OX-behaviour observed for Si-doped AlGaN with AI contents above 50% [10] only constitutes a problem at low temperatures (<200 K) and can be neglected for UV light emitters operated at room tem­perature and above. On the other hand, oxygen contamination during the growth process of n-type AlGaN is a more serious problem. Whereas oxygen is a shallow donor in GaN as well, it turns into a deep defect for Al-rich AIGaN alloys with AI contents exceeding 30% (dashed line in Fig. 6). Thus,

Novel A!GaN Heterostructures 151

the success with Si-doping of AlGaN appears to be limited by the cross con­tamination with oxygen, especially in samples grown by MOCVD with a larger background concentration ofO-containing impurities.

EvAc ----------~~--~----------

"!. = 0.3 ev t ...........••.•••.••...•

E0 (S1) = 320 meV

n-typeAIN

Eg- 6 2 eV (d1rect)

Wurtz1te

a 310A

z = 0.5 eV

p-type Diamond

Eg = 5.47 eV

(indirect)

Cubic

a= 3.56A

EA(B) = 360 meV ........................

Figure 7. Basic parameters for heterostructures consisting of boron-doped diamond and sili­con-doped AlN. Eg is the room temperature band gap, a and c are the fundamental lattice constants of the corresponding bulk crystals, and E0 (Si) as well as EA(B) denote the donor and acceptor ionization energies of substitutional Si-donors in AlN and B-acceptors in diamond, respectively. The electron affinity, z, of intrinsic AlN and diamond surfaces with respect to a common vacuum level provides a first-order approximation of the band line-up at this particu­lar heterointerface.

Figure 7 summarizes the most relevant material parameters of AIN and diamond for the envisaged application in UV LEDs. Both constituents have approximately the same small electron affinities of 0.3 eV (AlN) and 0.5 eV (diamond), so that to zero order, the conduction bands on both sides of the heterojunction are almost aligned [11,12]. A significant correction of the simple scheme in Fig. 7 will result from a potential jump at the interface due to a C-N- or C-Ga-dipole layer expected to form there. At present, nothing is known about the chemical nature of the interface layer, so that this consti­tutes a major issue for future device optimization. In addition, by using Al­GaN alloys instead of AlN, the bandgap of the Ill-nitride layer can be tuned to match that of diamond ( 5.4 7 e V, corresponding to an Al content of ap­proximately 85 at.%, cf. Figure 1), or to realize an AlGaN quantum well em­bedded between p-type diamond and n-type AIN contacts. Such a quantum

152 M Stutzmann

well structure (see Fig. 8) would have the additional advantages of spectral tunability and of shifting the recombination of injected carriers into the III­nitride side, where the direct band structure favors radiative transitions.

nergy E

n-AIN p-Diamond x AIGaN

Figure 8. Schematic band diagram of an AIN/AIGaN/diamond quantum well structure for UV LEDs. The n-type AIN and the p-type diamond are used to inject electrons and holes into an undoped AIGaN quantum well, where radiative recombination occurs.

Major issues of concern for the successful implementation of an LED structure as shown in Fig. 8 are the different lattice constants and crystal structures of AIN and diamond also indicated in Fig. 7. To address this issue, AIN was grown on (100) and (Ill) oriented diamond surfaces by plasma­enhanced MBE at a temperature of 815 °C. Subsequently, the heterostructure was analysed by high resolution X-ray diffraction in order to assess the structural quality and the heteroepitaxial relationship of the AIN layer. As demonstrated by Fig. 9, the structural quality of the AlN epilayer already

Novel A/GaN Heterostructures 153

AIN 002 (I)-scan AIN 002 20/., -scan

A ':." ~ :M subs0 'L x 10 _) L C(A11)8 _k

1 3° 0 12 x50

CA111t l_k_ 1 4° 0 17

X 100

c (100) A _) ~,. _105.

Al,03(0001)

-2 -1 0 1 2 -0.6 -0.3 0 0 0.3 0 6

'tv (. ) '2o(•)

Figure 9. High resolution X-ray diffraction spectra of different AlN/diamond heterostructures. Shown are rocking curves (left) and Bragg-Brentano scans (right) of the AlN 002 peak for two (Ill) and one (100) oriented diamond substrates. The same curves for AlN grown on sapphire are shown for comparison.

comes close to that of an AlN/ Ah03 reference system, even without any op­timization of the growth parameters. Moreover, from pole figures of the 101 reflection of AlN, the epitaxial relationship could be deduced. According to our results, AIN grows entirely in the wurtzite lattice structure both on (111) and ( 1 00) diamond surfaces, with the (000 1) c-axis perpendicular to the sub­strate. For the in-plane orientation, we have obained the following relation­ship for ( 111) diamond substrates [ 13]:

(0001) [10-10]AlN II (111) [01-1] diamond. For (100) diamond substrates, two different domains were observed:

(0001) [10-10]AlN II (100) [011] diamond (majority domain) (0001) [-12-10]AlN II (111) [011] diamond (minority domain).

For an investigation of the electronic properties of the AlN/diamond het­erostructures, Si-doped AlN ([Si] = 1019 cm-3) was grown on boron-doped (100) oriented diamond ([B] = 1017 cm-3). Then, Ti/Al contacts with a di­ameter of 150 ~m (AIN) and 300 ~m (diamond, backside) were evaporated

154 M Stutzmann

and the current-voltage characteristics were measured at room temperature. The results are shown in Fig. 10, both on a linear and semilogarithmic scale [14].

1<1' 250

'1101 --N 200 E ~10'1 (,)

4: c Q)

2; '0

z. 150 ~ 10'3

'iii l: :::1

c () Q) 10'5 -c 100 -3 -2 -1 0 1 2 3 c Voltage (V) Q) .... .... ::J 50 u

0

-3 -2 -1 0 1 2 3

Voltage {V)

Figure 10. Room temperature current- voltage characteristics of the first AlN/diamond bipolar heterojunction diode.

Already the first heterojunction diode exhibited a surprisingly good recti­fication ratio of about 1: 1000 at a bias of 2 V. The ideality factor n under forward bias was between n = 2 ... 3. However, the semilogarithmic plot also reveals a high reverse current indicative of a high density of structural de­fects at the interface, as well as a high series resistance in forward direction. The latter is partially due to non-ohmic contacts, but also is caused by the still too low conductivities of the AlN epilayer and the diamond substrates. We have already found out that the contact resistance can be reduced signifi­cantly by a rapid thermal annealing (e.g. laser annealing with an ArF exci­mer laser with a pulse energy density of about 2 J/cm2). Further improve­ment will be possible by the use of higher doping levels in the contact layers and optimized doping profiles close to the junction. In addition, we expect that optimized growth parameters will also allow us to reduce the density of deep electronic defects at the heterointerface. All these issues are currently under closer examination.

Another unexpected feature is the observation of a strong electro­luminescence from the heterojunction even at room temperature under the condition of forward biases of several tens of volts. Because of the above

Novel A!GaN Heterostructures 155

mentioned high series resistance, such voltages are necessary to obtain for­ward current densities in the range of 10 A/cm2, corresponding to a de cur­rent of about 1 rnA through one of the Til Al contact pads with 150 flill di­ameter. As demonstrated in Fig. 11, under these bias conditions, a strong

en c Q.) -c

Wavelength (nm)

600 400 300 250

442 nm

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

Photon energy (eV)

Figure 11. Room temperature electroluminescence spectra of the AlN/diamond heterojunction under strong forward bias. Dark circular spots on the surface correspond to the Ti/Al-contacts to the epitaxial AlN layer. The inset shows that light emission mainly occurs at the edges of the diamond substrate, because of total internal reflection.

e1ectroluminescence (EL) of the p-n diode is observed, with two emission peaks in the visible (440 nm) and the UV spectral range (255 nm). Whereas the visible peak obviously must come from a defect-related electronic transi-

156 M Stutzmann

tion at the interface, the second weaker UV -peak is compatible in energy with a radiative recombination between electrons in the silicon donor band of AlN and holes in the boron acceptor level in diamond. However, this pre­liminary assignment requires more detailed investigation. A particular fea­ture of LEDs on diamond is demonstrated by the inset in Fig. 11, showing the spatial distribution of light emission across the 2 x 2 mm2 diamond sub­strate. Although only the circular contact in the upper left comer of the AlN epilayer was used for current injection, most of the optical emission occurred at the edges of the diamond substrate. This effect is due to the total internal reflection of EL ~mission in a transparent substrate and is well known for usual LED devices.

Although these first results are very promising, the future potential for UV-LEDs based on AlGaN/diamond heterostructures remains to be investi­gated. At present, there are obvious routes to improve the performance of such devices: optimized growth conditions, optimized doping profiles and contact geometries of the AlN- and diamond contact layers, the use of AlGaN quantum wells as illustrated in Fig. 8, and more efficient light extrac­tion schemes. All of these efforts are probably very well justified, since AlN and diamond at present are the only materials with a sufficient electronic quality necessary for active optoelectronic devices in the UVC spectral range. In addition, the high thermal conductivity, chemical stability, and natural biocompatibility of diamond substrates provide further advantages for specific applications, particularly in the area of bioelectronics and bio­sensmg.

4. NOVEL BIOSENSOR CONCEPTS BASED ON AlGaN/GaN HETEROSTRUCTURES

The detection or processing of bioelectronic activity, e.g. in nerve cells cur­rently is very much constrained by the incompatibility of the most com­monly used semiconductor materials, silicon and gallium arsenide, with the chemical environment of living cells [ 15]. Thus, other semiconductors should be investigated to this end. Of particular interest in this context are wide bandgap semiconductors such as AlGaN or diamond, because they would in principle allow an attractive combination of the sensitive electronic detection of electrical activity in biological systems (e.g. the opening or clos­ing of transmembrane ion channels) with the well established fluorescence techniques widely used in biology and medicine.

Novel A!GaN Heterostructures 157

lipid bilayer

sapphire substrate

Fluorescence detection

Figure 12. Device concept of a novel bioelectronic sensor based on an AIGaN/GaN hetero­junction. Due to the spontaneously formed two-dimensional electron gas (2DEG) at the het­erointerface, this device is very sensitive to changes in the ion concentration at its free sur­face, e.g. caused by ion channels in a lipid bilayer membrane in close contact with the device. A possible combination with fluorescence detection of membrane processes is also indicated schematically.

The basic design of such a bioelectronic sensor, which is insensitive to il­lumination in the visible and near-UV spectral region, is shown in Fig. 12. The structure consists of a double-sided polished sapphire substrate onto which an AlGaN heterostructure is deposited. Depending on the excitation wavelength, the Al contents of the heterostructure can be chosen in order to avoid absorption in the active layer. Here, the same considerations apply as discussed in Section 2 above. For many fluorescent markers used in biology, an excitation wavelength above 370 nrn is sufficient, so that GaN can be used as one layer of the heterojunction. If, in addition, a very thin AlGaN­layer with a thickness between 10 and 30 nrn is deposited on the top of this GaN layer, a two-dimensional electron gas (2DEG) will form spontaneously at the GaN/ AlGaN interface because of the spontaneous polarization of the wurtzite Ill-nitrides [16]. With a sheet carrier density of about 1013 cm-2 and a room temperature mobility of about 1000 cm2N·s, this 2DEG provides a

158 M Stutzmann

very sensitive means of detecting ionic adsorption or desorption processes at the free surface of the device. Our investigations so far have shown that, due to the close proximity of surface ions and electrons in the 2DEG, changes in the ion concentration are mirrored almost 1:1 by changes in the 2DEG elec­tron density [17,18]. Since the mobility of electrons in the 2DEG is about a million times higher than typical ion mobilities in electrolytes, an ionic sig­nal at the surface of the heterostructure sensor will be amplified by about the same factor as well. This, together with the optical transparency, the chemi­cal inertness, and the natural biocompatibility of AlGaN, opens up exciting possibilities for the use of AlGaN/GaN heterostructures in novel biosensors [19].

ACKNOWLEDGEMENTS

The results and ideas summarized above are the outcome of many individual contributions of members of the III-nitride group at the Walter Schottky In­stitut over the last four years. It is a great pleasure to acknowledge in particu­lar Oliver Ambacher, Martin Eickhoff, and Jose Antonio Garrido as the prin­cipal scientists involved in this work. For their dedicated daily work in the lab I would like to thank the following PhD students: Barbara Baur, Sebas­tian Gonnenwein, Martin Hermann, Uwe Karrer, Claudio Miskys, Ralph Neuberger, Jan Schalwig, Georg Steinhoff, Olaf Weidemann, and Roland Zeisel. Special thanks go to GUnther Vogg (Fraunhofer Institut ZMI, Mu­nich) and Gerhard MUller (EADS Deutschland, Munich) for a very fruitful collaboration. Last but not least, financial support by the German Research Foundation (DFG) in the framework of the Priority Programme SPP 1032 is gratefully acknowledged.

REFERENCES

I. D. Brunner, H. Angerer, E. Bustarret, F. Freudenberg, R. Hopler, R. Dimitrov, 0. Ambacher, and M. Stutzmann, J. Appl. Phys. 82,5090 (1997).

2. M. Razeghi and A. Rogalski, J. Appl. Phys. 79, 7433 (1996). 3. B.W. Lim, J.Y. Yang, M.A. Khan, and Q.C. Chen, Appl. Phys. Lett. 68, 3761

(1996). 4. E. Monroy, F. Calle, E. Munoz, F. Omnes, B. Beaumont, P. Gibart, J.A. Munoz,

and F. Cusso, MRS Internet J. Nitride Semicond. Res. 3, 9 (1998). 5. D. Walker, E. Monroy, P. Kung, J. Wu, M. Hamilton, F.J. Sanchez, J. Diaz, and M.

Razeghi, Appl. Phys. Lett. 74, 762 (1999). 6. U. Karrer, A. Dohner, 0. Ambacher, and M. Stutzmann, J. Vac. Sci. Techno!. B 18,

757 (2000). 7. U. Karrer, PhD Thesis, Technische UniversiHit Mlinchen (2001).

Novel A/GaN Heterostructures 159

8. T. H. Borst and 0. Weis, Phys. Stat. Sol. (a) 154, 423 (1996). 9. S. Koizumi, K. Watanabe, M. Hasegawa, and H. Kanda, Science 292, 1899 (2001). 10. R. Zeisel, M. W. Bayer!, S. T. B. Gonnenwein, R. Dimitrov, 0. Ambacher, M. S.

Brandt, and M. Stutzmann, Phys. Rev. B 61, R16283 (2000). 11. S. P. Grabowski, M. Schneider, H. Nienhaus, W. Monch, R. Dimitrov, 0. Am­

bacher, and M. Stutzmann, Appl. Phys. Lett. 78,2503 (2001). 12. F. Maier, J. Ristein, and L. Ley, Phys. Rev. B 64, 165411 (2001). 13. G. Vogg, C. R. Miskys, J. A. Garrido, M. Hermann, M. Eickhoff, and M. Stutz­

mann, submitted to J. Appl. Phys. 14. C. R. Miskys, J. A. Garrido, C. E. Nebel, M. Hermann, 0. Ambacher, M. Eickhoff,

and M. Stutzmann, Appl. Phys. Lett. 82, 290 (2003). 15. P. Fromherz, Phys. Chern 3, 276 (2002). 16. 0. Ambacher, J. Majewski, C. Miskys, A. Link, M. Hermann, M. Eickhoff, M.

Stutzmann, F. Bernardini, V. Fiorentini, V. Tilak, B. Schaff, and L. F. Eastman, J. Phys.: Condens. Matter 14, 3399 (2002).

17. R. Neuberger, G. Muller, 0. Ambacher, and M. Stutzmann, Phys. Stat. Sol. (a) 183, RIO (2001).

18. G. Steinhoff, M. Hermann, W. J. Schaff, L. F. Eastman, M. Stutzmann, and M. Eickhoff, Appl. Phys. Lett. 83, 177 (2003).

19. G. Steinhoff, 0. Purrucker, M. Tanaka, M. Stutzmann, and M. Eickhoff, Adv. Funct. Mater. 13, 841 (2003).

NITRIDE PHOTODETECTORS IN UV BIOLOGICAL EFFECTS STUDIES

E. MuNOZ, J. L. PAU, and C. RIVERA Institute for Systems based on Optoelectronics and Microtechnology-ISOM and DIE ETSI Telecomunicaci6n. Univ. Politecnica de Madrid. 28040-Madrid, Spain E-mail: elias@die. upm. es

Abstract: The role of the solar UV radiation in our biosphere is highlighted. Action spec­tra of solar UV radiation in biological processes are summarized. The ery­thema-weighted response has become standard for UV biological action moni­toring. Fluorescence techniques and the requirements of AllnGaN-based UV detectors in biophotonics are described. AlGaN detectors fitting the erythema action spectrum are presented. QW nitride-based photodetectors, suitable for multifunctional integrated biochips, are introduced. Problem areas are dis­cussed.

Key words: UV photodetectors, GaN/nitrides, AlGalnN/nitride-alloys, erythema, action spectrum, solar radiation, UV damage, red skin, quantum wells, biophotonics

1. INTRODUCTION

The Sun is the most powerful natural UV source, and all living species of the biosphere are affected by the received solar UV radiation. The ozone layer and other atmospheric gases strongly absorb the UV emission from the Sun, and only light with wavelengths longer than 280 nm reaches the Earth sur­face. Ozone absorption is peaked at 250 nm and, in fact, life on Earth has been adapted to such spectrum because DNA damage is also peaked at 250 nm. Although there is a wide range of applications and studies around the UV region (UV astronomy, resin curing, combustion engineering, flame de­tection, etc.), water purification, biological effects studies, crosslinking of nucleic acids, DNA and protein biochips, phototherapy, detection of minute

161

M.S. Shur and A. Zukauskas ( eds.), UV Solid-State Light Emitters and Detectors, 161-177. © 2004 Kluwer Academic Publishers.

162 E. Munoz et a/

amounts of hazardous chemical and biological agents, etc. are new and inter­esting areas of activity.

Efficient UV detectors have been available since last century, mainly Si­based photodetectors and photomultiplier (PM) tubes. These solutions have significant limitations, either due to the need of filters to stop low energy photons (visible and IR light present), their degradation (solarization) and lower efficiency, or because of the need of a high-voltage supply (PM case). To avoid the use of filters, i.e. trying to obtain visible-blind operation, UV detectors based on wide gap semiconductors have been considered during the last decade. The turning point to make visible-blind UV detectors a real­ity has been the development of (ln,Al,Ga)N semiconductors [1]. They are direct gap alloys, with GaN showing its absorption edge at 360 nm (Eg = 3.4 eV), and covering from theIR (InN, Eg = 0.8 eV) to the 200 nm range (AlN). This tunability of the detection edge by just varying the (ln,Al) mole frac­tion, the fact that (ln,Al,Ga)N technology is already fully commercial for light emitting diodes (LEDs ), and the intrinsic radiation hardness of nitrides are important points that have made nitride-III alloys the most adequate solu­tion for UV detection in many applications.

In this chapter we first briefly review the role of the solar UV radiation in our biosphere. The astonishing matching between ozone absorption and DNA damage is presented. The biological action of UV is characterized by an action spectrum, indicating the relative effectiveness at each wavelength. Representative action spectra are presented. The second part of the presenta­tion describes some recent techniques in biophotonics, and the role of the UV detectors in such applications. Fluorescence is presently a key tool in medical and biological research, and III-nitrides offer the unique possibility to integrate a range of emitters and detectors from the green to the UV, open­ing new roads in biochip development. The third part of this chapter presents some considerations about performance and problems of UV detectors in biological studies. This information complements the chapter by M. S. Shur and A. Zukauskas, also in this book, where a review of the key issues in de­tector development is presented. We also describe here novel quantum well detectors based on (lnGaAl)N. They allow covering the whole spectrum from visible to near UV radiation. The need for internal gain in some fluo­rescence applications is pointed out, and various solutions to obtain GaN photodetectors with internal gain are reviewed. As one of the conclusions, erythema action detectors are reference or standard detectors for solar UV monitoring, in a variety of medical and biological effects studies, and their nitride implementation is commercially interesting. To fit any spectral weighting function (action spectra) multiband (ln,Ga,Al)N detectors are needed. DNA and protein microarrays will benefit from the integration of

Nitride Photodetectors in UV Biological Effects Studies 163

emitters and detectors based on nitrides. Problem areas m relation to photodetectors in biophotonics are finally discussed.

2. SOLAR UV RADIATION AND ITS BIOLOGICAL ACTION

In our Sun-dependent biosphere, the UV radiation reaching the Earth is usu­ally classified into three bands, UV A ( 400-320 nm), UVB (320-280 nm), and UVC (below 280 nm). To determine the effects of the various solar UV bands on the terrestrial ecosystem and human beings is an important subject, and it has been driving the needs for reliable and efficient visible-blind UV detectors since mid last century. On the other hand, to detect man-made UV sources (flames, missile plumes, sparks, etc.), UV photodetectors with solar­blind characteristics are required (cut-offwavelength below 280 nm) [1].

The solar spectral irradiance is shown in Fig. 1. While at ground level practically no photons shorter that 280 nm reach us, above the atmosphere UVC radiation is present (Table 1).

·e·• .--------.------------.,

TOP Or Hif ATMOSPI-IERE

.. E ·E·' ~ w 0

~ Zl

1E·2

~ •E·3 g;

a: :5 g 'E-4

•E·S 280 300 320 3<&0

WAVELENGTH (nm)

Figure I. Solar spectral irradiance above and below our atmosphere [2].

This important absorption result is due to the presence of various species in our upper atmosphere (03, 0, NO, OH, N02), that extend in the 15 to 60 km region of altitude. Ozone is the most important one, with a very strong peaked absorption at 250 nm and that tails down to the 300 nm. The ozone layer is thus directly responsible for blocking the shorter wavelength pho­tons. Specifically, there is a direct relation between the spectral irradiance of

164 E. Munoz et a/

Table I. Solar irradiance prior to absorption by earth's atmosphere and at sea level.

Wavelength Top of the at- Top of the at- Sea level (nm) mosphere mosphere

Irradiance %of total %of total (W/m2) Zenith angle = oo

uvc 6.4 0.5 UVB 21.1 1.5 0.5 UVA 85.7 6.3 6.3 VIS (400-760) 532 38.9 38.9 IR(>760-3000) 722 52.8 54.3

the UVB band (increasing) and the ozone concentration (decreasing), with the shortest wavelength significantly enhanced (amplification factor) if the ozone layer thins down. As an example, Fig. 2 shows that relationship, as determined by ground measurements in Thessaloniki [3].

r-400 -1

~ r

- 350 ~ m p ~

~ 300 ~

o~---.----r---,---~--------·~----~------------r 250

90 91 92 93 94 95 YEARS

Figure 2. Variation of total ozone (Dobson units) solar UV irradiance reaching the earth 's surface. They have been measured at Thessaloniki [3).

Very small changes in the ozone concentration cause significant in­creases in the UVB radiation reaching us [4]. This key role of ozone has been a matter of serious concern as the ozone column has been decreasing in the recent decades, and dramatically proven on the Antarctic continent. So­ciety has been alerted about ozone depleting substances, mainly halogens.

Nitride Photodetectors in UV Biological Effects Studies 165

This stratospheric ozone layer, if being under normal conditions of tempera­ture and pressure, would be a layer just about a 3-mm thick (300 Dobson units).

Table II summarizes the well-known risks and benefits in humans of the solar UV radiation. As in all biological processes, damage and healing mechanisms go together, in a very complicated scheme. Somewhere, there is a balance between too much sun and melanoma risk, or too little sun and autoimmune disease for instance.

Table IL Risks and benefits in humans of solar UV radiation.

Risks

Sunburn (red skin)

Suntanning (immediate and delayed pigmentation)

Photoaging

Solar kerastoses

Skin cancer (basal and squarnous cell carcinoma)

Malignant melanoma

Local and systemic inmune suppression

Photosensitivity diseases

Drug related phototoxic and photoallergic reactions

Cataracts

Benefits

Vitamin D synthesis (bones)

Phototherapy and photochemiotherapy

Psychological comforts

Lower risk of certain autoimmune system diseases

There is presently a significant concern about the solar UV radiation, de­rived from the important effects of such wavelengths on the biological eco­system. All the biological consequences of the solar UV radiation are wave­length dependent and are characterised by unique action spectra (describing the relative effectiveness of each wavelength). Thus action spectra depend on the specific biological process being studied (skin-sunburn or erythema, DNA damage, chloroplast activity, plant damage, germicidal effects, bacte­ria killing, skin cancer, etc.). The UV bioaction is the weighted product of the UV irradiance by the action spectrum of the specific species or biologi­cal process being illuminated, as illustrated in Fig. 3.

166

........ ...,., = = 0... ...,., ........ 0:: ........ ::> t= cC -' u..o 0::

= = cC

10°

10- 1

10-2

E. Munoz et al

SPECTRUM

BIOLOGICAL '--~--;;;.WEIGHTED

IRRADJANCE

Figure 3. The biological weighted effect of the UV radiation depends on the specific action spectrum and on the impinging solar irradiance [3].

Some representative biological actions of the solar UV are illustrated in Fig. 4. There are also well known cases where the UV exposure produces very beneficial effects, and again these positive UV effects are wavelength

10° .

10-1 -ERYTHEMA

UJ ••••• TYPHIMURIUM VI -·-PHYTOPLANKTON z 10-2 ----PLANT DAMAGE 0 0.. Vl ....., 0:: 10-3 UJ

> I-<{ -I UJ 0::

10-S

10-~80 300 320 340 360 380 400 WAVELENGTH (nm)

Figure 4. Some illustrative biological action spectra [3].

Nitride Photodetectors in UV Biological Effects Studies 167

dependent. Because of its practical importance for human beings, the ery­thema action (skin sunburning) has been extensively studied, and agreed internationally (International Commission on Illumination, CIE, Commis­sion Internationale de l'Eclairage). Such response is the most widely used UV biological action response. Commercial instruments were developed, and this availability made that other biological actions were also monitored by using such erythema weighted detectors. Current solutions and AlGaN­based devices are described in the last part of this chapter.

The key biological action is DNA damage. The various nucleotides have peak absorptions in the 240-270 nm range, resulting in DNA absorption maximum at about 250 nm. On the other hand, chloroplast activity deter­mines plant photosynthesis. Photosynthesis is due to the visible spectrum, being stronger for blue and green photons (photoactinic region, PAR). These two basic action spectra (benchmark spectra) are shown in Fig. 5 [5]. By similar considerations, the way UV radiation interferes plant photosynthesis does not follow a single pattern. The effects of the increasing UVB radiation on agriculture have received attention worldwide [6]. In many cases, an en­hanced UVB radiation causes photosynthesis to be inhibited.

..... ' -N

'E ::::.. -6

CIJ

10"2

10"3

10"4

10"5 ....

Chloroplast activity · ··· ·-· ....

280 300 320 340 360 380 400 Wavelength (nm)

Figure 5. Biological action spectra for DNA dimers and chloroplasts [5].

From Figs. 4 and 5 it is clear that although all the effects of the UV radia­tion are stronger at the shortest wavelengths, and these effects decrease as photon energy becomes smaller, the details depend markedly on the specific process, the specific species being studied and on their previous UV history.

168 E. Munoz et a/

Living species have developed their own UV protection schemes (furs, pig­ments, flavonoids in plants, mycosporinelike aminoacids in algae, etc.).

At the medical level, the main concerns about UV radiation refer to its ef­fects on the skin and the eyes. The importance of erythema action lies in the fact that it may be the first step towards skin cancer (Fig. 6) [7]. The action spectrum of human skin cancer has revealed to have a rather non-monotonic structure (Fig. 7), probably due to the complex interactions of the repairing mechanisms. Information about the daily erythema dose (for how long a sunbath can be taken), is being provided to the public, in several countries, through the UV index (8,9]. To give some numbers, but for each individual there are various modifying factors (skin type, frequency of UV dose, etc.), the minimum erythema dose for red skin is 210 J/m2 • Ranking from 1 to 16, this UV index is obtained from the weighted erythematic radiation, in steps of 25 m W/m2 , just multiplying by 40.

Ozone depletion

UVB

Dermatoheliosis

Figure 6. Skin cancer cascade [7].

The absorption of UV radiation in the various regions of the eye is dis­played in Fig. 8 [ 1 0]. The lens absorbs strongly the UV A and remaining UVB regions, leading to cataracts. In the search for new, efficient lighting sources, it is important to minimize any hazard coming from the residual UV emitted from the lamp. In this context, it is important to notice that semicon­ductor light sources do not follow any black body radiation scheme, and even white LEDs have a very much smaller UV hazard as compared to most traditional white light sources [ 11].

Nitride Photodetectors in UV Biological Effects Studies

l.TY

101

Erythema (red skin)

J 10~ l T\ .B

280 300 320 340 360

Wavelength (nm) 380 400

Figure 7 Important UV action spectra for human beings.

·~ 1-4-+-+-o.--.

t---:2 s.z

\, Lens_l~j ~Retina

AQueous humour

100-- <280

91-- JOO

45--- 320

31-- 340

34-- 360

Cornea

Figure 8. UV absorption of the cornea, aqueous humour and lens [I 0].

169

At this point, one may conclude that the UV radiation is a key one for life, its damage and healing. Since the big bang, in the long process for life generation, organisms have been adapting to the solar radiation, in fact, to Earth's atmosphere. By natural selection, genetic variants, repairing en­zymes, etc., life today has adapted to the radiation received from the Sun. There is a perfect coupling between ozone absorption and the wavelengths

170 E. Munoz et al

that severely damage DNA (250 run). Living species have developed their own protection and healing-repairing mechanisms; life has adapted [12]. One may also recall that clean water absorption has the lowest absorption region in the 580-380 run band, approximately [13]. This is important for marine life, amphibians, and also for detection of contaminants.

3. UV RADIATION IN BIOPHOTONICS TECHNIQUES

The role of optical techniques in biology and medical research has increased spectacularly in recent years. Being non-invasive in nature, and allowing all wonders of digital image processing, biophotonics is a leading field in life sciences. Fluorescence is a key technique today. Well known marking dyes have been joined by fluorescent proteins (initially, the green fluorescent pro­tein was found in aequora victoria fish), fluorescent microspheres (filled with dyes), and semiconductor quantum dots to allow imaging of nucleic acids, protein tagging, cell marking, immunodiagnostic assays, DNA dam­age measurements, cancer cell imaging, etc. (Fig. 9) [14]. Concerning UV, one is limited by its potential damage indicated in the previous section. In­trinsic UV fluorescence from DNA seems to be rather weak, but it has been shown that the presence of metallic particles enhances DNA UV emission, probably due to charge transfer mechanisms [15]. In tagging applications,

.2 6CI ii

'" :: " ...

Wavelength lnm) -------Figure 9. The excitation and emission spectra of enhanced blue, cyan, green, yellow and DsRed fluorescent proteins [14].

Near-UV photons, typically above 350 run, are used as excitation (absorp­tion) for blue-green markers. Its helps to significantly broaden the range of fluorescence colours, allowing multi-tagging applications and softening

Nitride Photodetectors in UV Biological Effects Studies 171

bandpass filter requirements. Hence, nitride UV and blue sources open new possibilities in biophotonics.

The use of semiconductor quantum dots (QD) as fluorophores has created a lot of expectations as reviewed in [16], and have started to be offered commercially. Quantum dots offer high photoluminescence response, long life, easy to excite, stability, and the possibility of tuning emission colours as needed (by size). Difficulties are seen as individual quantum dots have to be produced, later to be covered with a hydrophilic coating and carboxylic acid groups for bioconjugation, so they can be incorporated into the cells or used in DNA sequencing (Fig. I 0). Being initially prepared from II-VI semicon­ductors (CdSe, 3-6 nm for instance), other schemes like AlGaN/GaN nano­columns grown by MBE may open new possibilities for semiconductor nanotechnology in biophotonics [17] by extending the emission range into the UV.

• •• Figure 10. Unkown DNA containing fluorescent dyes is analyzed by being mixed with mi­crobeads containing a color code of quantum dots. DNA that matches the sequence on the outside of the microbead sticks to it [ 16].

Where both nitride UV emitters and detectors may have a strong role is in lab-on-a-chip, DNA/protein microarrays. The micro optics integration (arrays) of LEOs, LOs and photodetectors, from the near UV to the green region, all based on nitrides, will have benefits in multifunctional microar­rays, softening also optical filter requirements. Some views are given in [18]. Figure 11 shows AlGaN/GaN nanocolumns, with heights in the 200-nm range and 10- 20 nm in diameter, grown by MBE under nitrogen rich condi­tions [17]. Smaller sizes and proper coating of such individual nanocolumns seems to be plausible.

172 E. Munoz et a/

Figure 11. AIGaN/GaN/ AIGaN nanocolumns grown by MBE [ 17).

4. AllnGaN-BASED PHOTODETECTORS IN BIOPHOTONICS APPLICATIONS

In the two previous sections, a range of biophotonic applications for nitride UV and VIS detectors has been delineated: UV biological action studies, fluorescence from dyes and proteins as markers, photoluminescence from quantum dots, detection by fluorescence of hazardous species and contami­nants, etc. All they impose different requirements on the photodetectors to be used. From the wavelength viewpoint, detection of chemicals and bio­species by fluorescence goes down to 250 nm or even lower, while in tag­ging techniques, damage issues advice not to go lower than 320 nm. Solar UV biological action studies should cover the whole UV A, UVB bands (280 nm). In the UVB range, AlGaN detectors have been reported with very good responsivity and noise characteristics. Let us also remember that fluores­cence emission is usually made in the visible for marking applications. Other differences come from the detector frequency or time domain operation. So­lar biological effects studies are made under quasi-de conditions, being sta­bility and reproducibility for long periods of time the main requirements. Besides, sensitivity is usually not a problem, the solar signal is not difficult to detect. On the other hand, fluorescence imaging and fluorescence detec­tion of species require photodetectors working under ac conditions and a high sensitivity is needed. Time-domain fluorescence and fluorescence reso­nance energy transfer (FRET) have also revealed as very interesting tech­niques in biophotonics.

To consider detector structures for the above applications, let us summa­rize some issues about nitride UV photodetectors that would help to bind

Nitride Photodetectors in UV Biological Effects Studies 173

their applications in biophotonics. Various detailed reviews on the develop­ments of GaN UV photodetectors has been published recently [ 19]. Photo­conductive, p~n junction, p~i~n devices, Schottky barrier (SB), metal~

semiconductor~metal (MSM), metal~insulator~semiconductor structures (MIS), phototransistors, detector arrays, avalanche detectors, etc., have al­ready been reported. UV imaging using AIGaN arrays has also been clearly demonstrated. All these developments have been tightly linked with pro­gresses in the quality of the AIGaN materials, and with the development of reproducible processing technology.

The lack of appropriate substrates is a pending problem in the nitrides device effort. From the physics viewpoint, photodetectors work under very low currents and very low photon fluxes, usually, and they reflect even more pronouncedly some of the current problems in AlGaN layer quality, mainly the presence of dislocations. In AlGaN layers slow trapping processes are usually present at very low currents, giving rise to persistent photoconductiv­ity effects (PPC). This PPC is not yet fully understood, and it may show up in AlGaN photodetector devices, depending on their contact structure, mate­rials quality and device operation. In fact, while junction photodiodes are fast with a good UV/VIS contrast and a linear operation, AIGaN photocon­ductors are slow, have internal large gain, non-linear operation and a poorer UV!VIS contrast. A hybrid behaviour is found in many cases in photocon­ductive and MSM structures. Details of the metal contacts, free carrier con­centration and dislocation density cause to have photodetectors where some significant photoconductive gain coexist with a reasonable speed and a good UV!VIS contrast. On the other hand, Schottky, MSM and p~n junction de­tectors may show photoconductive gain and deviations from linearity under reverse bias conditions, reflecting also processing and layer quality condi­tions [19]. The (lnGaNAl)N detectors shown below are adequate to properly cover the UV A and the UVB region, to match any weighted action spectrum.

Fluorescence ac techniques require high sensitivity detectors. It is inter­esting to consider gain mechanisms in AlGaN detectors. Two mechanisms have been studied, photoconductive gain and avalanche multiplication. Photoconductive gain is easy to achieve in AlGaN photodetectors, although its control and reproducibility is poor. Photoconductors and MSM hybrid devices are thus adequate for fluorescence applications, although some ex­ternal optical filter may be required to increase UV !VIS contrast. Avalanche multiplication requires further developments, and only small area devices have been reported.

In biological effects studies, erythema weighted detectors are widely used for solar UV monitoring. One of the reasons has been the availability of commercial solutions presently achieved by using low gap photodiodes (Si, GaAs, GaAsP), and a series of filters and phosphor coatings to be inserted in

174 E. Munoz et a/

the optical path (Fig. 12) trying to match the erythema weighting function (Fig. 13) [20]. All these elements make the sensor system bulky, less reli­able, needing a temperature-controlled chamber, more expensive (although affordable) and prone to degradation. This technical solution for sun-burning UV detectors was already suggested in 1976.

Today, as shown in Fig. 13, by proper layer composition and shallow centers control, the erythema weight action is approximated by a single Al­GaN photodiode, with about 30% of AI composition, and no filters [20]. Note that a key region is the device response below the band edge. In this application, not the best AlGaN epilayer has to be used. This is the most im­portant point to insure spectral reproducibility in the erythema detector fab­rication. A key nitride advantage is its radiation hardness. In solar monitor­ing, UV detectors receive a significant UV A and UVB dose that tend to degrade optical filters (solarization), as required in erythema radiometers shown in Fig. 12.

The stability and reproducibility requirements of erythema detectors are very stringent in the UV A region. Besides, there is a need for nitride-based UV A and VIS detectors. (In,Ga,Al)N quantum well-based (QW) photodetec­tors are being developed at ISOM in order to benefit from the larger absorp­tion of QWs and from the compatibility with QW emitters. Figure 14 (b) shows the spectral responses of MOVPE QW photodetectors with detection edges in the blue and the UV A band, respectively. The layer structure shown in Fig. 14 (a) allows the emitter-detector integration in microarrays. The use of QW (ln,Ga,Al)N photodetectors allows developing multidetector (multi­band) sensors, able to match any spectral weighting function (action spec­trum) needed for quantifYing effects of UV radiation.

In nitride-based high-sensitivity detectors for fluorescence applications, significant advances in materials quality and device processing have been produced. As reviewed in [19], small area MSM and p-i-n detectors with time responses in the 300 ps range have been reported, internal quantum ef­ficiencies of 86% have been achieved, very low noise structures with detecti-

Nitride Photodetectors in UV Biological Effects Studies 175

SQAnr~teo uv tROM CLOUCCS. ANO Y

\

PRI~CIPlC Or 0P(f?A110

Figure 12. Optical filters and phosphorous layer required to achieve the erythema weighting action spectrum, due to the use of low bandgap semiconductor photodiodes [20].

10-4 260 280

UV-B -:~ UV-A

300 320

--CIE Erythema Action · · · · Commercial Detector

----- AIGaN Detector

340 360 380

Wavelength (nm) Figure 13. Erythema weighting action spectrum achieved with AlGaN photodetectors (Au semitransparent Schottky diodes), and no other optical component [21].

176

n-GaN

apphire

a)

0,1

0,01

~ 1E-3

-1. ~ 1E-4

·~ ::: ~ 1E-7

E. Munoz et a!

- A4Y ·7 - FU14582-5

1 e-a -~---r--.-.,.......,.~T"'"""'-.--.---,-.--,,---.--,---.-,--.....-~ 210 240 270 300 330 360 390 420 450 480

Wavelength (nm)

b)

Figure 14. a) The generic structure of InGaN/AIGaN MQW p-i- n diodes is shown. Three QWs have been mainly used. b) The spectral responsivity of devices, intended to have their detection edge in the blue region and at 400 nm, respectively.

vities in the 1013 cm·W-1 Hz112 range have been fabricated, and UV/visible contrasts between three and five orders of magnitude are today obtained in various laboratories. These devices are quite appropriate for biophotonics applications.

5. CONCLUSIONS

There is a perfect coupling between ozone absorption and the wavelengths that severely damage DNA (250 nm). All the biological consequences of the solar UV radiation are wavelength dependent and are characterised by unique action spectra (describing the relative effectiveness of each wave­length). Representative action spectra have been presented. Erythema action (red skin) detectors are reference detectors in a variety of medical and bio­logical effects studies. Their nitride implementation (AlGaN) is commer­cially interesting. For other action spectra, to fit any spectral weighting func­tion, multiband (ln,Ga,Al)N detectors, properly weighted channels, are needed. Radiation hardness offered by nitrides is an important point.

A range of biophotonic applications for nitride UV and VIS detectors has been delineated. Fluorescence is presently a key tool in medical and biologi­cal research, and III -nitrides offer the unique possibility to integrate a range of emitters and detectors from the green to the UV, softening also filter re­quirements. Lab-on-a-chip, DNA and protein microarrays will benefit from such integration based on nitrides. Significant advances in materials quality

Nitride Photodetectors in UV Biological Effects Studies 177

and device processing have been produced, but extra efforts are needed to achieve gain for high-sensitivity fluorescence detection. Nitrides nanotech­nology may also open new possibilities in fluorescent tagging applications.

REFERENCES

I. M. Razeghi and A. Rogalsky,J. Appl. Phys. 79,7433 (1996). 2. University ofThessaloniki, LAP; Greece. 3. P. C. Simon and C. S. Zerefos, "UV radiation at the Earth's surface", in The contri­

bution of EASOE and SESAME to our current understanding! of the ozone layer, European Commission, DG XII, EUR 16986 (1997).

4. G. Seckmeyer, Instruments to measure solar ultraviolet radiation. Part I, WMO TO No. I 066 (2000).

5. P. J. Neale, "Spectral weighting functions for quantifying effects of ultraviolet ra­diation in marine ecosystems" in S.J. de Mora eta!, Effects ofUV Radiation on Ma­rine Ecosystems (Cambridge Univ. Press, 2000).

6. D. S. Bigelow, J. R. Slusser, A. F. Beaubien, and J. H. Gobson, Bulletin of the American Meteorological Soc., 79, 601-615 (1998).

7. T. B. Fitzpatrick, J Dermatol, 23, 616-20 ( 1996). 8. Report on the WMO-WHO Meeting of Experts on Standardization of UV Indices

and their Dissemination to the Public, WMO/TD-No. 921, WMO, Geneva, 1998. 9. International Commission on Non-Ionizing radiation Protection, Global Solar In­

dex, ICNIRP-1/95 (1995). 10. World Health Organization, The effects of solar UV radiation on the eye (WHO,

Geneva, 1994 ). 12. A. Zukauskas, M. Shur, and R. Gaska, Introduction to Solid State Lighting (Wiley,

New York, 2002). 13. J. Withgott, Natural History 38 (2001). 14. R. A. J. Litjens, T.I. Quikenden, and C.G. Freeman, Appl. Optics 38 1216-1223

(1999). 15. Clontech Laboratories Inc., Palo Alto, California, USA. 16. J.R. Lakowicz, I. Gryczynski, Y. Shen, J. Malicka, and Z. Gryczynski, Photonics

Spectra 96-l 04 (200 I). 17. B. D. Butkus, editor, Biophotonics Inti., p. 60 (Dec. 2001) and p. 68 (Dec. 2002). 18. J. Ristic, M.A. Sanchez-Garcia, J.M. Ulloa, E. Calleja, J. Sanchez-Paramo, J.M.

Calleja, U. Jahn, A. Trampert, K.H. P1oog, Phys. Stat. Solidi (a) 234, 717 (2002). 18. N. D. Lamontagne, editor, Biophotonics Inti., 42-46 (Jan-Feb 2003). 19. E. Munoz, E. Monroy, J.L. Pau, F. Calle, F. Omnes, and P. Gibart, J Phys.: Con­

dens. Matter 13, 7115-7137 (2001). 20. Yankee Environmental Systems, Inc., Turner Falls, MA 01376 USA. 21. E. Munoz, E. Monroy, F. Calle, F. Omnes, and P. Gibart, J. Geophys. Res. 105,

4865 (2000).

PROMISING RESULTS OF PLASMA ASSISTED MBE FOR OPTOELECTRONIC APPLICATIONS

A. GEORGAKILAS, E. DIMAKIS, K. TSAGARAKI, and M. ANDROULIDAKI Microelectronics Research Group (MRG), Institute of Electronic Structure and Laser (IESL), Foundation/or Research and Technology-He/las (FORTH), P.O. Box 1527, 71110 Heraklion-Crete, Greece and University of Crete, Physics Department, Heraklion-Crete, Greece

Abstract: Plasma-assisted MBE (P AMBE) is not yet considered as an epitaxial growth method that could produce device quality material for optoelectronic applica­tions. However, several results suggest that PAMBE can grow high quality heterostructures on GaN templates and even directly on sapphire substrates. High quality N-face material can be probably grown on sapphire only by PAMBE. N-face GaN is grown after sapphire nitridation at low temperature and a layer-by-layer growth seems to occur immediately after the initiation of the GaN growth on the sapphire surface. Recently, high quality In-containing alloys, such as quaternary InAlGaN alloy thin films and quantum well het­erostructures, have also been grown by P AMBE with good control of the In incorporation. The layers exhibited strong PL up to room temperature and lasing under optical pumping.

Key words: GaN, III-nitrides, III-nitride semiconductors, MBE, plasma-assisted MBE, InGaN, InAlGaN, quaternary alloys, N-face GaN, sapphire nitridation, GaN polarity

1. INTRODUCTION

III-nitride devices for light emission and detection have been commercial­ized using heterostructure materials grown by metalorganic vapor phase epi­taxy (MOVPE). Molecular Beam Epitaxy (MBE) of III-nitrides has been investigated from the early stages of III-nitrides research [1-4] but the method has not yet been developed to the level required for device applica-

179

M.S. Shur and A. tukauskas ( eds. ), UV Solid-State Light Emitters and Detectors, 179-188. © 2004 Kluwer Academic Publishers.

180 A. Georgakilas et al

tions. Two different techniques of III-nitride MBE growth can be distin­guished, depending on the type of the source used to supply active nitrogen species to the substrate; the plasma assisted MBE (P AMBE) and the NH3 gas source MBE, also called as reactive MBE (RMBE).

P AMBE uses a compact plasma source to activate N2 gas, such as the electron cyclotron resonance (ECR) microwave source or the radio fre­quency (RF) source. ECR sources [1,2] were used frequently in the first MBE experiments but the inductively coupled RF plasma sources [3,4] soon dominated due to the reduced ion damage in the grown layers. An extensive review of the PAMBE growth of III-nitride materials has been given else­where [5]. This paper intends to present some important aspects ofPAMBE and recent results suggesting that P AMBE with nitrogen RF plasma source (RFMBE) could be used efficiently for the growth of state-of-the-art III­nitride heterostructure materials ·[5-14]. Three different growth tasks will be considered: (a) homoepitaxial GaN growth [5,6], (b) heteroepitaxial GaN growth on A}z03 (0001) [11-17], and (c) growth oflnxAlyGa1_x-yN alloys and quantum well heterostructures [12-14].

2. GALLIUM NITRIDE HOMOEPITAXY

GaN substrates have yet not been commercially available like in the conven­tional III-V semiconductors. However, significant experience for the GaN P AMBE growth has been gained by using templates consisting of GaN thin films grown on Ah03 (0001) or SiC (0001) substrates by MOVPE or hy­dride vapor phase epitaxy (HYPE) [5]. Such experiments have helped to un­derstand the fundamentals of GaN P AMBE growth.

It has been well documented both theoretically [15] and experimentally [ 16] that the GaN growth mode depends on the concentration of surface ada­toms. A surface Ga bilayer [15,16] can be formed using a GaiN flux ratio above unity and favors the layer-by-layer growth of GaN by the step-flow growth mechanism. Figure 1 shows an AFM micrograph for a 0.5-!lm GaN layer grown by RFMBE on a ~2-11m GaN/A}z03 (0001) MOVPE template. The GaiN flux ratio was ~ 1.3 and the substrate temperature was 700 °C. Un­der these conditions, step-flow growth occurs with no accumulation of any Ga droplets on the surface. The surface rms roughness was 0.26 nm, quite comparable to that of the initial MOVPE material.

It has been found that the crystalline defects of a RFMBE GaN film are continuation of defects propagating from the GaN template [5]. No addi­tional dislocations are formed at the interface, although several reports sug­gest that residual impurities may remain on the GaN surface after the usual substrate cleaning treatments [ 17].

Promising Results of Plasma Assisted MBE 181

Figure I. AFM micrograph of 0.5 J.lm GaN grown by PAMBE under step-flow growth mode on a - 2.0 J.lm GaN/A120 3 (0001) MOVPE template. The scan size is 2 x 2 J.lm and the z-axis full scale is 5 nm.

4.-----------------------, ,.__ (a) = ~ q, 3 "' = q,

"' "' q,

= ·a 2 = ~ 0 -=

MOVPE template

A. .. ~32Snrr T~I 7K

~ 1 !::::== ::::i:::==::;::=--.--...::c=:===:::::l 3.35 3.40 3.45 3.50 3.55 3.60

Energy(eV)

MBEsample

3.35 3.40 3.45 3.50 3.55 3.60 Energy(eV)

Figure 2. 17-K PL spectra for (a) - 2-J.lm GaN/AI20 3 (0001) MOVPE template and (b) 0.5-J.lm GaN grown by RFMBE on the MOVPE template.

Finally, the low temperature photoluminescence (PL) spectra of the RFMBE layers exhibit excitonic peaks with full width at half maximum (FWHM) values similar to those of the MOVPE templates, as shown in Fig. 2. This indicates that the RFMBE material quality is not limited by ion­induced damage.

182 A. Georgakilas et a/

3. HETEROEPITAXY ON (0001) SAPPHIRE

P AMBE is generally inferior of MOVPE in the field of direct GaN het­eroepitaxial growth on a different substrate, such as the commonly used Al20 3 (0001). The PAMBE weakness, according to the authors' opinion, is linked to two important stages of GaN/ Ah03 heteroepitaxy: (1) the GaN nu­cleation and (2) the buffer layer growth.

GaN films with the (0001) orientation or Ga-polarity are generally pref­erable than films with the (000-1) orientation or N-polarity, because they can be grown easily with smooth surface morphology (Fig. 1) and the direction of polarization fields is convenient for the fabrication of AlGaN/GaN high electron mobility transistors (HEMTs) [6].

MOVPE seems able to nucleate easily high purity Ga-face material, pos­sibly as a result of significant Ah03 nitridation under exposure in NH3 flux at high substrate temperature. In addition, large three-dimensional (3D) GaN islands may be initially nucleated on the Ah03 (0001) surface [18] but even­tually a flat GaN/ Ah03 buffer layer can be grown, apparently due to the high lateral growth speed and adatom surface mobility which is possible in MOVPE. This allows to increase the size of the initial crystalline grains (3D islands) and so to increase the percentage of the good crystallinity material that occurs within the grains.

In PAMBE, we have found [5,10,11] a significant difference in the amount of sapphire nitridation at the high nitridation temperature (HNT) of ~750 oc and the low nitridation temperature (LNT) of ~200 oc and that this controls the polarity of overgrown GaN layers, when a GaN buffer layer is used. A substantial surface nitride layer, with average thickness of 1.5 nm, was formed only after 100 min nitridation at HNT, while the nitridation should be limited to a surface atomic plane at LNT. The HNT results toGa­face material, while the LNT results toN-face GaN. The Ga-face polarity is attributed to the formation of an AlN/sapphire interface by nitridation at HNT (although a GaN buffer layer is then used), since Ga-face material is also grown when an AlN buffer layer is used on a substrate nitridated at LNT.

We have grown, by RFMBE, Ga-face GaN/Ah03 films with surface smoothness [5] and PL comparable to MOVPE. Figure 3 shows the 17-K PL spectrum of a 1.8-l.lm GaN/Ah03 sample. The FWHM of the excitonic peak is 7 meV and there is no deep levels' PL below 3.2 eV. However, it has been found rather difficult [5,7-9] to achieve reproducibly high purity, low defect density and smooth Ga-polar films, similar to MOVPE. The higher density of threading dislocations may be the result of the misfit strain relaxation dur­ing a nucleation-growth process that deviates from the ideal layer-by-layer MBEgrowth.

Promising Results of Plasma Assisted MBE

~6 = ~ ... ... a! 4 ~ ... = ·a 2 = ~

_ LmeV

).m=325nm T=l7K

f 0 '----..---.J.L......----L-....... =~=--.....~-'-'--...d 3.0 3.1 3.2 3.3 3.4 3.5

Energy(eV) 3.6

183

Figure 3. 17-K PL spectrum of a 1.8-J.lm Ga-face GaN layer grown by PAMBE on A}z03

(0001) using a 50-nm AlN buffer layer.

N-face GaN is grown on sapphire nitridated at LNT, using a GaN buffer layer [10,11]. A streaky RHEED pattern is observed continuously during GaN nucleation, indicating a layer-by-layer growth mode. Transmission electron microscopy (TEM) studies revealed [5,11] that the N-face layers have a low defect density compared to the usual structure of Ga-face GaN [8,9]. "Cubic pockets" were observed in the initial GaN buffer layer, while no such cubic phase material was monitored in all the Ga-face samples. However, a significantly lower density of threading dislocations and inver­sion domain boundaries (IDBs) appeared in theN-face samples, indicating that cubic GaN embedded in the hexagonal matrix is related to the reduction of the defect content ofthe overgrown layers.

The TEM results were in agreement with high-resolution X-ray diffrac­tion (HRXRD) measurements (Fig. 4). HRXRD rocking curves for the symmetric (00.2) and asymmetric (11.4) reflections of 1.3-!lm N-face GaN/A1203 (0001) exhibited record low values of FWHM, equal to 54 and 135 arcsec, respectively.

These results suggest that P AMBE is particularly suitable for the growth ofN-face GaN/Al203 and this has not yet been exploited.

184 A. Georgakilas et al

3.0

j2.5 ~"' ~ 2.0 ~

; 1.5

·~ 1.0 ~ .s 0.5

(a) (00.2) FWHM = 54 arcsec

0.01-------

-1000 -500 0 500 Aro (arcsec)

1000

2.5 (b) '""'

FWHM = 135 arcsec

e. 2.0 "'=

(11.4)

...... 1.5 -!! £ 1.0 "' = .; 0.5 ....

0.0

-1000 -500 0 500 1000 Aro (arcsec)

Figure 4. X-ray diffraction w-scans for the reflections: (a) symmetric (00.2) and (b) asymmet­ric (11.4) in grazing incidence, for a 1.3-!IDl N-face GaN/AI20 3 (0001) layer

4. GROWTH OF INDIUM-CONTAINING ALLOYS AND QUANTUM WELLS

The growth of In-containing alloys, InxAlyGa1_x-yN and quantum well het­erostructures may be the strongest point of P AMBE. The energetic nitrogen species that are supplied by the plasma source make possible to grow III­nitrides at very low substrate temperatures, where the thermal decomposition of NH3 gas would be impossible. This facilitates the incorporation of In, which is limited from the low thermal stability of InN.

We have investigated the temperature dependent incorporation of In at­oms in quaternary InxAlyGa1_x-yN alloys [12]. The fluxes of Al (FA1) and Ga (F oa) atoms were kept constant and corresponded to an AlGaN composition (without incorporation of In) of approximately Al0.40Gao.60N. The flux of ac­tive nitrogen species (FN) was also kept constant and it was higher than the growth rate of Alo.4oGao.60N. The incorporation of In atoms in the InxAlyGa1_x-yN alloys was a sensitive function of the growth temperature, as shown in Fig.5, which gives a plot of the In mole fraction (x) as determined by RBS versus the growth temperature in the range of 510-610 °C. There­sults show that the amount of incorporated In flux matches the excess nitro­gen flux {FN-(FA1+Faa)} at a substrate temperature of 510-530°C. The preferential incorporation of Al and Ga atoms compared to In atoms is ex­pected considering the relative strength of their bonds with N [ 19]. It should be noticed, however, that a recent careful analysis suggests that additional In atoms may be incorporated instead of Al and Ga atoms at temperatures lower than ~530 oc [20].

Promising Results of Plasma Assisted MBE 185

We have studied also the control of the InxAlyGat-x-yN composition by the flux of In atoms, which is controlled by the temperature of the In cell. As shown in Fig. 6, for a constant substrate temperature of 540 oc and an N flux of 2.6x10 13 cm-2s-' the percentage of the incoming In flux that is incorpo­rated is constant and equal to ~ 17%.

0.10

c 0.08

·! ~ 0.06 ..::

..!:l " 0.04 e .: 0.02

•

0.00'---...--L~....L.-~.L..-'-----'-~---'--~..J...::::..__,

500 520 540 560 580 600 620 640 Growth Temperature ("C)

Figure 5. Dependence of the In mole fraction in quaternary layers of lnxAlo.40(l-xPao.6o(J-x)N on the growth temperature. The alloy composition was determined by RBS. (Reprinted from Journal of Crystal Growth, Vol. 251, E. Dimakis, A. Georgakilas, M. Androulidaki, K. Tsaga­raki, G. Kittler, D. Cengher, E. Bellet-Amalric, D. Jalabert, N.T. Pelekanos, "Plasma-Assisted MBE Growth of Quaternary InAlGaN Quantum Well Heterostructures with Room Tempera­ture Luminescence", pages 476-480, Copyright 2003, with permission from Elsevier.)

Further work has leaded to the optimization of the growth of InxAlyGat-x-yN alloy thin films and quantum well heterostructures [12-14]. Ternary and quaternary alloy layers with smooth surfaces and excellent op­toelectronic properties were grown at 530-540 °C. Indicative 17-K PL re­sults for different InxAlyGat-x-yN alloy compositions are given in Fig. 7. Both ternary and quaternary alloys exhibited very strong band edge luminescence with the best FWHM values of 60-65 meV for emission at 3.0-3.15 eV, which are record low values for PAMBE grown alloys. As shown in Fig. 7, we have grown ternary InxGa1_xN alloys with x up to 0.26 without any evi­dence of phase separation, according to HRXRD measurements. The Ino.26Gao.14N layer was characterized by PL emission at 2.46 e V and FWHM of 127 meV. For quaternary InxAlyGat-x-yN alloys, we have reached repro­ducibly In composition up to x = 0.135 for y = 0.26-0.28 and these layers exhibited 17-K PL emission at ~3.1 eV with FWHM of 64-68 meV. The alloys also maintained strong PL emission at room temperature what indi­cates relatively weak nonradiative processes in the material.

186

6 • incorporated In flux _,-..., • non-incorporated In flux ·, 5 '~a ..... 4 -= 3-3 ~ ,s 2

.s 1

0 L-"'=====:::;::=::!:::::=~=._j 2 3 4 5 l3 62 I 7

Incident In flux (x 10 em-s-)

A. Georgakilas eta/

Figure 6. The incorporated In flux (squares) and the non-incorporated In flux (circles) have been plotted versus the incident In flux on the substrate surface, for constant N flux of 2.6x1013 cm-2s-1•

~OI,y=0.3

~07,y=0.33 ~.y=0.28

A."'=325nm T=l7K

2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.8 Energy(eV)

Figure 7. 17-K PL spectra of different lnxAlyGa1_x-yN samples grown by RFMBE.

The quaternary InxAlyGai-x-yN alloys exhibited a particularly large value for the In bowing coefficient [13]. This means that the bandgap of the InAl­GaN alloys could be larger or smaller than that of GaN, depending on the In mole fraction oflnAlGaN.

Multiple Quantum Well (MQW) GaN/InAlGaN structures were also grown, where the well material was either GaN or quaternary InAlGaN. The growth was optimized to ensure the optimum surface smoothness of all the layers and the abruptness of the interfaces [12]. HRXRD [12] and high­resolution TEM [21] measurements confirmed the good quality ofthe MQW structures.

The polarization-induced electric fields within the GaN/InAlGaN QWs were also investigated and the potential for zero field QWs was demon-

Promising Results of Plasma Assisted MBE 187

strated, as a consequence of polarization matching between the InAlGaN and GaN layers [13,14].

Finally, laser emission under optical pumping at room temperature has been obtained on structures containing several InAlGaN active QWs [12,14]. The threshold power density for the InAlGaN/GaN QWs was lower than that of comparable GaN/AlGaN QWs, indicating the positive effect of reduced internal electric field [ 14].

5. CONCLUSIONS

We have discussed several results suggesting that P AMBE is a technique with great potential for the development of state-of-the-art III-nitride het­erostructure-nanostructure materials, similarly to what the MBE method used to be in the development of other III-V semiconductors. However, full exploitation of its capabilities requires the availability of GaN substrates, which are under development with a variety of technological approaches. A significant advantage of P AMBE is anticipated in the area of In-containing alloys and heterostructures. In addition, heteroepitaxial N-face GaN/Alz03

(0001) of excellent structural quality can be probably grown only by PAMBE.

ACKNOWLEDGEMENTS

Our work has been funded from several programs of the General Secretariat for Research and Technology (GSRT) of the Greek Ministry of Develop­ment and from the European Commission projects IST-FET-26464 and 38982. Support from University of Crete is also acknowledged.

We are also grateful toN. Pelekanos, G. Konstantinidis, Ph. Komninou, Th. Karakostas, M. Calamiotou, E. Bellet-Amalric and D. Jalabert for their valuable collaboration.

REFERENCES

1. T. Lei, M. Fanciulli, R. J. Molnar, T. D. Moustakas, R. J. Graham, and J. Scanlon, Appl. Phys. Lett. 59, 944 (1991).

2. R. J. Molnar and T. D. Moustakas, J. Appl. Phys. 76,4587 (1994). 3. W. E. Hoke, P. J. Lemonias and D. G. Weir, J. Cryst. Growth 111, 1024 (1991). 4. J. M. Van Hove, G.J. Cosmini, E. Nelson, A. M. Wowchak, P. P. Chow, J. Cryst.

Growth 150, 908 (1995).

188 A. Georgakilas eta/

5. A.Georgakilas, H. M. Ng and Ph. Komninou, in Nitride Semiconductors, Handbook on Materials and Devices, ed. by P. Ruterana, M. Albrecht, and J. Neugebauer, Chapter 3 (2003, Wiley-VCH, Berlin, 2003), pp. 107-191.

6. M. Zervos, A. Kostopoulos, G. Constantinidis, M. Kayambaki, and A. Georgakilas, J. Appl. Phys. 91, pp. 4387-4393 (2002).

7. K. Amimer, A. Georgakilas, M. Androulidaki, K. Tsagaraki, M. Pavelescu, S. Mik­roulis, G. Constantinidis, J. Arbiol, F. Peiro, A. Cornet, M. Calamiotou, J. Kuzmik, and V. Yu. Davydov, Mater. Sci. Eng. B 80, pp. 304-308 (2001).

8. G. P. Dimitrakopoulos, Ph. Komninou, J. Kioseoglou, Th. Kehagias, E. Sarigianni­dou, A. Georgakilas, G. Nouet and Th. Karakostas, Phys. Rev. B 64, 245325 (2001).

9. Ph. Komninou, Th. Kehagias, Th. Karakostas, G. Nouet, P. Ruterana, K. Amimer, S. Mikroulis and A. Georgakilas, in Proceedings of the 2000 MRS Fall Meeting, Symposium G: GaN and Related Alloys (November 27- December l, 2000, Boston, USA) Mat. Res. Soc. Symp. Proc. 639, G3.47 (2001).

10. S. Mikroulis, A. Georgakilas, A. Kostopoulos, V. Cimalla, E. Dimakis, and Ph. Komninou, Appl. Phys. Lett. 80, pp. 2886-2888 (2002).

ll. A. Georgakilas, S. Mikroulis, V. Cimalla, M. Zervos, A. Kostopoulos, M. An­droulidaki, Ph. Komninou, Th. Kehagias and Th. Karakostas, Phys. Stat. Sol. (a) 188, pp. 567-570 (2001).

12. E. Dimakis, A. Georgakilas, M. Androulidaki, K. Tsagaraki, G. Kittler, D. Cengher, E. Bellet-Amalric, D. Jalabert, N.T. Pelekanos, J. Cryst. Growth 251, pp. 476-480 (2003).

13. M. Androulidaki, N. T. Pelekanos, E. Dimakis, F. Kalaitzakis, E. Aperathitis, F. Bellet-Amalric, D. Jalabert, K. Tsagaraki, and A. Georgakilas, Phys. Stat. Solidi (c) 0, pp. 504-507 (2002).

14. F. KalaYtzakis, M. Androulidaki, N. T. Pelekanos, E. Dimakis, E. Bellet-Amalric, D. Ja1abert, D. Cengher, K. Tsagaraki, E. Aperathitis, G. Konstantinidis, and A. Geor­gakilas, Phys. Stat. Sol. (a) 195, 2003 (in print).

15. J. E. Northrup, J. Neugebauer, R. M. Feenstra, and A. R. Smith, Phys. Rev. B 61, 9932 (2000).

16. B. Heying, R. Averbeck, L. F. Chen, E. Haus, H. Riechert, and J. S. Speck, J. Appl. Phys. 88, 1855 (2000).

17. S. W. King, J. R. Barnak, M.D. Bremser, K. M. Tracy, C. Ronning, R. F. Davis, and R. J. Nemanich, J. Appl. Phys. 84, 5248 (1998).

18. R. Chierchia, T. Bottcher, H. Heinke, S. Einfeldt, S. Figg, and D. Hommel, J. Appl. Phys. 93, 8918 (2003).

19. R. Averbeck and H. Riechert, Physica Status Solidi (a) 176 301 (1999). 20. E. Dimakis, Master Thesis, Physics Department, University of Crete, July 2003. 21. Ph. Komninou, unpublished results.

LOW DISLOCATIONS DENSITY GaN/SAPPHIRE FOR OPTOELECTRONIC DEVICES

B. BEAUMONT, J.-P. FAURIE, E. FRA YSSINET, E. AUJOL, and P. GIBART Lumilog, 2720, Chemin de Saint Bernard, Les Moulins I, 06220 Vallauris, FRANCE

Abstract: It is nowadays well established that threading dislocations (TDs) are degrading the performances and the operating lifetime of optoelectronic GaN-based de­vices (LDs and UV-LEDs). GaN/sapphire layers have been grown by Metal Organic Vapor Phase Epitaxy (MOVPE). An amorphous silicon nitride layer is deposited using a SiH4/NH3 mixture prior to the growth of the low temperature GaN buffer layer. Such a process induces a 3D nucleation at the early begin­ning of the growth, resulting in a kind of ELO process with intrinsic random opening sizes. This produces a significant decrease of the TDs density com­pared to the best GaN/sapphire templates. GaN layers with TD density as low as 7xl07 cm-2 were obtained, as measured by atomic force microscopy (AFM), cathodoluminescence (CL) and transmission electron microscopy (TEM). The two-step epitaxial lateral overgrowth technology (2S-ELO) allows decreasing the TDs around 107 cm-2• These templates are suitable for fabricating LDs. Regrowth by HVPE on this ELO GaN/sapphire further decreases the TDs den­sity below 106 cm-2

Key words: threading dislocations, epitaxial lateral overgrowth, MOVPE, HVPE, UV de­tectors

1. INTRODUCTION

Bulk GaN is intrinsically very difficult to grow because of the high vapour pressure of nitrogen at the melting point of GaN. Growth in molten metals (Ga, Na) is currently under development and has so far only produced small high quality crystals. Therefore, all the development of nitride based devices has been made on foreign substrates. GaN is grown in the form of epitaxial layers on either sapphire or 6H-SiC. The lattice parameters and the thermal

189

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190 B. Beaumont et al

expansion coefficients of sapphire and SiC are not well matched to GaN. The epitaxial growth therefore generates huge densities of dislocations (109

to 1011 cm-2). These dislocations propagate up to the surface, deteriorating the performance of optical and electronic devices.

Heteroepitaxy on sapphire requires several steps including the nitridation of the sapphire substrate, the deposition of a low-temperature buffer layer and the heat treatment of this nucleation layer. The main goal of heteroepi­taxy is to limit the generation of threading dislocations. But how far should we reduce the TDs in GaN? GaN epilayers with TDs densities in the mid 108

range can be produced easily by MOVPE. This quality has proven to be good enough for fabricating standard LEDs. However, new generation of LEDs or UV LEDs need dislocation densities much lower than 108 cm-2•

Furthermore, laser diodes (LDs) for Blue-Ray DVD require free standing GaN with the TDs density in the low 106 cm-2 range.

2. GaN/SAPPHIRE WITH TD DENSITY <109 cm-2

The exposure of the sapphire substrate prior to the deposition of a low­temperature GaN nucleation layer (NL) under simultaneous silane and am­monia flows fundamentally influences the quality of GaN epilayers grown by MOVPE [1]. During the annealing up to 1080 °C, this process induces a dramatic morphological change of the GaN NL from a flat layer fully cap­ping the substrate to a high density of 3D GaN islands (200--400-nm large and 1 00--200-nm high). Such a technology led to TDs density ranging in the mid 108 cm-2 (See Table I for other characteristics).

3. GaN/SAPPHIRE WITH TD DENSITY <108 cm-2

3.1 Experimental

The density of defects in materials grown with a 3D mode was recently re­duced down to the mid 107 cm-2 range by a longer silane+ ammonia treat­ment of 360 sec [1]. The whole process is monitored in real time by in situ laser reflectometry, Fig. 1. During the growth of this kind of samples, the full recovery ofthe reflectivity takes about 4 hours. As expected, the size of the 3D nuclei increases with the deposition time of SiN. This technology produces a TDs density of 7x107 cm-2 (other characteristics are in Table I) Delaying of the coalescence has been recently achieved using a different approach [1,2]. Tuning the VIlli ratio was used to control the coalescence of the island nucleation; TD densities of 1.5x108 cm-2 have been reported [3].

Low Dislocations Density GaN/Si Sapphire 191

(Compared to standard GaN, this quality is called Ultra Low Dislocation, ULD.)

Iii c::: Ol

25

20

"iii 15 z. "5 u Q) 10 ~ 0::

5

Standard GaN/Sapphire

0 250 Time (mn)

Figure 1. Comparison between reflectivity spectra recorded during the growth of GaN/sapphire standard epilayer and ULD GaN/sapphire. Arrows indicate where the growth starts.

3.2 Assessment by Transmission Electron Microscopy [1]

High resolution TEM images reveal that there is an amorphous region be­tween GaN and sapphire, most likely SiN, (EDS shows some silicon). How­ever, the amorphous material at the interface is a discontinuous layer. In­deed, the interface SiN layer is build of grains (20-nm height and 20--40-nm long). In Fig. 2, two small amorphous inclusions inside the GaN layer at the interface are seen. Indeed, the SiN layer acts as randomly distributed (in size and in location) openings, thereby inducing a lateral overgrowth process in the lateral windows. The GaN layer over these "masks" is hexagonal and follows the usual epitaxial relationship to sapphire (30° rotation around the 0001 axis). Sometimes the cubic sequence is also observed in grains of the NL between two SiN "inclusions." However, these regions are also over­grown by hexagonal GaN. As a further proof of the occurrence of an ELO

1g2 B. Beaumont et al

mechanism, bending of threading dislocations, occurs in this micro-ELO process as shown in Fig. 3. More precisely, Fig. 3 shows two TDs that propagate horizontally after bending.

100 nm -

Figure 2. High Resolution image of the interface in a GaN/sapphire sample

Figure 3. Cross-sectional bright field image of the GaN/sapphire sample show­ing the interface region.

4. GaN/SAPPHIRE WITH TD DENSITY< 107 cm-2

To further reduce the TDs density below 107 cm-2, the Epitaxial Lateral Overgrowth (ELO) technology must be implemented. The ELO process is described in several review papers [1-3]. So far, the two-step ELO allows to achieve the largest low defect density surfaces. In short, as schematically shown on Fig. 4, a standard GaN/sapphire template is covered by a thin layer of a dielectric mask. Then stripes opening along <1-100> axis are made by photolithography. Afterwards, growth is allowed to restart and GaN is first deposited on the stripe openings, but not on the mask (selective epitaxy). In the first step, growth conditions are tuned to get triangular stripes with { 11-22} lateral facets. During this first step, TDs are bent at goo as soon as they reach the lateral facets as evidenced in the TEM image. This bending prevents the propagation of TDs along the growth direction. A straightfor­ward explanation of the goo bending could be as follows: bending results from the general principle of minimization of the free enthalpy of the sys­tem. During growth, dislocations follow a direction leading to minimum en­thalpy; basically the axis of the dislocation tends to be perpendicular to the growing surface. As the energy of a dislocation depends also on its charac­ter, the energy of a screw being the lowest, goo-bending at the edge eventu­ally produces a screw dislocation, or introduces a screw component thus lowering the enthalpy of the system. In the second step, growth conditions are modified to enhance the lateral growth and to produce a flat surface. At

Low Dislocations Density GaN/Si Sapphire 193

the end, TDs merge only at the coalescence boundaries of the two laterally growing GaN stripes, as seen by catholuminescence (Table 1).

3 J.lm wide window stripes

7 J.lm wide SiN mask stripes

Figure 4. 2S-ELO, from left to right, mask, first step, the TDs behavior is schematically shown by red lines, second step of the ELO (planerisation), TEM image showing TDs bending at 90°, then propagating laterally to either end up in voids eventually formed at the coalescence boundaries or merge at the surface.

5. GaN/SAPPHIRE WITH TD DENSITY <106 cm-2

Whatever the improvements of the ELO technology - multistep ELO, pendeoepitaxy, cantilever epitaxy - the intrinsic limitations (coalescence boundaries, starting sapphire substrate) will not make ELO template suitable for the fabrication ofthe next generation ofLDs for Blu-Ray DVD. 2" free­standing epiready GaN wafers with the TDs density of ~106 cm6 are needed for the fabrication of these LDs structures. Several routes are currently de­veloped to obtain free-standing GaN. First, thick layers are required; since MOVPE is not appropriate to grow thick layers, other growth technologies like Halide Vapor Phase Epitaxy (HYPE) or sublimation should be imple­mented. However, the mechanisms of reduction of TDs when growing thick layers are very inefficient, therefore several hundred microns are needed to reach ~ 107 cm-2 TDs densities. Current technologies use HYPE either di­rectly on sapphire or on MOVPE GaN/sapphire templates. Then, after growth of several hundred microns, the GaN layer is separated from the sap­phire either by laser lift off (LLO), grinding, hydrogen implantation, or even by strain-induced self-separation. To get better quality GaN, in the present work the template was of ELO quality. Henceforth, free-standing GaN with

194 B. Beaumont et a/

<106 cm-2 TDs density was obtained by strain-induced separation (see Ta­ble 1).

Table I. Basic characteristics of different GaN/sapphire: FWHM of the PL near-bandgap emission, time resolved photoluminescence (TRPL) decay time of the free exciton A, TDs density and comparison of the CL images at the same scale.

Sample FWHM TRPL

(PL) (XA)

Standard <3 me V . 80 ps

ULD <2meV

2S-ELO <1 meV

Free­

standing

220 ps

375 ps

TDs density CL images

between

stripes

full wafer

Low Dislocations Density GaN/Si Sapphire 195

6. UV PHOTODETECTORS

Even though Si- or GaAs-based UV detectors are present on the market, the nitride alloys allow fabrication of the detectors without the stringent re­quirement of filters that are usually necessary to cut the parasitic infrared and visible light in front of the small band gap semiconductor-based UV commercial detectors. The large band gap of nitride-based compounds also results in a much better linearity of the optical response of junction-based detectors in the high-energy photon spectral region, and it is clearly proven today that the noise figure of nitride-based UV photo detectors very favora­bly compares to that of Si-based UV visible-blind photodiodes, despite a quite large dislocation density that is commonly observed in nitride-based compounds grown on sapphire. Furthermore, a very high level of radiation hardness of nitride-based materials makes expectable a much longer lifetime for nitride-based UV photo detectors than silicon-based UV photodiodes, especially when immersed in a harsh environment. In the present work, Schottky diode UV detectors were made on GaN/sapphire grown along dif­ferent technologies, i.e. with different densities of TDs.

Currently Schottky photodetectors exhibit a visible rejection ratio of 3 to 4 orders of magnitude. Figure 5 displays the spectral response for three dif­ferent qualities of GaN layers, an increase of one order of magnitude is ob­served for ELO or micro-ELO compared to standard GaN/sapphire.

10

~ 10 .,

"> u; 5 10 .;t

c. til Cll

et:: 10 ... "0 Cll -~ -- ELOG GaN

~ 10 • -- Mlcro-ELO GaN

0 - -- - GaN/Sapphire z

10

250 300 350 400

Wavelength, nm

450

Figure 5. Normalized spectral response of ELO Schotttky photodiodes compared to the spec­tral response of the same devices on standard GaN/sapphire and ULD (labeled as micro­ELO).

196 B. Beaumont et al

Although other mechanisms (like incorporation of impurities) can limit the rejection ratio, there is a direct relationship between the rejection ratio and the TDs density [1,2].

7. DISCUSSION AND CONCLUSIONS

Table I summarizes the basic characteristics of GaN/sapphire with different TDs densities. It is rather difficult to reduce the TDs densities in GaN and the conventional methods used in heteroepitaxial system like SiGe/Si were found to be inefficient for GaN/sapphire.

An analysis of the reduction mechanisms in GaN proposed by Mathis et al. [1], the TDs dislocation density decreases as h-213 , where h is the thick­ness. Indeed, interactions between dislocations are more efficient close to the interface, where more mixed dislocations are present. As the layer becomes thicker, this leaves primarily edge dislocations with larger separation, there­fore interactions become less and less likely and a further decrease of the TDs density becomes difficult. The ELO technology allowed to reduce this TDs density to some extend by bending TDs. Further progress is required to produce epiready free-standing GaN at an affordable cost.

REFERENCES

1. S. Haffouz, B. Beaumont, P. Vennegues, and P. Gibart, Phys. Stat. Sol. (a) 176,677 (1999) (and references therein).

2. E. Frayssinet, B. Beaumont, J. P. Faurie, Pierre Gibart, Zs. Makkai, B. Pecz, P. Le­febvre, P. Va1vin, MRS Internet J. Nitride Semicond. Res. 7, 8 (2002).

3. S. Figge, T. Bottcher, S. Einfe1dt, and D. Hommel, J. Cryst. Growth 221, 262 (2000).

4. D. D. Kokele et al., Appl. Phys. Lett. 81, 1940 (2002). 5. Courtesy Bela Pecz, Research Institute for Technical Physics and Mat!. Sci., Buda­

pest, Hungary. 6. B. Beaumont, P. Vennegues, and P. Gibart,"Epitaxial Lateral Overgrowth ofGaN,"

in Special Issue: Interfaces and Defects at Atomic Level, Phys. Stat. Sol (b) 227, 1-43 (2001).

7. P. Gibart and B. Beaumont, "Epitaxial lateral overgrowth and other approaches for low-defect-density GaN/sapphire," in Proc. SPIE: Gallium-nitride-based technolo­gies, ed. by M. Osinski, 112-139 (2002).

8. P. Gibart, B. Beaumont, and P. Vennegues, "Epitaxial lateral overgrowth of GaN" in Nitride Semiconductors Handbook on Materials and Devices, ed. by P.Ruterana, M. Albrecht, and J. Neugebauer, 45-106 (2003).

9. E. Monroy, F. Calle, E. Muiloz, B. Beaumont, F. Omnes, P. Gibart, Electron. Lett. 35, 1488 (1999).

10. Elias Mufioz, private communication.

Low Dislocations Density GaN/Si Sapphire 197

11. S. K. Mathis, A. E. Romanov, L. F. Chen, G. E. Beltz, W. Pompe, and J. S. Speck, J. Cryst. Growth 231,371 (2001).

STIMULATED EMISSION AND GAIN IN GaN EPILAYERS GROWN ON Si

A. L. GURSKI! 1, E. V. LUTSENKO 1, V. Z. ZUBIALEVICH 1,

V. N. PAVLOVSKII 1, G. P. YABLONSKI! 1, K. KAZLAUSKAS 2,

G. TAMULAITIS 2, S. WRSENAS 2, A. ZUKAUSKAS 2, Y. DIKME 3,

H. KAUSCH 3, A. SZYMAKOWSKI 3, R. H. JANSEN 3,

B. SCHINELLER \ and M. HEUKEN 4

1 Stepanov Institute of Physics of NAS Belarus, F. Skaryna Ave. 68, 220072 Minsk, Belarus E-mail: gurskii@dragon.bas-net.by 2 Institute of Materials Science and Applied Research, Vilnius University, Lithuania 3 Institutfor Theoretische Elektrotechnik, RWTH, Aachen, Germany 4AIXTRON AG, Aachen, Germany

Abstract: Optical and lasing properties .of GaN/Si and GaN/Si/SiO/Si structures with a strain reducing AlGaN/ AlN layer stack were investigated in a wide range of optical excitation densities and within a temperature interval of 10-290 K. Both the structure containing the SiO interlayer and that without the interlayer had the same laser threshold (700 kW/cm2) at room temperature. However, structures with the interlayer had a 2-3 times higher efficiency of photolumi­nescence and lasing. The peculiarities of the PL and lasing spectra in two types of samples are discussed in terms of a difference in carrier lifetime resulting in difference in excited volume of the epitaxial layers. The net-gain spectra ex­hibited lower optical losses in structures containing the interlayer, apparently due to an improved waveguide structure.

Key words: lasing, GaN, Si substrate, SiO interlayer, optical pumping, lasing threshold, optical gain, MOCVD.

1. INTRODUCTION

Silicon is a promising alternative substrate for GaN growth because of its low cost, excellent quality, large-area availability and the possibility to inte­grate GaN based light emitting devices and high power electronics with Si

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M.S. Shur and A. "tukauskas (eds.), UV Solid-State Light Emitters and Detectors, 199-206. © 2004 Kluwer Academic Publishers.

200 A. L. Gurskii et a!

based photodetectors and logical circuits. However, the main issue in using silicon as a substrate for nitride-based structures is a high value of lattice mismatch. Progress in the growth ofhigh-quality crack-free GaN on silicon can be achieved by introduction of appropriate nucleation/buffer layers [1-8], such as AlN grown at different temperatures [1-4], AlAs [5,6], amor­phous GaN [7] as well as SiC interlayers [8,9]. Promising results have been obtained by using of a sophisticated MOCVD growth procedure including combination of high- and low-temperature GaN and AlGaN layers as well as more than one nucleation/recrystallisation step [1]. This technology resulted in room-temperature (RT) laser action in GaN/Si epilayers [8]. Another way to improve the GaN/Si heterointerface is the modification of Si substrate. In this paper, we report on the results of investigation of the optical properties and lasing in GaN structures grown by MOCVD on Si substrates, including those containing a silicon monoxide interlayer (Si/SiO/Si).

1.1 EXPERIMENTAL

The samples were grown in an AIXTRON reactor on Si/SiO/Si substrates at low pressures (200 mbar or 50 mbar). Trimethylgallium (TMGa), trimethy­laluminum (TMAl), trimethylindium (TMin), ammonia (NH3) and silane (SiH4) were used as precursors; H2 and N2 were used as carrier gases.

Prior to the growth process, the substrates were cleaned by a two-step etching process. First, to remove organic contaminations and to oxidize the surface, the substrate was etched for one minute with a solution consisting of H20, H20 2 and H2S04 (1: 1 :3). In a second step, the created thin layer of ox­ide was removed by HF (2%). After each step, the substrate was rinsed with deionized water. The etching process was repeated and the cleaning proce­dure was accomplished by drying the substrate with N2• The procedure ap­plied resulted in a deoxidized hydrogen-terminated Si surface without use of high-temperature annealing.

To facilitate the growth of GaN on Si, a low-temperature AlN nucleation layer [1] was deposited on the substrate at 720 °C. The nucleation layer was followed by a 50-nm thick AlN layer grown at higher temperature. After this, TMGa was started to gradually intoduce into the reactor up to the value of the flow required for the growth of GaN. Simultaneously, the TMAl flow was gradually decreased to zero. As a result, a graded AlGaN layer was grown. On the top of the graded layer, a 1.25-f..lm thick GaN layer was de­posited. For comparison, some samples were grown also on Si substrate without SiO using the same epitaxy process.

Photoluminescence (PL) and stimulated emission were excited by radia­tion of a cw He-Cd laser (325 nm) at T = 17 K and by a pulsed nitrogen la­ser radiation (337.1 nm, Iexc = 500 kW/cm2, fp = 8 ns) at room temperature.

Stimulated Emission and Gain in GaN 201

The 3rd harmonic of a pulsed Y AG:Nd3+ laser was used for PL measure­ments in the temperature range of 17-300 K and for optical gain measure­ments. Low-temperature reflectivity spectra were also recorded in order to estimate the free exciton position. To achieve lasing, cavities were formed from the samples by cleaving. Nitrogen laser radiation was focused on the sample surface in a thin stripe with Iexc varied from 200 to 1000 kW/cm2•

The output radiation spectra were registered in a time-integrated mode using a polychromator and a CCD-camera. The output power of lasing was meas­ured by an Rm-3700 radiometer (LaserProbe Inc.) with the RjP-668 detector. Optical gain spectra were measured by the variable stripe length method at room temperature. The stripe length was up to 2 mm, while its width was about 40-60 ).lm.

2. RESULTS AND DISCUSSION

Energy(eV) 355 35 345 34 3.35

A+B (3.461 eV) T=18K

12 (3.454 eV)

PL (A+B)-LO -

. 1a !·

--- 2a ~_, 350 355 360 365 370

Wavelength ( rvn)

Figure 1. Low temperature PL (a) and reflectivity spectra (b) of GaN/Si (I) and GaN/Si/SiO/Si (2)

In Fig. 1, low temperature PL spectra (labeled a) and reflectivity spectra (la­beled b) of both GaN/Si (1) and GaN/Si/SiO/Si (2) are shown. Excitonic emission (assigned to h type bound exci­ton) was dominant in the PL spectra at T = 18 K. Near-band-edge bands related to the impurity recombinatiop were not observed. An important result is a negli­gible shift of both PL maxima and re­flectivity features in these spectra. (The shift of the excitonic features in reflec­tivity spectra is not greater than 1 meV.) The half-widths of main band in both PL spectra are also almost equal, although the PL spectrum of GaN/Si/SiO/Si is slightly narrower. These small spectral differences mean that the values of in­ternal strain in GaN epilayers are almost equal, and the presence of the SiO inter­layer near the substrate surface does not lead to the relaxation of the lattice mis-

match in GaN. The value of strain estimated by the comparison of reflectiv­ity data with the known literature data [8,9] was found to be about 7 kbar.

Figure 2a shows room-temperature near-band-edge (NBE) photolumines­cence (PL) spectra of GaN/Si (curve 1) and GaN/Si/SiO/Si (curves 2 and 3,

202 A. L. Gurskii eta/

which correspond to the samples grown under equal conditions but in differ­ent epitaxy runs). The samples grown on Si with an additional SiO layer are seen to have 1.5-3 times higher NBE PL intensity. The structure of the spec­tra in Fig. 2a may be caused by the contribution of impurity recombination channels formed, for example, by extended defects or complexes of point defects as discussed in [1 0]. Figure 2b presents the temperature dependence of the intensity ratio between the sample grown with and without SiO inter­layer. The sample with the SiO interlayer is seen to have a higher PL inten­sity in the whole temperature region under study, but the highest ratio (about 2.7) was observed around the liquid nitrogen temperature. At room tempera­ture, this ratio is lower (about 1.3). Figure 2c demonstrates the temperature dependencies of the PL peak position and half-width for both samples, showing no significant differences between them.

Wavelert,Jth (rvn)

'h:1r! ,_......;:J!IO~J80;;..;_..:;.370.;..._:360.;..;._...., T.::IOOK

'• ' • . , ;r. : 0

a) 0 3..,..1 ~-,...· c:' '='=----:'7--':"3.5~

2_1 l(with SIOI/I(withoot SIO)

..., 2'0 1~\ Integral intensity

~ ... ~~l\\/ = 2.2 •

~ 2.0 l\ -~~ § IIi E. 1.1

~ Pe..ak intensity 1.1

,.. b) ~::tt~ 1'2 -~-. __,..50__,.11)11---.150___,200___,250......-1300

Tomporoturo [KJ

Figure 2. (a) Room temperature PL spectra of GaN/Si (solid curve) and GaN/Si/SiO/Si (dashed curves); (b) Temperature dependence ofthe PL intensity ratio for two kinds of sam­ples; (c) The temperature dependences of the peak position and half-width for samples grown with and without SiO interlayer.

A modification of the NBE emission spectra of GaN/Si/SiO/Si with in­creasing excitation intensity from 175 to 1000 kW/cm2 is shown on Fig. 3. The observed changes are typical for an appearance of stimulated emission and lasing. The laser threshold was estimated to be of 700 kW/cm2 (see the inset in Fig. 3).

As in the case of other GaN/Si epilayers, the laser line position in the GaN/Si/SiO/Si (375.6 nm) lasers is shifted towards the low-energy side in comparison with that in the GaN/Ah03 lasers (373 nm), what is attributed to the effect of strain caused by different lattice mismatch.

Stimulated Emission and Gain in GaN 203

It is worth noting that the values of the threshold in both samples grown with and without SiO interlayer are almost equal, however, in the sample with the SiO interlayer, the differential efficiency of lasing (estimated from the slope of the output characteristics above threshold) is 3.5 times higher than in the sample containing no interlayer. As a result, the lasing intensity well above the threshold is also 3.5 times higher in GaN grown on Si/SiO/Si compared to GaN/Si (see the inset in Fig. 3). This is in line with the ratio of the integral PL intensity in two samples (see Fig. 2b).

The gain spectra (Fig. 4) show that the samples with the SiO interlayer have a higher net gain and feature no significant losses in the low-energy region, while a significant absorption (of about 300 cm-1) is present in the sample grown on Si without SiO. These losses are probably due to coupling of stimulated emission to the thick absorptive Si substrate. Meanwhile, in­troduction of the SiO interlayer seems to result in an improved waveguiding structure, which prevents from unwanted absorption in the Si substrate.

,; .e ;?:>

Wavelength (nm) 440 420 400 300 360

T<lOO K Ermsim ITtlm edge

GiWSIO'SI ~,!:' 1.5x1o'

1.0x1o'

-~ E l_(kW/cm )

1_ (kW/cm' ): 5.0x1a' 1000

I 175

.~~~~~~~0-0 2.8 2.9 10 11 12 13 14 15

Energy (eV)

Figure J Evolution of stimulated emission spec­tra in GaM/Si/SiO/Si with increasing N2-laser excita­tion. Inset: output power dependence on pump in­tensity.

I GaN without S iO j

~~

~ --a twl}ll;•:ura.v o600 \:.c• &..-31 WWicm

T•300K

a) _,,.-~-,-..-,-..-,----'r-~~~

~~ 3~ )~ lU l~ 3. lM

Photon Enorgy (oV)

I GaN with SIC f

t.otcc •J.60.V l._ •t.31 111Wicm1

T•lOO~

b)

Photon Energy (eVJ

Figure 4. Gain spectra of GaN/Si (a) and GaN/SiO/Si (b) measured under excitation by Y AG:Nd3+ laser radiation.

A striking equality in lasing thresholds with simultaneous differences in PL intensity, gain values, and differential efficiency of lasing in the two samples can be explained as follows. Such a behavior is possible provided that in the sample with a higher PL efficiency, the carrier mobility (and, cor-

204 A. L. Gurskii et al

respondingly, the diffusion length) is higher. This leads to an increased depth of the excited area, and, consequently, to a decrease of the effective carrier generation rate. Thus in the "better" sample, the threshold carrier concentra­tion is lower, although the threshold excitation intensity has almost the same value as in the "worse" sample. Higher values of diffusion length and carrier mobility evidence better structural quality of the GaN/Si/SiO/Si samples compared to the samples grown without SiO interlayer.

Actually, the lasing power P1, the differential quantum efficiency l]D, and the laser threshold 11hr are given by the following equations

(1)

_ S hv1 ___5_ . lJD- 'lt k ' e r +a

k = _!_ ln (_!_) · r L R '

(2)

(3)

respectively. Here L is the cavity length, e is the elementary charge, R is the mirror reflection coefficient, a is the internal losses coefficient, k, is the mir­ror losses coefficient, h v1 is the lasing quantum energy, L1 vis the band half­width, n is the refractive index, A, is the laser wavelength, d is the depth of excited area, 'li and ry1 are the radiative recombination efficiency and the lasing efficiency, respectively, Sis the excited area, Cis a constant. Assum­ing S1 ::::: S2, L1 ::::: L2, R1 ::::: R2, and a1 ::::: a2 for samples 1 and 2, respectively, Eqs. (2) yields

l]Dl - lJn ---. (4) 'lm 'ltz

The equality of the thresholds [Eq. (3)] is only possible if we assumed d2/d1 ~ ry1211]n that is a reasonable assumption in view of the considerations presented above.

Another possible explanation of the observed threshold behavior is the difference in the waveguide properties of the structures, since the presence of the SiO interlayer significantly changes the refractive properties of the GaN/Si interface. This fact also may increase the differential efficiency of

Stimulated Emission and Gain in GaN 205

lasing. However, it cannot explain satisfactory the differences in integral PL intensity. Therefore, the first explanation seems to be more adequate.

Figure 5 shows the temperature dependence of the spontaneous-PL and stimulated-emission (SE) peak positions. At low temperature, the separation between the PL and SE peaks is 45 meV, which is much higher than that expected for excitonic recombination mechanisms. Therefore, we attribute the lasing mechanism in the samples under study to electron-hole plasma recombination. With increasing temperature, the separation gradually in­creases up to 100 me V at room temperature. The additional red shift of the SE band in the high-temperature region can be explained by the band edge smearing (increased phonon-assisted absorption below the band gap), which leads to the corresponding modification of the optical gain spectra.

I GaN without SiO l h'·tJ • 325 nm (Heed)

.... ....

1·-I, •1S WIC:tn1 3.45 ....

3.45 ·-360 .... '"'' PL .... ..... ..... ·--- ... _ .....

365 E' 3.40 365 E' Otllt

3.40 ' ,..,.

' S' =. ..... > ' =.

"" Ot1•

~ ' ' "" .!!. c. ·-' c. El ·->o c 6f.IM c ., 01 ..

370 1 0 4.12 ... SE 370 l

ouu Q) ·-Q) ·- c: 3.35 ..... c: 3.35 5:

1 4121 w SE 3: "'" w .,71, ... ~ "'" ·-hvoe •3..50eY ·- ...,1:1111: . 3.5 .v ""' "'" .....

1,. •3.06111Niem' ..... •u::: • 3.01 MWian' '""' ..... 01111

375 ltUII 375 .,, ''"' 3.30 I1Cill

3.30 .,G.ll ·-· ·- ..... ...... . . 50 100 150 200 250

50 100 150 200 250

Temperature [K] Temperature [K]

Figure 5. Temperature dependencies of the spontaneous PL and SE maxima positions for GaN epilayers grown without and with SiO interlayer. The bars on the right-hand side of the pictures show the SE intensity scale.

3. CONCLUSIONS

The use of a set of AlN and AlGaN buffer layers allows growing of monocrystalline GaN layers of high quality on Si substrates. The value of tensile strain in GaN epilayer was estimated to be about 7 kBar. Deposition of a SiO interlayer on the substrate surface does not significantly decrease the strain value in the GaN layer. However, the samples with the SiO inter-

206 A. L. Gurskii eta/

layer have higher PL intensity in the whole temperature region, what evi­dences a lower concentration of nonradiative centers. In both GaN layers grown with and without SiO interlayer, lasing at room temperature with the almost same threshold of 700 kW/cm2 was achieved. However, the samples grown on Si/SiO/Si have a higher differential quantum efficiency, higher lasing power, higher gain and lower optical losses. The equality of thresh­olds can be explained by larger diffusion coefficient in the samples grown on Si/SiO/Si samples, what results in an increased volume of the excited region. Lasing is attributed to electron-hole plasma recombination in the whole temperature region of 17-300 K.

ACKNOWLEDGEMENTS

This work was partially supported by the ISTC grant B-176.

REFERENCES

1. A. Dadgar, J. Blaesing, A. Diez, A. Alam, M. Heuken, and A. Krost, Jpn. J. Appl. Phys. 39, Lll83-Lll85 (2000).

2. H. Lahn\che, P. Venm\gues, 0. Totterau, M. Laiigt, P. Lorezini, M .Leroux, B. Beaumont, P. Gibart, J. Cryst. Growth 217, 13-25 (2000).

3. S. Zamir, B. Meyler, E. Zolotoyabko, J. Salzman, J. Cryst. Growth 218, 181-190 (2000).

4. Z. M. Zhao, R. L. Jiang, P. Chen, D. J. Xi, Z. Y. Luo, R. Zhang, B. Shen, Z. Z. Chen, andY. D. Zheng, Appl. Phys. Lett. 77 44~46 (2000).

5. A. Strittmatter, D. Bimberg, A. Krost, J. Blasing, and P. Veit, J. Cryst. Growth 221, 293-296 (2000).

6. A. Strittmatter, A. Krost, M. StraBburg, V. Tiirck, D. Bimberg, J. Blasing, and J.Christen, Appl. Phys. Lett. 74, 1242-1244 (1999).

7. P. Chen, S. Y. Xie, Z. Z. Chen, Y. G. Zhou, B. Shen, R. Zhang, andY. D. Zheng, J. Cryst. Growth 213, 27-32 (2000).

8. H. M. Liaw, R. Venugopal, J. Wan, R. Doyle, P. L. Fejes, and M. R. Melloch, Solid State Electron. 44, 685--690 (2000).

9. J. H. Hang, C. I. Park, S. L. Chon, G. M. Yang, K. Y. Lim, and K. S. Nahm, in Proc. Int. Workshop on Nitride Semiconductors (IWN2000), IPAP Conf. Series I, pp. 363-366.

10. G. P. Yablonskii, E. V.Lutsenko, V. N. Pavlovskii, I. P. Marko, V. Z. Zubialevich, A. L. Gurskii, H. Kalisch, A. Szymakowskii, R. A. Jansen, A. Alam, Y. Dikme, B. Schineller, and M. Heuken, Phys. Stat. Sol. (a) 192, 54-59 (2002).

11. K. Kornitzer, T. Ebner, K. Thonke, R. Sauer, C. Kirchner, V. Schwegler, M. Kamp, M. Leszczynski, I. Grzegory, S. Porowski, Phys. Rev. B 60 1471 (1999).

12. B. Gil and 0. Briot, Phys. Rev. B 55 2530 (1997).

MATERIALS CHARACTERIZATION OF GROUP-III NITRIDES UNDER HIGH-POWER PHOTOEXCITATION

S. JURSENAS, G. KURILCIK, S. MIASOJEDOVAS, and A. ZUKAUSKAS Institute of Materials Science and Applied Research, Vilnius University, Sauletekio 9-///, LT-2040 Vilnius, Lithuania

Abstract: Results on group-III nitrides materials study by luminescence spectroscopy at high-density laser excitation are presented. Impact of carrier heating on re­combination dynamics of degenerated electron-hole plasma is discussed. GaN epilayer quality characterization method based on luminescence transient stud­ies under deep-trap saturation regime is demonstrated. Advances of application of high-density excitation for characterization of InGaN/GaN multiple quan­tum wells under screened built-in electric field are shown.

Key words: GaN, InGaN/GaN MQWs, photoluminescence, luminescence decay, electron­hole plasma, electron capture time, screening of built-in field

1. INTRODUCTION

Group-III nitride materials are widely used for fabrication of green to UV light-emitting diodes and lasers [1,2]. One of the most powerful tolls used for materials characterization is luminescence spectroscopy [3,4]. Here we report on new group-III nitride materials properties that can be revealed by using luminescence spectroscopy at high-density laser excitation.

In a highly excited semiconductor, new phenomena that are related to many-body interactions in a dense system of nonequilibrium quasiparticles occur: heating of the electron and phonon systems, screening of Coulomb interaction by free carriers, band-gap renormalization, degeneration of car­rier system and inverse population, enhancement of nonlinear recombina-

207

M.S. Shur and A. Zukauskas (eds.), UV Solid-State Light Emitters and Detectors, 207-214. © 2004 Kluwer Academic Publishers.

208 S. Jursenas et al

tion, etc. Some of these phenomena can be applied for materials characteri­zation.

In the following Section 2, we describe a nonthermalized high-density electron-hole plasma (EHP) in GaN epilayers. In Section 3, we introduce materials quality characterization method based on luminescence transient studies under a deep-trap saturation regime. In Section 4, we show advances of application of high-density excitation for characterization of InGaN/GaN multiple quantum wells (MQWs) under conditions of screening of built-in electric field by a high-density carrier system.

2. DENSE ELECTRON-HOLE PLASMA IN GaN

For highly excited semiconductors, defect states are usually saturated and excitonic states are screened by nonequilibrium carriers, thus EHP is the main electronic state.

Figure 1 displays some typical time-resolved luminescence spectra of GaN epilayers grown on sapphire (a) and GaN (b) substrates for a high den­sity of the excitation energy (/g = 1.1 mJ/cm2). The spectra obtained for the both samples are seen to contain a broad emission band peaked in the vicin­ity of the bandgap energy ( ~ 3.4 e V). The emission band becomes narrower and redshifts with time. The observed luminescence dynamics is typical for radiative recombination of high-density EHP [5-12]. Our experimental con­ditions, namely high carrier temperature (k8 Tc > Ex. Ex is the exciton binding energy) and intense photoexcitation (estimated carrier density of the order of 1019 cm-3, thus essentially higher than the Mott density [6,10]), predetermine formation of free electron-hole pair system, where excitonic states are ther­mally ionized and/or screened by carriers. Radiation emitted in the backward direction is mainly due to spontaneous emission of plasma, since the thick­ness of the excited region in GaN is very small (dg:::::: 0.1 !liD) and stimulated emission in the backward direction is inefficient [ 6]. However at high excita­tion, carrier system becomes degenerated and laterally stimulated emission usually is strong [8]. Some traces of scattered stimulated emission at 3.3 eV can be resolved at an early delay time [Fig. 1 (b)].

At high optical excitation, the system of nonequilibrium carriers can be brought out of thermal equilibrium with the lattice due to an excess energy supplied and/or due to many-body recombination processes. Room tem­perature experiments with resonant and off-resonant excitation of GaN have demonstrated that broadening of the high-energy tail of the EHP lumines­cence band is due to the excess photon energy supplied to nonequilibrium­carrier and longitudinal optical phonon system [7]. Analysis of the high­energy wing of the recorded EHP band within a simple one-particle ap-

Materials Characterization of Group Ill-Nitrides 209

proach [13] (simulated spectra are shown by lines in Fig. 1) indicates that the carrier temperature reaches a value of 750 K at zero delay and relaxes ex­actly to the equilibrium value within the first 100 ps. Carrier heating up to 1200 K has been observed in GaN epilayers [7,8].

104

(a)

,. .. -e 103 .,· ~ ..

1/o .. ..

• 26.7 ps • 66.7 ps .a. 173 ps

GaN/GaN

• 435K .. .. 298 K

3.2 a3 3A 3~ a6 3J 3B

Photon energy (eV)

:§' ·c ::l

-e ~ z. iii c

~ ~ c

~ Q) c E ::l _J

104 (b)

.. . . . 103

# . <#" : .. . . .

102 .... ... .. .... .. ., .. 700 K

101

• 26.7 ps 375K

• 86.7 ps .;. 206.7 ps

10° 3.2 3.4 3.6

Photon energy (eV)

3.8

Figure 1. Luminescence spectra in homo- (a) and heteroepitaxial (b) GaN layers for the excitation density of Ig = 1.1 mJ/cm2 recorded for backward geometry at different delay time. The spectra are arbitrary shifted along the vertical axis. Points, experiment; solid lines, calculation. The deduced carrier temperature is indicated at each spectrum.

Carrier temperature is one of the crucial parameters of a dense nonequi­librium quasiparticle system. It determines the degree of degeneration of the carrier system and controls the rates of stimulated and spontaneous emission as well as the radiative and nonradiative recombination rates [8,14]. Varia­tion in plasma temperature can also result in changes in the peak position of the emission band due to temperature-dependent band-gap renormalization [7] and band-filling effect.

Cooling of the initially hot EHP results in a nonlinear carrier recombina­tion dynamics [8]. In particular, the rate of lateral stimulated emission is highly sensitive to the carrier temperature. Usually at room temperature, a rapid carrier thermalization process takes place on the initial stage of relaxa­tion. Owing to a decrease in plasma temperature, the carrier system becomes degenerated, thus stimulated emission rises up and results in a rapid decrease of the plasma density. After the stimulated emission is exhausted, spontane­ous radiation is observed [8]. To characterize a material by time-resolved luminescence at high excitation conditions, carrier heating and laterally stimulated emission effects have to be considered [8].

210 S. Jursenas et al

3. MATERIALS CHARACTERIZATION BY ELECTRON-HOLE PLASMA LUMINESCENCE DECAY

Transients of EHP luminescence can provide important information on mate­rials quality. In the later stage of the relaxation (> 1 00 ps ), when carrier and nonequilibrium phonon thermalization is completed and the inverse popula­tion is exhausted, one usually observes an exponential decay, which is re­lated to capture of carriers by deep centers of nonradiative recombination [8,9]. The number of nonradiative traps is one of the crucial parameters that controls the efficiency of semiconductor light emitters.

Under intense photoexcitation when the nonequilibrium electron and hole concentrations (Lln and Ll.p, respectively) are large compared to the den­sity of deep traps, N~> and the equilibrium carrier density, n0, (Lln, !1p >> N~> n0), for typical asymmetry of the electron and hole capture cross sections (say, bh > be), the traps are saturated by holes. Thus a deep-trap saturation regime is established [9]. In this regime, carrier recombination is controlled by the electron capture time re = (beN1r 1 provided that the bimolecular re­combination rate, bn is negligible (brn << beN1). Since holes are captured only by empty traps, the hole lifetime, rh, is increased up to the electron life­time, re. This means that an EHP can be characterized by a common carrier density n z p and a common lifetime re z rh. Consequently, an exponential decay of the luminescence intensity with a time constant of rw ~ r/2 is ob­served. With further increase of the plasma density (brn >> beNJ, the lumi­nescence decay becomes nonexponential and plasma-density dependent [8].

Figure 2 shows luminescence transients obtained in various GaN epilay­ers at the peak position of the emission band (~3.4 eV). It is evident that lu­minescence decays almost exponentially and the characteristic decay time varies in samples prepared by different growth procedures. The largest lumi­nescence decay time rw = 445 ps is characteristic of a homoepitaxial GaN epilayer grown by metalorganic chemical vapor deposition (MOCVD, (solid points). A GaN epilayer grown under similar conditions over a sapphire sub­strate shows a significantly lower value of rw = 195 ps (open points). Low­quality GaN epilayers can have a luminescence decay time below 10 ps (squares). Our study shows that thick GaN epilayers (~1 )liD) grown by hy­dride vapor-phase epitaxy (HVPE) can have rather large luminescence decay time of rw = 205 ps.

Lines in Fig. 2 show results of calculation of the EHP spontaneous lumi­nescence decay transients obtained by solving a system of three coupled rate equations for nonequilibrium electrons, holes, and recombination centers, respectively, under standard assumptions [9]. To fit the experimental points, different electron capture times were used: re = 195 ps (dotted line), 440 ps

Materials Characterization of Group III-Nitrides 211

(dashed line), and 970 ps (dot-dashed line). (Note that the values of 'Z"e are slightly larger than 2 rw, because of bimolecular recombination taken into account.) For a reasonable value of the electron recombination coefficient be= lxl0-8 cm3/s [9], the density of deep traps can be estimated: N1 = 5.lxl017 cm-3 for a GaN epilayer grown on sapphire, N1 = 2.lxl017 cm-3

for HYPE-grown GaN and N1 = l.Oxl017 cm-3 for homoepitaxial GaN. In GaN grown on sapphire, these deep traps are most likely related to a larger number of threading dislocations occurring at the epilayer-substrate inter­face.

en 1 o 3 n-----r----.---.-l :!::::: c :::::l

.0 ..... ro :; 102 c Q) (.) c Q) (.)

~ 101 c E :::::l

__J

-no detects <1 GaN homoepitaxial l> GaN HVPE o GaN on sapphire

-o- low·litetime GaN on sapphire

bimolecular rec .

'LU = 95 ps

0 200 400 600 800 1 000

Delay time (ps)

Figure 2. Transient behavior of the normalized ltuninescence intensity at the peak position of emission band (- 3.4 eV) in various GaN crystals. Points, experiment; lines, calculation for various electron capture times: dotted line, Te = 195 ps; dashed line, Te = 440 ps; dot-dashed line, Te = 970 ps; solid line, intrinsic band-to-band recombination (no nonradiative capture).

Solid line in Fig. 2 shows a luminescence decay transient for intrinsic band-to-band recombination with the bimolecular coefficient of br = 3xl0-11 cm3/s (an average of the data reviewed in Ref. 15). It is evident that although homoepitaxy significantly reduces the number of threading disloca­tions, the quality of crystals can still be improved to reach a luminescence decay time of about 1.6 ns expected for purely bimolecular recombination at a carrier density of 1019 cm-3•

212 S. Jursenas et al

4. CHARACTERIZATION OF InGaN/GaN MULTIPLE QUANTUM WELLS UNDER SCREENED BUIT-IN ELECTRIC FIELD

Ternary InGaN-based multiple quantum wells (MQWs) are the key struc­tures for near-UV to green light emitting diodes and laser diodes [1,2]. Lu­minescence spectroscopy is one of the most widely used methods for charac­terization of the MQWs. However, the characterization is usually ambiguous because of a difficulty in distinguishing between blue shifts due to the quan­tum-confined Stark effect caused by built-in electric field and filling of localized-states in a partially disordered InGaN alloy [ 16, 17]. Here, we applied a high-excitation regime to screen the built-in electric field by free carriers and to reveal the impact ofln-segregation related disorder.

InGaN/GaN MQW structures of various well thicknesses have been characterized by backward and lateral luminescence at low and high excita­tion density [18,19]. The undoped MQWs consisted of five 10-nm thick GaN barriers and In0.15Ga0.85N wells with the thickness of 2 to 4 nm in dif­ferent samples. Figure 3 shows the well-width variation of the peak position of luminescence obtained at various excitation conditions.

3.1

>3.0 <ll -§ 2.9

:;:::; 'iii g_ 2.8

:.c. co 8:. 2.7

• stimulated lateral (high exc.) 0 spontaneous backward (high ex c.) (I spontaneous backward (low exc.)

~ • lnGaN/GaN MQWs

2 3 4 Well layer thickness (nm)

Figure 3. Well-width dependence ofthe emission peak position at different excitation condi­tions. Solid points, backward spontaneous emission at low intensity cw excitation; open squares, backward spontaneous emission at high excitation obtained after I 00 ps time delay introduced for exhaustion of stimulated emission; solid squares, lateral stimulated emission at the threshold of stimulation. Lines are guides for eye.

Solid points in Fig. 3 show the luminescence peak position obtained at room temperature for low-intensity cw excitation. With increasing well width from 2 nm to 4 nm, a red shift of 310 me V is observed. The shift re-

Materials Characterization of Group III-Nitrides 213

suits not only from the quantum size effect but also from the quantum­confined Stark effect and from intricate thermodynamics of In segregation that is due to lattice mismatch between InN and GaN [1,2,16]. To eliminate the built-in electric field, high-excitation conditions were applied (Ig = 1 mJ/cm-2). To avoid distortion of the spectra by stimulated emission that occurs at high excitation, backward spontaneous luminescence was measured at a 100-ps time delay, i.e. when stimulated emission is exhausted [19]. Open squares show the obtained variation of the peak position with well width. The red shift is seen to be reduced to 210 me V but it is still lar­ger than 125 meV, the value expected to be due to finite-barrier quantum confinement in our MQW structures. We attribute the excess red shift of 85 me V to a difference in magnitude of the potential fluctuations that are due to formation of spatially separated In-rich and In-poor regions in the quan­tum well layers. Such fluctuations might be larger in thicker layers, since a longer growth time facilitates In segregation in strained InGaN system [20].

This assumption is confirmed by measurements of the peak position of the stimulated-emission band (solid squares in Fig. 3). With increasing well width, the energy separation between the stimulated and spontaneous emis­sion bands is seen to increase from 25 meV to 220 meV. Since stimulated emission originates from carriers in the vicinity of the mobility edge [3,4] and spontaneous emission usually appears from the lowest occupied local­ized states, the energy separation between the peak energies of stimulated and spontaneous emission bands is close to the double magnitude of the po­tential fluctuation. The difference in potential fluctuation estimated from the well-width dependence of the separation ( ~ 1 00 me V) is in reasonable agreement with that deduced earlier from the excess red shift of the sponta­neous band (85 me V).

5. CONCLUSIONS

Group-III nitride materials properties have been studied by time-resolved luminescence spectroscopy under high-power photoexcitation conditions that are close to semiconductor-laser operation regime.

An approach for estimation of GaN crystal quality by carrier lifetime un­der a deep-trap saturation regime has been demonstrated. The method was applied forGaN epilayers "trown on sapphire, HYPE grown GaN, and high­quality homoepitaxial GaN layers.

Advantages of a high-excitation regime that enabled us to characterize In-segregation related disorder in InGaN/GaN MQWs under conditions of built-in electric field screening by a high-density carrier system were dem­onstrated.

214 S. Jursenas et al

ACKNOWLWDGEMENTS

The authors would like to thank to S. Porowskii, P.R. Hageman, and C. C. Yang for high quality GaN epilayers and InGaN/GaN MQWs. The research was partially supported by the Lithuanian State Science and Educa­tion Foundation, by the joint Lithuanian-Latvian-Taiwan grant, by and Euro­pean Commission supported SELITEC center Contract No.G5MA-CT -2002-04047. A. Z. acknowledges the Lithuanian Ministry of Education and Sci­ence for his Fellowship.

REFERENCES

I. S. Nakamura, G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and La­sers (Springer, Berlin, 1997).

2. Zukauskas, M. S. Shur, and R. Gaska, Introduction to Solid-State Lighting (Wiley, New York, 2002).

3. S. Chichibu, T. Sota, K. Wada, and S. Nakamura, J. Vac. Sci. Techno!. B 16, 2204 (1998).

4. Y.-H. Cho, T. J. Schmidt, S. Bidnyk, G. H. Gainer, J. J. Song, S. Keller, U.K. Mishra, and S. P. DenBaars, Phys. Rev. B 61, 7571 (2000).

5. S. Jursenas, G. Kurilcik, and A. Zukauskas, Phys. Rev. B 58, 12937 (1998). 6. F. Binet, J. Y. Duboz, J. Off, and F. Scholz, Phys. Rev. B 60,4715 (1999). 7. S. Jursenas, G. Kuri1cik, G. Tamulaitis, A. Zukauskas, R. Gaska, M.S. Shur,

M.A. Khan, and J. W. Yang, Appl. Phys. Lett. 76, 2388 (2000). 8. S. Jursenas, G. Kurilcik, N. KuriiCik, A. Zukauskas, P. Prystawko, M. Leszczynski,

T. Suski, P. Perlin, I. Grzegory, and S. Porowski, Appl. Phys. Lett. 78, 3776 (2001). 9. S. Jursenas, G. Kuri1cik, N. Kurilcik, A. Zukauskas, and P. R. Hageman, Appl.

Phys. Lett. 83, 66 (2003). 10. S. Hess, R. A. Taylor, J. F. Ryan, B. Beaumont, and P. Gibart, Appl. Phys. Lett. 73,

199 (1998). 11. J. F. Muller and H. Haug, J. Luminescence 37, 97 (1987). 12. E. A. Meneses, N. Jannuzzi, J. G. P. Ramos, R. Luzzi, and R. C. C. Leite, Phys.

Rev. B 11, 2213, (1975). 13. G. Lasher and F. Stem, Phys. Rev. 133, 553 (1964). 14. S. Jursenas, G. Kurilcik, and A. Zukauskas, Phys. Rev. B 54, 16706 (1996). 15. A. Dmitriev and A. Oruzheinikov, J. Appl. Phys. 86, 3241 (1999). 16. N. A. Shapiro, P. Perlin, C. Kisielowski, L. S. Mattos, J. W. Yang, and E. R. Weber,

MRS Internet J. Nitride Semicond. Res. 5, I (2000). 17. E. Kuokstis, J. W. Yang, G. Simin, M.A. Khan, R. Gaska, and M.S. Shur, Appl.

Phys. Lett. 80, 977 (2002). 18. S. Miasojedovas, S. Jursenas, G. Kurilcik, A. Zukauskas, S.-W. Feng, C. C. Yang,

H.-W. Chuang, C.-T. Kuo, and J.-S. Tsang, Phys. Status Solidi C 0, 483 (2002). 19. S. Jursenas, S. Miasojedovas, G. KuriiCik, A. Zukauskas, S.-W. Feng, Y.-C. Cheng,

C. C. Yang, C.-T. Kuo, and J.-S. Tsang, Phys. Satus Solidi C 0 (2003), in press. 20. D. Doppa1apudi, S. N. Basu, K. F. Ludvig, Jr., and T. D. Moustakas, J. Appl. Phys.

84, 1389 (1998).

SMALL INTERNAL ELECTRIC FIELDS IN QUATERNARY InAIGaN HETEROSTRUCTURES

S. ANCEAU 1'2, S. P. LEPKOWSKI 1·*, H. TEISSEYRE \ T. SUSKI\ P. PERLIN 1 P. LEFEBVRE 2, L. KONCZEWICZ 2, H. HIRAYAMA 3,

andY. AOY AGI 3

1 UN/PRESS, Polish Academy of Sciences, Sokolowska 29/37, Warszawa, Poland 2 GES, Universite Montpellier 1L F-34095 Montpellier Cedex 5. France 3 RIKEN (Institute of Physical and Chemical Research) 2-1Hirosawa, Wako-shi, Saitama 351-0198, Japan * Corresponding author; e-mail: slawek@unipress.waw.pl

Abstract: A study of the internal electric field contribution to the light emission mecha­nism in InAIGaN-based multiple quantum wells was performed. We used two sets of structures with different quantum well width and different AI content in the barriers (series A with 30% of AI in the barriers and series B with 60% of AI in the barriers). To determine the magnitude of the built-in electric field we employed several methods: i) theoretical estimation of piezoelectric and spon­taneous polarizations, ii) analysis of the emission energy as a function of quan­tum well width, iii) hydrostatic pressure experiments, and finally iv) measure­ments of photoluminescence decay. The performed calculations gave high magnitudes of the built-in electric field for both series of samples. On the con­trary, from the experimental results, we concluded that the built-in electric field is negligible in the samples of series A and is rather weak in those of se­ries B. Possible reasons for the controversies between theory and experiment are suggested.

Key words: quaternary InAlGaN compounds, quantum wells, piezoelectric polarization, built-in electric fields, time-resolved spectroscopy, high pressure experiments

1. INTRODUCTION

Recently, there is an increasing demand for light-emitting diodes and semi­conductor lasers operating at around 350 nm. Such emitters have wide appli­cations in many fields, in particular, environment control and medical in-

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M.S. Shur and A. tukauskas (eds.), UV Solid-State Light Emitters and Detectors, 215-222. © 2004 Kluwer Academic Publishers.

216 S. Anceau, S. P. Lepkowski eta!

struments as well as illumination. Highly efficient emitters in the UV range were recently developed using quaternary InAlGaN alloys or GaN/ AlGaN quantum wells (QWs) [1-3].

In the case oflnGaN/GaN or GaN/AlGaN QWs [grown along the (0001) direction], we always deal with internal strain in the system, which results in a piezoelectric polarization and thus in a built-in electric field in the QW layers. Moreover, spontaneous polarization, which exists in hexagonal ni­trides, has different values for AlN, InN and GaN [4]. This leads to an addi­tional increase in the built-in electric field in both InGaN/GaN and GaN/AlGaN QWs. The resulting large internal electric field invokes the Quantum Confined Stark Effect (QCSE), which results in the separation of the electron and hole wavefunctions, and reduces the optical transition ma­trix element. On the contrary, in InAlGaN-based QWs compressive or ten­sile strain can be engineered. Particularly, one may expect that, for properly chosen compositions of barriers and QWs, the internal electric field is negli­gible since a term coming from spontaneous polarization cancels the term originating from the piezoelectric effect. In such a case no reduction of the optical transition probability due to QCSE occurs.

In practice, it is difficult to realize any arbitrary composition of the qua­ternary alloys using epitaxial growth. There are some interrelations between In- and At-incorporation in these compounds (see, e.g. Ref. 3). Thus, the condition for the cancellation of built-in electric field (which theoretically is close to lattice matching between barriers and wells) is not easy to achieve.

The purpose of this work is to determine the magnitude of built-in elec­tric fields in quaternary InAlGaN QWs showing intense light emission. We realize this task by performing time-resolved photoluminescence experi­ments and high-pressure studies of the light emission on two series of sam­ples containing InAlGaN QWs of different width. The structures in the first set of samples (series A) have lower Al content in the barriers than the sam­ples belonging to the second series (B). With increasing Al content in the barriers, we increase both the lattice mismatch between the barriers and the wells (i.e. piezoelectric effect) and the difference in spontaneous polariza­tions between the barriers and the wells. Both effects should lead to a higher magnitude of the built-in electric field in the samples from series B com­pared to those from series A.

2. SAMPLES

The studied InAlGaN structures were grown by metalorganic vapor phase epitaxy (MOVPE) on the Si-face of an on-axis 6H -SiC(OOO 1) substrate. Each sample consists of 3 quantum wells separated by barriers of 5-nm width. In

Small Internal Electric Fields in Quaternary InA!GaN 217

series A, the composition of QWs and barriers is In0.05Alo.2oGao.75N and In0.02Al0.3Gao.68N, respectively. The QW width varies from 1.3 nm up to 4.0 nm in different samples. In series B, the composition of QWs is the same as in series A while the composition of barriers is different, i.e. Ino.o2Alo.6Gao.JsN. The QW width varies from 0.9 nm up to 4.7 nm in differ­ent samples of series B.

In each ofthe samples, the MQWs were grown on a structure consisting of: i) a 400-nm thick Al0.2Ga0.8N buffer layer, ii) a 30-nm thick In0.02Al03Ga0.68N layer (barrier composition), iii) a strain reduction layer, and iv) a 20-nm thick In0.02Al0.3Ga0.68N layer. Additionally, a reference sample containing a thick epitaxial layer of ln0.05Al0.20Ga0.75N (120 nm) was also grown on a 400-nm thick Al0.2Ga0.8N buffer layer deposited on on-axis 6H­SiC(OOOl). The alloys composition in the samples of series A was measured by Rutherford backscattering spectrometry, while for series B, Al and In contents were estimated from the technological procedure. More details of the growth procedure are given elsewhere [3].

3. DETERMINATION OF BUILT-IN ELECTRIC FIELD

3.1 Emission Energy versus QW Width

Figure 1 shows the dependence of the PL peak position on QW width L for series A (squares) and B (circles). The emission energy for the thick refer­ence layer corresponding to the QW material is also shown (triangle).

In hexagonal ternary nitride QWs, the dependence of the emission en­ergy, EE vs. L has been often used to determine the magnitude of the internal electric field. However, there exist three factors, which can contribute to this dependence. Namely, i) quantum confinement, which in narrow QWs causes a blue shift of EE in respect ofthe Sand gap, Ea, of the well material, ii) the built-in electric field, which leads to a red shift of EE in respect of Ea with the effect more pronounced in wider wells, and iii) in InGaN based struc­tures, In fluctuations, which cause a decrease of a "local" value of Ea and EE in respect to the average band gap of the QW material.

In the case of quaternary nitride structures, the above-described analysis is even more complicated. In contrary to ternary QWs, determination of the quantum confinement term is presently impossible. This is due to the lack of knowledge of the band offsets and the effective masses for quaternary well­and barrier materials. Moreover, though there are strong suggestions in favor of the importance of In-fluctuations in the material studied, their structural details require further investigations.

218

4.0

3.9

> 3.8 ~

5 3.7 :;::; "iii g_ 3.6 -"'

"' ~ 3.5

3.4

3.3

S. Anceau, S. P. Lepkowski eta/

• \ T=80K --series A -.-series B

·~ ... thick layer

• ·~.~

~.~ ·-· ~ ...

2 3 4 120

Quantum well width [nm]

Figure I. Photoluminescence peak positions taken at 80 K, as a function of quantum well width for samples of series A (squares) and B (circles) and for 120-nm thick layer of I11o.osAlo.2oGao. 75N (triangle).

Taking these arguments into account for the case of quaternary QWs, a reliable determination of the magnitude of the built-in electric field from the dependence of EE vs. L should be treated with caution. However, one may suggest that from the fact that EE is weakly dependent on L, what is observed particularly for the series A, the magnitude of the built-in electric field is small. Additionally for all samples from series A, EE is higher than the emis­sion energy for the thick layer (see Fig. 1), thus this suggests that the de­crease of EE with increasing L is mainly due to quantum confinement. For the samples from series B we observe a stronger decrease of EE with increas­ing L, and for the widest QW (4.7 nm) EE is smaller than that obtained for the thick reference sample. This may indicate a presence of the built-in elec­tric field in the latter case. However, in both cases the decrease of EE with increasing L is rather small comparing with the values obtained for ternary GaN/AlGaN and InGaN/GaN QWs [5,6]. It may suggest that in the case of the studied quaternary based QWs (series A and B) we deal with the pres­ence of rather small internal electric fields.

A contrary observation comes from theoretical estimation of the magni­tude of internal electric field in our samples. Surprisingly, application of the recently published results of ab initio calculations for spontaneous and pie­zoelectric polarizations in ternary nitride compounds [ 4] to our quaternary QWs (assuming Vegard-like behavior, i.e. no additional bowing for quater­nary alloys) brings the magnitude of the internal electric field between 1.3 and 1.7 MV/cm for series A and between 3.2 and 3.7 MV/cm for series B, respectively.

Small Internal Electric Fields in Quaternary InAlGaN 219

In order to clarify the situation, we analyze in considered structures: i) the pressure behavior of the PL peak position and ii) the PL decay times. Both effects are very sensitive to the presence of the internal electric field in the QW systems. The PL decay time, T, and the pressure coefficient of the peak position, dEt!dP are known to show a drastic variation (an increase ofT and a decrease of dEEfdP) with QW width in a QW system with strong built­in electric field. This refers to wurtzite InGaN/GaN and GaN/AlGaN QWs [5,6]. On the contrary, T and dEt!dP remain almost independent of QW width when the internal electric field in QWs is negligible [7].

3.2 Time-Resolved Photoluminescence Measurements

Time-resolved photoluminescence was measured at T = 8 K by using fre­quency tripled laser pulses from a Ti-sapphire cavity (hv= 4.77 eV) with a typical pulse width of 2 ps and a repetition rate of 82 MHz. The typical power density for the luminescence excitation was of the order of 100 W/cm2, subject to some changes for different samples.

• Series A ' ~ 10 r • Series B C/)

.s Q)

E i= T = 8K ' >- • ro (.) Q)

0 _J •

' 0.. i ~ r

.1. • • • • 1 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Quantum well width [nm]

Figure 2. PL decay time as a function of QW width for samples of series A and B.

For the studied samples, the PL decays show nonexponential character. For this reason, the PL decay time, T, is determined as a time after which the maximum of PL intensity drops by factor of 10. Figure 2 depicts the meas­ured values ofT for the samples from series A and B. For series A, the val­ues of T are seen to locate between 1 and 2 ns, and show no sensitivity to QW width. This observation supports the finding of weak internal fields in samples of series A. The estimated magnitude of the built-in electric field is below 0.1 MV/cm [7]. For the samples of series B , T is seen to increase by

220 S. Anceau, S. P. Lepkowski et al

one order of magnitude with increasing L from 0.9 nm to 4.7 nm. In this se­ries, the estimated magnitude of the internal electric field is of about 0.6 MV/cm [7].

3.3 High Pressure Experiments

High pressure experiments were performed for the samples from ,series A. The measurements were carried out at T = 80 K in a low-temperature dia­mond anvil cell filled with liquid argon, which served as a pressure transmit­ting medium. PL was excited by a He-Cd laser (hv= 3.8 eV) with the power of 2 m W. The emission from the sample was collected in a backscattering geometry, dispersed by a SPEX500M spectrometer and detected by a GaAs photomultiplier.

In Fig. 3, we compare the measured dEEidP for the samples of series A (solid circles) with the pressure coefficients of wurtzite (solid triangles) and cubic (solid squares) InGaN/GaN QWs taken from Refs. 5 and 8, respec­tively. For InAlGaN-based QWs, dEEidP varies from 34 meV/GPa to 36 meV/GPa. Taking into account that the experimental error of dEEidP deter­mination is ±2 meV/GPa, the obtained values of dEEidP can be treated as almost independent of quantum well width. A weak dependence of dEEidP on QW width (or a small nonlinear dependence associated with the quantum confinement effect) was observed in the case of cubic InGaN/GaN QWs where internal electric field is absent, for symmetry reasons. A very small variation of dEEidP in quaternary InAlGaN-based QWs (wurtzite structure) is in contrast to dramatic changes of dEEidP in the case of hexagonal InGaN/GaN QWs. In the latter case, the internal electric field can reach 2.4 MV/cm (at ambient pressure) for the In content of 20% [5]. The observed strong, linear decrease of dEEidP with QW width in these structures was ex­plained by the pressure-induced increase of the magnitude of piezoelectric field [9].

The described results suggest that in the studied quaternary QWs of se­ries A, the built-in electric field is very small.

Small Internal Electric Fields in Quaternary InAlGaN

Cii' 35 a.. (9 30

~ 25 E ~ 20 c: -~ 15 IE Q) 10 0

·----•-----. .,;:A--------

',,, . •

0 5 • cubic lnGaN/GaN QWs '4 ~ • wurtzite lnGaN/GaN QWs ~ 0 • wurtzite lnAIGaN (series A) -., ~ -5+-~-.--~-.~~--~--.-~--~ a.. 0 1 2 3 4 5

Quantum well width [nm]

221

Figure 3. Pressure coefficients of the peak position as a function ofQW width for the samples of series A (circles) compared to results obtained for hexagonal (triangles) and cubic (squares) InGaN/GaN QWs (Refs. 5 and 8, respectively).

Our preliminary studies of dEFidP in the series-B samples show that there is some tendency to a reduction of the pressure coefficient with quantum well width. The sample with L = 3.3 nm shows dEE/dP=32±2 meV/GPa whereas the sample with L = 4.7 nm is characterized by dEE/dP = 25±2 meV/GPa. This result supports the conclusion drown from the time resolved measurements that in the samples of series B, a small built-in electric field is present.

4. CONCLUSIONS

Measurements of the PL decay time and evolution of PL with pressure have been used to determine the magnitude ofbuilt-in electric field in two sets of InAlGaN-based QWs showing intense light emission. For the first set of samples having lower Al content in the barriers (series A), both the pressure coefficient of the PL peak energies, dEEidP, and the PL decay time, r, do not change with the thickness of the QW. This observation can be explained by assuming a very weak (<0.1 MV/cm) built-in electric field in these struc­tures. For the samples of series B, with the widest QWs, we observed a small increase of rand a small decrease of dEEidP, as compared to the ternary In­GaN/GaN QWs. We estimated the magnitude of the internal electric field in these structures to be of about 0.6 MV/cm. The small values of the built-in fields observed in both series are in contradiction to the theoretical predic-

222 S. Anceau, S. P. Lepkowski et a/

tions. There are few possible reasons for this disagreement. For example, the internal electric field can be screened by an unintentional doping. Another problem is the poor knowledge of piezoelectric and spontaneous polariza­tions for quaternary nitride alloys, what may lead to an overestimation of the calculated values of the built-in electric field.

REFERENCES

1. E. Kuokstis, J. Zhang, M.-Y. Ryu, J.W. Yang, G. Simin, M. AsifKhan, R. Gaska, and M.S. Shur, Appl. Phys. Lett. 79, 4375 (2001 ).

2. M. E. Aumer, S. F. LeBoeuf, B. F. Moody, and S. M. Bedair, Appl. Phys. Lett. 79, 3803 (2001).

3. H. Hirayama, Y. Enomoto, A. Kinoshita, A. Hirata, and Y. Aoyagi, Appl. Phys. Lett. 80, 1589 (2002),

4. V. Fiorentini, F. Bernardini, 0. Ambacher, Appl. Phys. Lett. 80, 1204 (2002). 5. P. Perlin, I. Gorczyca, T. Suski, P. Wisniewski, S. Lepkowski, N.E. Christensen, A.

Svane, M. Hansen, S.P. DenBaars, B. Dami1ano, N. Grandjean, and J. Massie, Phys. Rev. B 64, 115319 (2001); S.P. Lepkowski, H. Teisseyre, T. Suski, P. Perlin, N. Grandjean, and J. Massie, Appl. Phys. Lett. 79, 1483 (2001).

6. P. Lefebvre, A. Morel, M. Gallart, T. Taliercio, J. Allegre, B. Gil, H. Mathieu, B. Damilano, N. Grandjean, and J. Massies, Appl. Phys. Lett. 78, 1252, (2001).

7. H. Teisseyre, T. Suski, S.P. Lepkowski, S. Anceau, P. Perlin, P. Lefebvre, L. Konc­zewicz, H. Hirayama, and Y.Aoyagi, Appl. Phys. Lett. 82, 1541, (2003).

8. T. Suski, H. Teisseyre, S.P. Lepkowski, P. Perlin, T. Kitamura, Y. Ishida, H. Oku­mura, and S.F. Chichibu, Appl. Phys. Lett. 81, 232, (2002).

9. G. Vaschenko, D. Patel, C.S. Menoni, S. Keller, U.K. Mishra, and S.P. DenBaars, Appl. Phys. Lett. 78, 640, (2001).

MOCVD GROWTH OF AlGaN EPILAYERS AND AlGaN/GaN SLS IN A WIDE COMPOSITION RANGE

W. V. LUNDIN, A. V. SAKHAROV, A. F. TSATSUL'NIKOV, E. E. ZAVARIN, A. I. BESULKIN, A. V. FOMIN, and D. S. SIZOV A.F.Ioffe Physico-Technical Institute of the Russian Academy of Science, 194021 St.Petersburg, Russia E-mail: Iundin. vpegroup@mail. ioffe.ru

Abstract: Peculiarities of A!GaN epilayer and AIGaN/GaN superlattice (SL) growth were investigated using R&D-scale (Epiquip VP-50 RP, I x 2 inch) and pro­duction-scale (AIX2000HT, 6 x 2 inch) MOCVD reactors. Structures with AIGaN in a whole composition range were grown. For both reactors, it was re­vealed that dependence of AIGaN composition on TMA flow has a strong trend for saturation (more pronounced effect is for the larger reactor). The saturation seems to be a manifestation of parasitic reactions and a critical pa­rameter responsible for it is the TMA partial pressure in the reactor. In addi­tion to a reduction of reactor pressure, the AIN mole fraction in AIGaN layers may be increased by magnification of the total flow in the reactor and a reduc­tion of the TMG flow. Up to 20% AIN mole fraction was reached in AIX2000HT at 400 mbar reactor pressure (up to 40% at 200 mbar) using this practice. Growth of AIGaN with the AIN mole fraction above 60-70% (smaller reactor) was achieved using a reduced ammonia flow to suppress parasitic reactions.

Key words: MOCVD, AIGaN, superlattice

1. INTRODUCTION

AlGaN and AlGaN/GaN heterostructures are under intensive investigation due to their possible utilization in various electronic and optoelectronic de­vices operating in the UV and blue-green range [ 1].

223

M.S. Shur and A. tukauskas ( eds. ), UV Solid-State Light Emitters and Detectors, 223-231. © 2004 Kluwer Academic Publishers.

224 W V. Lundin et a/

Due to strong build-in electric fields, significant problems with doping, and complex nature of strain relaxation processes in these structures, more investigations are required to realize all potential of this materials system. At the same time, the first problem on this way is a development of regimes of epitaxial growth of AlGaN with high AI contents.

While growth of low-Al-content layers is a routine procedure, growth of epilayers and multylayer structures with a high AI content is a more complex procedure, especially in large industrial-scale reactors.

2. EXPERIMENT

In this work, peculiarities of AlGaN epilayer and AlGaN/GaN superlattice (SL) growth were investigated using two types ofMOCVD reactors.

The first one is a horizontal flow, reduced-pressure MOCDV system Epiquip VP-50 RP redesigned for III-N growth with a quartz reactor, a joint group-III and -V inlet, and an inductively heated graphite susceptor for one 2-inch substrate. The second one is an AIX2000HT system with a planetary reactor for 6 x 2 inch substrates and a separate group-III and-V inlet.

Epitaxial growth was performed on (0001) Ab03 substrates using NH3,

trimethylgallium (TMG), and trimethylaluminum (TMA) as precursors and hydrogen as carrier gas. Standard two-step growth procedure with low­temperature GaN or AlGaN nucleation layer was utilized.

Composition of AlGaN epilayers was measured using photoluminescence (<15% of AIN mole fraction), Raman spectroscopy [2] and X-ray diffracto­metry.

2.1 AIGaN Growth in Epiquip VP-50 PR Reactor

In the Epiquip VP-50 RP system, AlGaN growth was carried out at 200 mbar reactor pressure. Due to the absence of substrate rotation, it was difficult to obtain a reasonable epilayer thickness uniformity at a higher reac­tor pressure. Some experiments were carried out at lower reactor pressure -no change in AI content was observed. The growth temperature was 1050-1070 °C, within the optimal temperature range for GaN growth in this sys­tem.

Figure 1 shows the dependence of AlN mole fraction in the layers on TMA/(TMA+TMG) mole flow ratio. If the growth rate of AlGaN is kept the same as for GaN (open symbols), aluminum incorporation into the layers saturates at relatively low levels [3]. This effect is especially strong for thin AlGaN layers grown on thick GaN buffers. It was found that AI incorpora­tion might be increased by reduction of the growth rate (closed symbols).

MOCVD Growth of A!GaN Epilayers 225

Thus for AlGaN, a low growth rate is required, while forGaN, a too low growth rate results in the degradation of surface morphology. To realize the growth sequence of SL in which GaN layers are grown much faster than AlGaN layers, two TMG sources (TMG-1 and TMG-2) and one TMA source were used. While TMG-2 was supplied to reactor continuously, TMG-1 and TMA were supplied in the anti-phase scheme [4]. Up to 50% AlN mole fraction in the barriers was reached in AlGaN/GaN SLs using this technique.

s::::: 0.5

0 ;

0.4 (J C'(S '-'+-

..! 0.3 0 E

0.2 z <

0.1

0.1 0.2 0.3 0.4

TMAI(TMA+ TMG) mole flow ratio

Figure 1. Data for Epiquip VP 50 RP growth system: AlGaN composition vs. TMA/(TMA+TMG) mole flow ratio. Open symbols, high AlGaN growth rate; closed sym­bols, reduced AlGaN growth rate. Triangles, thin AIGaN on GaN buffers; circles, thick Al­GaN layers and SLs. Arrows show the change in epilayer composition as a result of the TMG flow reduction keeping all other flows constant.

Some preliminary experiments have shown that AlN epitaxial growth is possible with the same NH3 and main H2 flows as for GaN growth. A reduc­tion NH3 and an increase of the main H2 and TMA flows results in AlN quality improvement. It means that for AlN/GaN heterostructures growth, a fast change of the NH3 and main H2 flows is necessary, what requires a spe­cific design of gas blending and manifold system to avoid pressure unstabil­ity in the reactor. Growth of high-Al-content AlGaN is possible at very low TMG flows. The NH3 and main H2 flows should be precisely determined in accordance with Ga content. AlGaN epilayers in all composition range were fabricated in Epiquip VP-50 RP system using the described above methods.

226 W V. Lundin et al

2.2 AIGaN Growth in AIX2000HT Reactor

In the case of the AIX2000HT system, AIGaN growth was carried out at the reactor pressure range of 10~00 mbar. The growth temperature was 1050-1180 °C. It would be mentioned that due to different methods of reactor temperature measurement in Epiquip VP-50 RP and AIX2000HT systems, these data could not be compared directly. Generally, the nominal reactor temperature in AIX2000HT should be higher than in Epiquip VP-50 RP, to reach the same substrate temperature (the optimal temperature for GaN growth in Epiquip VP-50 RP is 1050-1070 oc while in AIX2000HT it is 1150-1180 oc and the properties of epilayers grown at this conditions with the same growth rates are practically identical).

Figure 2 shows the dependence of AIN mole fraction in AIGaN epilayers on reactor pressure for AIX2000HT at 1050 °C and 16-slm total flow in the reactor. The growth rate was 500-600 nmlh. A reduction of the AI content in the epilayers at higher reactor pressures is very pronounced.

While single AIGaN layers may be grown at low reactor pressure, in some cases growth of GaN layers requires higher pressures. In the large­volume AIX2000HT reactor, pressure cannot be changed in a few seconds. Thus, regimes of higher-pressure AlGaN growth have to significantly extend the degree of freedom for AIGaN/GaN heterostructures epitaxy.

The AI content in the epilayers grown at 400 mbar may be increased by a change of the total flow through the reactor (the H2 flow was increased) as it can be seen in Fig. 3. On the one hand, the observed 3-times increase of the AI content is a pronounced effect. However, the reached value of 5% of the AIN mole fraction is too low for the most of applications. What is even more pronounced, an increase in TMA flow at 1 00 mbar reactor pressure results in a linear increase of the AIN mole fraction in the epilayers, while at 400 mbar, AI incorporation saturates at very low TMA flows (see Fig. 4).

The effect described above may be considered as the following experi­mental fact: concentration of Al-containing active gas-phase components participating in epigrowth (TMA or intermediate reaction products) above the substrate can not be increased above some certain value by magnification of the TMA flow. In this situation, the obvious way to increase the AI con­tent is a decrease of the TMG flow (as in the case of smaller reactor). The effect of TMG flow on Al content in AIGaN epilayers can be seen in Fig. 5. The AIN mole fraction is seen to be proportional to 1/(TMG flow) and AlxGa1_xN epilayers with x up to 20% (which is enough for most of applica­tions) can be grown at 400 mbar reactor pressure. At 200 mbar reactor pres­sure, the AIN mole fraction reached by the same technique was as high as 40%.

MOCVD Growth of A/GaN Epilayers

s:::: 0 ·--CJ ca ~

'; 0.10 0 E z < 0.05

Total flow 16 slm

100 200 300 400 Reactor pressure, m bar

227

Figure 2. Data for AIX2000HT growth system: AIGaN composition vs. reactor pressure (all other growth conditions are identical).

s:::: 0.05 0 Pressure 400 mbar .. (.) ca ~ 'I-

Q) 0.03 0 E z <

0.01

15 20 25 30 Total gas flow through the reactor, slm

Figure 3. Data for AIX2000HT growth system: AlGaN composition vs. gas flow through the reactor (all other growth conditions are identical).

228 W V Lundin et a/

0.30 c: 0

; 0.25 Pressure 100 mbar u ca Total flow 16 slm .... .... 0.20 C1)

0 E 0.15 z -< 0.10 Pressure 400 mbar

Total flow 27 slm 0.05

10 20 30 H2 flow through TMA bubbler, seem

Figure 4. Data for AIX2000HT growth system: AIGaN composition vs. TMA flow.

0.25 c: 0 ·- Pressure 400 mbar, .... u 0.20 ca Total flow 27 slm .... .... C1)

0.15 0 E z 0.10 <

0.05

0.05 0.10 0.15 1/(H2 flow through TMG bubbler), seem·1

Figure 5. Data for AIX2000HT growth system: AIGaN composition vs. TMG flow.

MOCVD Growth of AlGaN Epilayers

3. CONCLUSIONS: COMMON FEATURES OF AlGaN EPITAXY IN BOTH REACTORS

229

While AlGaN growth regimes for Epiquip VP-50 RP and AIX2000HT are different, there are some common effects observed for both investigated MOCVD systems.

First of all, there is a saturation of Al content in the epilayers with in­crease of TMA flow above some certain value, indicating strong parasitic reactions. Summarizing all data for both systems, the following conclusions may be done. An increase of the Al content in the epilayers may be reached by a reduction of reactor dimensions (compare two growth systems), a re­duction of reactor pressure, magnification of the total flow, a reduction of TMG flow, and changing the reactor temperature (effect of temperature may be different depending on other parameters). But there exists a general pa­rameter, which is responsible for the rate of Al-related parasitic reactions: the TMA partial pressure in the reactor.

In Fig. 6, one can see the dependence of X/Xv on TMA partial pressure in the reactor (X/Xv is the parameter, which corresponds to the effect of para­sitic reactions on the epigrowth results and is calculated as a ratio of the AlN mole fraction in the epilayer to TMA/(TMA+TMG) ratio in the reactor inlet).

From data in Fig. 6, it may be concluded that generally, Al incorporation is more effective in the smaller system, but for the both systems X/Xv drops when TMA partial pressure exceeds the level of 1-2x 1 0--{j bar. At the same time, X/Xv is not directly correlated withAl content in the epilayers (see data labels in Fig. 6). A strong dependence of the rate of these parasitic reactions on TMA partial pressure indicates the high order of these reactions: a mole­cule of the product contains more than one atom of aluminum.

Very generally, it is possible to distinguish two modes of AlGaN epi­growth: the growth under high TMA partial pressure when Al content is strongly affected by TMA gas-phase chemistry, and the growth under low TMA partial pressure when Al content is mostly influenced by Ga-related effects (re-evaporation of Ga from the surface and parasitic reactions reduc­ing TMG concentration in the gas phase). Typical features of these two modes are given in Table I.

230 W V. Lundin et al

2.5 0 0.20

2.0 0 0.40

> 1.5 <> 0.11 :» X - 0 0.29

Ill

X 1.0 0 0.067

~ 0.19 0.137 0.097 0.072

0.5 0.048

+ 0.029 + 0.018 + 0.05

1*10-6 2*10-6 3*10-6 4*10-6

TMA partial pressure, bar

Figure 6. Data for Epiquip VP-50 RP (upper group of symbols) and AIX2000HT growth systems: Aluminum incorporation efficiency (the ratio of solid-phase composition to vapor­phase composition) vs. TMA partial pressure in the reactor. Diamonds, 100 mbar; squares, 200 mbar; crosses, 400 mbar reactor pressure, respectively. Data labels show the AIN mole fraction in the epilayers. Points marked with star and arrows correspond to structures, grown at the same growth conditions except with the reactor temperature of 1050 °C for the structure with 29% AIN and of 1180 °C for the structure with 40% AIN, respectively

Table I. Typical features of AIGaN epigrowth depending on TMA partial pressure.

Case of high TMA partial Case of low TMA partial pressure pressure

Reduction of reactor pres- AI content is increased No influence sure Reduction of reactor tern- AI content is increased AI content is decreased perature AI content distribution Increase towards the reactor Decrease towards the reactor across the substrate inlet inlet

MOCVD Growth of AlGaN Epilayers 231

ACKNOWLEDGEMENTS

This work was supported by the NATO Science for Peace program (SfP-972614) and Russian Foundation for Basic Research.

REFERENCES

1. Presentations at NATO Advanced Research Workshop on UV Solid-State Light Emitters and Detectors (June 17-21, 2003, Vilnius, Lithuania).

2. V. Yu. Davydov, A. A. Klochikhin, I. N. Goncharuk, A. N. Smimov, A. S. Usikov, W. V. Lundin, E. E. Zavarin, A. V. Sakharov, M. V. Baidakova, J. Stemmer, H. Klausing, and D. Mistele, Proc. Int. Workshop On Nitride Semiconductors, IPAP Conf. Series 1, 665 (2000).

3. W. V. Lundin, A. V. Sakharov, A. F. Tsatsu1nikov, E. E. Zavarin, A. I. Besu1kin, M. F. Kokorev, R. N. Kyutt, V. Yu. Davydov, V. V. Tretyakov, D. V. Pakhnin, A. S. Usikov, Phys. Stat. Sol. (a) 188, 885-888 (2001).

4. W. V. Lundin, A. S. Usikov, I. L. Krestnikov, A. V. Sakharov, A. F. Tsatsul'nikov, M. V. Baidakova, D. V. Poloskin, V. V. Tret'iakov, and N. N. Ledentsov in Booklet of the gh EW-MOVPE (June 8-11, 1999, Prague, Czech Republic), pp. 49-52.

GALLIUM NITRIDE SCHOTTKY BARRIERS AND MSM UV DETECTORS

B. BORA TYNSKI and M. TLACZALA Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology, Janiszewskiego 11117, 50-372 Wroclaw, Poland E-mail: bborat@wemif.pwr.wroc.pl, tel. 48-71-320 3507

Abstract: GaN Schottky diodes and MSM UV photodetectors were fabricated on GaN layers grown by MOVPE technique on sapphire substrates. Process technology of temperature-stable Schottky barrier contacts to GaN and AlGaN epitaxial layers was elaborated. Electrical parameters of the Schottky barriers were evaluated in a range of temperatures up to 260 °C. The Schottky barrier height, calculated from I-V measurements was 0.72 eV; the ideality factor value was 1.7. The MSM photodetectors of two Pt/Au contact pattern geometry: 2 !!ml 3!-tm and l!!m/2!-lm were used. The response to UV light was measured using a mercury lamp, a chopper and lock-in technique. The output ac signal dependence on photodetector bias up to 25 V was evaluated. The responsivity of 0.3 A/W was obtained. The detectors were insensitive to wavelengths above 400 nm. The MSM photodetector response to 305 nm-laser beam was meas­ured. In a parallel study, test structures of AlGaN/GaN HFETs were fabricated and their electrical parameters with the transconductance of 45 mS/mm have been evaluated.

Key words: GaN, AlGaN, Schottky barriers, UV detectors, MSM, HFET

1. INTRODUCTION

Group III-N of semiconductor materials, that includes GaN and Ga(ln,Al)N alloys, finds wide application in the areas of high power, high frequency de­vices such as Schottky diodes, HEMTs, HBTs, and also in optoelectronic devices such as lasers, LED emitters and photodetectors. Recently, MMIC circuits based on GaN material were developed. Gallium nitride, as a wide­bandgap material is very robust to harsh environments; it is radiation hard

233

M.S. Shur and A. Zukauskas (eds.), UV Solid-State Light Emitters and Detectors, 233-238. © 2004 Kluwer Academic Publishers.

234 B. Boratynski and M Tlaczala

and temperature resistant. The large energy gap of GaN and its alloys make them good materials for construction of photodetectors in the 200--400 nm wavelength region, i.e. in the UV light spectrum. Of particular interest there are so called solar blind detectors with a high UV /solar rejection ratio. A principal application interest of such detectors lies in the defense area such as missile launch detection, UV-wavelength short-range communication, biological agents detection systems. Others include industrial application for engine test or flame control, where a high temperature environment is en­countered. Several types of photodetectors: PIN, avalanche photodiode and MSM have been reported. In this work, we study Schottky diodes and metal-semiconductor-metal (MSM) photodetectors fabricated on GaN epi­taxial layers grown on sapphire substrate. The Schottky contact is also a ba­sic element of amplifying devices such as MESFETs or HEMTs, so it is im­portant to have reliable Schottky contact technology for other devices designed to operate at elevated temperatures. The fabricated MSM structures were of planar configuration, and are suitable for future monolithic integra­tion within GaN/sapphire MMIC technology. In the first part of the paper results on Schottky contact parameters and the Schottky diode 1-V character­istics measured at elevated temperatures are discussed. The second part con­cerns with characterization of MSM photodetectors and measurements of their response to UV light excitation. Finally, some results of parameter evaluation of the fabricated AlGaN/GaN HFETs are presented.

2. DEVICE STRUCTURES FABRICATION

To investigate Schottky contact parameters, the Schottky diodes were fabri­cated on 2-J..lm thick undoped GaN epitaxial layers grown by Metal Organic Vapour Phase Epitaxy (MOVPE) on sapphire substrates. The free-electron concentration in the GaN layer was measured to be 5xl016 cm-3• The diode structure was of circular geometry with the inner Schottky contact of 160 J..lm in diameter and the surrounding ohmic contact ring of 80 J..lm width and with a 30-J..lm spacing between them. First, the ohmic contacts: Til Al/Ni/ Au (20/200/40/150) nm were formed by a single evaporation process and lift­off, followed by alloyed process at 850 oc for 30 s in N2• Next, a Pt/Au (20/150) nm Schottky metalization was deposited using e-beam evaporation and lift-off. The wafer was annealed in N2 at 300 oc for 20 min.

The MSM detector structure consisted of two planar Pt/ Au interdigitated Schottky contacts. Two configurations of a finger width to finger spacing: 2 J..lm/3 J..lm and 1 J..lm/2 J..lm were used in the design. The active area of the photodetector was 50 x 80 J..lm2• In addition to the diode and MSM device structures, a number of test structures including circular TLM test patterns

Gallium Nitride Schottky Barriers and MSM Detectors 235

and large-area Schottky contacts for C-V measurements were fabricated on the same substrates.

The HFET structures of circular geometry were fabricated using Alo.2Gao.?N/GaN undoped structures. The Al0.2Ga0.7N undoped layer, 30-nm thick, was grown on top of the GaN/sapphire wafer. In the design, a circular contact electrode topology has been used to avoid mesa etch process. In this case, the gate electrode is completely surrounded by the source electrode; thus the parasitic shunt drain current is eliminated. The gate length was 4 f.!m and the drain-gate and source-gate separation was 2.5 f.!m. The drain and source ohmic contacts were made using Ti/ Al/Ni/ Au metalization deposited in a single UHV process. After the lift-off, the wafer was annealed in nitro­gen ambient at 800 oc for 90s. To obtain good contact surface morphology a second metalization of Ti/ Au (301150) nm was applied. The gate Schottky contact has been made of Pt/ Au metalization in the same processing se­quence as for the Schottky diodes.

3. DEVICE CHARACTERIZATION

The parameters of the fabricated device structures - Schottky diodes, MSM detectors, and HFETs - have been evaluated for the purpose of future mono­lithic integration in a photoreceiver circuit.

3.1 Schottky Diode Parameters

The Schottky diode current-voltage characteristics have been measured at room temperature (RT) and at the temperatures up to 250 °C. At RT, the Schottky barrier height of0.72 eV was calculated for the Pt-Au metalization that is a typical value for Pt barrier to GaN. The junction ideality factor val­ues were in the range of 1.7-2.0. The 1-V measurements revealed large se­ries resistance of the diode resulting from the planar, circular topology, ex­tension of the ohmic contact and moderately high resistivity of the GaN layer. As shown in Fig.1 the 1-V characteristics measured at elevated tem­peratures revealed increasing value of the diode series resistance (from 107 ohm at 21 °C to 21 0 ohm at 264 °C). The Rs values for the appropriate tem­peratures are shown in the figure inset. The temperature coefficient of the series resistance had almost constant value and was calculated to be 0.38 ohm/K. This value corresponds to the temperature coefficient of resistance, TCR value of(0.2-0.4)% K-1 within the used temperature range.

In Fig.2, the 1-V curves are plotted after subtracting a voltage drop across the series resistance. As it is seen, the slope of the log(I)--V curve does not change with temperature. This suggests no major change in the junction ide-

236 B. Boratynski and M Tlaczala

ality factor and thus, no change in the dominant current flow mechanism at elevated temperatures.

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0.4

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500 1000 1500

voltage (mV)

Figure 1. Forward 1-V characteristics as measured at various temperatures. The Rs values shown in the inset.

., 5 ~

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--+- 254 c - +--204C ~-155C

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200 400

vdtage (mV)

600

Figure 2. Forward 1-V characteristics for different temperatures after sub­traction of the voltage drop due toRs.

The temperature coefficient of forward voltage above 100 oc is negative and varies with increasing temperature. Its value however, does not depend on the forward current value. The plot of the forward voltage drop versus temperature is shown in Fig. 3. The I-V characteristics for the reverse bias taken at different temperatures are shown in Fig. 4. There is a considerably large value of the reverse current, however, the structures were unpassivated.

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Figure 3. Forward voltage drop versus temperature.

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reverse voltage [V)

Figure 4. Reverse 1- V characteristics measured at different temperatures.

Gallium Nitride Schottky Barriers and MSM Detectors 237

3.2 MSM Photodetector Parameters

Two types of MSM detector geometry were fabricated and measured, namely with 2-J.!m and 3-J.!m spacing between the contact fingers. The ac response to UV light was measured using a filtered mercury lamp, a low fre­quency chopper and lock-in technique. The photocurrent signal versus detec­tor bias is shown in Fig. 5 for the two detector geometries. It is seen that the signal saturates at 12 V bias for 2-J.!m geometry and at 20 V for 3-J.!m ge­ometry, respectively, at approximately the same level. The onset of signal saturation is related to the depletion region width of the reverse biased Schottky contact. For the 2-J.!m finger spacing the high electric field region fills the whole gap between the contact fingers at about 12 V bias.

25

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~ 15 of ' ~ 10 / I ·· • · msm-<3> ~

If - -• ··msm<2>

~ 0 5 ." -a i , 0 ...... ~ 0

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bias volage M

Figure 5. ac photocurrent signal re­sponse of the MSM to UV light ver­sus de bias.

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Figure 6. MSM response to 305 nm la­ser light. Optical power 70 J.!W; load IOkOhm.

The dynamic response was measured using a 305 nm wavelength of Ar laser. The UV laser beam was focused at the spot of 30 J.!m diameter that was smaller then the active area of the MSM. The incident optical power was estimated to be 70 J.!W. The signal was measured using a lock-in amplifier and recorded using a digital scope. The detector responsivity was found to be 0.3 A/W. The response obtained for the photodetector biased at 8 Vis shown in Fig. 6. The rise-time and fall-time of the pulse depended on the optical power level and improved with increasing optical power. It is important to note, that the detectors were insensitive to the light wavelengths longer than 400 nm, i.e. practically to all visible and IR light. The exact value of the op­tical absorption edge has yet to be determined from the spectral response measurements. The photoluminescence measurements has been done for the same GaN epitaxial structure and resulted in a peak PL spectrum at 368-nm wavelength.

238 B. Boratynski and M Tlaczala

3.3 HFET Parameters

The fabricated HFET topology is shown in Fig. 7. The measured output 1-V characteristics of the GaN HFET device are presented in Fig. 8.

. -\~)

Figure 7. Top view of the fabri­cated round A!GaN/GaN HFET

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The saturated drain current was 55 rnA for the gate width of 400 J.lm. The gate pinch-off voltage was -3.6 V. The small signal transconductance of the HFET was 18 mS, that is 45 mA/mm. The preliminary measurements of high-frequency parameters revealed fT about 700 MHz, which is a reasonable value for the gate length of 4 J.lm used in this design.

4. CONCLUSION

Pt/Au Schottky contacts to GaN, reliable at RT and elevated operating tem­peratures, have been developed. Using this technology, MSM UV photodetectors on GaN/sapphire wafers were fabricated and tested. The UV response increased with the increasing detector bias and saturated at bias (12-20) V depending on the contact geometry. The response to 305 nm laser wavelength has been measured with the responsivity of 0.3 A/W. Test struc­tures of AlGaN/GaN HFETs were also fabricated and their electrical pa­rameters with the transconductance of 45 mS/mm have been evaluated.

ACKNOWLEDGEMENTS

Some of the AlGaN/GaN structures were kindly supplied by W. Strupinski from the Inst. of Electronic Materials Technology in Warsaw. This work was partially supported by the KBN Grants: 8Tl1B00618, 8Tl1B05718 and FNP Project TECHNE 6/2001.

III-NITRIDE BASED ULTRAVIOLET SURFACE ACOUSTIC WAVE SENSORS

D. CIPLYS 1'3, A. SEREIKA 1, R. RIMEIKA 1, R. GASKA 2, M. SHUR 3,

J. YANG 4, and M. ASIF KHAN 4

1 Department of Radiophysics, Vilnius University, Sauletekio 9, Vilnius 2040, Lithuania E-mail: daumantas.ciplvs@tfvu.lt; tel. (3705) 233 6034 2 Sensor Electronic Technology, Inc., Columbia, SC, USA 3 Rensselaer Polytechnic Institute, Dept. of Electrical, Computer, and Systems Engineering, Troy, NY, USA 4 University of South Carolina, Dept. of Electrical Engineering, Columbia, SC, USA

Abstract: Large piezoelectric constants of AlN, GaN, and their alloys make these mate­rials attractive for applications involving surface acoustic waves (SAW). The electrical conductivity and, thus, SAW velocity in these materials is affected by UV radiation. This allowed us to develop a SAW-based UV GaN sensor by placing a SAW element into an oscillator feedback loop. The output of such a sensor is a radio signal with UV radiation-dependent frequency, which makes this sensor especially attractive for remote sensing applications. In addition, the spectrum line width depends on the noise of the UV source and is espe­cially narrow for solar radiation allowing for a solar-blind detection. We pre­sent the basic principles of the III-nitride based SAW UV sensors, summarize the results of theoretical simulations and experimental investigations of their properties, and discuss possible applications.

Key words: aluminum-gallium nitride, surface acoustic wave, ultraviolet sensor

1. INTRODUCTION

Due to a wide energy band gap, AlN, GaN, and their alloys are well suited for the fabrication of ultraviolet (UV) sensors, particularly, of visible-blind and solar-blind photodetectors. These materials possess strong piezoelectric

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M.S. Shur and A. Zukauskas (eds.), UV Solid-State Light Emitters and Detectors, 239-246. © 2004 Kluwer Academic Publishers.

240 D. Ciplys et a!

properties making them attractive for surface acoustic wave (SAW) device applications.

For sensing purposes, it is very convenient to use the SAW delay-line os­cillator, which has been first demonstrated in 1969 [ 1] as temperature­sensitive device. Since then, various SAW sensors have been developed but not those for UV. Making use of the unique combination of wide energy gap and piezoelectric properties, we were first to implement the GaN-based sur­face acoustic wave oscillator and to apply it for UV sensing [2--4]. We pre­sent here the basic principles of the III-nitride based SAW UV sensors, summarize the results of theoretical simulations and experimental investiga­tions of their properties, and discuss possible applications.

2. EXPERIMENTAL

2.1 Basic Principles

The oscillator consists of a broadband RF amplifier with the SAW delay line connected to the feedback loop. The schematic diagram of the device is shown in Fig. 1.

&TRA VIOLET RADIATION

SAW DELAY LINE

Output IDT GaN layer Input IDT

AMPLIFIER

RF SPECTRUM ANALYZER

Remote signal

pickup is possible

Figure 1. Schematics of the SAW-based UV sensor.

When the amplifier gain is set to overcome the losses in the feedback loop, the device oscillates at frequency fo determined by the phase condition

2tr fo L/V +rp =2mtr , (1)

Ultraviolet Surface Acoustic Wave Sensors 241

where L is the acoustic path length, Vis the SAW velocity, m is an integer and rp is the additional phase shift due to the connections and the amplifier. Any change in the SAW velocity, path length, or phase shift values leads to the correspondent shift of the oscillator frequency.

In our experiments, we used GaN and AlGaN layers grown by low­pressure metal-organic chemical vapor deposition (MOCVD) on (0001) sap­phire substrates. The SAW delay lines were formed by depositing pairs of the AI interdigital transducers on the layer surface. The layer thickness was in the range of 1-2 Jlm, and the transducer period was 16-24 Jlm, corre­sponding to the oscillator operating frequencies 200-300 MHz. An example ofthe fabricated SAW oscillator is shown in Fig. 2.

Figure 2 . View of the SAW-based UV sensor.

2.2 UV-Induced Oscillator Frequency Shift

The frequency of the output signal of the SAW oscillator was measured us­ing a frequency meter or a spectrum analyzer. It should be noted that the sig­nal could be fed to the measuring device not only by the cable connection, but also by the wireless transmission. This possibility reveals a very impor­tant advantage of the SAW oscillator to be used for remote sensor operation.

The influence of illumination by ultraviolet light on the oscillator fre­quency was investigated. In the first experiments [2], a mercury lamp was used as an UV source that illuminated the entire surface of the sample, in­cluding transducer area, for few seconds. The UV -induced frequency in­crease as large as 0.1 MHz, i.e. about 0.05% of the fundamental frequency, has been observed (Fig. 3). An increase in the ambient temperature due to the heat received from the mercury lamp and investigator's body led to the slow decay of the frequency pedestal. When the transducers were shielded

242

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if 220.92 c Q) :J

~ lL 220.88

on off on off

220.84 -'--T~-.--~-.-~-.--~---i 0 200 400 600 800

Time,s

D. Ciplys et a/

Figure 3. Abrupt changes in the SAW oscillator frequency due to switching on and off the GaN surface illumination by UV radiation from mercury lamp (after Ref. 2).

from the UV light so that only the area between the transducers was illumi­nated, the UV-induced oscillator frequency change decreased by an order of magnitude. The observed high sensitivity of transducer area to the ultraviolet radiation and the subsequent oscillator frequency change can be attributed to the variation of the phase term in Eq. 1 due to the variation in transducer im­pedance under illumination.

Next, we performed a more detailed investigation of the oscillator re­sponse to UV illumination of the SAW propagation path between transduc­ers, what led to the change of the first term in Eq. 1 [3]. The GaN surface was illuminated by Xenon lamp through the filter with the transmittance maximum around 360 nm. The RF spectral output of the oscillator in the dark is shown in Fig. 4 [see curve (a)]. When the SAW propagation path on the GaN surface was illuminated, the downshift of the oscillation frequency was observed [curve (b) in Fig. 4]. It should be noted that we took precau­tions to avoid the temperature-related effects, since the oscillator frequency should also decrease with growing temperature due to the temperature de­pendencies both of the acoustic velocity and the propagation path length. We attribute the change in the oscillator frequency to the change in the SAW velocity, which can be explained in terms of the acoustoelectronic interac­tion: the photoconductivity electrons screen the piezoelectric fields of the acoustic wave, reducing the acoustic velocity by the amount of K212, where K2 is the electromechanical coupling coefficient. The typical value of the latter in our samples is 0.1% , hence the relevant change in oscillator fre­quency of 0.05% can be attained.

Ultraviolet Surface Acoustic Wave Sensors

-20

E Ol :!:!. -40

~ g_ -60

"' c

"' iii -80

-100

b c a

221.28 221 .30 221.32 221 .34 221 .36

Frequency (MHz)

243

Figure 4. RF spectra of SAW oscillator output in dark (a) and under illumination from Xenon lamp: with filter, UV around 360 nm (b); and without filter (c), visible and near-infrared light present (after Ref. 3 ).

The SAW oscillator frequency downshift was measured as a function of optical wavelength in the range from 330 nm to 600 nm using the mono­chromator of 1-nm resolution. The result is plotted in Fig. 5. The maximum response is observed at the optical wavelength of 365 nm. This corresponds to the photon energy equal to the band gap of GaN 3.4 eV. At longer wave

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LL.

320

• • • • • ......

340 360

• • • • ••

380

•• 400

Optical wavelength (nm)

420

Figure 5. Dependence of the GaN-based SAW oscillator frequency downshift on the optical wavelength (after Ref. 3).

lengths, we observed a drastic decrease in the oscillator frequency shift. No frequency shift (with accuracy 11100) was observed at optical wavelengths above 400 nm. A slight decrease of the SAW frequency shift in the short-

244 D. Ciplys et al

wavelength part of the spectrum can be attributed to the increased light ab­sorption in the layer. The results obtained by the SAW oscillator method are in a good agreement with the spectral characteristics of photoconductivity in GaN measured using other techniques [5].

It should be noted that the frequency downshift was considerably reduced when the illumination by the Xenon lamp was performed without UV filter [curve (c) in Fig.4]. We attribute this effect to the quenching of the photo­conductivity by the visible and near- infrared light [6]. In order to achieve the visible-blind UV sensing, it is important to eliminate the observed optical quenching effect, which affects the response of the SAW oscillator to UV radiation in the presence of longer-wavelength light.

2.3 Oscillator Line Widths under Illumination by Different UV Sources

We have observed considerable differences in the oscillator output signal spectra under illumination by various sources. The linewidth for the Sun light remained almost the same as for the dark signal, whereas it was much broader for the illumination using a Xenon lamp (Fig. 6). We attribute the

· 10 ·5 . 0 5 10

Frequency deviation (kHz)

Figure 6. Spectral lines of the SAW oscillator under different conditions of acoustic path illumination.

differences in linewidth to the different noise spectra of the artificial and natural UV sources. Evidently, the Sun radiation is much more stable than

Ultraviolet Surface Acoustic Wave Sensors 245

that of the Xenon lamp. Based on this result, we speculate that the measure­ments of the oscillator noise might identify different artificial UV sources. The observed differences in linewidths may serve as a basis for the devel­opment of solar-blind UV sensor.

3. SUMMARY

In conclusion, a gallium nitride based surface acoustic wave delay-line oscil­lator and its response to ultraviolet radiation have been demonstrated. Both the increase and decrease in oscillator frequency with increasing UV illumi­nation has been observed. We attribute the frequency increase to the UV­induced change in SAW transducer impedance, and the frequency decrease is explained by the SAW velocity reduction due to acoustoelectronic interac­tion with photoconductivity electrons. Our results show that III-nitride based structures offer the possibility of integrating the acoustic and electronic ele­ments on a single chip capable to serve as an UV sensor. The output signal in the RF format enables a remote wireless operation of the device. The spectral characteristics of the SAW oscillator response showed a large visi­ble/UV rejection ratio, which make these devices promising for the devel­opment of visible-blind remote sensor. The sensitivity rejection at 300 nm for solar-blind operation can be achieved by using the AlxGa1_xN layer with the proper choice of AI molar amount, x, and, consequently, the band gap. The considerable difference between SAW oscillator line widths under illu­mination by the Sun and the Xe lamp is observed, which may be applied for the development of a source-selective solar-blind remote UV sensor.

REFERENCES

I. J. D. Maines, E. G. S. Paige, A. F. Saunders, and A. S. Young, "Simple technique for the accurate determination of delay-time variations in acoustic-surface-wave structures," Electron. Lett. 26, pp. 678-680 (1969).

2. D. Ciplys, R. Rimeika, A. Sereika, R. Gaska, M. S. Shur, J. W. Yang, and M. A. Khan, "GaN-based SAW delay-line oscillator," Electron. Lett. 37, pp. 545-546 (2001).

3. D. Ciplys, R. Rimeika, M. S. Shur, S. Rumyantsev, R. Gaska, A. Sereika, J. Yang, and M. Asif Khan, "Visible-blind photoresponse of GaN-based surface acoustic wave oscillator," Appl. Phys. Lett. 80, pp. 2020-2022 (2002).

4. D. Ciplys, R. Rimeika, M. S. Shur, R. Gaska, A. Sereika, J. Yang, and M. Asif Khan, "Radio-frequency response ofGaN-based SAW oscillator to UV illumination by the Sun and man-made source," Electron. Lett. 38, pp. 134-1356 (2002).

5. D. Walker, X. Zhang, P. Kung, A. Saxler, S. Javadpour, J. Xu, and M. Razeghi, "AlGaN ultraviolet photoconductors grown on sapphire," Appl. Phys. Lett. 68, pp. 2100--2101 (1996).

246 D. Ciplys et a!

6. Z. C. Huang, D. B. Mott, P. K. Shu, R. Zhang, J. C. Chen, and D. K. Wickenden, "Optical quenching of photoconductivity in GaN photoconductors," J Appl. Phys. 82, pp. 2707-2709 (1997).

OPTICALLY PUMPED InGaN/GaN/AlGaN MQW LASER STRUCTURES

V. YU.IVANOV \ M. GODLEWSKI 1'2, H. TEISSEYRE 3, P. PERLIN 3,

R. CZERNECKI 3, P. PRYSTAWKO 3, M. LESZCZYNSKI 3,

I. GRZEGORY 3, T. SUSKI 3 , and S. POROWSKI 3

1 Institute of Physics, Polish Acad. of Sciences, AI. Lotnik6w 32/46, 02-668 Warsaw, Poland 2 College of Science, Dept. of Mathematics and Natural Sciences, CardinalS. Wyszynski University, Warsaw, Poland 3 High Pressure Research Center, Unipress, ul. Sokolowska 29/37, 01-142 Warsaw, Poland

Abstract: We report ultra low threshold pump powers for optically excited stimulated emission from homoepitaxial (grown on bulk GaN substrates) In­GaN/GaN/AlGaN laser diodes. These threshold powers are as low as 2.4 to 5.8 kW/cm2, depending on a cavity length, i.e., are the lowest ever-reported. Ca­thodoluminescence .studies indicate possibility of further reduction of the threshold power in these structures.

Key words: GaN, laser diode, threshold powers, cathodoluminescence

1. INTRODUCTION

Even though InGaN-based laser diodes (LDs) were commercialized already 4 years ago [1,2] and there are wide spread possible applications for these LDs emitting in a blue-violet spectral region, recent progress in this field is surprisingly slow. After 4 years of development, InGaN-based LDs are still very expensive, have low output powers and relatively short lifetimes. Defi­nitely this is the consequence of a high dislocation density in structures grown on lattice-mismatched sapphire. Dislocations act as efficient centers ofnonradiative recombination in nitrides [3). In light-emitting diodes, excep­tionally large magnitude of potential fluctuations present in quantum wells (QWs) limits the role of dislocations as centers of nonradiative recombina-

247

M.S. Shur and A. Zukauskas (eds.), UV Solid-State Light Emitters and Detectors, 247-252. © 2004 Kluwer Academic Publishers.

248 V Yu. Ivanov et al

tion [3]. Diffusion lengths of carriers/excitons are too low for most of them to approach dislocations and decay there nonradiatively.

Figure 1. Room temperature cathodoluminescence image taken at 30 kV and at 6000 magni­fication for a current density above the threshold value.

It was assumed that at high excitation densities, required to obtain stimu­lated emission, potential fluctuations present in QW planes could be at least partly screened by free carriers. CL investigation indicates however that stimulated emission still in-plane fluctuates and is very sensitive to micro­structure details, as is shown in Fig. 1 for the LD structure with 500-f.lm laser cavity. Dislocations become even more effective centers of nonradiative re­combination, since the diffusion length of carriers/excitons increases due to a partial screening of potential fluctuations. This explains difficulties in achieving laser emission, despite of a massive production of light emitting diodes (LEDs).

The density of dislocations can be reduced by two-three orders in magni­tude when using epitaxial lateral overgrowth (ELO) technique (see Ref. [4] for the explanation). Use of the ELO method was essential for achieving stimulated emission [1,2] . However, use of this process still results in struc­tures with relatively large dislocation densities of about 106-107 cm-2• More­over, ELO considerably increases costs of the LD devices and limits the

Optically Pumped MQ W Laser Structures 249

width of a laser cavity, due to a limited size of regions with an improved morphology. The latter considerably limits light emission power.

In this work, we demonstrate advantages of a competing approach and report on a superior performance of homoepitaxial LD structures grown on bulk GaN substrates. Use of bulk GaNas a substrate material allows tore­duce dislocation density to about 102 cm-2 or even less, i.e., by about 104 to 105 times, as compared to heteroepitaxial LD structures. For such low densi­ties of dislocations, the active parts of lasers (laser cavities) are statistically dislocations free. This enabled us to achieve laser action in homoepitaxial LDs under carrier injection conditions [5]. We demonstrate that such LD structures show superior properties, which result in laser action at record-low threshold powers under optical pumping [6].

2. SUBSTRATES AND LD STRUCTURES

GaN crystals, in the form of hexagonal platelets of typically 100 mm2 size, were synthesized by high-pressure, high-temperature method (p ;::: 20 GPa, T;::: 1500°C) [7]. Growth of LD structures was performed in a home-built MOCVD reactor using TMGa, TMAl, TMin, CP2Mg, SiH4 and ammonia as precursors. The first step of the growth was annealing of GaN substrate in ammonia, what was followed by growth of GaN epilayer at 1050 oc on the Ga-face of the GaN crystal. The growth procedures of InGaN and AlGaN layers were similar to growth conditions used for analogous heteroepitaxial processes.

The LD structure was a Separate Confinement Heterostructure Device (SCHD) structure. The only difference was that for optical pumping, the pre­sent structures were undoped and that their GaN cap layer was made thinner, to reduce unwanted light absorption. The active region of the structures con­sisted of five In0.09Gao.91NIIn0.01 Gao.99N QWs. This active part of the LDs was embedded between two 0.1-f..lm thick GaN wave-guiding layers and two cladding layers. The lower cladding layer consisted of GaN/Al0.15Gao.ssN superlattice (2.5 nm/2.5 nm) with 120 repetitions. The upper cladding layer was in the form of 0.36-f.!m thick Al0.08Gao.92N covered with a thin GaN cap layer. We studied a series of LD structures cleaved with a different cavity length of L = 300, 500, 800 and 1000 J..lm.

3. RESULTS AND DISCUSSION

In Fig. 2 we show the results of optical pumping experiments performed at room temperature for the LD structure with 300-f..lm long cavity. At low ex-

250 V. Yu. Ivanov eta!

citation powers a relatively broad spontaneous emission is observed. This emission gradually is replaced by a laser emission at higher excitation densi­ties. Sharp laser modes (with 0.25-nm width) were polarized (about 100%, see Fig. 3) perpendicular to the active layer plane, i.e., the TE cavity modes were excited. Laser emission appears at the high-energy wing of the sponta­neous emission, indicating some contribution of localization effects.

,...-...... L=3001-lM en ? • =355 nm ....., exc

c RT

:::J

..0 ~

ro ..._... c 2 kW/cm 2

0 en en ·- 2 E 1.6 kW/cm

w +-' :::J a. +-' :::J 0

3.00 3.05 3.10 3.15 3.20

Photon Energy ( eV)

Figure 2. Room t emperature spontaneous and stimulated emission from LD structure with 300 f1m laser cavity.

Optically Pumped MQW Laser Structures 251

0 30 60 90 120 150 180 210

Angle (deg.)

Figure 3. Room temperature linear polarization of stimulated emission from a LD structure with 300-j.!m laser cavity.

The threshold powers for optical pwnping at room temperature are 5.8 kW/cm2 for the LD with 300-Jlm cavity, 5.6 kW/cm2 for the LD with 500-Jlm cavity, 2.5 kW/cm2 for the LD with 800-Jlm cavity, and 2.4 kW/cm2 for the LD with 1 000-f..Lm cavity, respectively. These, to our best knowledge, are the record low threshold powers, which are by a factor of 5-6 lower than the lowest threshold powers reported in literature for LDs grown on lattice mis­matched substrates (see references given in [6]).

Typical threshold powers for most of heteroepitaxial LD structures are in the order of MW/cm2 [8], i.e., one to two orders of magnitude larger than those observed by us. We expect that such differences in threshold power are the consequences of three factors: (a) elimination of dislocation and related nonradiative recombination centers, (b) improvement in the cleaved mirror quality, and (c) very low internal wave-guide losses.

We have recently reported the observation of stimulated emission in QW structures of GaN!InGaN under electron beam pwnping [9]. This observa­tion, obtained for laser structures but without p-type doping, allowed us to evaluate in-plane fluctuations of stimulated emission (as shown in Fig. 1) and relate them to microstructure details of the samples studied. From ca­thodoluminescence measurements, we could also evaluate the threshold den­sity for stimulated emission. For such evaluation, we took as an excitation radius, the radius of a cloud of primary and secondary electrons. This radius in GaN and in related materials is fairly small, of about 50 nm, since diffu-

252 V. Yu. Ivanov eta/

sion of carriers and excitons is limited by strong localization effects [3]. The estimated threshold density is then in the range of I 00 A/cm2• The equivalent threshold value, when estimated from experiments under optical pumping, is of about 800 A/cm2• Both these values are considerably lower than that achieved under carrier injection, which is of about 5.6 kW/cm2• Apparently, problems with n- and p-type doping, materials homogeneity, and technology of contacts lead to much larger threshold densities in laser structures operat­ing under current injection conditions.

Concluding, homoepitaxial LD structures show superior properties under optical pumping, with respect to heteroepitaxial structures even with an ELO growth step.

ACKNOWLEDGMENTS

This work was partly supported by the grant number 5 P03B 007 20 of KBN for the years 2001-2003 and European Project DENIS (G5RD-CT-2001-00566).

REFERENCES

1. S. Nakamura, M. Senoh, S. Nagahama, T. Matsishita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, and T. Mukai, Jpn. J. Appl. Phys., Part 2, 38, L226 (1999).

2. S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Matsushita, and T. Mukai, Appl. Phys. Lett. 76, 22 (2000).

3. M. Godlewski and E. M. Goldys, in III-Nitride Semiconductors: Optical Properties, Vol. II ed. by H. Jiang and M. 0. Manasreh, Optoelectronic Properties of Semicon­ductors and Superlattices (Taylor & Francis, New York, 2002), pp. 259.

4. Z. R. Zytkiewicz, Thin Solid Films 412, 64 (2002). 5. I. Grzegory, M. Bockowski, S. Krukowski, B. Lucznik, M. Wroblewski, J. L. Wey­

her, M. Leszczynski, P. Prystawko, R. Czemecki, J. Lehnert, G. Nowak, P. Perlin, H. Teisseyre, W. Purgal, W. Krupczyilski, T. Suski, L. H. Dmowski, E. Litwin­Staszewska, C. Skierbiszewski, S. Lepkowski, and S. Porowski, Acta Phys. Polon. (a) 100, Supplement, 229 (2001).

6. V.Yu. Ivanov, M. Godlewski, H. Teisseyre, P. Perlin, R. Czemecki, P. Prystawko, M. Leszczynski, I. Grzegory, T. Suski, and S. Porowski, Appl. Phys. Letters 81, 3735 (2002).

7. I. Grzegory, S. Krukowski, M. Leszczynski, P. Perlin, T. Suski, and S. Porowski, Acta Phys. Polon. (a) 100, Supplement, 57 (2001).

8. D. A. Stocker, E. F. Schubert, and J. M. Redwing, Appl. Phys. Lett. 77, 4253 (2000).

9. M. Godlewski, V.Yu. Ivanov, E. M. Goldys, M. Phillips, T. Bottcher, S. Figge, D. Hommel, R. Czemecki, P. Prystawko, M. Leszczynski, P. Perlin, I. Grzegory, and S. Porowski, Acta Physica Polonica A (in press).

HIGH POWER LED AND THERMAL MANAGEMENT

A.MAHLKOW Optotransmitter Umweltschutz Technologie (OUT) e. V., Berlin, Germany

Abstract: A high-power SMD-LED (HL-LED) outline (3.2 x 2.8 mm2) was developed, with a chip-size up to 1.4 mm2 and power dissipation up to 1500 mW (at 400 rnA for UV-InGaN) in a corresponding thermal ambient. The thermal resis­tance is 12 K/W. For high-integration applications (spotlights, general light­ing), special PCBs with isolating layers thinner than 10 )liD (commercial solu­tions: !50 )liD) were developed also. Modules on I mm copper (area 40 x 40 mm2) with 100 HL-LEDs, P101 = 50 W, Papr = 8 Win amber (595 nm) and with thermal resistance of 6 K/W were demonstrated.

Key words: high-power SMD-LED, chip-size, power dissipation, thermal management, integration density, optical output, LED-module, thin isolating layer, thermal resistance, general lighting, ambient lighting, illumination, light source, medi­cal application, solid-state lighting

1. THE HIGH-POWER LED

1.1 Principles

A High-Power LED (HL-LED) for the spectral region of 405 to 980 run cov­ered today from commercially available semiconductor chips was developed. This device allows bias currents as high as 600 rnA, in comparison to 70 rnA until now. The HL-LED outline is 2.8 x 3.2 mm2 • The key feature of the de­vice is a very low thermal resistance, less than 12 KIW, due to a core made of copper. Thus the chip can operate as light emitter with a very high performance. The actual chip size is in the range of 200 to 1200 1.1m in square. In conventional surface-mounted LEDs (SMD-LEDs), power dissipation up to 150 m W at room temperature is possible; therefore a maximum current of approximately 70 rnA is allowed for most chips. The

253

M.S. Shur and A. tukauskas ( eds.), UV Solid-State Light Emitters and Detectors, 253-260. © 2004 Kluwer Academic Publishers.

254 A. Mahlkow

approximately 70 rnA is allowed for most chips. The HL-LED can handle power dissipation up to 1500 m W, so cw operation with 600-mA and higher current becomes possible. Owing to the 1 0-times higher current, a 1 0-times higher light flux from a semiconductor chip is possible. The chip size can be increased to reduce the current density (i.e., the current per chip surface area) to minimize aging of the semiconductor material also.

1.2 Structure of a High-Power LED

Figure I. Principal construction of a High-Power LED (HL-LED)

For mechanical base of the HL-LED, a standard PCB-substrate (FR4, FRS, Kaplan, etc.) is used (3 in Fig. 1). A new feature in comparison to con­ventional devices is that a thick inner layer made of copper (2) is added to the upper (5 and 8) and lower (1 and 4) conductive layers. So, an excellent contact with a very low thermal resistance to the peripherical PCB is estab­lished and the heat generated by the LED-chip (7) is taken away. For con­ventional constructions, this is the main limiting point for power dissipation and current. The chip sits in a cavity (9) that serves as a reflector and heat sink as well as a back contact. Thus a very good electric and thermal contact

High Power LED and Thermal Management 255

is formed. Upward, a clear or toned potting compound forms the mechanical conclusion of the device made of epoxy ( 6). By selection of the walls and/or the depth of the sack-hole and the thickness of the metallization, the optical features can be adapted to the chip in its expansion, thickness, emission wavelength, shape, aspect relationship, position of the transition layer, splendor description, etc. and the external splendor behavior is influenced positively.

For an operation lifetime of 100 000 hours and more and for operation at maximum ratings of the HL-LED, a secure derivation of the heat at the me­chanical boundary of the HL-LED where it is soldered on the PCB trough a good thermal conduction via the solders of the device to a fitting periphery is essential. Almost all kind of soldering (iron, reflow, etc.) established an ef­fective binding with low thermal resistance and very good conductivity. Fig­ures 2a,b depict the temperature distribution in top view for a HL-LED oper­ating at 1000 m W in comparison to a microscopy image for orientation.

Figure 2a, b. Left: microscope image in top view. Right, with the same magnification: micro thermal image taken with a resolution of 50 f..!m. The chip (red rectangle) operates at 1000 mW, the maximum temperature that appears is 54 °C at ambient temperature of25°C.

1.3 Simulation of Heat Distribution

To investigate the principle limits of this design, we simulated the thermal distribution with a complex 3D-model with a few million lines using a pro­fessional software solution CFX RC. Some results are shown in Fig. 3.

256

~~::iir:.~}¥¥:" ~=;~~;;~-~§§~ --.--····--­...... --····111"-----·-··--­..... --···· .. ·-­·--···-·==--~ ·--····-...-...--

--···~---'-"' --··~~------~··---, ·-····---*

A. Mahlkow

...

.. ,

Figure 3. 3D-image of results in a thermal equilibrium after some virtual hours (top). Cut parallel to x-axis through the data-matrix (bottom). The thermal resistance from the chip (white area) to the pad (dark gray) is about 24 KIW (Pdisp = 1000 mW).

Using the results of the simulation, we redesigned the basic-material and the mechanical structure to optimize the thermal resistance. The resistance of the pure device is 12 K/W. In the simulation, the resistance of the chip sub­strate is also added. For InGaN on sapphire, we obtained in total less than 14-K/W resistance. This result corresponds to our measurements.

1.4 Measurements of Thermal Distribution

We measured the thermal distribution with a spatial resolution of about 50 Jlm first to optimize the materials parameters used for simulation and second to check the results of the simulation. The simulation matches with reality better than within 5% for different dissipation powers and ambient media. Figure 4 shows the experimental setup we used for taking pictures like that shown in Fig. 2b.

High Power LED and Thermal Management

·~ ,.;a - '•..-.::._

257

Figure 4. Experimental setup for thermal imaging. On the right: the liquid-nitrogen cooled camera with macro optics made of CdS mounted on an optical rail. In the background: some power supplies. The High Power LED soldered on copper is shining in green (for the photo, with a few milliamps).

All thermal images in this paper are taken with this setup. To get a realis­tic thermal equilibrium even at lower power, a thermal management is very important and for good results, the total ambient resistance should be lower than 2 KIW. To drive the high-power LEDs at their limit and to get the maximum output using the advantage of the small size, we developed an efficient thermal management for this device also.

2. THERMAL MANAGEMENT

2.1 Idea

There are a lot of ideas and technical solutions for heat spreading, dissipation and cooling for devices with high power in the region of some dozens of watts, e.g. processors, power diodes, etc. These techniques are active solu­tions with fans and/or water flow. We tried to find solutions that were all passive and contained no moving parts. The idea is given by metal-core

258 A. Mahlkow

PCBs some ten years ago but the commercially available products are not satisfYing. So we optimize them.

2.2 Principles

A metal-core PCB is very simple. The first layer is a sheet of metal (copper, aluminum) followed by an electrical isolating layer. The isolation is neces­sary for a structured board with different electrical potentials. This isolating layer is followed by a structured layer or layers with the power dissipators soldered or glued on the top. There are two ways to reduce the thermal resis­tance of the isolating layer. First one is the use of materials with excellent thermal conductivity, e.g. diamond, sapphire, etc. The second one is the use of conventional materials with reduced thickness. The first way is very ex­pensive and not applicable for large-scale industrial production. Fortunately, the high-power LED is a device driven with a few volts, so the dielectric breakdown can be below several tens of volts. This allowed us to reduce the thickness of the isolating layer drastically, by one order of magnitude in comparison to about 150-J..Lm thickness typical of present commercially available metal-core PCBs. The following microscopy image (Fig. 5) shows a cross section of our material with a HL-LED on top.

Figure 5. Cross section of metal-core PCB on an aluminwn sink and single HL-LED on top. The isolating layer is about l 0 J.lm, one order of magnitude thinner than other commercial materials.

2.3 Simulation of Different Designs

The second idea for efficient heat sinking is to use thermal through-hole platings (THP). Using simulation, we compared these concepts with real models and prepared design rules for both. Figures 6 and 7 show the simu­lated and measured heat distribution pictures of the same structure.

High Power LED and Thermal Management 259

Figure 6. Results of thermo-simulation (first, with the inset of microscopy image) in compari­son to the measurements (second). The solder pad for the HL-LED shows 72 through-hole platings (0 0.2 mm) in a very narrow array. The first two pictures both show a bottom view. In the cross section (right), the heat distribution through all the important structures is shown.

The comparison to the results for metal-core PCBs (Fig. 7) shows the ex­pected fact that the equilibrium temperature is much lower than for thermal THP.

Figure 7. Simulated thermal distribution of a HL-LED with P101 = 1000 mW on a metal-core PCB with the isolating layer thickness of 10 jlm. (left). Right picture shows the density of weight for clarification of the structure.

2.4 Thermal Imaging of Optimized Structures

With different design rules for optimized structures, different HL-LED­modules were constructed. The first one is a linear module with thermal THP with a normal density of devices (Fig. 8, top) and the second one is a highly integrated HL-LED spotlight with 100 devices mounted nearest to each other on an area of ( 4 cm)2 (Fig. 8, bottom).

260

.v

5

0

5

0 0 5 10

Xfmm]

A. Mahlkow

15 . 20 0 2 3 4 5 6 Xfmml

Figure 8. Thermal image of a linear module (top) with HL-LEDs and thennal THP for a light­line. The maximum temperature is about 37° C. The second image (bottom) is taken from a HL-LED spotlight integrating 100 devices with maximum density. The maximum temperature is 42°C and the total power dissipation about 80 W (400 rnA per LED) with an optical flux 2500 Im (AIInGaP, f..p = 595 nm).

Using the developed thermal management, one has industrially scalable technique to construct HL-LED-based lamps for general lighting.

DETECTION OF BLUE LIGHT BY SELF-ASSEMBLED MONOLAYER OF DIPOLAR MOLECULES

0. NEILANDS 1, N. KIRICHENKO 1, I. MUZIKANTE 2, E. FONAVS 2,

L. GERCA 2, S. JURSENAS 3, R. VALIOKAS 4, R. KARPICZ 5, and L. VALKUNAS 5'6

1 Riga Technical University, Azenes Str.14, Riga LV 1048, Latvia 2 Institute of Physical Energetics, Aizkraukles Str.21, Riga LV 1006, Latvia 3 IMSAR, Vilnius University, Sauletekio Ave. 9, build. 3, LT-2040 Vilnius, Lithuania 4 Institute of Technology, Linkopings University, SE-581 83 Linkoping, Sweden 5 Institute of Physics, Savanoriu Ave. 231, LT-2053 Vilnius, Lithuania 6 Department of Theoretical Physics, Faculty of Physics, Vilnius University,Sauletekio Ave. 9, build. 3, LT-2040 Vilnius, Lithuania

Abstract: Dipolar donor-acceptor molecules show pronounced changes in the dipole moment upon photoexcitation, resulting in transformation of their optical and electrical properties. A monolayer of 4'-(3H-1,2,5-dithiazepan-5-yl)benzylidene indan-1 ,3-dione (DMABI-2S), possessing high electric dipole moment is self-assembled (SAM) on Au layer. Analysis of the IR vibrational and fluorescence spectra of SAMs supports the conclusion about the specific tight packing of the molecules in the SAM structure. The changes of the sur­face potential on irradiation with blue-light is measured by Kelvin probe tech­nique. The reversible changes of surface potential of the order several tenths of millivolts is induced by irradiation of the sample. The relationship between ab­sorption spectra and spectral dependence of the surface potential is observed. Such SAM structures can be applied for design of molecular sensors and 2D recognition devices.

Key words: donor-acceptor molecule, self-assembled monolayers, fluorescence, surface potential, switching effect

261

M.S. Shur and A. iukauskas (eds.), UV Solid-State Light Emitters and Detectors, 261-269. © 2004 Kluwer Academic Publishers.

262 0. Neilands, N. Kirichenko, I. Muzikante et al

1. INTRODUCTION

2-( 4' -Dialkylaminobenzylidene )indan-1 ,3-diones (DMABI) are coloured molecular compounds built up by molecules of high electric dipole moment and, therefore, manifesting well-expressed nonlinear optical (NLO) proper­ties [ 1 ,2].

The nonlinear optical and electrical properties of the solid state are mainly predetermined by asymmetry in molecular orientations. Thus, highly ordered films of the molecules containing donor-acceptor moieties seem to be very promissing in their various applications. Several methods are applied by manufacturing such ordered solid films. It has been shown that Langmuir-Blodgett films can be composed by the amphiphilic derivative of DMABI [3]. Adsorption of the donor-acceptor molecules on a solid inorganic surface is the other possibility to design the stable molecular monolayer. The organic molecules with the thiol group (SH) at the end demonstrate strong affinity for gold and, therefore, are strongly bound to a gold surface via a slightly polar covalent Au-S bond. Formation of self­assembled monolayers (SAMs) as a result of thiols bound to gold surfaces has disclosed a new scientific activity with important interdisciplinary implications ranging from the corrosion science to molecular recognition and nanotechnology. Among substances being used for adsorption on the gold surface, compounds with disulfide moieties have been described [ 4].

New derivative of 4'-(3H-1,2,5-dithiazepan-5-yl) benzylidene indan-1 ,3-dione (DMABI-2S) comprising the disul­fide moiety, is synthesized to prepare a monolayer self-assembled on a surface by the sulfur bonds [ 5]. The indan­dione-1 ,3 fragment . corresponds to the electron acceptor (A) and the dimethyl­aminophenyl fragment relates as the elec­

0 DMABI-2S

tron donor (D). Dipolar donor-acceptor molecules show pronounced changes in the dipole moment upon photoexcitation, resulting in transforma­tion of their optical and electrical properties.

Detection of Blue Light by Monolayer of Dipolar Molecules 263

2. EXPERIMENTAL

2.2. Synthesis of 4'- (3H-1,2,5-dithiazepan-5-yl)benzyli­dene indan-1,3-dione (DMABI-2S)

Synthesis of DMABI-2S has been performed starting from indan-1,3-dione (1) and 4-[bis-(2'-chloroethyl)amino]benzaldehyde (2). We have performed a condensation reaction in the presence of a strong acid, thus, finally obtain­ing 4' -[bis-(2" -chloroethyl)amino ]benzylidene indan-1 ,3-dione (3).

C-S bond formation is the next step. For this aim, a reaction of the chloro substituted compound 3 with thiourea in excess was chosen.

Cl

o c1 (__rei I 0 I N

+ ~Nr- CH3COO~ H~\ HCI 0

\ ~ /, ~ ~ 0 Cl 7'

I 2 I -::,...

NH

==< 2 DMF 3 + s -

100 "c NH2

We have isolated a mixture of thiourea and salt 4. Interaction ofthis mix­ture with potassium hydroxide in water-ethanol solution resulted in a cleav­age product of bis-isothiuronium salt, namely in dipotassium salt of dithiole 5. This salt was not isolated, but oxidized by solution of potassium hexa­cyano-ferrate(III). Dithiolate dianion was transformed by oxidation in cyclic disulfide 6 (DMABI-2S), our target product. Compound 3, 0.65 g (1.7 mmol) and thiourea, 1.07 g (14 mmol) were dissolved in DMA, 6 mL, and heated at 100 oc on water bath for 12 h. After cooling the solution was diluted with methylene chloride, 50 mL, and kept in refrigerator overnight. Red precipitate was filtered off. The yield of a mixture, containing thiourea and expected isothiuronium salt 4 was obtained finally in the amount of 1.27 g. The red precipitate was dissolved in the mixture of water, 65 mL, and ethanol, 65 mL, and heated until boiling. The 1M solution of potassium hy­droxide, 15 mL (15 mmol), was added. After 1 min. a solution of 1.25 g (3.8 mmol) potassium hexacyanoferrate(III) in water, 65 mL, was added. A

264 0. Neilands, N. Kirichenko, I. Muzikante eta/

red very fine hardly filter­able precipitate was ob-tained. The yield of crude targeted compound 6 was obtained by the amount of 0.4 g. The red substance was refluxed in chloro-form, 40 mL, for 5 min., the insoluble part (0.08 g) was filtered off, and the s K3Fe(CN\s

H 20 orange solution was sub-jected to column chroma-tography on silicagel, elu-

/"'s + otigorners

N I

\.-..../s

ent toluene-methylene chloride 1 :3. The isolated intermediate fraction of the orange substance is a mixture of two compounds. This we can see in 1H NMR spectra and on thin layer chromatogram. It was possible to isolate two compounds by the repeated chromatography. The first and most orange frac­tion is our target compound 6. The second orange fraction could be a stable conformer of compound 6. Both fractions are positioned in the column very closely. We did not study the second compound in details except its 1H NMR spectra.

DMABI-2S forms orange crystals, m.p. 286-288 °C. 1H NMR, (CDCb), 8: 3.11, t, 4H (CH2S), 4.08, t, 4H (CH2N), 6.72, d, 2H (3',5'-H), 7.83, m, 5H (4,5,6,7-H, H-C=), 8.54, d, 2H (2',6'-H). IR-spectra, em -I (absorpt.%): 1710 (29), 1666 (60), 1564 (60), 1522 (59). UV Vis spectra, CHCb ,nm (e): 251 (24290), 266 (18100), 472 (79700). Elemental analysis. Found,%: S 17.3; C20H17N02S2. Calculated, %: S 17.5. Isomer ofDMABI-2S forms orange crystals. 1H NMR, (CDCh), 0 : 2.71, t, 4H (CH2S), 3.89, t, 4H (CH2N), 6.87, d, 2H (3',5'-H), 7.83, m, 5H (4,5,6,7-H, H-C=), 8.51, d, 2H (2',6'-H).

2.2 Preparation of Self-Assembled Monolayers

The self-assembled monolayers (SAMs) were produced by adsorption of DMABI-2S on a polycrystalline or Au (111) surface. The glass substrate covered with Au (111) or a poly crystalline Au layer deposited by vacuum evaporation was immersed in the chloroform solution of the DMABI-2S with concentration of the order of c = 1 o-3 mol/1. The time of deposition was 69 hours. After deposition the samples were rinsed by the corresponding solvent and dried in N2 stream.

Detection of Blue Light by Monolayer of Dipolar Molecules 265

2.3 Measurements of Absorption and Fluorescence Spec­tra and of Surface Potential

Absorption spectra of DMABI-2S in chloroform solution were measured in the spectral region between 200 and 800 nm by a computer controlled UV­Vis Specord. The IR spectra were recorded by the Broker IFS88 spectrome­ter. Luminescence spectra were obtained by the SPEX Fluorolog 2F111A1 spectrometer. The surface potential of monomolecular layer of DMABI-2S has been proved by Kelvin probe technique. The experimental set-up is de­scribed elsewhere [ 6]. The changes of the surface potential on irradiation were measured in spectral region between 300 and 600 nm by Besocke Delta PHI GmbH/ Kelvin control equipment [7]. The samples were irradiated by a xenon lamp using interference filters. The density of light flux was in the range of2-5 mW/cm2•

3. OPTICAL PROPERTIES

Analysis of the IR vibrational and fluorescence spectra of SAMs supports the conclusion about the specific tight packing of the molecules in the SAM structure. Fluorescence spectra of SAMs on polycrystalline Au layer (1) and Au (111) layer are shown on Fig. 1. Weak fluorescence with the maximum at 520 nm, what is typical for molecular fluorescence ofDMABI, is recorded from SAMs. Spectra are remarkably broadened due to the intermolecular interaction.

IR absorbance spectra of the SAMs on Au ( 111) layer and DMABI crys­tal a-form are shown in Fig. 2. It is evident that the main features of molecu­lar vibration modes are similar for both samples. The vibration spectrum of the DMABI molecules and crystal was studied recently [8]. The dominating vibrations are at 1554 cm-1 and 1515 cm-1, which are typical ofthe benzene ring and C=C stretches [8]. Vibrational spectrum of the SAM ofDMABI-2S is blue-shifted by about 20 cm-1 in comparison with DMABI crystal what is evidently seen by comparing C=O anti-symmetric and symmetric stretches at 1707 cm-1 and 1662 cm-1 in both samples. This is because the C=O combine is involved in the intramolecular charge-transfer from the DMAB nitrogen to the indandione oxygen. The shift is also sensitive to the dipole moments of the surrounding molecules [8]. The same could be said concerning the shift of the C=C combine vibration at 15 54 em-\ which bounds polar donor and acceptor fragments of the DMABI molecule.

266

0.10 ~ c: ~ 1/)

~ 0 if 0.05

0. Neilands, N. Kirichenko, I. Muzikante eta/

460 480 500 520 540 560 580 600 620 640 660 680 700

'A (nm)

Figure I. Fluorescence spectra of DMABI-2S monolayer on polycrystalline Au layer (I) and Au (Ill) layer (2).

c 0.1 0 e. 0 (/)

..c rn

~ 0.01

1 E-3 1600 1400 1200 1000

Figure 2. Vibrational spectra ofSAMs on Au (Ill) layer (I) and DMABI crystal a-form (2).

Thus, the optical properties of SAM predominantly follow those of the single molecule with a larger influence of the intermolecular interaction, what is an indication of successful deposition of DMABI-2S molecules on Au surface.

Detection of Blue Light by Monolayer of Dipolar Molecules 267

4. SURFACE POTENTIAL

Surface potential studies provide useful information regarding both struc­tural and electronic properties of oriented films [9]. The surface potential of the film depends on both the packing density and the orientation of mole­cules. The film is treated usually as a uniform assembly of molecular dipoles giving rise to polarisation of the layer.

The surface potential of SAMs increased by 0.14 V in comparison with the bottom gold electrode. The surface potential was uniform across the layer, indicating that the DMABI-2S molecules are absorbed on the Au sur­face.

Donor-acceptor molecules undergo a reversible optically induced change of the value of the dipole moment due to intramolecular charge transfer. Quantum chemical calculations indicate a substantial change of the dipole moment of the DMABI molecule in the excited state (J.Jo = 2.3D and Jlex = 14.2D) [ 1 0], which yields changes of the surface potential. Changes of the surface potential !J.U5 were observed at irradiation of the SAM of DMABI-2S molecules with light between 300 nm and 550 nm. In darkness, a reversible change of the surface potential is observed (see Fig.3). The sur­face potential was recorded at different time intervals during irradiation with light and after switching off the light. The best fit of the experimental data was obtained with an exponential function. The changes of the surface po­tential were between 4 and 25 m V and time constant characterizing response to irradiation was between 10 s and 1.5 min., respectively. The response of the surface potential remained the same after several repeated cycles of irra­diation.

Spectral dependence of the changes of the surface potential to irradiation is in correlation with the absorption spectrum ofDMABI-2S (see Fig. 4).

It is well known, that the optically induced intramolecular charge transfer is an ultrafast process of the order of femtoseconds. In our case response of surface potential to irradiation close to a minute were observed. Thus, the change of surface potential is a macroscopic response of the film. Switching off the dipole moment of a single molecule yields only an extremely small change of the surface potential. We might monitor only the ratio of the molecules in the ground and excited states until the equilibrium state is achieved. It is shown that thin solid films of DMABI molecules are highly photoconducting with photogeneration threshold energy at 1.95 eV (636 nm) [ 11]. Consequently, the photo generation of charge carriers in spectral region 300-600 nm also take place. Both, the dipole moment of the excited mole­cule and the photogenerated charge carriers may change the value of surface potential during irradiation. For better understanding of the photoinduced

268 0. Neilands, N. Kirichenko, I. Muzikante et al

process in monolayers of DMABI-2S molecules, further experimental stud­ies are needed.

10

5

0

> .§.. "' -5 ::::>

<l

-10

-15 0 4 8 12 16 20 24

!(min)

Figure 3. Changes of surface potential !J..Us of SAM on a polycrystalline Au layer under irra­diation with light at 475 nrn in respect to darkness.

8

6 8.0x104

• 4 6.0x104

> E s 2 4.0x104 §. If)

:J <I 0 d

-2 2.0x104

-4 0.0 250 300 350 400 450 500 550 600

A. (nm)

Figure 4. Spectral dependence of changes of surface potential I':.. Us of SAM on a polycrystal­line Au layer (I) and absorption spectra in chloroform with c = 1 o-3 mol!I (2) of DMABI-2S. The values of surface potential are estimated for the light power density of I m W/cm2•

Detection of Blue Light by Monolayer of Dipolar Molecules 269

5. CONCLUSIONS

The DMABI-2S molecules were chemically adsorpted on gold surface com­prising monolayer.

Reversible changes of the surface potential at irradiation in spectral region 300--550 nm with maximum around 475 nm are observed. The changes of the surface potential may be caused by the photoinduced intramolecular charge transfer and by the photogeneration of charge carriers.

ACKNOWLEDMENTS

This research was supported by Lithuanian-Latvian-Taiwan joint grant. The authors (I.M., E.F.) are grateful to B.Stiller and Prof. L.Brehmer of the Pots­dam University for the possibility to measure photoinduced changes of sur­face potential by the Kelvin probe technique.

REFERENCES

I. S. Jursenas, A. Gruodis, G. Kodis, V. Gulbinas, E. A. Silinsh, L. Valkunas, J. Phys. Chern. B 102, 1086 (1998).

2. H. Schwartz, R. Mazor, V. Khodorkosky, L. Shapiro, J. T. Klug, E. Kovalev, G. Meshulam, G. Berkovic, Z. Kotler, S. Efrima, J. Phys. Chern. B 105, 5914 (2001).

3. M. Rutkis, E. Wistus, S. E. Lindquist, Proc. SPIE 2968, 34 (1997). 4. S-G. Liu, H. Liu, K. Bandyopadhyay, Z. Gao, L. Echegoyen, J. Org. Chern. 65,

3292 (2000). 5. 0. Neilands, I. Muzikante, E. Fonavs, L. Gerca, COST Chemistry Action Dl9 Int.

Workshop on Nanochemistry (September 26~28, 2002, Vienna, Austria), Abstract P12.

6. 0. Vilitis, E. Fonavs, I. Muzikante, Latv. J. Phys. Techn. Sci. 5, 38 (2001). 7. I. Muzikante, L. Gerca, E. Fonavs, M. Rutkis, D. Gustina, E. Markava, B. Stiller, L.

Brehmer, G. Knochenhauer, Mat. Sci. and Engineering C 22, 339 (2003). 8. A. Gruodis, S. Jursenas, V. Aleksa, V. Kovalevskij, L. Valkiinas, Lith. J. Phys. 38,

237 (1998). 9. D. M. Taylor, Adv. Colloid and Interface Science, 87, 183 (2000). 10. V. Gulbinas, G. Kodis, S. Jursenas, L. Valkunas, A. Gruodis, J.-C. Mialocq, S.

Pommeret, T. Gustavsson, J. Phys. Chern. A 103, 3969 (1999). 11. I. Kaulach, E. A. Silinsh, Latv. J. Phys. Techn. Sci. 3, 3 (1997).

ATOMIC AND MOLECULAR SPECTROSCOPY WITH UV AND VISIBLE SUPERBRIGHT LEDS

G. PICHLER, T. BAN, H. SKENDEROVIC, and D. AUMILER Institute of Physics, Bijenicka cesta 46, P. 0. Box 304, HR-10001 Zagreb, Croatia

Abstract: Our most recent absorption and emission experiments with heavy alkali vapor using super bright light-emitting diodes from NICHIA, LUMILED and OSRAM will be presented. UV LEDs together with LEDs in the visible spec­tral region present invaluable narrow-band continuum light source for very precise absorption measurements. We used all-sapphire cells and heat-pipe oven cells in usual absorption arrangement with high resolution scanning monochromators. Beside self-broadened alkali resonance lines, we investi­gated molecular bands and diffuse bands of alkali dimers. A number of appli­cations, especially in dentistry will be also presented.

Key words: UV LEDs, atomic spectroscopy, molecular spectroscopy

1. INTRODUCTION

Ultraviolet spectral region is a very special window into the structure of many atoms and molecules. It is attractive because the experiments in the UV region still do not require complicated vacuum equipment and special spectral instrumentation. Absorption spectroscopy in the UV spectral region could bring about much information on discrete spectral lines and structured continua of molecular origin. Beer-Lambert law for the absorption process at low intensities could reveal absolute values of the absorption coefficient in the useful spectral region, provided the absorptive species have uniform den­sities. However, a stable and reliable spectral source that serves as a back­ground continuum is difficult to find. Usually for this purpose, a tungsten filament lamp, filled with iodine to allow higher working temperatures, served quite well throughout the visible spectral region. Unfortunately, in the

271

M.S. Shur and A. Zukauskas (eds.), UV Solid-State Light Emitters and Detectors, 271-278. © 2004 Kluwer Academic Publishers.

272 G. Pichler et al

ultraviolet region the halogen lamp spectral distribution rapidly fades out. In addition to this, deuterium discharge lamp offers the background continuum in the UV spectral region, but the lamp requires a very stable power supply. In all these and similar cases, the continuum radiation extends over wide spectral region, usually much broader than the region of interest. This causes an increased influence of the scattered radiation inside spectrometers and monochromators used for absorption measurements. Light-emitting diodes (LEDs) provide the source of the background continuum radiation in rela­tively narrow spectral region and at high stability. Recent development of InGaN LEDs shifted the low wavelength limit from deep blue to about 360 nm. UV LEDs can be modulated by modulating their driving current; this simplifies the use of the lock-in technique in absorption measurements, thereby increasing the accuracy of the obtained values for the absorption coefficient. We shall present several cases were UV and visible-light LEDs enable excellent and precise absorption measurements of line wings of broadened atomic spectral lines and structured continua belonging to simple diatomic molecules. Alkali atoms and diatoms offer a perfect example to test the use of these new spectral light sources with unique features. Nowadays the LEDs can be obtained with the peak emission from 365 up to 920 with the separation of only 5 nm. We could imagine a carousel or a linear array of all these LEDs with slightly incremented peak maxima, and they would span the entire visible spectrum with near UV and near infrared (IR) spectrum. If at each spectral region an appropriate LED would be in use, we could sup­press the scattered radiation from distant spectral regions appreciably. In Fig. 1 we present the spectra of three LEDs at the currents of 30 rnA and 100 rnA. It is interesting that at higher current, the blue LED develops an UV component and the red LED exhibits a red shift of its peak value.

2. ABSORPTION EXPERIMENTS

In all our absorption experiments, we use both heat-pipe ovens and all sap­phire cells. By heating the ovens and cells, we generate alkali vapor, which consists of about 99% atomic particles and about 1% of alkali dimers. By using the Beer-Lambert law,

(1)

Atomic and Molecular Spectroscopy with LEDs

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273

600 650 700

Figure 1. Spectra ofthree different Light Emitting Diodes at driving currents of30 and 100 rnA. Note a well-developed UV component at 100 rnA of the NICHIA blue LED.

where 10 (A) is the incoming intensity, !(A) the transmitted intensity, k(A) the absorption coefficient and L the length of the cell with hot metal vapor, it is possible to determine the total absorption coefficient which is proportional to the particle concentration in question:

(2)

In Fig. 2 we show a simple UV LED based absorption experiment in which the laser induced fluorescence (LIF) could be almost simultaneously performed. The combination of both LIF and absorption measurement could bring about many useful information on the nature of dense metal vapor. The main advantage of the absorption experiment is in the fact that it reveals ab­solute values of the absorption coefficients which can be directly connected to atomic and molecular transition probabilities, provided the particle con­centrations could be independently determined.

274

MONOCHROMATOR WITH PHOTOMlA.TIPLIER TUBE

LOCK~N

DOVE PRISM

I------~--­s FM2

G. Pichler et a/

\;lOLET LASER ECDL

Figure 2. Absorption experiment in rubidium vapor with a NICHIA UV-LED as a back­ground source with a possibility to measure fluorescence induced by a NICHIA violet laser at 420 nm.

3. RESULTS AND DISCUSSION

Principal-series spectral lines of alkali atoms have been attractive subject for the research over almost entire twentieth century. Recently, they became again the subject of research because of possible applications in the field of ultracold molecules at large internuclear distances [1,2]. We performed new measurements with blue/UV LED (at 100 rnA) as a continuum source using saturated and superheated cesium vapor. In Fig. 3, we present transmission spectra at a temperature below and temperatures above the critical point where superheating starts. The second and the third principal-series doublet of cesium can be readily seen at 455 and 390 nm. There are also impurity lines of potassium and rubidium second doublets of their principal series lines, and the electric quadrupole lines of cesium.

Using Eq. (2), the absolute value of the absorption coefficient was ob­tained and plotted in Fig. 4. We present only superheated vapor cases in or­der to emphasize the contributions from the diatomic molecules in compari­son to collision induced features. Increasing the temperature of superheated cesium vapor thermally destroys cesium dimers leaving almost intact the collision induced molecular features. In this way, it is possible to

Atomic and Molecular Spectroscopy with LEDs

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Transmission spectrum of UV/blue LED through Cs vapor

53s •c

380 390 400 410 420 430 440 450 460 470 480

J. /nm

275

Figure 3. Transmission spectra at four different temperatures in the all-sapphire cell filled with cesium saturated vapor (T = 291 "C), and superheated vapor (T = 424, 476, 53-5 °C).

conclude directly from the spectral observation which spectral phenomenon belongs to singlet manifold (true molecule) and which one- to triplet mani­fold (collision induced feature) . The band peaking at about 395 nm certainly belongs to the singlet manifold, because it decreases in intensity with in­creasing the temperature of superheated vapor [3].

It is of considerable interest to search for collision induced features, es­pecially if they are heavily overlapped by dimers, as it is nicely shown in the spectral region above 460 nm in Fig. 4. The satellite band peaking at 472 nm could be observable only when the cesium dimer concentration was reduced due to thermal destruction.

In Fig. 5 we show the spectral distributions of two LEDs peaking in UV and violet spectral region. The UV LED would be ideal for absorption meas­urements of dense thallium vapor and its resonance line at 377.6 nm [4] . The other resonance line of thallium at 535 nm is already in the visible spectral region, but it could be ideally measured by the green LED. Other Tl reso­nance lines connecting lowest p and first excited d levels are situated below 300 nm, and therefore they are still not attainable by commercial UV LEDs.

The LED peaking at 405 is ideal for the second doublet of atomic potas­sium. This doublet is also interesting for the satellite bands search, which could lead to discovery of the ion-pair long range states (2). In Fig. 6, we

276

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G. Pichler et al

Cs2 UV/blue LED absorption

424 °C

380 390 400 410 420 430 440 450 460 470 480

),/ nm

Figure 4. Absorption coefficient in the 380-480 run spectral range at four different tempera­tures in the all-sapphire cell filled with cesium saturated vapor (T = 291 °C), and superheated vapor (T = 424, 476, 535 °C).

present the measured absolute values of the blue satellite band of the second doublet of its principal series and a spectrum simulated using ab-initio potential curves.

Blue, violet and UV LEDs have a very broad application, but we pursued the photopolymerization of composite materials in dentistry [5,6]. The blue LED peaking at about 470 nm reveals the best results because it coincides with a peak of absorption coefficient of camphorquinone, which is used as a trigger for the polymerization process. However, there are other materials, which could serve for the same purpose, but peaking at 410 nm, or even in the ultraviolet spectral region.

4. CONCLUSION

We have shown how versatile and useful are UV and visible LEDs in simple absorption spectroscopy of atoms and molecules in dense metal vapor of alkali elements. We believe that these narrow-continuum spectra offer an

Atomic and Molecular Spectroscopy with LEDs

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277

Figure 5. Spectral distribution of two LEDs peaking in UV and violet spectral region.

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400 40 I 402 403 404

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Figure 6. The measured absorption coefficient (circles) of the blue satellite band of the potassium second doublet. Theoretical simulation well describes only the main peak of the satellite band

excellent background source for the studies of spectral line broadening, in which the peculiar satellite bands may point out to the existence of alkali molecules at large interatomic distances. The latter subject is of considerable interest for making ultracold molecules, and certainly a possible channel for

278 G. Pichler et al

quantum control of chemical interactions with shaped femtosecond laser pulses. We intend to extend the present experiments to pulsed mode UV­LEDs, where the pulses in the nanoseconds or even picoseconds time region are already available [7,8]. The main advantage of absorption measurement is in revealing of an absolute value of the absorption coefficient. This is di­rectly connected to atomic and molecular transition probabilities provided the particle concentrations could be independently determined [9].

ACKNOWLEDGMENT

We are grateful for the support from the Ministry of Science and Technology of Republic Croatia and Alexander von Humboldt Stiftung, Germany. Fruit­ful discussions with D. Veza, S. Milosevic, R. Beuc, B. Resan and S. Ter­A vetisyan are gratefully acknowledged.

REFERENCES

I. R. Beuc, H. Skenderovic, T. Ban, D. Veza, G. Pichler, and W. Meyer, "Cesium satellite band at 875.2 nm from the Cs2 0/(6p 2P112+6s 2S112) state," European Physical Journal D 25, pp. 209-214 (200 I).

2. T. Ban, H. Skenderovic, R. Beuc, I. Krajcar Bronic, S. Rousseau, A. R. Allouche, M. Aubert-Fnkon, and G. Pichler, "Pure long-range ion-pair Cs2 molecules," Chemical Physics Letters 345, pp. 423-428 (2001).

3. T. Ban, H. Skenderovic, S. Ter-Avetisyan, and G. Pichler, "Absorption measure­ments in dense cesium vapor using UV-blue light emitting diode," Applied Physics B 72, pp. 337-341 (2000).

4. G. Pichler and J. L. Carlsten, "Self-broadening of Tl 377.6 nm resonance line," J. Phys. B: At. Mol. Phys. 11, pp. L483-L488 (1978).

5. A. Knezevic, Z. Tarle, A. Meniga, J. ~Malo, G. Pichler, M. Ristic, "Degree of con­version and temperature rise during polymerization of composite resin samples with blue diodes," Journal of Oral Rehabilitation 28, 586-591 (2001).

6. Z. Tarle, A. Meniga, A. Knezevic, J. Sutalo, M. Ristic, G. Pichler, "Composite con­version and temperature rise using a conventional, plasma arc, and an experimental blue LED curing unit," Journal of Oral Rehabilitation 29, pp. 662--667 (2002).

7. T. Araki, H. Misawa,"Light emitting diode-based nanosecond ultraviolet light source for fluorescence lifetime measurements," Rev. Sci. Instrum. 66, 5469 (1995).

8. http://www.ibh.co.uk/products/light sources/nanoled main.htm. 9. D. Aumiler, T. Ban, R. Beuc, and G. Pichler, "Simultaneous temperature and den­

sity determination of rubidium vapor," Applied Physics B 16, 859- 867 (2003).

SEMI-INSULATING GaN AND ITS FIRST TESTS FOR RADIATION HARDNESS AS AN IONIZING RADIATION DETECTOR

I 2 2 J. V. VAITKUS , W. CUNNINGHAM , M. RAHMAN ,

2 3 K. M. SMITH , and S. SAKAI 1 Institute of Materials Science and Applied Research, Vilnius University, Vilnius, Lithuania 2 Department of Physics and Astronomy, University of Glasgow, Glasgow, UK 3 Satellite Venture Business Laboratory, University ofTokushima, Tokushima, Japan

Abstract: The first results of charged particle response from thin epitaxial GaN radiation detectors are presented. Semi-insulating epitaxial GaN is a promising material for X-ray imaging detectors, and for hard-particle tracking radiation detectors. Electrical characterization of the devices was carried using /-V measurements, that demonstrated good rectifying behaviour of the Schottky contact, with a room temperature reverse leakage current density of 1 x 1 o-8 A-em -2 at V = 10 V. The charge collection efficiency (CCE) was investigated using 5.49 MeV alpha particles from a 241 Am radioisotope source and was more than 90% in non-irradiated samples. The response of samples irradiated with 600 Mrad of 10 keY X-rays and with 5xl014 cm-2 of>IOO keY neutrons was investigated. No significant degradation in CCE was seen in the X-ray irradiated device, whilst a reduction in CCE to 78% was observed in the neutron-irradiated sam­ple.

Key words: semi-insulating GaN, ionizing radiation detectors, charge collection efficiency, radiation hardness

1. INTRODUCTION

Rapid progress of epitaxial growth techniques for GaN makes this material attractive for applications in high-temperature/high-power electronic devices operating at high frequencies as well as in blue-UV optoelectronic devices [ 1]. Semi-insulating GaN is a promising material for application in the detec­tion of UV radiation but also, due to its high density, stability and high

279

M.S. Shur and A. Zukauskas ( eds. ), UV Solid-State Light Emitters and Detectors, 279-286. © 2004 Kluwer Academic Publishers.

280 J V Vaitkus et al

threshold voltage, seems to be promising for ionising radiation detectors (comparison of different materials for fast ionising radiation detectors is pre­sented in Table 1). The proposed upgrade of the CERN Large Hadron Col­lider to ten times brighter luminosity (1035 cm-2/s) poses severe challenges to semiconductor detectors within the CERN experiments.

Table I. Comparison of materials for fast radiation hard detectors at 300 K.

Si SiC c GaAs GaN AlN z 14 14/6 6 31/33 31/7 13/7 Density, g/cm3 2.33 3.2 [2] 3.5 5.32 [2] 6.15 [4] 3.23

[2] [2] [4] Bandgap !!.E, e V 1.1 [2] 3.26 (4H) 5.45 1.43 3.39 [3] 6.2

3.0 (6H) [2] [2] [4] [2]

Electron mobility, 1500 1000 (4H) 2200 4000 1000 [3] 135 cm2N·s [2] 370(6H) [2] f2] [3] [41 Hole mobility, 600 [2] 50 [2] 1600 400 [2] 30 [3] 14 [4] cm2N·s [2] Breakdown field, 0.3 [2] 2.4 (6H) 10 0.6 17.3** 25.3 ** MV/cm 2.0 (4H) [2] [2] [5] [5]

[2] Displacement en- 13-20 22 [2] 43 9.8 > 15 [7] ergy, eV [6] [6] [4] 19±2 [81 e-h pair energy, 3.6 [6] 7.8 (4H) [2] 13 4.21 eV [2] [2] **predicted values

Recently the epitaxial layers of semi-insulating GaN (SI-GaN) were grown [9] and some peculiarities of differently compensated samples proper­ties, dark current temporal dependence and photoconductivity transient be­haviour, were investigated in our recent paper [10]. In the present work, par­ticular attention is paid to a review of first measurements of the charge collection efficiency (CCE) in as-grown material and in samples irradiated by different types of ionising radiation samples.

2. EXPERIMENTAL

The epitaxial GaN layers used in this study were grown by MOCVD on Ah03 (0001) substrates. The properties ofthe layers were changed by varia­tion of the substrate temperature and the trimethylgallium (TMGa) flow rate during growth [9]. The buffer n*-GaN layer was thin, low temperature grown GaN (carrier concentration 3.3x1016 cm-3, electron mobility 610 cm2N·s). An epitaxial 2-2.5-f.lm thick capping layer was grown at a TMGa

Semi-Insulating GaNas an Ionizing Radiation Detector 281

flow rate of 88 J..lmol/min, at 925 °C. Pad detector test structures were fabri­cated using 1.5-mm diameter evaporated gold Schottky contacts. The I-V characteristics were measured by contacting one Schottky contact, deposited on top of the sample and highly doped GaN by conductive glue (Fig. I). The devices demonstrated good rectifying behaviour of the Schottky contact, with a room temperature reverse leakage current density of lxl0-8 A·cm-2 at V = 10 V in the as-grown samples.

The CCE measurements were performed by applying bias to the two Au contact areas, so that the real sample structure was "Schottky contact-(SI­

Figure 1. A schematic section of the sample.

cross-

GaN)-(highly doped GaN)-(SI-GaN}­Schottky contact". A 241Am alpha parti­cle source was used for non-equilibrium carrier excitation. The 241 Am emits 5.48 MeV alpha particles. The detector and source were housed in a vacuum cham­ber in which the pressure was less 20 mbar, to ensure negligible particle en­ergy loss. The measurement setup con­sisted of a charge-sensitive preamplifier and a shaper amplifier with a shaping time of 1 flS connected to a pulse-height

analyser. Energy calibration of the detection system was carried out using a Si surface barrier diode assumed to have 100% CCE. By using the same gain and shaping time, it is possible to convert from channel number to energy for the Si diode. Further, by correcting for the difference in the mean energy to create an electron-hole pair be-tween Si and GaN, it is possible to assign energies to the peaks of 0.30

the observed GaN spectra. In ~a order to calculate the energy de- 8

> posited within the ~2 microme- ~ 0_25

ters thick SI layer, the SRIM program [ 11] was used to calcu- ~ late the energy deposition versus depth dependence. This integra-tion gave a total energy deposi-

Energy deposited by ionisation

-~ .,.

0.0 0.5 1.0 1.5 2.0 2.5 Depth (~m)

tion of 553 keV within the 2 J..lm detection layer (Fig.2). From this, it was possible to calculate the CCE of the detector.

Figure 2. Energy deposition vs. penetration depth of 5.48 MeV alpha particles in GaN, calculated using the SRIM code.

282

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RESULTS AND DISCUSSION

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J. V Vaitkus et al

The 1-V dependencies presented in Fig. 3 for the Au-SI-GaN-n*-GaN samples irradiated by dif­ferent ionising radiation way show a few peculi­arities of the changes re­lated to the irradiation. One is the very noisy re­verse current and the ini­tial part of the forward current in as-grown sam­ples. This noise current can be related to tunnel­ing through the compli­cated layer structure that creates a percolation in the SI-GaN conductivity. The properties of these samples are analyzed in [12]. The irradiation by X-rays created a load re­sistance that shifted the region of the carriers in­jection to higher forward bias and created defects. Polarization effects are seen in Fig. 3 and cause the I-V characteristics to be time dependent. The irradiation by neutrons transforms the Schottky barrier type of sample into a photoresistor but the introduced defects maintain the high resistiv­ity state of the sample. The analysis of radiation­induced defects and radia­tion damage properties

Semi-Insulating GaNas an Ionizing Radiation Detector 283

will be published elsewhere. All samples were of high enough resistivity to permit testing of the alpha particle induced signals.

Exposure of the GaN samples to 241Am alpha particles resulted in spectra given in Fig. 4-6 for non-irradiated, X-rays- and neutron-irradiated samples,

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respectively. The spectra were recorded at various bias voltages as indicated in the insets.

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Well-defined signals were observed in all unbiased samples, showing that the samples are polarized and an internal field exists. The half-width and

284 J. V V aitkus et al

2000 Fluence 5 x 1014 em·' of fast neutrons

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0 0.0 0.1 0.2 0.3 0.4 0.5 0.6

E MeV Figure 6. Alpha particle pulse height spectr'a from the SI-GaN sample irradiated by fast neutrons (E > 100 keV), fluence Sx 1014 cm-2• The bias voltages are indicated in the inset.

peak position values of the pulse height spectra increase with bias. An in­crease of the bias voltage also leads to growth of the CCE, which in non­irradiated and X-rays irradiated samples reached a value of approximately 93%. In neutron-irradiated sample the CCE was smaller, at around 78%, what is still much better than in other promising materials at the same level of irradiation. A comparison of our results and those presented in [13] is shown in Fig. 7.

100

80

~ 60

~ 0

u.i N <..) 40 (.)

• 20

0 0 1 2 3 4 5 6

F, 1014/cm2

Figure 7. Schematic comparison of CCE dependence on neutron fluence in GaAs according [13] and GaN. The lines are only a guide to the eye.

Semi-Insulating GaNas an Ionizing Radiation Detector 285

So far, the GaN detectors that we have tested have shown very good CCE (~93%) with a small reduction only after very high fluences of neutrons.

The observed change of~ 15% is to be compared to the ~60% reduction observed for GaAs at a similar neutron fluence [13]. These first tests of charge collection in irradiated GaN open a desire to grow thicker (0.05 - 0.2 mm thick) SI-GaN epitaxial layers or wafers for testing material correspond­ing to the requirements of the big experiments related to the Large Hadron Collider (LHC), ATLAS, CMS, LHCb and ALICE [14). The GaN parame­ters (as well as AlN) listed in Table I suggest the possibility of developing a detector system with the working parameters required for an upgrade of the LHC [6,14). The pixel readout time should be of the order of 12 ns. The sig­nal will not be worse than in silicon detectors due to the higher breakdown field and saturation velocity [2]. Also, the radiation hardness of material has to be tested up to 1 MeV neutron equivalent lxl016 cm-2 fluence. The pixel sizes, e.g. for ATLAS detector, have to be 0.04 x 0.04 mm2 and a standard microstrip sensor chip is around 6 x 5 cm2• The total area of ionising radia­tion detectors for the LHC detectors combined has to be more than hundreds square meters. Therefore the design of large-area GaN wafer growth tech­nology and reduction of the wafer cost become important.

4. CONCLUSION

Semi-insulating GaN has to be shown to be promising as a radiation-hard detector of ionizing radiation.

AKNOWLEDGEMENTS

This work has been partly performed in the framework of the CERN RD50 collaboration. The authors express their gratitude to the Royal Society and to PPARC for a long-term grant that supported this work. We thank M.Mikuz and E.Noah for their assistance with the sample irradiations.

REFERENCES

1. Nakamura S. and Chichibu S.F. (Ed.). Introduction to nitride semiconductor blue lasers and light emitting diodes. London: Taylor and Francis, 2000.

2. Bertuccio G., Casiraghi R., Study of SiC for X-ray detection and spectroscopy. IEEE Transactions on Nuclear Science 2003; 50:175-185.

3. Shur M. S. GaN based transistors for high field power application. Solid State Elec­tronics 1998; 42:2131-8.

286

4.

5.

6.

7.

8.

9.

J. V. Vaitkus eta/

Levinshtein M. E., Rumyantsev S. L. and Shur M.S. (Ed.). Properties of Advanced semiconductor materials: GaN, AIN, InN, EN, SiC, SiGe. New York: John Wiley and Sons, 2001. Allam J. "Universal" dependence of avalanche breakdown on bandstructure: choos­ing materials for high-power devices. Jpn. J. Appl. Phys. 1997; 36:1529--42. Moll M., on behalf of the RD50 collaboration. Development of radiation hard sen­sors for very high luminosity colliders - CERN - RD50 project. Computer physics communications, 2003 (to be published). Look D. S., Reynolds D. S., Hemsky J. W., Sizelove J. R., Jones R. L., and Molnar R. 1. Defect donor and acceptor in GaN. Phys. Rev. Lett. 1997; 79:2273-6. Ionascut-Nedlcescu A., Carlone C., Haudayer A., von Bardeleben H.J., Cantin J.-L., Raymond S. Radiation Hardness of Gallium Nitride. IEEE Transactions of Nuclear Science 2002; 49:2733-8. Wang T., Shirahama T., Sun H. B., Wang H. X., Bai J., and Sakai S. Influence of buffer layer and growth temperature on the properties of an undoped GaN layer grown on sapphire substrate by metalorganic vapor deposition. Appl. Phys. Lett. 2000; 76:2220-2.

10. Vaitkus J. V., Gaubas E., Sakai S., Lacroix Y., Wang T., Smith K. M., Rahman M., and Cunningham W. Role of potential barriers in epitaxial layers of semi-insulating GaN layers. Solid State Phenomena 2003; 93-93:301--6.

11. Zeigler J. F., Biersack J.P., IBM-Research, Yorktown Heights, NY, USA, 1996. 12. Vaitkus J., Cunningham W., Gaubas E., Rahman M., Sakai S., Smith K. M., and

Wang T. Semi-insulating GaN and its evaluation for alpha particle detection. Nu­clear Instruments and Methods in Physics Research A 2003; 509:60--4.

13. Bates R. L., DaVia C., D'Auria S., O'Shea V., Raine C., Smith K. M., and RD8 col­laboration. The effects of radiation on gallium arsenide radiation detectors. Nuclear Instruments and Methods in Physics Research A, 1997; 395:54-59.

14. Price M. J. The LHC project. NIMA, 2002; 478: 46--61.

TOWARDS THE HYBRID BIOSENSORS BASED ON BIOCOMPATIBLE CONDUCTING POLYMERS

A. RAMANA VICIENE 1 and A. RAMANA VICIUS * 2•3.4 1 Laboratory of Ecological Immunology, Institute of Immunology of Vilnius University, Moletl{pl. 29, Vilnius, Lithuania 2 Department of Analytical and Environmental Chemistry, Vilnius University, Naugarduko 24, Vilnius, Lithuania 3 Sector ofimmunoanalysis and Informatics, Institute of Immunology of Vilnius University, Moletl{pl. 29, Vilnius, Lithuania 4 Laboratory of Bioanalysis, Institute of Biochemistry, Mokslininkl{ 12, Vilnius Lithuania. *Corresponding author e-mail: arunas@imi.lt and/or arunas.r@bchi.lt

Abstract: The effective combination of biological and physical methods in analytical device could provide the basis for direct detection of wide range of analytes with great sensitivity and specificity. The main aim of this study is to briefly present the current state in construction of biosensors based on conducting polymers. Current state of research on ;r-;r conjugated polymer polypyrrole based biosensors is presented. The biological recognition parts of reviewed biosensors were based on polypyrrole doped or covalently modified by en­zymes and other proteins. Future perspectives of polypyrrole application in hybrid biosensors based on dual (optical and electrochemical) detection system are discussed. Investigations of polypyrrole show that florescence of this tr-tr

conjugated polymer is undetectable. It can be successfully exploited as the immobilization matrix in fluorescence based biosensors, however.

Key words: biosensors, conducting polymers, polypyrrole

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288 A. Ramanaviciene and A. Ramanavicius

1. INTRODUCTION

The most powerful alternative to conventional analytical techniques, har­nessing the specificity and selectivity of biological systems in small, low­cost devices is biosensor technology. Biosensor is described as a compact analytical device, incorporating a biological or biomimetic sensing element to, or integrated within, a transducer system [1]. The detection is based on specific complementary binding or catalytic conversion of analyte of interest by bio-recognition element immobilized on the suitable signal transducer. The specific interaction of analyte with bio-recognition element results in a change of one ore more physicochemical properties (electron transfer, capac­ity, optical properties, etc.); those are detected and can by measured by the signal transducer.

Biosensors and affinity-sensors are usually defined as sensing devices consisting of a biological recognition element in intimate contact with a suit­able transducer, which is able to convert biological recognition reaction or, eventually, the biocatalytic process into a measurable electronic signal. Ma­jor indispensable condition during the creation of affinity-sensors is that one of the able-to-bind reagents is immobilized and at least one must be found in the sample [2]. It means that immobilisation of biologically sensitive com­pounds is one of the main questions during the creation of affinity- and bio­sensors [3]. Here, conducting polymers can be considered as effective im­mobilization materials [ 4]. Polyaniline and polythiophene are often used as electrocatalysts and immobilizers for biomolecules [5]. However, the neces­sity to detect bio-analytes at neutral pH range leads to electro-inactivity of the deposited films, discouraging the use of polyaniline and polythiophene as biosensing materials. Polyacetylene can be synthesized from gas phase only in the presence of the catalysts. Highly sophisticated procedure and instabil­ity of polyacetylene in humid environment makes this polymer less attractive for application in sensor design. Among the other conducting polymers, polypyrrole (Ppy) is one of the most extensively studied materials. Ppy can be easily synthesized by chemical and electrochemical polymerization ap­proaches. This polymer has attractive features, such as excellent conductivity and stability on various substrate materials, even in a neutral pH region. The electrochemical properties of Ppy strongly depend on their redox states, and overoxidation of Ppy occurring at positive potentials leads to lowering of its conductivity as well as to dedoping of anionic molecules. Overoxidized Ppy has been used in some electroanalytical applications that utilize its permse­lectivity. Polypyrrole-based selective and stimulus-responsive biopolymers prove to be promising as new materials in affinity-sensors especially for biomedical application where direct detection of analyte is desirable [6]. Polypyrrole is often used in biosensors and affinity-sensors because of the

Hybrid Biosensors Based on Biocompatible Conducting Polymers 289

best biocompatibility and the easy ways for immobilization of various bio­logically active compounds [7]. From the analytical point of view, Ppy has some very attractive characteristics: (i) is biocompatible and, hence, causes minimal and reversible disturbance to the working environment; (ii) is capa­ble of transducing the energy arising from interaction between immune re­agents into electrical signals that are easily monitored; (iii) protects elec­trodes from fouling and interfering materials such as electroactive anions; (iv) can be modified in situ in a controlled fashion. Characteristics men­tioned above possess great application possibilities of Ppy in affinity-sensors devoted to direct detection of analyte ( 4).

Depending on the method of signal transduction, biosensors are divided into different groups: electrochemical, optical, thermometric, piezoelectric or magnetic. Electrochemical biosensors are the most commonly reported class of biosensors [8]. The main advantages of electrochemical transduction sys­tems are low cost, simple operation and the use of disposable electrodes. Limitations of electrochemical transducers include interference from electro­active compounds and, as usually, low sensitivity. The problems mentioned here can be solved if alternative detection method is applied additionally. The most powerful alternative is optical signal transduction. The optical transducers may be involved to detect the analytical signal simultaneously with the electrical transducer to extend quality/quantity off data received.

The main aim of the work presented here. is to show: (i) current state in application of conducting (;r-;r conjugated) polymers and especially polypyrrole in biosensor design, (ii) perspectives of polypyrrole application in biosensors based on dual detection (optical and electrochemical) system.

2. DISCUSSION

2.1 Discovery and Structure of Conducting Polymers

Conducting polymer polyacetylene was discovered by MacDiarmid, Shira­kawa, and Heeger. They brought the unique properties of conjugated poly­mers to the fore in 1977 when they discovered that chemical doping of these materials resulted in increases in electronic conductivity over several orders of magnitude [9]. Since then, electronically conducting materials based on conjugated (conducting) polymers have been applied in diverse items such as sensors, biomaterials, light-emitting diodes, polymeric actuators, and cor­rosion protection agents. Some conducting polymers like polyaniline, polytiophene or polypyrrole are biocompatible and cause minimal and re­versible disturbance to the working environment and protect electrodes from

290 A. Ramanaviciene and A. Ramanavicius

fouling and/or interfering with electrochemically active materials [1 0]. Cur­rently the potential of these new materials for a wide range of nano-science and nano-technological applications is being demonstrated, as well [11].

General structure of conducting polymers is based on a framework of al­ternating single and double carbon-carbon (sometimes carbon-nitrogen) bonds. Single bonds are referred to as a--bonds, and double bonds contain a a--bond and a Jr-bond. All conjugated polymers have a a--bond backbone of overlapping sp2 hybrid orbitals. The remaining out-of-plane Pz orbitals on the carbon (or nitrogen) atoms overlapped with neighboring Pz orbitals to give IT­

bonds. The chemical structures of these materials usually are represented as sequences of consecutively alternating single and double bonds. In reality, the electrons that constitute the Jr-bonds are delocalized over the entire molecule [12].

2.2 Methods Used for Fabrication of Polypyrrole Films

Polypyrrole is usually synthesized by electrochemical and chemical oxida­tive polymerization techniques.

Chemical polymerization occurs after oxidation of pyrrole monomers and oligomers by oxidators to the cation-radicals those are recombining and forming polymeric structure of polypyrrole. Chemical methods are difficult to use for miniaturization, construction of sensor arrays, or optimization of surface microenvironments.

Electropolymerization is a more modem and elegant method of poly­meric film deposition. It is achieved if an initial electron transfer takes place that permits coupling reactions to occur leading to additional chain growth as additional electrons are transferred. Polypyrrole films are generally formed by electrochemical anodic oxidation of a monomer, and are insoluble, con­ducting or, in some cases, insulating polymer films that coat the electrode surface [13]. This is especially attractive, since the oxidation of monomer solution under the appropriate conditions results in a film deposited on the surface of the electrode, and enables control of growth rate and film thick­ness. The electrochemical formation of conducting-polymer films has found increasing interest in the development of bio- and immuno-sensors since they allow non-manual reproducible formation of modified electrode sur­faces with integrated biological recognition elements [14]. The use of con­ducting polymers for immobilizing a biological compound in sensor applica­tions has the advantage, compared . to conventional immobilization procedures, because the amount of deposited material can be readily con­trolled and the immobilizing matrix can conduct electricity allowing switch­ing between the conducting and isolating states. Moreover, useful copoly­meric structures can be developed if differently modified monomers are

Hybrid Biosensors Based on Biocompatible Conducting Polymers 291

copolymerized [15,16]. Electrochemical Ppy film formation can be per­formed using potential cycling methods, fixed potential techniques, pulsed potential approaches, and galvanostatic techniques.

2.3 Application of Conducting Polymers for Immobilization of Biologically Active Compounds

A number of techniques to immobilize biologically active compounds (BCs) on the electrode surface are available including adsorption, covalent attach­ment, cross-linking and entrapment within polymeric chain. Conducting polymers such as polypyrrole, polythiophene and polyaniline are proved to be the most useful molecular structures for these applications.

Adsorption is the simplest way to immobilize a BC on the surface of an electrode coated by polypyrrole. The electrode can be electrochemically coated by polypyrrole using one of the mostly used electropolymerization methods. By using this method, BC from single proteins (antigens or anti­bodies) uptol whole cells can be adsorbed on the CP surface.

Covalent attachment is the next approach that has been used for immobi­lization of BC on the surface of conducting polymer. Covalent attachment of BC on the surface of CP results in BC activity higher than that obtained pre­viously by adsorption and is responsible for enhanced stability of analytical system during continous measurements. The next very important advantage of this method is ordered BC orientation, which is especially important for efficiency of affinity sensors. In many cases, irreversible adsorption or cova­lent binding of an antigen or antibody does not lead to the effective bio­sensing system due to the instability of BC molecules, low BC loading, or the potential loss of activity during covalent immobilization.

Entrapment of BC molecules within conducting polymer backbone is more favourable approach for sensor design. Polypyrrole is suitable for this purpose because it can be easily prepared on miniaturized components; be­sides, it has a high conductivity and is relatively stable. Usually entrapment of biologically active compounds BC within the polymeric chains of con­ducting polymer is carried out during electrochemical polymerization of monomer in the presence of conducting polymer [ 17]. This method is very attractive because the formation of biologically active layer can be per­formed during "one-step" procedure and is promising for the formation of multi-array biosensors and affinity sensors. On the other hand, polypyrrole is biocompatible and, hence, causes minimal and reversible disturbance to the working environment. Ppy is also capable to transfer energy as electro­chemical transducer. Stabilization of the biological response currently is the major problem, with almost every reported sensor exhibiting a gradual deg-

292 A. Ramanaviciene and A. Ramanavicius

radation in the electrical signal during continuous measurements. This is due to instability ofbiomolecules used in design ofbiosensors. Resolution of this problem and the production of robust designs, vital for medical and envi­ronmental monitoring applications, can be based on creation of synthetic molecular recognition systems. Artificial receptors have been gaining in im­portance as a possible alternative to immobilized biomolecules based sys­tems.

Molecular imprinting is increasingly becoming recognized as a versatile technique for the preparation of artificial receptors based on molecularly im­printed conducting polymers (MIPs) containing tailor-made recognition sites. MIP is another class of substances of great interest in the field of chemical sensor technology. These highly stable synthetic polymers possess molecular recognition properties due to cavities in the polymer matrix that are complementary to the analyte (ligand) both in shape and in positioning of functional groups [18]. It is the reason why development of synthetic recog­nition systems is of great interest to workers in the field of sensor technol­ogy. Moreover, some of these polymers have shown very high selectivity and affinity constants fully comparable to naturally occurring recognition systems, such as antibodies, what makes them especially suitable for use in artificial receptors. Overoxidized polypyrrole exhibits an improved selectiv­ity, which is attributed to the removal of positive charges from Ppy films due to introduction of oxygen functionality, such as carbonyl groups. The nano­pores and nano-cavities complementary to removed dopant can arise during dedoping process. Sensors based on mPpy for serotonin and 1-naphthalensufonate were reported [19]. Molecular imprinting is a technology for the manufacture of synthetic polymers with predetermined molecular recognition properties. The preparation of molecularly imprinted polymers requires polymerization around print species using monomers those are se­lected for their capacity to form specific and definable interactions with the print species. BC molecules entrapped within Ppy can be removed by sol­vent extraction and the molecularly imprinted polymer is ready for use. Cavities are formed in the polymer matrix, which are images of the size and shape of the print molecules. Furthermore, chemical functionalities of the monomer residues become spatially positioned around the cavity in a pat­tern, which is complementary to the chemical structure of the print molecule [20]. These imprints constitute a permanent memory for the print species and enable the imprinted polymer to selectively rebind the print molecule from a mixture of closely related compounds. Finally, the print molecules are re­moved by solvent extraction and the molecularly imprinted polymer is ready to be used. In some instances very high selectivities and affinity constants have been reported, fully comparable to naturally occurring recognition sys­tem such as antibodies. Some of these synthetic polymers have been shown

Hybrid Biosensors Based on Biocompatible Conducting Polymers 293

to be useful in sensor applications, exhibiting tolerance towards acid, base, high temperature, and organic phases.

2.4 General Detection Methods Used in Biosensors

The conversion of the binding event into a measurable signal at low concen­trations of analyte, the regenerability, and the reusability are, among other topics, major challenges in sensor development research. According to ap­plication of additional labels, two main detection types are applied in elec­trochemical sensor design: (i) indirect electrochemical detection - the bind­ing reaction is visualized indirectly via an auxiliary reaction by a labeled compound; (ii) direct detection - no additional electrochemical labels are required. The effective combination of bio-chemistry and electrochemistry in an analytical device could provide the basis of direct electrical detection of wide range of analytes with great sensitivity and specificity [21]. Direct­detection affinity sensors are the most attractive because they require no ad­ditional chemicals. Moreover, it allows real-time measurement without any additional hazardous reagents. For direct conversion of the binding event into measurable signal, one can use: (i) optical, for example, surface plas­mon resonance; (ii) piezoelectric, for instance, quartz crystal microbalance; (iii) surface scanning, like atomic force microscopy (AFM), and (iv) electro­chemical transducers. Because of possibilities to operate in non-transparent solutions, miniaturization, and simple signal transduction, the electrochemi­cal affinity sensors are the most suitable for direct detection of the analyte. In direct affinity sensors, the electrochemical detection of the analyte bind­ing to a biologically active layer can be performed with alternating current (ac), impedance or potential-step methods by measuring differences in capacitance and/or resistance of electrode.

Pulsed amperometric detection (PAD) techniques are such techniques where sensor can be used for analyte detection in static or flow injection mode by applying pulsed potentials between the working electrode (sensor surface) and the reference electrode. In this method changes in charge densi­ties or conductivities are used for transduction and no auxiliary reaction is needed. It seems that the mechanism of the analyte binding reaction at mPpy based electrodes involves the variation in the capacitive properties of the polymer. During interactions of analyte with biologically-active layer, dif­ferences in capacitance and/or resistance arising in electrochemical system can be converted into electrical signals that are easily monitored [22]. The current obtained can be directly related to the concentration of the analyte in solution [23]. Over the past years advances in the application ofPpy for con­struction of bio-sensing components and the fast response times offered by electrochemical techniques has created a demand for fast bio-sensing infor-

294 A. Ramanaviciene and A. Ramanavicius

mation-collecting systems that can be easily integrated into instruments, such as microprocessor based electronics. The PAD detection seems to be one of the most promising among the other electrochemical techniques ap­plied in Ppy based affinity sensors.

2.5 Application Potential of LED Based Transducers in Design of Hybrid Biosensors

Multiple-transducer based biosensors offer great advantages over conven­tional mono-transducers based techniques. The multi-functionality of trans­ducing part of biosensor offers the opportunity for development of highly specific devices for real-time analysis in complex mixtures, without the need for extensive sample pre-treatment or .large sample volumes. Since those biosensors promise to be highly sensitive, rapid, reproducible, simple to­operate and multipurpose analytical tools, they will find great application not only in analytical fields but in scientific investigations as well. Such systems are especially required in the fields of proteomics and bioinformatics where a wide spectrum of information about interacting biological objects is de­sired to be collected in real-time.

In recent years, a key stimulus for the development of optical biosensors has been the availability of high-quality and diverse LED's, fibres and other optoelectronic components. The electrochemical/optical biosensor format may involve direct detection of analyte of interest or indirect detection through labeled probes. The optical transducers may detect changes in fluo­rescence, luminescence, absorbance, polarisation, refractive index etc. The advantages of optical transducers are their speed, the immunity of signal to electrical or magnetic interference and the potential for higher sensitivity and advanced information content. However, several years ago the main draw­back was high cost of some optical instrumentation. Currently, the costs of optical components dramatically decreased, what offers a great opportunity for construction ofbiosensors suitable for mass production.

3. CONCLUSIONS AND OUTLOOK

The background presented illustrates that polypyrrole is very attractive, ver­satile material suitable for preparation of various enzymatic-, immuno-, DNA-sensors. The presented overview of experimental results shows that electrochemical affinity sensor based on molecularly imprinted polypyrrole could have a great potential for direct electrochemical sensing. Since, ac­cording to our initial investigations, fluorescence of polypyrrole is undetect­able, we believe that it can be successfully exploited as the immobilization

Hybrid Biosensors Based on Biocompatible Conducting Polymers 295

matrix in fluorescence based biosensors as well. Registration of bind­ing/desorption of template molecule and quantification of analyte can be per­formed by very simple PAD method. Optical transducers based on fluores­cence detection of protein-protein and DNA-DNA binding events are under development.

ACKNOWLEDGEMENT

This work was financially supported by Lithuanian State Science and Stud­ies Foundation project number C 03047, and COST program Project number 853.

REFERENCES

1. Warsinke A., Benkert A., Scheler F. W. Electrochemical immunoassays. Fresenius J Ana.! Chem. 2000; 366: 622--634.

2. Cook C. J. Nature Biotechnol. 1997; 15: 467-471. 3. Sargent A., Sadik 0. A Electrochim. Acta 1999; 44: 4667-4675. 4. Ramanaviciene A., Ramanavicius A. Crit. Rev. Anal. Chem. 2002; 32: 245-252. 5. Geise R. J., Adams J. M., Barone N. J., Yacynych A. M. Biosens. Bioelectron.

1991; 6:51-160. 6. Wang J., Jiang M., Fortes A., Mukherjee B. Anal. Chim. Acta 1999; 402 (1-2): 7-

12. 7. Kwon I. C., Bae Y. H., Kim S. W. Nature 1991; 28:291-293. 8. Knichel M., Heiduschka P., Beck W., Jung G., Gopel W. Sens. Actuators 1995;

B28: 85-94. 9. Chiang C. K., Fincher C. R., Park Y. W., Heeger A. J., Shirakawa H., Louis E. J.,

Gau S.C., MacDiarmid A. G. Phys. Rev. Lett. 1977; 39: 1098-1101. 10. Ramanavicius A. Biologija 2000; 2: 64--66. 11. Shiigi H., Okamura, K., Kijima, D., Deore, B., Sree, U., Nagaoka, T. J. Electro­

chem. Soc. 2003; 150: H119-H123. 12. Wallace G. G., Dastoor P. C., Officer D. L., Too C. 0. Chem. Innov. 2000; 30: 14-

22. 13. Ramanavicius A., Habermiiller K., Razumiene J., Meskys R., Marcinkeviciene L.,

Bachmatova 1., Csoregi E., Laurinavicius V., Schuhmann W. Prog. Colloid Polym. Sci 2000; 116: 143-148.

14. Ramanavicius A., Habermiiller K., Csoregi E., Laurinavicius V., Schuhmann W. Anal. Chem. 1999; 71:3581-3586.

15. Habermuller K., Ramanavicius A., Laurinavicius V., Schuhmann W. Electroanal. 2000; 12: 1383-1389.

16. Mieliauskiene R.; Kurtinaitiene B., Bachmatova 1., Ramanavicius A. Biologija 2000; 2: 42-44.

17. Ramanavicius A., Kurtinaitiene B., Razumiene J., Laurinavicius V., Meskys R., Rudomanskis R., Bachmatova I., Marcinkeviciene L. Biologija, 1998; Suppl. 1: 15-17.

296

18.

19.

20.

21.

22.

23.

A. Ramanaviciene and A. Ramanavicius

Kroger S., Turner A. P. F., Mosbach K., Haupt K. Anal. Chern. 1999: 71: 3698-3702. Shiigi H., Okamura K., Kijima D., Hironaka A., Deore B., Sree U., Nagaoka T. Electrochem. Solid St. 2003; 6: Hl-H3. Spurlock L. D., Jaramillo A., Praserthdam A., Lewis J., Braither-Toth A. Anal. Chim. Acta 1996: 336: 37-40. Laurinavicius V., Razumiene J., Kurtinaitiene B., Lapenaite 1., Bachmatova 1., Mar­cinkeviciene L., Meskys R., Ramanavicius A. Bioelectrochemistry 2002; 55: 29-32. Ramanaviciene A., Vilkanauskyte A., Acaite J., Ramanavicius, A. Biologija 2000; 2: 67--69. Ramanaviciene A., Vilkanauskyte A., Acaite J., Ramanavicius A. Acta Medica Li­tuanica 2000; 5: 49-59.

OPTICALLY PUMPED UV -BLUE LASERS BASED ON InGaN/GaN/Ah03 AND InGaN/GaN/Si HETEROSTRUCTURES

G. P. YABLONSKII 1, A. L. GURSKI! 1, E. V. LUTSENKO \ V. Z. ZUBIALEVICH 1, V. N. PAVLOVSKII 1, A. S. ANUFRYK \ Y. DIKME 2, H. KAUSCH 2, R. H. JANSEN 2 , B. SCHINELLER 3, and M.HEUKEN 3

1 Stepanov Institute of Physics of NAS Belarus, F. Skaryna Ave. 68, 220072 Minsk, Belarus E-mail: yablon@dragon. bas-net. by 2 lnstitutfur Theoretische Elektrotechnik, RWTH, Aachen, Germany 3 AIXTRON AG, Aachen, Germany

Abstract: Optically pumped lasing in GaN epitaxial layers and InGaN single, multiple and electroluminescence-test quantum well heterostructures grown on sapphire and silicon substrates are investigated as functions of temperature (80-650 K), the excitation density of the nitrogen- and dye laser radiation (10-1100 kW/cm2), excitation and operation wavelengths, and MOVPE growth condi­tions. Laser action was achieved in all types of the heterostructures from the near ultraviolet (370 nm) up to the blue spectral region (470 nm). The lowest laser threshold at room temperature was 35 kW/cm2, the maximal laser power was 80 W, and the half width of the laser line was 0.04 nm. The maximal op­erating temperature of 630 K was for InGaN/GaN/Si lasers. On the base of in­vestigation of the temperature dependence of the laser threshold and photolu­minescence spectra, photoluminescence and laser excitation spectra, conclusions about the role of localized and delocalized states in the optical gain mechanisms were made.

Key words: lasing, InGaN, heterostructures, optical pumping, threshold, gain, buffer lay­ers, MOCVD, sapphire substrate, silicon substrate

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298 G. P. Yablonskii eta!

1. INTRODUCTION

The fabrication of GaN based lasers for green and UV spectral region still remains a problem, in contrast to the light emitting diodes, which already occupy a much more wide spectral region than lasers [1]. The main problem in fabrication of green lasers is the phase separation in InGaN alloy [2,3], while UV lasers suffer mostly from low AlGaN dopability and high activa­tion energies of acceptors [4]. While the violet lasers have lifetime up to 15000 hours and output power of about 30 mW [5], true blue junction lasers have been created only recently and have lifetime only about 2000 hours [ 6]. Thus, another problem connected with the two above-mentioned ones is the short lifetime of devices. In addition, especially for automotive and other applications in high-volume low-cost market segments, the choice of sub­strate is still an open question. Sapphire and SiC are commonly used as sub­strates because of the lack of large-area GaN bulk crystals. Sapphire suffers from a low thermal conductivity worsening the high-power operation of la­sers, light emitting diodes and transistor devices due to heat-removal prob­lems. Silicon carbide has a lower lattice mismatch to GaN and a higher thermal conductivity than sapphire, but is hampered by a very high price and a limited availability for the electrically insulating crystal modification. Sili­con is a promising alternative substrate for GaN growth because of its low cost, excellent quality, large-area availability and the possibility to integrate GaN-based light emitting devices and high-power electronics with Si-based photodetectors and logical circuits. However, the main challenge connected with the use of silicon is the high lattice mismatch. Whereas the growth of GaN on sapphire requires one nucleation layer to form the crossover from sapphire to GaN, the growth of high-quality crack-free GaN on silicon re­quires a sophisticated growth procedure employing a combination of high­and low-temperature GaN and AlGaN layers as well as more than one nu­cleation/recrystallization step [7]. Tensile strain, especially during the cool­down phase after the epitaxial deposition, causes cracks in the nitride layer stack. As a result, the devices grown on Si show usually worse characteris­tics compared to those grown on sapphire substrate. Study of the spontane­ous and stimulated radiation of the structures at optical excitation is an excellent tool to understand the weak points of material and heterostructures and to establish a feedback between the growth technology and laser proper­ties. In this paper, we report on the creation of the optically pumped lasers based on InGaN/GaN MQW heterostructures grown both on sapphire and Si and covering a wide spectral range, and on the study of the properties of these lasers.

Optically Pumped UV-Blue Lasers 299

2. EXPERIMENTAL

All structures were grown in AIXTRON MOVPE reactors on 2-inch (0001)­oriented Ah03 and on (111 )-oriented n-type Si substrates at low pressures (200 mbar or 50 mbar). Trimethylgallium (TMGa), Trimethylaluminum (TMAl), Trimethylindium (TMin), ammonia (NH3) and silane (SiH4) were used as precursors. The thickness of the InGaN active layer was 3 nm, the thickness of the barriers was 4 nm and the thickness of the upper cap layer was 10-50 nm for the MQWs grown on sapphire (MQW/Ab03). The struc­tures grown on Si were free of cracks. Photoluminescence (PL) and lasing were excited by the radiation of a N2 laser (hv= 3.68 eV, l exc = 102-

106 W/cm2, f= 1000Hz, Tp = 8 ns), a He-Cd laser (hv= 3.81 eV) and by radiation of a dye laser with tuning frequency for direct excitation of the quantum wells. The quantum energy of the dye laser was lower than the band gap value of GaN. Figures 1 shows the typical design of the InGaN MQW/Ah03 and InGaN MQW/Si heterostructures under study, respec­tively.

Jt GaN cap layer

~~~~~~]f~~ lnGaN barrier layers

lnGaN quantum AI,O, substrate well layers

a)

Jt G N cap layer ~==-====sri lnGaN

b)

arrier layers lnGaN quantum

well layers strain-reducing stack of layers

Figure 1. Design oflnGaN/GaN heterostructures grown on sapphire (a) and silicon (b) sub­strate

3. RESULTS AND DISCUSSION

Figure 2 shows the laser spectra of all studied heterostructures (both grown on Si and sapphire) excited by N2 laser radiation at room temperature. The spectra are seen to span over the spectral interval from the near-ultraviolet up to the blue region. The lasers based on InGaN/GaN SQW and GaN/AlGaN single heterostructures grown on sapphire cover the spectral region from A.= 370 nm up to A. = 470 nm. The lasers based on the structures grown on Si cover the region from A.= 383 nm up to A. = 462 nm. The best laser parameters were reached for the InGaN/GaN MQW/Ah03 "violet"

300 G. P. Yablonskii et al

(A.= 400-450 run) lasers. The laser action was achieved up to very high tem­perature Tmax = 580 K. The minimal laser threshold at room temperature was lthr = 35 kW/cm2, the full width at half maximum (FWHM) of the laser line near the laser threshold was 0.04 run, the pulse energy was E = 630 nJ, the pulsed power was P = 80 W and the characteristic temperature was T0 = 164 Kat T= 200-500 K and T0 = 530 Kat T= 80-200 K. The laser parameters of the "blue" lasers were the following: A,= 450-470 run, FWHM = 0.05 run, Tmax = 460 K, lthr = 50 kW/cm2, E = 300 nJ, P = 40 W.

Lasers based on the MQWs grown on Si have operating temperatures from 300 K to 630 K, characteristic temperatures T0 = 200 K at T < 500 K and T0 =55 Kat T> 500 K, 11hr = 30-50 kW/cm2 and P = 30 W at 300 K. It should be noted that the obtained threshold values for the structures grown on Si are only by an order of magnitude higher than the lowest values ob­tained for optimized structures grown on bulk GaN substrates (2.4-5.8 kW/cm2 [8]).

On the base of measurements of the lasing, stimulated emission and PL spectra as functions of temperature and excitation intensity, it was concluded that at high excitation intensity Uexc > 1000 kW/cm2) a considerable thermal overheating !!.T of about 100 K of the active region takes place. It was shown that it is exclusively due to inherent InGaN/GaN laser radiation which power density on the laser mirrors was evaluated to be I> 5 MW/cm2•

The spectral-angular distributions of laser emission in the InGaN/GaN MQW on sapphire substrates at T = 300 K and at the excitation density Iexc zlthr (the threshold value) and at Iexc = 3.2Irhr are shown in Fig. 3. In this case, the cavity length was about 230 run. From these patterns one can con­clude, based on the calculations of the optical confinement factor r, that lasing takes place in high-order transverse modes. Numerical modeling showed that for heterostructures containing 5 QWs, the optical confinement factor r has its maximal value (about 4%) for closed and leaked modes. With increasing the wavelength A., the value of r increases for the fixed mode order. For heterostructures with 10 QWs, the value of r is about 6% for modes of the 9-1 Oth order at A, = 3 90 nm. For longer wavelengths, the maximum corresponds to the lower-order modes (the 7th order at A,= 470 nm). From the spectral-angular distribution oflaser emission for InGaN/GaN MQW grown on Si (Fig. 4) one may conclude (and it is confirmed by the preliminary calculations) that the optimum conditions for lasing take place for low-order modes (1st-4th order). Figure 5 shows the power characteris­tics of the 449-run InGaN/GaN/Si MQW laser at different excitation levels. The maximum pulse energy achieved was 240 nJ, and the external laser quantum efficiency was 5%. This value is even higher than the maximal val­ues we observed for InGaN/GaN MQWs grown on sapphire (about 3-4% for lasing wavelengths around 430 nm, for longer wavelengths the values were

Optically Pumped UV-Blue Lasers 301

smaller). This is evidence that the use of the set of several strain-reducing layer stacks between the Si substrate and GaN heterostructure permits achieving of the results comparable or even better than those for laser het­erostructures grown on sapphire.

:::::1

~ >.

:!::: (/) c Q.) -c c 0

' (j) .!a E Q.) .... Q.) (/) ro _J

Wavelength [nm] 460 440 420 400 380 360

Blue I Vlolet I UVA

Jl IT=300 Kl \.

l lnGaN/GaN MQW J GaN -!!! :c a. a. tiS

II "' c: 0

.11111\..J .\l -2.6 2 .7 2 .8 2 .9 3.0 3.1 3.2 3.3 3.4

Energy [eV]

Figure 2. Laser spectra of the studied MQW heterostructures grown on Si (top) and on sap­phire (bottom).

I =I e:ac: thr I = 3.2*1

eac thr

FWHM = 0.04 nm

a) b)

Figure 3. 3D-plots of the spectral-angular distribution of the intensity of laser radiation from lnGaN/GaN MQW structure grown on sapphire at the threshold value of the excitation den­sity (a) and above the threshold (b).

302 G. P. Yablonskii et al

l lnGaN/GaN/Si MQW I 439

E' .:.. 2.83 ~ .s::. 438 - ...... Cl >-c: Cl Q) ...

Q) Q) c:

~ 437 w 2.84

436 -40 -20 0 20 40

Angle[ degree]

Figure 4. Spectral-angular distribution of output radiation of the InGaN/GaN MQW laser grown on Si. 0 degrees corresponds to radiation in the plane ofMQW.

Figure 5. Laser pulse energy (triangles) and external quantum efficiency (circles) of the InGaN/GaN/Si MQW laser at different excitation levels.

4. CONCLUSIONS

Optically pumped lasing was achieved and investigated in a large number of InGaN/GaN MQW heterostructures grown both on sapphire and silicon sub­strates. The spectral range of lasing was 3 70-4 70 nm for heterostructures grown on sapphire and 385-460 nm for structures grown on Si. Lasing was

Optically Pumped UV-Blue Lasers 303

observed at T= 80-550 K for sapphire-grown structures, and at T= 300-630 K for structures grown on Si. The maximum observed output power at room temperature was 40 Wand 30 W, respectively.

While in MQW heterostructures grown on sapphire lasing takes place on the high order transverse and leaky modes, in heterostructures grown on Si the main operating modes were first-third order transverse modes.

The external quantum efficiency of MQW lnGaN/GaN heterostructure grown on Si was about 5% which is higher than that measured for MQW heterostructures grown on sapphire. Thus, the use of the set of strain­reducing layer stacks containing low temperature AlN layers and graded composition layers for growth of MQW heterostructures on Si allows to achieve laser parameters comparable or better than those for analogous structures grown on sapphire. The achieved values of thresholds were 30-50 kW/cm2, the estimated characteristic temperatures were 55 K above 500 K and about 200 K at lower temperatures.

ACKNOWLEDGEMENTS

This work was partially supported by the ISTC grant B-176.

REFERENCES

I. Morkoc H., Nitride Semiconductors And Devices. Berlin/Heidelberg/New York: Springer, 1999, Chapter II.

2. 3. Singh R., Dappalapudi D., Maussakas T. D., Romano L. T. Phase separation in In­

GaN thick films and formation of InGaN/GaN double heterostructures in the entire alloy composition. Appl. Phys. Lett. 1997; 70: 1089-1091.

4. Romano L.T., McCluskey M. D., Van de Walle C. G., Northrup J. E., Bour D. P., Kneissl M., Suski T., Jun J. Phase separation in InGaN multiple quantum wells an­nealed at high nitrogen pressures. Appl. Phys. Lett. 1999; 75: 3950-3952.

5. Kozodoy P., Hansen M., DenBaars S. P., Mishra U.K. Enhanced Mg doping effi­ciency in Al0.2Gao.8N/GaN superlattices. Appl. Phys. Lett. 1999; 74: 3681-3683.

6. Nagahama S., Yanamoto T., Sano M., Mukai T. Characteristics of Laser Diodes Composed ofGaN-Based Semiconductor. Phys. Stat. Sol. (a) 2002 190: 235-246.

7. Nagahama S., Yanamoto T., Sano M., Mukai T. Blue-violet nitride lasers. Phys. Stat. Sol. (a) 2002; 194: 423-427.

8. Dadgar, A., Blasing, J., Diez, A., Alam, A., Heuken, M., Krost, A. Metalorganic chemical vapour phase epitaxy of crack-free GaN on Si (Ill) exceeding I f.!m in thickness. Jpn. J. Appl. Phys. 2000; 39: Lll83-Lll85.

9. Ivanov V.Yu., Godlevski M., Teisseyre H., Perlin P., Czemecki R., Prystavko P., Leszczynski M., Grzegory 1., Suski T., Porovski S. Ultralow threshold powers for optical pumping of homoepitaxial InGaN/GaN/AlGaN lasers. Appl. Phys. Lett. 2002;81: 3735-3737.

Key Word Index

action spectrum, 161 AlGalnN/nitride-alloys, 161 AlGaN, 223, 233 AlGaN/GaN biosensors, 143 AIGaN/GaN heterostructures, 15 AllnGaN, 41 aluminum gallium nitride, I, 59, 239 ambient lighting, 253 atomic spectroscopy, 271 biophotonics, 161 biosensors, 287 buffer layers, 199, 297 built-in electric field, 41 built-in electric fields, 215 cathodoluminescence, 247 CdTe, 93 charge collection efficiency, 279 chip-size, 253 conducting polymers, 287 diffusion, 93 donor-acceptor molecule, 261 electron capture time, 207 electron-hole plasma, 207 epitaxial growth, 59 epitaxial lateral overgrowth, 18 9 erythema, 161 exciton localization, 41 fluorescence, 261 fluorescence sensing, spectroscopy, 127 four-wave mixing, 93

305

GaAs, 93 GaN,93, 179,199,207,233,247 GaN polarity, 179 GaN/nitrides, 161 general lighting, 253 group-III nitrides, 31 heat dissipation, 31 heterostructures, 41, 93,297 HFET, 233 high pressure experiments, 215 high-power-SMD-LED, 253 host materials, 111 HVPE, 15, 189 IBICC, 77 III -nitride semiconductors, 179 III -nitrides, 41, 179 illumination, 253 InAlGaN, 179 InGaN, 93, 179, 297 InGaN/GaN MQWs, 207 integration density, 253 ionizing radiation detectors, 279 laser diode, 24 7 laser-diodes, 31 lasing, 199, 297 LED-module, 253 light source, 253 light-emitting diodes, 127 luminescence decay, 207 MBE, 179

306

medical application, 253 Metal-Semiconductor-Metal detectors, 77 MOCVD, 199, 223, 297 molecular spectroscopy, 271 MOVPE, 189 MSM, 233 narrow band UV detectors, 143 N-face GaN, 179 nonequilibrium carriers, 93 optical measurements, 127 optical nonlinearities, 93 optical output, 253 optical pumping, 199, 297 optical waveguiding, 31 PALE, 59 photoluminescence, 207 photon cascade emission process, Ill piezoelectric polarization, 215 plasma-assisted MBE, 179 polypyrrole, 287 power dissipation, 253 Pr3,111 quantum cutting, Ill quantum wells, 161,215 quaternary alloys, 179 quaternary compounds, 41 quaternary InAIGaN compounds, 215 radiation hardness, 279 recombination, 93 red skin, 161 sapphire nitridation, 179 sapphire substrate, 297 Schottky barriers, 233 screening of built-in field, 207

self-assembled mono layers, 261 semi-insulating GaN, 279 silicon substrate, 297 SiO, 199 solar blind photodetectors, 1 solar radiation, 161 solid-state lighting, 253 strain relief, 59 stress management in (Al,Ga)N, 77 superlattice, 223 surface acoustic wave, 239 surface potential, 261 surface recombination velocity, 93 switching effect, 261 thermal management, 253 thermal resistance, 253 thin isolating layer, 253 threading dislocations, 189 threshold powers, 24 7 threshold, gain, 199, 297 time-resolved spectroscopy, 215 TL lighting, Ill ultraviolet LEOs, I ultraviolet sensor, 239 UV damage, 161 UV detectors, 189, 233 UV emitter, 59 UV LED, 15,271 UV light emitting diodes, 143 UV photodetectors, 161 UV solar blind detectors, 77 wide-band-gap semiconductors, 41 ZnTe, 93

Author Index

Anceau, S., 215 Androulidaki, M., 179 Anufryk, A. S., 297 Aoyagi, Y., 215 AsifKhan, M., 41, 59, 239 Aujol, E., 189 Aumiler, D., 271 Ban, T., 271 Beaumont, B., 189 Besulkin, A. 1., 223 Boratynski, B., 233 Bottcher, T., 31 Ciplys, D., 239 Cunningham, W., 279 Czemecki, R., 24 7 Dikme, Y., 199, 297 Dimakis, E., 179 Dmitriev, A., 15 Dorenbos, P ., Ill Duboz, J- Y., 77 Einfeldt, S., 31 Faurie, J-P., 189 Figge, S., 31 Fomin, A. V., 223 Fonavs, E., 261 Frayssinet, E., 189

307

Gaska, R., 59, 127, 239 Georgakilas, A., 179 Gerca, L., 261 Gibart, P., 189 Godlewski, M., 247 Grandjean, N., 77 Grzegory, 1., 247 Gurskii, A. L., 199, 297 Heuken, M., 199,297 Hirayama, H., 215 Hirsch, L., 77 Hommel, D., 31 Ivanov, V. Yu., 247 Jansen, R. H., 199, 297 Jarasiiinas, K., 93 Jursenas, S., 199,207, 261 Kalisch, H., 199, 297 Karpicz, R., 261 Karpov, S. Yu., 15 Kazlauskas, K., 199 Kirichenko, N., 261 Konczewicz, L., 215 Kovalenkov, 0. V., 15 Kuokstis, E., 41 KurilCik, G., 207 Lefebvre, P., 215

308

Lepkowski, S. P., 215 Leszczynski, M., 247 Lundin, W. V., 223 Lutsenko, E. V., 199, 297 Mahlkow, A., 253 Melnik, Yu., 15 Miasojedovas, S., 207 Mosca, M., 77 Munoz, E., 161 Muzikante, 1., 261 Neilands, 0., 261 Omnes, F., 77 Pau, J. L., 161 Pavlovskii, V. N., 199, 297 Pechnikov, A. 1., 15 Perlin, P., 215, 247 Pichler, G., 271 Porowski, S., 247 Prystawko, P ., 24 7 Rahman, M., 279 Ramanaviciene, A., 287 Ramanavicius, A., 287 Reverchon, J- L., 77 Rimeika, R., 239 Rivera, C., 161 Sakai, S., 279 Sakharov, A. V., 223 Schineller, B., 199,297

Semond, F., 77 Sereika, A., 239 Shapovalova, E., 15 Shur, M.S., 1, 59, 127, 239 Sizov, D. S., 223 Skenderovic, H., 271 Smith, K. M., 279 Soukhoveev, V. A., 15 Stutzmann, M., 143 Suski, T., 215, 247 Szymakowski, A., 199 Tamulaitis, G., 41, 199 Teisseyre, H., 215,247 Tlaczala, M., 233 Tsagaraki, K., 179 Tsatsul'nikov, A. F., 223 Usikov, A. S., 15 Vaitkus, J. V., 279 Valiokas, R., 261 Valkunas, L., 261 VanderKolk, E., 111 Van Eijk, C. W. E., 111 Vink, A. P., 111 Yablonskii, G. P., 199,297 Yang, J., 239 Zavarin, E. E., 223 Zubialevich, V. Z., 199, 297 Zukauskas, A., 1, 127, 199,207