ISSN 1063�7826, Semiconductors, 2010, Vol. 44, No. 6, pp. 808–811. © Pleiades Publishing, Ltd., 2010.Original Russian Text © A.F. Tsatsulnikov, W.V. Lundin, A.V. Sakharov, E.E. Zavarin, S.O. Usov, A.E. Nikolaev, N.V. Kryzhanovskaya, M.A. Synitsin, V.S. Sizov, A.L. Zakgeim,M.N. Mizerov, 2010, published in Fizika i Tekhnika Poluprovodnikov, 2010, Vol. 44, No. 6, pp. 837–840.

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1. INTRODUCTION

The increased brightness and efficiency of galliumnitride light�emitting diodes and the fact that thesedevices cover almost the entire visible spectral rangehave resulted in their application as effective lightemitters not only in indication and backlight systemsor signaling apparatus, but also for general lightingpurposes. To date, solid�state light sources have beenfabricated on the basis of phosphor�coated LEDs witha luminous efficacy (LE) of more than 160 lm W–1,which exceeds the luminous efficacy of the conven�tional incandescent lamps by more than a factor of 10and is twice that of mercury fluorescent lamps. Thedisadvantages of light sources of this kind include theStokes loss in phosphors, which limits the luminousefficacy, and the impossibility of dynamic (in thecourse of operation) control over the color parametersof white light.

Of interest in this context is development of a newgeneration of high�efficiency light sources by mixingemission from different�color LEDs (polychromicsources). Using this approach, it is possible to obtainnearly all really existing colors with multiple�crystalLED sources and, which is particularly important, ahigh�quality white light (color�rendering indexCRI > 90) in a wide range of color temperatures(2500–20000 K). Development of monolithic poly�chromatic white�light sources is a further elaborationof this approach. Because of the simpler fabricationtechnology (no need for hybrid assembly of severalcrystals), light sources of this kind must have, in addi�

tion to a higher efficiency, also a lower cost at a betterreliability.

Several approaches to fabrication of monolithicwhite�light sources have been suggested. The mostwidely occurring variant is that with an active regionbased on two or three InGaN quantum wells (QWs)emitting at different wavelengths [1–5]. Studies ofstructures containing a set of InGaN QWs emitting inthe blue and green (510–530 nm) spectral ranges haveshown that this combination can produce white lightwith an unacceptably low CRI [2, 5]. In [3, 4], dichro�matic white�light sources were produced based onmixing of light with wavelengths of 450 and 560 nm,which made it possible to raise the CRI, but the emis�sion efficiency was low. The main reason for the pooroverall efficiency of monolithic white LEDs is the lowinternal quantum efficiency of emission from InGaNQWs with high content of In. Another problem to besolved when developing monolithic white�lightsources consists in providing an effective carrier injec�tion into various QWs.

In this study, a new approach was suggested toimproving the uniformity of carrier injection into theactive region of a monolithic white LED, which con�tains several InGaN QWs emitting in the blue(440 nm) and green (540–570 nm) spectral ranges.This approach is based on the use of a short�periodInGaN/GaN superlattice as a barrier layer betweenthe InGaN QWs. Experiments were carried out withepitaxial growth of structures of this kind and their optical

A Monolithic White LED with an Active Region Based on InGaN QWs Separated by Short�Period InGaN/GaN SuperlatticesA. F. Tsatsulnikova, b^, W. V. Lundina, b, A. V. Sakharova, b, E. E. Zavarina, b, S. O. Usova, b,

A. E. Nikolaeva, b, N. V. Kryzhanovskayab, c, M. A. Synitsinb, c, V. S. Sizova, b, A. L. Zakgeimb, and M. N. Mizerovb

aIoffe Physicotechnical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia^e�mail: andrew@beam.ioffe.ru

bScientific–Technological Center for Microelectronics and Submicrometer Heterostructures, Ioffe Physicotechnical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia

cSt.�Petersburg Physics and Technology Centre for Research and Education, Russian Academy of Sciences, St. Petersburg, 195220 Russia

Submitted November 17, 2009; accepted for publication November 23, 2009

Abstract—A new approach to development of effective monolithic white�light emitters is described based onusing a short�period InGaN/GaN superlattice as a barrier layer in the active region of LED structuresbetween InGaN quantum wells emitting in the blue and yellow�green spectral ranges. The optical propertiesof structures of this kind have been studied, and it is demonstrated that the use of such a superlattice makes itpossible to obtain effective emission from the active region.

DOI: 10.1134/S1063782610060205

PHYSICS OF SEMICONDUCTOR DEVICES

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A MONOLITHIC WHITE LED WITH AN ACTIVE REGION 809

properties were examined by the methods of photolumi�nescence (PL) and electroluminescence (EL).

2. EXPERIMENTAL

The epitaxial structures were grown by MOVPE on(0001) sapphire substrates. Ammonia, trimethylgal�lium (TMG), triethylgallium (TEG), trimethylin�dium (TMI), trimethylaluminum, monosilane, andmagnesium bispentacyclodienyl were used as sourcecompounds. Nitrogen, hydrogen, and mixturesthereof served as carrier gases. The gases were purifiedby the corresponding Aeronex purifiers.

The active regions of the structures contained two3�nm�wide InGaN QWs emitting in the blue spectralrange (440–450 nm) separated by a GaN barrier witha thickness of ~10 nm and an ~3�nm�wide InGaN QWemitting in the green spectral range (540–570 nm).The QWs emitting in the blue and green spectral rangeswere separated by a 12�period InGaN/GaN superlat�tice with InGaN and GaN layer thicknesses of ~1 nm.The active region of the LED structure is detailed inFig. 1. The procedure used to form the short�periodInGaN/GaN superlattice and its properties and theeffect of this superlattice on the properties of LEDstructures were specified in [6].

3. RESULTS AND DISCUSSION

Figure 2a shows EL spectra of the structures.Structures 1 and 2 differ in growth conditions, whichleads to different positions of the “green” band in theemission spectrum. It can be seen in Fig. 2a that theratio between the intensities of the “blue” and “green”bands appreciably varies with the position of the“green” band. The external quantum efficiencies ofboth structures are close (Fig. 2b). For comparison,Fig. 2b also shows dependences of the external quan�tum efficiency on current for a “blue” (430–450 nm)LED structure (curve 3) with an active region contain�ing two InGaN QWs similar to those grown in the

structure under study and for a “green” (540–560 nm)LED structure (curve 4) with an active region contain�ing a single InGaN QW also similar to that grown inthe structure under study. It can be seen from thedependences in Fig. 2b that the emission intensity inthe monolithic LED structure under study is approxi�mately half the intensity of emission from the LEDstructure for the green spectral range. This effect is dueto a substantial increase in the total content of indiumin the active region, which presumably leads to addi�tional defect formation. This conclusion is indirectlyconfirmed by the fact that, for structure 1, the maxi�mum quantum efficiency is observed at low currents,whereas, for structure 2, in which the In content of the“green” QW is higher, the external quantum efficiencyincreases with the current, which may be indicative ofa higher defect density. It should, however, be men�tioned that no catastrophic decrease in the quantum

p�AlGaN

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Fig. 1. Schematic of the active region of the structuresunder study. 700650600550500450400

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TSATSULNIKOV et al.

efficiency of emission is observed, despite the increasein the total content of indium in the active region ofthe monolithic LEDs under study. Upon the process�ing and assembly, the external quantum efficiency ofthe blue and green LEDs was ~25 and ~8%, respec�tively. With this circumstance taken into account, itshould be expected that, without using technologicalprocedures that can raise the extraction of light, theexternal quantum efficiency of the structure understudy should be 4–5%.

As already noted, to create monolithic light sourcescontaining several InGaN QWs, it is necessary to pro�vide carrier injection into all the QWs. In the case ofa nonuniform injection, the emission intensitydecreases as the distance between the InGaN QW andthe p�doped region becomes larger. This effect resultsin the fact that the emission intensity from these layersdepends on current and, consequently, so do the colorparameters of the monolithic white�light sources. Inthis study, a short�period InGaN/GaN superlatticewas deposited as a barrier between the InGaN QWsemitting in the blue and green spectral ranges. Asshown in [6], such a superlattice can improve the effi�ciency of carrier injection into InGaN QWs situatedfar from the p�doped region, with the emission effi�ciency from LED structures raised. Figure 3 showshow the ratio between the emission intensities of the“green” and “blue” bands depends on current for thestructures under study. It can be seen that, for structure 1the emission spectrum of which is dominated by a“green” band peaked at 540 nm, a rather fast levelingoff of the “green” band emission intensity is observedwith increasing current. This results in the fact that, asthe current increases, the ratio between the intensitiesof the “blue” and “green” bands varies rather widely:from ~80 at low currents to ~5 in the range of high cur�rents. For structure 1b, this ratio is nearly current�

independent and has values of 1–1.2 at currents in therange 10–100 mA. Finding the causes of such a pro�nounced difference between the dependences of theemission intensities of the “blue” and “green” bandson current requires further study of the crystalline,optical, and electrical properties of such structures.

That the intensity ratio of the “blue” and “green”bands depends on current results in a current depen�dence of the chromaticity coordinates (Fig. 4). It canbe seen that the emission from sample 2 lies in thewhite�light region. The correlated color temperature(CCT) varies from 10000 to 6000 K as the current israised to 20 mA and further depends on the currentonly slightly (Fig. 5). The optical efficacy (lumen�equivalent) of the resulting white light is as high as300 lm W–1.

100806040200Current, mA

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Fig. 3. Ratio between the peak intensities of the “green”and “blue” emission lines vs. current for structures 1aand 1b. Dashed line, ratio equal to unity.

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Fig. 4. 1931 CIE chromaticity diagrams. Points show chro�maticity coordinates at various currents for the emissionspectra of the samples under study: (1) sample 1a and(2) sample 1b.

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SEMICONDUCTORS Vol. 44 No. 6 2010

A MONOLITHIC WHITE LED WITH AN ACTIVE REGION 811

The emission from structure 2 has a small CRIbecause of the absence of emission bands located inthe red and near�green (~500 nm) spectral ranges. TheCRI can be raised by fabricating additional InGaNQWs that emit in the green spectral range, and also byshifting the emission of one of InGaN QWs to the yel�low�red spectral range (either by increasing the Incontent of the layer, or by making the emission bandbroader).

In this study, LED structures for monolithic white�light sources with an active region containing InGaNQWs emitting in the blue and green spectral rangesseparated by a short�period InGaN/GaN superlatticewere grown and studied. It was demonstrated thatwhite light with a CCT of ~6000 K can be obtained foran active region containing InGaN QWs with emissionbands peaked at 430 and 570 nm. Variation of the cur�rent through such a structure in the range 20–100 mAdoes not lead to significant changes in the intensityratio of the “blue” and “green” bands, which mani�fests itself in a comparatively weak dependence of theCCT on current.

ACKNOWLEDGMENTS

The study was supported by the Russian Founda�tion for Basic Research, project no. 09�02�12449�ofi_m,

“Monolithic Injection Sources of White Light Basedon Self�Organized InGaN Quantum Dots.”

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6. A. F. Tsatsul’nikov, V. V. Lundin, A. V. Sakharov,E. E. Zavarin, S. O. Usov, A. E. Nikolaev, N. A. Cher�kashin, B. Ya. Ber, D. Yu. Kazantsev, M. N. Mizerov,H. S. Park, M. Hytch, and F. Hue, Fiz. Tekh. Polupro�vodn. 44, 96 (2010) [Semiconductors 44, 93 (2010)].

Translated by M. Tagirdzhanov