Journal of Crystal Growth 244 (2002) 6–11

Growth of high-quality GaN films on epitaxial AlN/sapphiretemplates by MOVPE

Masahiro Sakaia,*,1, Hiroyasu Ishikawaa, Takashi Egawaa, Takashi Jimboa,Masayoshi Umenoa,2, Tomohiko Shibatab, Keiichiro Asaib, Shigeaki Sumiyab,

Yoshitaka Kuraokab, Mitsuhiro Tanakab, Osamu Odab

aResearch Center for Micro-Structure Devices, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, JapanbThin Film Application Project, NGK INSULATORS, LTD., 2-56 Suda-cho, Mizuho-ku, Nagoya 467-8530, Japan

Received 14 January 2002; accepted 17 June 2002

Communicated by D.T.J. Hurle

Abstract

High-quality GaN films were grown on epitaxial AlN/sapphire templates by metal-organic vapor phase epitaxy. The

Hall mobility as high as 790 cm2/V s with the carrier concentration of 7.6� 1016 cm�3 at 300K along with low

dislocation density of 5� 107 cm�2 have been achieved. The X-ray rocking curve full-width at half-maximum were 61

and 232 arcsec for the (0 0 0 4) and (2 0 %2 4) reflections, respectively. Atomic force microscopy images showed smooth

surface morphology and clear step formation. Additionally, pit terminations could be hardly found. Photoluminescence

measurement revealed a higher intensity near the band edge.

r 2002 Elsevier Science B.V. All rights reserved.

PACS: 81.15

Keywords: A1. Characterization; A3. Metalorganic vapor phase epitaxy; B1. Nitrides; B2. Semiconducting III–V materials

1. Introduction

Gallium nitride (GaN) is an attractive materialfor the applications in visible to ultravioletoptoelectronics, high-power, high-temperatureand high-frequency electron devices. Sapphiresubstrates have been widely used for the growth

of GaN films. It is, however, difficult to grow high-quality GaN films because of lattice constant andthermal expansion coefficient mismatches betweenGaN and sapphire. Currently, the most popularmethod of epitaxial growth of GaN is to use thinaluminum nitride (AlN) or GaN films deposited atlow temperatures on sapphire substrates as thebuffer layer [1–5]. The GaN films grown onsapphire substrates with low temperature bufferlayers (LT-BLs) show high dislocation densitiesand residual stresses.

In this study, we show that high-qualityepitaxial AlN films grown on sapphire substrate

*Corresponding author.

E-mail address: m-sakai@ngk.co.jp (M. Sakai).1On leave from NGK INSULATORS, LTD.2Present address: Department of Electronic Engineering,

Chubu University, Matsumoto-cho, Kasugai 487-8501, Japan.

0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 5 7 3 - 7

at high temperature higher than 10001C arepromising templates for GaN epitaxial growth.When epitaxial AlN/sapphire templates are usedas substrates instead of LT-BLs, the overgrownGaN layers are expected to show higher crystalqualities than those grown on LT-BLs because theepitaxial AlN/sapphire templates are single crys-talline.

Yoshida et al. have first grown GaN film onAlN films which had been grown at high tem-peratures by molecular beam epitaxy (MBE), andreported that Hall mobilities and peak intensity ofcathodoluminescence was improved [6]. However,the crystal quality was not yet satisfactory. Ohbaet al. have grown GaN films on AlN films bymetal-organic vapor phase epitaxy (MOVPE) andreported the atomic force microscopy (AFM)images [7]. However, pits were often observed onthe surface of AlN films and the crystal quality ofthe GaN films was not precisely studied.

We have recently reported that the crystalquality of epitaxial AlN films on sapphire can belargely improved and have realized high-qualityepitaxial AlN films with the atomically flat surfacewhich have no pit and with the dislocation densityof approximately 1010 cm�2 [8]. In this paper, wehave for the first time clarified that electrical,crystal, optical qualities of GaN films grown onthe high-quality epitaxial AlN films can be largelyimproved.

2. Experimental procedure

Epitaxial AlN films on two-inch-diameter C-plane sapphire substrates were grown by horizon-tal low-pressure MOVPE. Trimethylaluminum(TMA) and ammonia (NH3) were used as Al andN sources, respectively. The substrates were heatedat a temperature higher than 10001C to growapproximately 1-mm-thick AlN films [8]. Subse-quently, Si-doped approximately 2.8-mm-thickGaN films were grown in another horizontalatmospheric-pressure MOVPE system on theseAlN/sapphire templates without any low-tempera-ture buffer layers. Trimethylgallium (TMG) andammonia were used as Ga and N sources, andmonosilane (SiH4) was used as the n-type dopant.

The growth conditions were the same as whenGaN films were grown on sapphire substrates withLT-BLs. For comparison, GaN films were grownon two-inch-diameter C-plane sapphire substrateswith low-temperature GaN buffer layers by thesame horizontal atmospheric-pressure MOVPE asthat used for GaN growth on AlN/sapphiretemplates. The epitaxial layers consist of a 30-nm-thick GaN nucleation layer grown at 5001Cand a Si-doped approximately 2.8-mm-thick GaNlayer.

Hall effect measurement was performed usingthe van der Pauw technique. X-ray diffraction(XRD) measurement was performed to confirmthe crystal quality. The surface morphology wasanalyzed using AFM. For investigating the struc-tural quality of the films cross-sectional and plantransmission electron microscopy (TEM) wasapplied. For characterizing the optical properties,room-temperature photoluminescence (PL) wasmeasured using the 325 nm line of a He–Cd laseras an excitation light source and a photomultipliertube with a GaAs photocathode as the detector.

3. Results and discussion

3.1. Electrical properties

Fig. 1 shows the Hall mobilities of undoped orimpurity-doped GaN films as a function of thecarrier concentration at room temperature. Rec-tangle dots, which are our data, show the Hallmobilities of Si-doped GaN films grown on AlN/sapphire templates. The depth profile of carrierconcentrations was confirmed to be constant bythe C–V method. The highest Hall mobility were790 cm2/V s with an electron carrier concentrationnc of 7.6� 1016 cm�3 at 300K, and 1454 cm2/V swith nc of 1.8� 1016 cm�3 at 77K. Diamond andcircle dots show Nakamura et al.’s data [9,10]. TheHall mobilities of GaN films grown on AlN/sapphire templates were higher than those of GaNfilms grown on sapphire substrates with LT-BLs ateach carrier concentration. These mobilities arethe largest compared to previously reported valueswith the similar electron carrier concentrations.Thus, the crystal quality of GaN films grown on

M. Sakai et al. / Journal of Crystal Growth 244 (2002) 6–11 7

high-quality AlN/sapphire templates can be con-cluded to be superior to those grown on sapphiresubstrates with conventional LT-BLs.

Hall mobilities and carrier concentrations ofGaN films grown on AlN/sapphire templates areshown as a function of temperature in Fig. 2(a)and (b), respectively. The maximum Hall mobilitywas 1797 cm2/V s with the electron carrier concen-tration of 3.8� 1016 cm�3 at 127K. The Hallmobility first increased and then decreased as thetemperature decreased from 300 to 77K. The Hallmobility varies approximately as m ¼ m0T

�1:5

between 170 and 300K, where m is Hall mobility,m0 is a constant practically being independent oftemperature, and T is the absolute temperature.Based on this temperature dependence, the mainscattering mechanism which contributes to theHall mobility was found to be acoustic phononscattering above 127K. The Hall mobility de-creases below 127K because the influence ofionized impurity scattering becomes larger. Wecalculated the thermal activation energy of elec-trons between 77 and 300K from the data inFig. 2(b). The value was 26.5meV when the carrierconcentration is assumed to follow the Arrheniusformula, N ¼ Ncð�E=2kTÞ; where N is the carrierconcentration, Nc is the valence electron concen-

tration, E is the thermal activation energy, k is theBoltzmann constant and T is the absolutetemperature.

3.2. X-ray diffraction

The FWHM of the X-ray rocking curve (XRC)peak was 61 arcsec for the (0 0 0 4) reflection and

1017 1018 10190

200

400

600

800

1000

Carrier Concentration [cm-3]

Mob

ility

[cm

2 /Vs]

Nakamura et al.(9)

Nakamura et al.(10)

This results (on AlN/sapphire templates)

Fig. 1. Hall mobility as a function of the carrier concentration.

70 80 90 100 200 300700

800

900

1000

2000

Temperature [K]M

obili

ty [c

m2 /V

s]

0.004 0.006 0.008 0.01 0.012

1016

1017

Reciprocal of Temperature [1/K]

Car

rier

Con

cent

ratio

n [c

m−3

](a)

(b)

Fig. 2. (a) Hall mobility for the GaN film grown on a AlN/

sapphire template as a function of temperature. (b) Carrier

concentration for the GaN film grown on a AlN/sapphire

template as a function of the reciprocal temperature.

M. Sakai et al. / Journal of Crystal Growth 244 (2002) 6–118

232 arcsec for the (2 0 %2 4) reflection. The crystalquality of the present GaN film grown on the AlN/sapphire templates was found to be excellent. Onthe other hand, FWHMs of GaN films grown onsapphire substrates with LT-BLs were 216 and466 arcsec for (0 0 0 4) and (2 0 %2 4) reflections,respectively. The XRC FWHMs of the GaN filmsgrown on AlN/sapphire templates are superior, asin the case of the Hall mobilities. So far, thedifferent results have been reported about therelationship between Hall mobility and XRCFWHM. For example, Nakamura et al. reportedthat the XRC FWHM was not directly related toHall mobility [10] but X. Du et al. showed that theXRC FWHM for (0 0 0 2) reflection was correlatedwith Hall mobility [11]. In the present work, it isfound that the sample with higher Hall mobilityshowed better crystal qualities with narrower XRCFWHMs for (0 0 0 4) reflection and for (2 0 %2 4)reflection. The correlation between the Hallmobility and XRC FWHM was clearly confirmed,that is clearer than the result of X. Du et al.who show the correlation between Hall mobilityand XRC FWHM for (0 0 0 2) reflection butnot for twist range. Our result may be because ofthe good crystallinity of epitaxial AlN/sapphiretemplates.

3.3. Surface morphology

Furthermore, AFM images also showed thepreferable effect of high-quality AlN/sapphiretemplates. Fig. 3(a) shows the AFM image of theGaN film grown on an AlN/sapphire template andFig. 3(b) shows the GaN film grown on a sapphiresubstrate with a LT-BL. As shown in Fig. 3(a),smooth surface morphology and clear step forma-tion were observed. Additionally, pit terminationscould be hardly found. The root mean square(rms) roughness values of the GaN film shown inFig. 3(a) was 0.21 nm. GaN films grown on AlN/sapphire templates exhibited superior morphologycompared with those grown on sapphire substrateswith LT-BLs. Fig. 3(b) showed nonparallel stepsand some pit terminations. The rms value of theGaN film shown in Fig. 3(b) was 0.22 nm. The pitdensity estimated only by the number of visiblepits in Fig. 3(a) and (b) were less than 1� 107 and

4� 108 cm�2, respectively. It is known that the pitdensity has a strong correlation with the disloca-tion density, so that it is inferred that GaN filmsgrown on epitaxial AlN/sapphire templates havemuch lower dislocation densities than those grownon sapphire substrate with LT-BLs.

(b)

(a)

Fig. 3. AFM images of (a) the GaN film grown on a AlN/

sapphire template and (b) the GaN film grown on a sapphire

substrate with a LT-BL.

M. Sakai et al. / Journal of Crystal Growth 244 (2002) 6–11 9

3.4. TEM image

Fig. 4 shows a cross-sectional TEM image of theGaN film grown on an AlN/sapphire template.The interface structure between the AlN/sapphiretemplate and GaN layer was sharp. Densedislocations were observed in the epitaxial AlNfilm layer. However, dislocations did not thread tothe GaN film layer. Thus, AlN/sapphire templateswere found to play an important role in growinghigh-quality GaN films. From the observation ofthe cross-sectional and plan TEM images, totaldislocation density of the GaN film grown on AlN/sapphire templates was estimated to be approxi-mately 5� 107 cm�2 which is lower than thepreviously reported value of the GaN film withoutepitaxial lateral overgrowth (ELO) technique.

3.5. Photoluminescence

Fig. 5 shows the room-temperature photolumi-nescence (PL) spectrum of the GaN film grown onan AlN/sapphire template and on a sapphiresubstrate with a LT-BL. The intensity of deep-level emission (yellow luminescence) of both filmswas almost the same. These deep-level emissionsare postulated to be associated with Ga vacancies[12]. Even though the intensity of yellow lumines-cence is very high compared with that of the nearthe band edge emission, the reason is simplybecause the PL system has a higher detectionefficiency in the yellow luminescence region. The

intensity of the near the band edge emission ofGaN films grown on AlN/sapphire templates wastwice as high as that of GaN films grown onsapphire substrates with LT-BLs. This resultsuggests that GaN films grown on AlN/sapphiretemplates have superior crystal quality comparedwith GaN films grown on sapphire substrates withLT-BLs.

4. Conclusions

We have demonstrated that the MOVPEgrowth of high-quality GaN films using high-quality AlN/sapphire templates. Hall mobilities ofthe GaN films were 790 and 1454 cm2/V s withelectron carrier concentrations of 7.6� 1016 and1.8� 1016 cm�3 at 300 and 77K, respectively. TheXRC FWHMs were 61 and 232 arcsec for the(0 0 0 4) and (2 0 %2 4) reflections, respectively. AFMand cross-sectional TEM images showed that thecrystal quality was excellent. The dislocationdensity was 5� 107 cm�2. Higher PL intensity nearthe band edge was observed. Every result wassuperior to those of GaN films grown on sapphiresubstrates with LT-BLs by conventional method.Present AlN/sapphire templates are promising inthe growth of high-quality GaN films and thefabrication of high-performance devices.

AlN

GaN

sapphire

1µm

Fig. 4. Cross-sectional TEM image of the GaN film grown on a

AlN/sapphire template.

2.25 2.5 2.75 3 3.25 3.5

PL

Inte

nsity

[a.u

.]

Photon Energy [eV]

on an AlN/sapphire templateon a sapphire substrate

with a low temperature buffer layer

Fig. 5. Photoluminescence of GaN films grown on a AlN/

sapphire template and on a sapphire substrate with a LT-BL.

M. Sakai et al. / Journal of Crystal Growth 244 (2002) 6–1110

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