Polarity Inversion during Halide VPE Growth of GaNon GaAs(111)B-As Surface at High Temperatures

F. Hasegawa (a), M. Namerikawa (a), O. Takahashi (a), and R. Souda (b)

(a) Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba,Ibaraki 3058573, Japan

(b) National Institute for Research in Inorganic Materials, Tsukuba, Japan

(Received June 19, 2001; accepted July 12, 2001)

Subject classification: 68.55.Ac; 68.55.Jk; 81.15.Kk; S7.14

Halide Vapor Phase Epitaxy (HVPE) is a promising growth method of bulk GaN. Polarity of GaNgrown on GaAs(111)B-As surface was investigated. Depending on whether the GaAs substratewas exposed to GaCl or NH3 ambient prior to growth, polarity of the low temperature buffer layerexhibited Ga polarity or N polarity. However, GaN layers grown on it at 850 �C always showedGa polarity independently of the polarity of the buffer layer, suggesting that HVPE prefers Gapolar growth at high temperatures.

Introduction Currently, a practical GaN substrate has been obtained by growing athick GaN layer with halide vapor phase epitaxy (HVPE) on a sapphire substrate andremoving the sapphire substrate by laser resolution or by lapping [1]. It is, however,neither easy nor very reproducible to remove a very hard sapphire substrate. An alter-native method is to grow a thick HVPE GaN on a GaAs(111) substrate, which can beeasily removed by lapping and etching. We demonstrated that GaN could be grown onGaAs(111) substrates at 1000 �C by covering the whole GaAs substrate with GaNgrown at intermediate temperature [2]. Kumagai et al. [3] reported that they couldgrow GaN at 1000 �C on GaAs(111)A without an intermediate layer. Sumitomo Elec-tric group reported recently that they could obtain 2 inch diameter free standing GaNby growing GaN on GaAs(111)A substrates by HVPE [4].

It is well known that the crystal quality greatly depends on the polarity of hexagonalGaN, but it is not understood how the polarity of the grown layer is governed by thesubstrate, surface and growth conditions [5]. Since a (111) GaAs substrate has its ownpolarity, GaN growth on (111) GaAs looks to be the best way to investigate theseproblems.

The purpose of this work is to investigate the polarity of GaN grown by HVPE on aGaAs(111) substrate, especially, on GaAs(111)B-As surface. It was found that polarityof the buffer layer is determined by the substrate polarity but also by the surface condi-tion of the substrate, but GaN grown by HVPE at high temperature always gives Gapolarity.

Experimental GaN was grown by conventional HVPE using Ga metal, HCl, and NH3

with H2 carrier gas. GaCl/H2 produced by reaction between HCl gas and the metal Gasource was supplied via an injection pipe located at the center of the reactor tube tothe deposition zone with NH3 ambient. The HCl and NH3 partial pressures during thegrowth were 8 � 10–4 and 1.6 � 10–1 atm, respectively. After degreasing the sample,(111) GaAs substrates were etched in an etchant of NH4OH: H2O2 : H2O : = 1 :1 : 5 at

phys. stat. sol. (a) 188, No. 1, 443–446 (2001)

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0 �C for 2 min and dipped into HF solution for 15 min. The substrates were slightly gasetched (about 20 nm) at 550 �C by HCl gas prior to the growth to remove the surfaceoxide.

After removing the surface oxide, the substrate was kept in either GaCl or NH3

ambient for 45 min at 550�C, then a buffer layer was grown at 550 �C for 5 min with V/IIIratio of 400. The thickness of the buffer layer was about 50 nm. High temperaturegrowth was performed at 850 �C as an intermediate layer to protect GaAs and at1000 �C to grow a final layer with a smooth surface.

The polarity was characterized by CAICISS (Coaxial Impact Collision Ion ScatteringSpectroscopy) [6]. The polarity of the buffer layer was measured after heat treatment at850 �C for 10 min to crystallize the deposited GaN.

Results and Discussion The GaN buffer layer grown after exposing the GaAs(111)Bsubstrate to NH3 gave a smoother surface than that grown after GaCl exposure, asshown in reflection high energy electron diffraction (RHEED) patterns of Fig. 1. Thiswas also confirmed by images obtained by an atomic force microscopy (AFM).

The CAICISS spectra of the buffer layers grown with exposure to GaCl or NH3

are shown in Fig. 2 in comparison with simulated CAICISS spectra of pure Ga orN polarity. The polarity of the layer grown after exposing the GaAs(111)B sub-

444 F. Hasegawa et al.: Polarity Inversion during Halide VPE Growth of GaN

Fig. 1. RHEED patterns of the GaN buffer layer grown on GaAs(111)B substrate after NH3 expo-sure and GaCl exposure for 45 min

Fig. 2. CAICISS spectra of the bufferlayers grown on GaAs(111)B after NH3

and GaCl exposure. The buffer layerswere annealed at 850 �C for 10 min inNH3 and H2 ambients

strate to NH3 ambient for 45 min wasN dominant as expected from the sub-strate polarity, but the layer grownafter exposing to GaCl gave Ga domi-nant polarity. These results indicatethat the polarity of the buffer layer is

governed not only by the substrate polarity but also by the surface condition ofthe substrate.

Although the polarity of the buffer layers is different for different surface treatments ofthe GaAs(111)B substrate, that of GaN grown at 850 �C for 60 min was Ga dominant asshown in CAICISS spectra of Fig. 3. The thickness of these intermediate layers was about3 mm, and the surface was rough as shown in Fig. 1 of our previous paper [2], still CAI-CISS spectra could be clearly obtained as shown in Fig. 3. The fact that GaN grown on thebuffer layer with N dominant polarity has Ga dominant polarity means that the polarityof GaN changed during the high temperature HVPE growth of GaN. This polarity inver-sion can be explained as follows. It is well known that HVPE growth of GaN in this tem-perature range is surface kinetic limited [7]. Since the surface kinetic reaction leads to areduction of GaCl with H2 or NH3 [8], growth on the Ga polar surface should be muchfaster than that on the N polar surface.

Even if the buffer layer is N polarity dominant, it is reasonable to think that thereare some columns with Ga polarity. When GaN is grown at high temperatures on thiskind of N polarity dominant buffer layer, GaN would grow only on columns with Gapolarity, and finaly cover the whole surface with Ga polarity layer. The growth at550 �C should be also surface kinetic. The reason why a buffer layer with N polaritydominant grows still remains as a future work to be investigated.

phys. stat. sol. (a) 188, No. 1 (2001) 445

Fig. 3. CAICISS spectra of the GaN layersgrown at 850 oC on the buffer layers whichwere grown after NH3 and GaCl exposure for45 min

Fig. 4. SEM surface and PL spectrum of GaN grown at 1000 oC for 1 h on GaAs(111)B surfacewith the Ga polar buffer layer

A GaAs(111)B substrate with an N polar buffer layer could not stand 1000 �Cgrowth even if it is covered with an 850 �C intermediate layer. The SEM surface andphotoluminescence (PL) of GaN grown at 1000 �C on a GaAs(111)B surface with Gapolar buffer layer are shown in Figs. 4a and b. The surface was reasonably flat and aband edge emission without a deep PL was obtained.

Summary In order to investigate how the polarity of GaN is controlled, GaN wasgrown on GaAs(111)B-As surface by HVPE. The polarity of the low temperature buf-fer layer depended on whether the GaAs substrate was exposed to GaCl or NH3 ambi-ent prior to the growth, and exhibited Ga polarity or N polarity. However, GaN layersgrown on it at 850 �C always showed Ga polarity independent of the polarity of thebuffer layer, probably due to the surface kinetic growth of HVPE of GaN.

Acknowledgements The author would like to express their sincere thanks to Mr.T. Sato for his help of HVPE growth of GaN and Dr. T. Suemasu for his support to theexperiment and for discussions of the experimental results.

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446 F. Hasegawa et al.: Polarity Inversion during Halide VPE Growth of GaN