High rate GaN epitaxial growth by sublimation sandwich method

JOURNALO'

ELSEVIER

CRYSTAL GROWTH

Journal of Crystal Growth 183 (1998) 10-14

High rate GaN epitaxial growth by sublimation sandwich method Yu.A. Vodakov*, E.N. Mokhov, A.D. Roenkov, M.E. Boiko, P.G. Baranov A.F. Ioffe Physico-Technical Institute, Russian Academy of Science, 194021 St. Petersburg, Russian Federation

Received 4 April 1997; accepted 7 June 1997

Abstract Thick GaN epitaxial layers were grown by the sublimation "sandwich method" (SSM) on SiC substrates at temperatures from llOO°C to 1250°C in ammonia flow. Metallic Ga or GaN powder was used as the vapor source. The possibility of growing of monocrystalline GaN layers with growth rates as high as 1 mm/h was demonstrated. The dependence of the growth kinetics on temperature, source to substrate distance and input ammonia flow rate was studied. Various characterization techniques show the high quality of the GaN layers. Keywords: GaN; Epitaxial layers; Sublimation; Growth

1. Introduction GaN is known to be one of the most promising wide band-gap semiconductor materials for optoelectronic devices operating in the blue spectrum, e.g. highly effective blue-light emitting diodes and short wavelength lasers for high optical storage systems [1]. However, these bright perspective appeared only recently, when low resistive p-type GaN layers have been prepared successfully [2]. One of the main problems of GaN semiconductor technology has been the difficulty to grow high-quality bulk crystal and epitaxial layers. GaN layers are usually grown on sapphire substrates. There exists severe mismatch both in their lattice

* Corresponding author. 0022-0248/98/$19.00 (C) 1998 Elsevier Science B.V. All rights reserved Pll 50022-0248(97)00413-2

parameters (more than 14%) and thermal expansion coefficients which result in a high misfit strain and, as a consequence, in a high density of dislocations and microcracks in the epitaxial layers. In this respect, SiC substrates are preferable, because of the smaller lattice mismatch (about 3.4%). Residual thermal stresses remain, though buffer layer deposition allows improvement in the crystal quality of the GaN layers. GaN substrates having the same lattice parameters have been shown to be the best for the design of GaN device structures [3]. Up to now, however, large free-standing GaN crystals are not available. Additionally, the growth rate of GaN layers obtained by conventional methods (MOCVD or MBE) is very low, which prevents the growth of bulk GaN crystals. Only recently, Asaki and co-workers [4J successfully grew by the

Yu.A. Vodakov et al.] Journal of Crystal Growth 183 (]998) 10-14

method of hydride vapor phase epitaxy thick (800 urn) GaN layers. We proposed the sublimation sandwich method (SSM) for the growth of GaN epitaxial layers [S, 6] and applied this technique for the growth of thick SiC layers of various polytypes both pure and doped [7]. Some features of growth mechanisms of GaN and SiC epitaxial layers on the (0 0 0 I)C and (OOOI)Si faces of SiC substrates were presented [8]. Later, the optical properties of GaN epilayers grown by SSM were studied [9]. In this report we show that high-quality very thick GaN layers can be prepared by SSM with growth rates as high as 1 mm/h, The influence of the main parameters of SSM growth on crystallization rate of GaN layers is analyzed. Some characteristics of GaN layers are discussed.

2. Experimental procedure GaN epitaxial layers were grown in a horizontal quartz-type reactor with RF heating (Fig. 1). The growth cell consists of the vapor source and substrate with narrow clearance, located in a temperature-gradient zone. The source temperature was heated to a higher temperature than the substrate. The temperatures of substrate and source are measured by using a W-Re thermocouple. The temperature difference was varied from 10"C to SO"e. SiC (000 1) crystals of 6H polytype grown by the Lely method were used as substrates. The area of the substrates was about O.S 1 cnr'. Metallic Ga or GaN powder has been used as source. The distance between substrate and source was varied from 2 to S mm. Before the growth experiments the substrates were etched in KOH melt for removal of surface layers damaged by grinding. The reactor was first evacuated at a temperature of SOO"e. The GaN epitaxial layers were deposited at atmospheric pressure in ammonia gas flow in a temperature range 10S0-l300°e. The input ammonia flow rate was varied from 10 to SO lIh. Monocrystalline GaN layers of 70-S00 urn thickness were prepared during a deposition time from S to 40 min. The growth rate was 0.2-1.1 mm/h,

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Fig. 1. Experimental set-up for growth of GaN layers by SSM: 1 - induction coil; 2 - susceptor; 3 - vapor source; 4 - container; 5 - SiC-substrate.

3. Results and discussion

3.1. Growth kinetic and morphology Good-quality GaN layers were grown by SSM in the temperature range l1S0-1230°e. The GaN layers obtained at high temperature ( > 1230°C) or at low input ammonia flow rates often contain Ga inclusions which usually appeared near structural or morphological defects. Decreasing of the growth temperature below IIS0°C promotes the formation of structural defects such as voids and micropipes. As a rule, GaN layers grown at a temperature < lOS0'C are polycrystalline. In this case, a great number of separate GaN crystals with platelet or prismatic morphology may be formed. Obviously, the polycrystalline growth of GaN is caused by a high flux of Ga-containing molecules to the substrate. We found that it was possible to grow highquality GaN layers without buffer layers only at temperatures higher than 1180"e. The surface morphology ofGaN layers grown on the polar {O 0 0 I}SiC substrate faces differed drastically. A smooth plane surface without morphological defects may be achieved by growth on (000 1)Si substrates in contrast to (000 I)C substrates which gave an irregular surface covered with hexagonal-shaped hillocks. This phenomenon is typical for the island-growth mechanism. The variation of GaN growth rate versus growth temperature is shown in Fig. 2. It demonstrates that the growth rate is increasing with temperature only up to 1200°e. Further temperature increase reduced the growth rate drastically because of

Yu.A. Vodako» et al. j Journal of"Crystal Growth 183 (/998) 10-14

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3.2. Characterization of the GaN layers

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