Properties of GaN Homoepitaxial Layers Grown on GaN Epitaxial Wafers
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2 DMITRIEV 2' 4"',A. NIKOLAEV 2, A. CHERENKOV1, D. TSVETKOV', S. STEPANOV1" , 2 3 3 KUZNETSOV 2, I. NIKITINA 2, A. KOVARSKY , M. YAGOVKINA , V. DAVIDOV Crystal Growth Research Center, St. Petersburg, Russia loffe Institute, St. Petersburg, Russia Mekhanobr-Analit Company, St. Petersburg, Russia MSRCE, Howard University, Washington, DC, USA TDI, Inc., Gaithersburg, MD, USA
ABSTRACT The lack of GaN substrates is a limiting factor for the development of III-V nitride devices. Recently we proposed to use GaN/SiC epitaxial wafers, consisting of thin GaN layer deposited by hydride vapor phase epitaxy (HVPE) on SiC wafer, as substrates for subsequent growth of IIIV nitrides and devices development. These wafers are attractive to be used as substrates for GaN device fabrication because the GaN-based device structures can be grown on these wafers by homoepitaxy without any buffer layer. Due to high SiC thermoconductivity and cleavage possibility, these wafers are especially attractive for high-power electronic and optoelectronic applications. In this paper, we focus on crystal structure, optical and electrical properties of GaN homoepitaxial layers and p-n structures grown by HVPE on GaN/SiC epitaxial wafers. New types of III-V nitride epitaxial wafers are described, insulating GaN/SiC epitaxial wafers and AIN/SiC epitaxial wafers. INTRODUCTION Currently, III-V nitride device structures are being grown by heteroepitaxial methods on sapphire or SiC substrates. Progress in the growth of bulk III-V nitride materials is slow, and
GaN wafers are not commercially available yet. As alternative substrates for 111-V nitride homoepitaxy, TDI, Inc. proposed to use GaN/SiC epitaxial wafers [1]. The HVPE technology has proved to be a powerful approach for growth of group III nitrides. Recent accomplishments include 1) the first 30 mm diameter GaN free-standing wafers were fabricated by HVPE (2], 2) GaN p-n structures grown by using Mg doping during HVPE deposition [3], 3) AIGaN and InGaN alloys grown by the HVPE method [4,5]. For HVPE technology, GaN growth rate can be controlled from 5 to 100 itm/hr [6]. Background concentration Nd-Na for undoped GaN deposited by HVPE was reduced to the 10'1 cm' range [7], and dislocation density of less than 108 cm"2 has been reported for GaN layers grown by HVPE [8]. Currently, we are using the HVPE technology for the manufacture of GaN/SiC epitaxial wafers. Commercial GaN/SiC epitaxial wafers [9] consist of approximately 0.5 jim thick GaN layers grown on (0001) on-axis 6H-SiC substrates (Fig. 1). Properties of these layers were described elsewhere [10,11,12]. It was shown that if the thickness of GaN is less than 0.1 - 0.3 pim, the dislocation density in GaN is high (>5x10 9 cm' 2). On the other hand, if the thickness of GaN is larger than 1.5 - 2 gLm, cracks may occur in GaN. If the GaN thickness is about 0.5 gim, the dislocation density in the top portion of GaN layers ranges from 108 to 109 cm 2 . The GaN layers are n-type with the concentration Nd-N, ranging from 1017 to 1011 cm"3 . When the G
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