Lattice Relaxation of AlN Buffer on Surface-Treated SiC in Molecular-Beam Epitaxy for Growth of High-Quality GaN
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Lattice Relaxation of AlN Buffer on Surface-Treated SiC in Molecular-Beam Epitaxy for Growth of High-Quality GaN Jun Suda, Kouhei Miura, Misako Honaga, Norio Onojima, Yusuke Nishi and Hiroyuki Matsunami Department of Electronic Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan ABSTRACT The effects of SiC surface treatment on the lattice relaxation of AlN buffer layers and the crystalline quality of GaN layers grown on the buffer layers were studied. AlN buffer layers and GaN main layers were grown by plasma-assisted molecular-beam epitaxy on on-axis 6H-SiC (0001)Si substrates. High-temperature HCl-gas etching resulted in an atomically flat SiC surface with (√3×√3)R30° surface reconstruction, while HCl-gas etching followed by HF chemical treatment resulted in an atomically flat surface with (1×1) structure. The AlN layer grown on the (1×1) surface showed slower lattice relaxation. GaN grown on the AlN buffer layer exhibited a (0002) X-ray rocking curve of 70 arcsec and 107 cm-2 of screw-type dislocation density, which was superior than that of GaN grown on (√3×√3)R30° surface.
INTRODUCTION Growth of high-quality GaN on SiC is one of key issues to realize high-frequency high-power transistors. We have reported molecular-beam epitaxial (MBE) growth of AlN on 6H-SiC (0001)Si substrates pretreated by high-temperature HCl-gas etching [1,2]. HCl-gas etching resulted in an atomically flat SiC surface [3]. The crystalline quality and surface roughness of AlN grown layers were greatly improved compared to AlN grown on an as-received SiC substrate, suggesting that HCl-gas etching is an effective pretreatment to grow high-quality AlN. In this study, the AlN layer was used as a buffer layer for GaN growth. We discuss the correlation between the lattice relaxation process of AlN buffer, which depends on surface reconstruction of SiC substrates, and the quality of GaN layer grown on the AlN buffer. It was revealed that surface reconstruction of 6H-SiC has strong effects on the lattice relaxation process of AlN buffer layers and the crystalline quality of GaN.
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EXPERIMENTAL DETAILS AlN and GaN layers were grown by plasma-assisted MBE using elemental Ga and Al, and active nitrogen (N*) generated by an EPI Unibulb radio-frequency plasma cell. Commercially available on-axis 6H-SiC (0001)Si substrates were used. In this study, three different pretreatments for 6H-SiC substrates were investigated. They were combinations of two methods, a wet chemical process and high-temperature HCl-gas etching. First, all substrates were cleaned by a wet chemical process: dipping into aqua regia, HCl solution and HF solution for removal of contamination and natural oxide. These substrates were referred as as-received substrates. As shown in figure 1(a), many polishing scratches were observed by AFM on the as-received substrate. Second, some of the substrates were etched by HCl/H2 gas at 1300°C to remove the damaged layer caused by polishing, which were referred as HCl-treated substrates. The surface exhibited
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