Influence of Substrate Misorientation Angle and Direction in Growth of GaN on Off-axis SiC (0001)
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Influence of Substrate Misorientation Angle and Direction in Growth of GaN on Off-axis SiC (0001) Jun Suda1,2, Yuki Nakano2 and Tsunenobu Kimoto2 1 PRESTO Nanostructure and Material Property, Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan 2 Department of Electronic Science and Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ABSTRACT Growth of GaN on 4H- or 6H-SiC (0001) Si-face substrates with various misorientation angles and directions is presented. GaN layers were directly grown on the SiC substrates by molecular-beam epitaxy using elemental Ga and rf plasma-excited active nitrogen. First, 4H-SiC (0001) 8º-off toward the [11-20] direction was investigated. Before the growth of GaN, ex-situ high-temperature gas etching was carried out, resulting in a smooth SiC initial surface. However, the surface of the subsequently grown GaN layer has wavy features with peak-to-valley height of 30 nm. Since the direction of the undulations is parallel to the misorientation direction, this feature must originate from the substrate misorientation. Step bunching and large faceting along and directions occurred during the growth of GaN. Lowering the growth temperature suppresses large faceting, and results in reduction of the peak-to-valley height to 3 nm. However, the surface still has the same undulating features on a smaller length scale. On the other hand such morphology was not observed for GaN grown on (0001) on-axis SiC substrates (misorientation < 0.3º). The influence of the polytype of the SiC substrate and the misorientation angle and direction are also discussed. INTRODUCTION SiC is one of the most promising materials for high-temperature, high-power and high-frequency devices due to its wide bandgap, high breakdown field, high thermal conductivity and high saturation electron velocity. However, bandgap engineering cannot be applied to SiC devices, since it is impossible to grow SixC1-x solid solutions with x near 0.5. Group-III nitrides (III-Ns), AlN and GaN have small lattice mismatch to SiC, and the bandgap energy can be varied from 3.4 - 6.2 eV by using AlxGa1-xN solid solutions. Integration of III-N materials with SiC is an interesting approach to realize
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high-performance wide-bandgap semiconductor devices. This approach can make it possible to realize SiC-based heterojunction devices, such as AlN/SiC metal-insulator-semiconductor field-effect transistors (MISFETs) [1,2] and GaN/SiC heterojunction bipolar transistors (HBTs) [3]. At present, successful SiC homoepitaxial growth is established only for off-axis SiC substrates, where the polytype mixing problem is avoided through step-controlled epitaxy [4]. Therefore the III-N semiconductor layer must be grown over an off-axis SiC homoepilayer in order to realize heterojunction devices (it is impractical to apply bulk SiC in the device active region). Although III-N heteroepitaxy on SiC substrates has been studied extensively, most prior work involved on-axis SiC substrates. Understanding the g
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