Impact of H 2 -preannealing of the Sapphire Substrate on the Crystallization of Low-Temperature-Deposited AlN Buffer Lay
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Impact of H2-preannealing of the Sapphire Substrate on the Crystallization of Low-Temperature-Deposited AlN Buffer Layer Michinobu Tsuda1, 2, Krishnan Balakrishnan2, Motoaki Iwaya2, Satoshi Kamiyama2, Hiroshi Amano2 and Isamu Akasaki2 1 Single Crystal Division, Kyocera Corporation, 1166-6, Nagatanino, Hebimizo-cho, Youkaichi-city, Shiga 527-8555, Japan 2 Faculty of Science and Technology, 21st Century-COE “Nano-Factory”, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan ABSTRACT The effect of H2-preannealing of sapphire substrate on low-temperature (LT-) AlN buffer layer deposited by metalorganic vapor phase epitaxy is investigated. Crystallinity of LT-AlN drastically changes with preannealing temperature variation. It is found that H2-preannealing of sapphire substrate is a requisite to get a better quality LT-AlN and as a consequence it leads to growth of better quality GaN epi-layer on it. INTRODUCTION Success of the growth of device-quality GaN on a sapphire substrate by metalorganic vapor phase epitaxy (MOVPE) depends on the technology of a low-temperature (LT-) buffer layer, which can solve the problems on a large lattice mismatch between GaN and sapphire [1, 2]. In general, control of substrate surface is essential to get high quality layers of crystal. It is well-known that, before the deposition of the LT-buffer layer, the sapphire substrate is preannealed in a H2 ambient [1, 2], although its mechanism is not well-understood. Some groups have reported the influence of other treatments, such as the injection of ammonia (NH3) at a high temperature before deposition of a LT-buffer layer [3, 4] and annealing the surface (0001) plane sapphire in air [5]. We have reported that, as for the (0001) plane sapphire substrate, H2-preannealing roughens the surface and the surface composition becomes Al-rich [6]. And the preannealing temperature Ta should be higher than 900°C to obtain high quality GaN epi-layer, otherwise, polycrystalline GaN are grown [6]. However, the effect of H2-preannealing on the LT-buffer layer, which is in direct contact with the sapphire surface, was not clarified. In this study, the effect of H2-preannealing on the LT-AlN buffer layer is studied.
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EXPERIMENTAL Mirror-polished (0001) plane sapphire is used as the substrate. After degreasing by organic solvents and etching by acids, the sapphire substrate is placed in the reactor. A LT-AlN layer is deposited on the sapphire substrate by MOVPE by following procedure. Figure 1 shows the temperature sequence used in this study. The H2-preannealing is performed for 5min at three different preannealing temperatures, 400, 900 and 1,200°C. The flow rate of H2 gas is fixed at 3.5slm and the pressure is atmospheric. After the H2-preannealing, the substrate is cooled to 400°C. Subsequently, an AlN layer of 20nm thickness is deposited as a LT-buffer on the substrate annealed at each temperature using trimethylaluminum (TMA) and NH3. In this study, two sets of sample are prepared; 1) as-deposited: the sample is coo
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