Fabrication of single crystalline bulk GaN substrates using GaAs as seed crystals

GaN衬底是InGaN/GaN基紫外激光二极管研制的迫切需要。本工作的目的是通过卤化物VPE在GaAs衬底上生长厚GaN来获得自支撑GaN衬底。（锌层结构）GaN对于基于氮化物的激光二极管的开发是实用的，在GaAs（001）衬底上用HVPE法生长了厚的立方相GaN，2 μ m厚的立方相纯度为98%，5 μ m厚的立方相纯度为90%，这是当时的世界纪录。然而，1998年在六方晶系（纤锌矿）GaN上实现了紫外激光二极管，因此课题转向在GaAs（111）衬底上生长厚的六方GaN。而在850℃左右生长的GaN中间层覆盖整个GaAs衬底，GaAs可以经受1000℃的生长。在1000℃以上生长的六方GaN存在表面平坦化生长机制，虽然GaAs（111）衬底具有极性，但HVPE生长的GaN始终具有Ga极性，与衬底或低温缓冲层的极性无关。这可能是由于HVPE的生长机制是表面动力学限制的事实，这是GaN晶体质量非常高的原因。

A GaN substrate is desperately needed for development of practical InGaN/GaN based ultraviolet laser diodes. Purpose of this work is to obtain a free standing GaN substrate by growth of thick GaN on GaAs substrates by halide VPE.At the beginning of the project, it was thought that cubic (zinc blende structure) GaN was practical for development of nitride based laser diodes, and growth of thick cubic GaN on GaAs(001) substrates was pursued by HVPE.Cubic phase purity of 98% for 2um thick layer and 90% for 5 um thick layer, which were the world record at that time, was obtained. An ultra violet laser diode, however, was realized in 1998on hexagonal (Wurtzite) GaN, so the subject was changed to growth of thick hexagonal GaN on GaAs (111) substrates.The biggest problem of GaN HVPE growth on GaAs(111) was endurance of GaAs at high temperatures. It was found, however, that GaAs could stand 1000℃ growth by covering whole GaAs substrate with intermediate GaN layer grown at about 850℃. Furthermore, it was found that there was some surface flattening growth mechanism for growth of hexagonal GaN at above 1000℃.Although a GaAs (111) substrate has polarity, GaN grown by HVPE has always Ga polarity, independently of the polarity of the substrate or the low temperature buffer layer. This is probably due to the fact that growth mechanism of HVPE is surface kinetic limited, and this is the reason why crystal quality of GaN is very high.