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

实用化的InGaN/GaN基紫外激光二极管的研制迫切需要GaN衬底。本工作的目的是用卤化物真空电子束外延技术在GaN衬底上生长厚GaN以获得自支撑的GaN衬底。项目初期，立方(闪锌矿结构)GaN被认为是开发氮化物基激光二极管的实用材料，而在GaN(001)衬底上用高压电子束外延生长厚立方体GaN，获得了当时的世界纪录，2微米厚的立方相纯度可达98%，5微米厚的立方相纯度可达90%。然而，1998年在六方(纤锌矿)GaN上实现了紫外光半导体激光器，于是课题转向在GaAs(111)衬底上生长厚六方GaN。GaN在GaN(111)衬底上生长的最大问题是GaN在高温下的耐受性。然而，通过在大约850℃的GaN衬底上生长的中间GaN层覆盖整个GaN衬底，GaAs可以承受1000℃的生长。此外，研究还发现，在1000℃以上生长六方GaN时存在某种表面平坦化生长机制。尽管GaAs111衬底具有极性，但高压外延生长的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的生长机制是受表面动力学限制的，这也是GaN晶体质量很高的原因。

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.