THE AMMONIA- MBE APPROACH FOR HIGH ELECTRON MOBILITY IN GaN EPILAYERS AND AlGaN/GaN HFETs

摘要

The electron mobility in GaN epitaxial films is not only a figure-of-merit parameter for many device applications, but also an increasingly accepted reliable measure of the material quality, due to the strong defect scattering discovered in this material system. Reduction of defect density and improvement of transport properties of MBE-grown GaN epilayers has been quite dramatic recently. In this article, we report on the progress achieved using the ammonia- MBE technique. The mobilities of GaN bulk layers as well as AlGaN/GaN HFET structures grown by this technique have been substantially increased, and now are comparable with the results achieved by MOCVD. Critical growth procedures that enhance electron mobility and growth reproducibility are reported. Effects of electron scattering by threading dislocations and interface roughness are discussed. The merits as well as further challenges of the GaN MBE technology for certain device applications such as the microwave power devices are identified.