GaN Growth Process by MOCVD Revisited: TEM Study of Microstructural Evolution Presented

Microstructure evolution of GaN grown on c-sapphire by MOCVD has been systematically studied using transmission electron microscopy based techniques. Individual samples have been derived by interrupting the GaN growth at various steps, starting from nitridation at 530° C, followed by the standard two step growth comprising of first deposition of low temperature GaN nucleation layer (LT-GaN NL) and then ramping up the temperature followed by high temperature GaN epilayer growth. Effect of nitridation, and the microstructure of the nitride layer for various nitridation temperatures has been recently reported by our group [1], where we have shown that the nitride layer formed at this nitridation temperature is cubic spinel AlxOyNz. In this paper it will be shown, that the LT-GaN grown on this AlxOyNz (after nitridation at 530° C) is primarily cubic zinc blende (zb) in structure, with multiple twin variants existing about various {111} planes. Its crystallographic orientation relationships with the underlying nitride layer and the sapphire substrate will be shown. The transformation of LT-GaN and the regions of transformation from cubic zb phase to the wurtzite (w) phase during the annealing step will be presented. Subsequent effects on the GaN epilayer growth due to the microstructural evolution of these underlying layers along with the evolution of defects will also be discussed.