Nucleation layer microstructure, grain size, and electrical properties in GaN grown on a-plane sapphire

We have investigated the microstructural origins of center-to-edge differen ces in the electrical properties of GaN films grown on a-plane sapphire via metal-organic vapor-phase epitaxy. Using cross-sectional transmission elec tron microscopy, we have observed that the grain size at the wafer edge is approximately 1 mu m, whereas the grain size in the wafer center ranges fro m 0.1 to 0.5 mu m. The smaller grain size at the wafer center is traced to a higher density of extended defects in the AlN nucleation layer: defects w hich, in turn, act as nucleation sites for GaN grain growth. (C) 1999 Ameri can Institute of Physics. [S0003-6951(99)03831-0].