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We have investigated the microstructural origins of center-to-edge differences in the electrical properties of GaN films grown on a-plane sapphire via metal–organic vapor-phase epitaxy. Using cross-sectional transmission electron microscopy, we have observed that the grain size at the wafer edge is approximately 1 μm, whereas the grain size in the wafer center ranges from 0.1 to 0.5 μm. The smaller grain size at the wafer center is traced to a higher density of extended defects in the AlN nucleation layer: defects which, in turn, act as nucleation sites for GaN grain growth. © 1999 American Institute of Physics.