Nonuniformities in free-standing GaN substrates

In this study, we report that free-standing GaN substrates grown by the hyd ride vapor-phase epitaxy (HVPE) are found to contain nonuniform regions wit h low crystal and optical quality located close to the top (near as-grown s urface) and bottom (near interface between GaN/sapphire) regions of substra te cross-section. We considered that the origins of these nonuniformities w ere surface reconstruction by undesired residual gas reaction after crystal growth on the top regions and the individual columns forming an irregular layer in the bottom regions by lattice mismatch and difference of thermal e xpansion coefficient between GaN films and sapphire substrate. We used cath odoluminescence imaging and spectroscopy for analyzing these nonuniform reg ions. The low quality regions with high electron concentration are easily v isualized using cathodoluminescence (CL). The coexistence of regions with l ow- and high quality allows us to explain the concurrent evidence of high s ubstrate quality in double crystal X-ray diffraction and photoluminescence (PL). (C) 2001 Elsevier Science S.A. All rights reserved.