Nonuniformities in free-standing GaN substrates

In this study, we report that free-standing GaN substrates grown by the hydride vapor-phase epitaxy (HVPE) are found to contain nonuniform regions with low crystal and optical quality located close to the top (near as-grown surface) and bottom (near interface between GaN/sapphire) regions of substrate cross-section. We considered that the origins of these nonuniformities were 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 expansion coefficient between GaN films and sapphire substrate. We used cathodoluminescence imaging and spectroscopy for analyzing these nonuniform regions. The low quality regions with high electron concentration are easily visualized using cathodoluminescence (CL). The coexistence of regions with low- and high quality allows us to explain the concurrent evidence of high substrate quality in double crystal X-ray diffraction and photoluminescence (PL).