Impact of Substrate Morphology and Structural Defects in Freestanding Gallium Nitride on the Breakdown Characteristics of GaN-on-GaN Vertical Diodes

The morphology of GaN substrates grown by hydride vapor-phase epitaxy (HVPE) and by ammonothermal methods has been correlated with reverse-bias stress testing applied to GaN-on-GaN p-i-n diodes. GaN substrates grown by HVPE showed ordered, well-separated arrays of surface features when observed using x-ray topography (XRT). All fabricated diodes that overlapped with these features had reverse-bias voltages typically of less than 100 V before reaching a critical leakage current limit that was set at 10(-6) A. In contrast, diodes not overlapping with such features reached reverse-bias voltages exceeding 300 V for the same leakage current limit. After surface etching, the HVPE substrate showed evidence for defect clusters and macro-pits. XRT images of the ammonothermal GaN substrate revealed no visible features. However, some diodes fabricated on the ammonothermal substrate still failed to reach reverse-bias voltages comparable to those of the HVPE-grown samples. Diodes on HVPE and ammonothermal substrates with low breakdown voltage showed crater-like surface damage. Progressive ion milling across such failed devices revealed the presence of voids and threading dislocations that penetrated deep into the substrate (similar to 25 mu m); these features were not observed in diodes with high reverse-bias voltages and low leakage current. This work emphasizes the potential unsuspected impact of substrate morphology in limiting the performance of vertical GaN devices.