Suppression of Inclusions in GaN Crystals Caused by Giant Steps During Na-Flux Growth through the Flux-Film-Coated Technique

Mass production of large-diameter, low-dislocation-density GaN substrates is desired for the realization of vertically structured GaN transistors. One promising approach to achieving such ideal substrates is oxide vapor phase epitaxy (OVPE) growth on GaN crystals grown by the Na-flux method (Na-flux GaN crystals). However, we found that inclusions of the Ga–Na melt within the Na-flux GaN crystals burst during OVPE growth because the OVPE growth temperature (approximately 1200 °C) is significantly higher than that of the Na-flux method (approximately 900 °C). In this study, we describe the cause of these inclusions as the development of bunched steps exceeding 10 μm in height, referred to as giant steps. Furthermore, we propose a new growth process to suppress giant steps using the flux-film-coated (FFC) technique, which utilizes the residual Ga–Na melt remaining on the crystal surface after the crystal is extracted from the melt. The burst during OVPE growth caused by inclusions was successfully suppressed by growth using the FFC process. This result enables the use of Na-flux GaN crystals as seeds for high-speed vapor phase epitaxy at high temperatures, contributing to the cost-effective production of high-quality GaN substrates.