Effect of Ga composition of a Ga–Na melt on the dislocation density in the coalescence region of GaN crystals grown by the Na-flux multi point seed technique

Low threading dislocation density (TDD) GaN substrates are essential for fabricating vertical GaN power devices. In this study, we fabricated large-diameter, low-TDD GaN wafers using the Na-flux multipoint seed (MPS) method. However, the MPS method leads to regions with high-TDD values (>105 cm−2) at the crystal coalescence regions. We focused on the Ga composition of a Ga–Na melt and investigated TDD reduction by modifying the crystal growth mode. As the Ga composition was reduced from 27 to 19 mol. %, crystal shape uniformity improved, and the maximum TDD above the coalescence regions decreased from 4.41 × 105 to 3.01 × 105 cm−2. This reduction suggests that the improved shape uniformity promotes the efficient concentration of dislocations toward the coalescence boundaries. We also found that reducing the Ga composition effectively suppressed the formation of inclusions in the GaN crystals, contributing to the reduction of inclusion-induced dislocations. At 19 mol. % Ga composition, the mean TDD in regions excluding coalescence boundaries was in the low 104 cm−2 range. These findings are expected to contribute to the fabrication of high-quality GaN substrates using the Na-flux method.