Status and perspectives of the ammonothermal growth of GaN substrates

The seeded growth of GaN by the ammonothermal method is updated. By using a polycrystalline GaN nutrient together with higher concentration of NaNH2, we have attained a long-term ammonothermal growth to 90 days. The as-grown crystal showed an improved structural quality to the previously reported GaN bulk crystals grown by the seeded ammonothermal growth, demonstrating the feasibility of the ammonothermal method for a high-quality bulk GaN growth. The threading dislocation density estimated by plan-view transmission electron microscopy (TEM) observations was <1×106 cm−2 for the Ga-face and 1×107 cm−2 for the N-face. There was no dislocation generation observed at the interface on the Ga-face although a few dislocations were generated at the interface on the N-face. The chemical etching together with X-ray diffraction measurements revealed macroscopic grains in the grown crystals.