Optical spectroscopy of bulk GaN crystals grown from a Na–Ga melt

Colorless transparent platelet and prismatic GaN crystals up to 3–4 mm, grown from a Na–Ga melt (0.6–0.7 mol fraction of Na) at temperatures of 700–800 °C in a modest (5 MPa) pressure of N2, are characterized using Raman scattering, room and low temperature photoluminescence, and reflectance. They exhibit sharp free and bound exciton luminescence features (down to 0.22 meV full width at half maximum), including multiple excited states. Residual Mg and Zn acceptors and a 33.6 meV donor (possibly ON) are identified. Raman spectra suggest free carrier concentrations down to the low to mid 1016 cm−3 range.