Luminescence of GaN single crystals prepared by heating a Ga melt in Na-N₂ atmosphere

Colorless transparent prismatic crystals (0.5-2.0 mm long) and hopper crystals (1.0-2.5 mm long) of GaN were prepared by heating a Ga melt at 800°C in Na vapor under N₂ pressures of 7.0 MPa for 300 h. The photoluminescence (PL) spectrum of a prismatic crystal at 4 K showed the emission peaks of neutral donor-bound excitan (D⁰-X) and free exciton (X_A) at 3.472 eV and 3.478 eV, respectively, in the near band edge region. The full-width at half-maximum (FWHM) of (D⁰-X) peak was 1.9 meV. The emission peaks of a donor-acceptor pair transition (D ⁰-A⁰) and its phonon replicas were observed in a lower energy range (2.9-3.3 eV). The emission peaks of the D⁰-A⁰ and phonon replicas were also observed in the cathodoluminescence (CL) spectrum at 20 K. The (D⁰-X) PL peak of a hopper crystal at 4 K was at 3.474 eV (2.1 meV higher), having a FWHM of 6.1 meV which was over 3 times larger than that of the prismatic crystal. A strong broad band with a maximum intensity around 1.96 eV was observed for the hopper crystals in the CL spectrum at room temperature.