Optical characterization of bulk GaN grown by a Na-Ga melt technique

Colorless platelet and prismatic GaN crystals grown from a Na-Ga melt are characterized using room and low temperature photoluminescence (PL), reflectance, and micro-Raman scattering. The largest (∼ 3-4 mm) platelet shows a neutral donor-bound exciton (D⁰,X) PL peak with a full-width at half-maximum (FWHM) of 2.2 meV at 1.7 K. Raman scattering reveals an A₁(LO) phonon mode in this sample at 739 cm^-1, implying a free electron concentration (n) around 2-3 × 10¹⁷ cm^-3. Smaller platelets grown in a pyrolytic BN crucible show even sharper exciton peaks, down to 0.22 meV FWHM. The stronger A₁(LO) Raman peak lies at 733 cm^-1 in this case, virtually unshifted by plasmon interactions. This observation implies n is in the mid 10¹⁶ cm^-3 range or below. Residual Zn acceptors are frequently observed, and two-electron transitions identify the binding energy of the residual donor species as 33.6 meV, which may be O_N. The 2.2 eV yellow PL band is generally weak or absent. The high purity and excellent optical properties may be due to gettering of donors such as O by the Na in the melt.