Nature of the 2.8-eV photoluminescence band in Si-doped GaN

Investigation on the defect-related luminescence centered around 2.8 eV in Si-doped GaN epifilms is presented. It is found that the mechanism of this blue emission (BL) is quite different from that of the yellow luminescence (YL). By comparing the photoluminescence (PL) spectra obtained from either front-side or backside excitation and combining with the results of the BL and YL-related PL excitation and the secondary-ion-mass spectroscopy measurements, we propose that the 2.8-eV emission is the transition from the substitutional oxygen donor (O_N) level at 0.25 eV above the conduction band to the V_Ga-O_N complex acceptor. The energy level of the V_Ga-O_N complex is found to be at 0.8 eV above the valence-band edge which is consistent with the theoretical calculation. In addition, we show that the VGa defect plays a key role in the link between the time evolution of the YL and BL spectra in GaN samples.