Thermal treatment effect of the GaN buffer layer on the photoluminescence characteristics of the GaN epilayer

Photoluminescence properties of undoped wurtzite GaN epilayers grown on sapphire substrates with different buffer layer treatment conditions in metalorganic chemical vapor deposition (MOCVD) growth have been studied as a function of temperature. At low temperatures, very well resolved spectral features associated with the GaN band structure were observed. From the photoluminescence (PL) data for free excitons, an accurate value of the A exciton binding energy was found. The localization energies of the excitons bound to neutral acceptor are .found to agree with Haynes' rule with the proportionality factor close to 0.1. The longitudinal optical (LO) phonon assisted photoluminescence associated with both the bound and free excitons has been observed. The characteristics of free excitons and their LO phonon replica have been studied in detail with the temperature variation and related to the point defects. The behavior of the peak energy and the full width at half maximum of the exciton band as a function of temperature reveals the change of the dominant recombination mechanism of the exciton with the temperature. Besides, all samples show peaks in the energy range of 3.15-3.19 eV, whose intensities vary with the annealing time of the buffer layer. We found that these peaks are related to the Zn impurity, which is unintentionally incorporated due to the memory effect in the MOCVD system. Intensities of Zn impurity related recombination and yellow band luminescence are also examined as functions of the temperature and annealing time of the buffer layer. Based on these temperature dependent PL results, the optimum growth conditions have been suggested.