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 sap phire substrates with different buffer layer treatment conditions in metalo rganic chemical vapor deposition (MOCVD) growth have been studied as a func tion of temperature. At low temperatures, very well resolved spectral featu res associated with the GaN band structure were observed. From the photolum inescence (PL) data for free excitons, an accurate value of the A exciton b inding energy was found. The localization energies of the excitons bound to neutral acceptor are found to agree with Haynes' rule with the proportiona lity factor close to 0.1. The longitudinal optical (LO) phonon assisted pho toluminescence associated with both the bound and free excitons has been ob served. The characteristics of free excitons and their LO phonon replica ha ve 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 temperatur e. Besides, all samples show peaks in the energy range of 3.15-3.19 eV, who se intensities vary with the annealing time of the buffer layer. We found t hat these peaks are related to the Zn impurity, which is unintentionally in corporated due to the memory effect in the MOCVD system. Intensities of Zn impurity related recombination and yellow band luminescence are also examin ed as functions of the temperature and annealing time of the buffer layer. Based on these temperature dependent PL results, the optimum growth conditi ons have been suggested. (C) 1999 American Institute of Physics. [S0021-897 9(99)03405-2].