Effect of the band-tail states on the exciton peaks in GaN epilayers grownon sapphire substrates

Photoluminescence (PL) measurements have been carried out to investigate th e effects of the band-tail states on the exciton lines in unintentionally d oped and Mg-doped GaN epilayers grown on sapphire substrates by using plasm a-assisted molecular beam epitaxy. The results of the PL spectra for the Mg -doped epilayers show that the peak positions of the bound exciton lines sh ift to higher energy with increasing temperature within the low-temperature region. The radiative recombinations of the carriers are related to the ba nd-tail states, and the temperature-dependent blueshifts are analyzed by us ing a Gaussian distribution of charged impurities. The calculated thermal a ctivation energies of the band-edge emission lines show that those lines in Mg-doped GaN epilayers are related to ionized donor bound exciton recombin ations. These results indicate that the positions and the intensities of th e exciton peaks observed in Mg-doped GaN films are significantly affected b y the concentration of the magnesium dopant. (C) 2000 American Institute of Physics. [S0021-8979(00)00114-6].