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

Photoluminescence (PL) measurements have been carried out to investigate the effects of the band-tail states on the exciton lines in unintentionally doped and Mg-doped GaN epilayers grown on sapphire substrates by using plasma-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 shift to higher energy with increasing temperature within the low-temperature region. The radiative recombinations of the carriers are related to the band-tail states, and the temperature-dependent blueshifts are analyzed by using a Gaussian distribution of charged impurities. The calculated thermal activation energies of the band-edge emission lines show that those lines in Mg-doped GaN epilayers are related to ionized donor bound exciton recombinations. These results indicate that the positions and the intensities of the exciton peaks observed in Mg-doped GaN films are significantly affected by the concentration of the magnesium dopant.