Photoluminescence of GaN epitaxial layers

The characteristic photoluminescence lines in GaN undoped epitaxial layers have been investigated in a large number of samples at low temperature. The lines observed in the high-energy part of the photoluminescence spectra are called I₁, I₂, I₃ and I₄ with decreasing photon energy. I₁, situated at 3.476+or-0.002 eV and I₄ at 3.311+or-0.002 eV are reported here for the first time. Their dependences on temperature and excitation intensity have also been measured. These lines are proposed to be due to bound exciton emission at localised centres and to donor-acceptor pair recombination caused by imperfections and nitrogen vacancies in the crystal. The thermal activation energies are determined as Delta E₁=1.9 meV, Delta E₃=3.4 meV and Delta E₄=14.4 meV. A new value for the lowest transverse free exciton in GaN at 3.479+or-0.002 eV is suggested.