Intrinsic photoluminescence of M-plane GaN films on LiAlO2 substrates

Polarized photoluminescence spectra of M-plane GaN films grown on LiAlO2 substrates demonstrate the existence of a large polarization anisotropy in the near-band-gap excitonic transitions, which is consistent with the reduction of the in-plane crystal symmetry and the anisotropic strain generated by the lattice mismatch between GaN and LiAlO2. Band structure calculations based on the k∙p formalism support the observed experimental results and allow us to explain the temperature dependence of the photoluminescence energy. From the results of these calculations and the measured excitonic transition energies, we accurately determined the in-plane strain. The strain relaxation as a function of temperature and the role of the film thickness in the in-plane strain anisotropy are also discussed.