Temperature and polarization dependence of the near-band-edge photoluminescence in a non-polar ZnO film grown by using molecular beam epitaxy

We investigated the temperature and the polarization dependence of the near-band-edge photoluminescence (PL) in non-polar (A-plane) ZnO films on R-plane sapphire grown by using plasma-assisted molecular beam epitaxy. We observed the band-edge emissions from the non-polar ZnO film at 3.449 and 3.386 eV, which were identified as a recombination of free excitons and as excitons bound to neutral donors (D⁰X) respectively. By analyzing the temperature dependent PL spectra, we attributed the transitions at 3.326 eV to the transition of free electrons in the conduction band to carriers bound to acceptors (FB). All the transitions in non-polar ZnO were blueshifted significantly by the anisotropic compressive strain. The polarized PL showed strong in-plane anisotropy of excitonic transitions even with the existence of strong strain. At low temperature, all the observed excitonic PL of non-polar ZnO was more than 90% polarized perpendicular to c-axis which lay in the film's plane. We report that the D⁰X and the FB transitions show 93% and 90% polarizations at 12 K and 82% and 80% polarization at 130 K, respectively.