Radiative and nonradiative excitonic transitions in nonpolar (1120) and polar (0001) and (0001) ZnO epilayers

Polarized optical reflectance and photoreflectance spectra of an out-plane nonpolar (112̄0) ZnO epilayer grown by laser molecular-beam epitaxy exhibited anisotropic exciton resonance structures according to the polarization selection rules for anisotropically strained hexagonal material. Consistently, the electric field component of its excitonic photoluminescence (PL) peak was polarized perpendicular to the [0001] axis. Different from the case for GaN, nonradiative PL lifetime at 293 K and the S parameter, which is a measure of Zn vacancy-related defect density obtained by positron annihilation spectroscopy, of the (112̄0) ZnO were comparable to those of state-of-the-art polar (0001̄) and (0001) epilayers. Since the polar epilayers exhibited pronounced exciton–polariton emissions, the negligible impact of growth direction on the defect incorporation suggests a potential use of epitaxial (112̄0) ZnO as polarization-sensitive optoelectronic devices operating in ultraviolet spectral regions.