Optical properties of non-polar a-plane ZnO single crystal

In this study, photoluminescence (PL) spectroscopy is used to track the temperature dependence of the exciton energy in non-polar a-plane ZnO single crystals grown by using the hydrothermal method. The PL spectrum exhibits the bound-exciton emission and the free-exciton emission, along with their multi-phonon replicas at low temperatures. The study showed the dominant presence of the bound excitonic transition at temperatures below 85 K or free excitonic transitions at higher temperatures for the observed PL spectrum whereas the visible emission bands show anomalous temperature-dependent behaviors. The free exciton emission can be observed at temperatures up to room temperature. The observed features exhibit redshifts with increasing temperature, and the temperature dependence of the peak positions was analyzed by using the empirical Varshni empirical equation.