Comparison of photoluminescence properties of Zn-and O-polar ZnO thin films

Abstract

Optical properties of Zn- and O-polar Zinc Oxide (ZnO) thin films grown by molecular beam epitaxy (MBE) were investigated by the temperature and excitation intensity dependent photoluminescence (PL) measurements. The behavior of excitonic transitions and the transitions originating from the deep-level defect centers were studied in detail. It was observed that the peak energy value of the transition, which dominates the low-temperature (10K) PL spectra for both samples, belonged to the neural donor-bound exciton (D0XA) centered at about 3.362 eV. The peak energy of the free exciton (FXA) transition for both samples was observed at 3.376 eV and 3.28 eV at low temperature and room temperature, respectively, with a total redshift of approximately 96 meV, which is compatible with the Varshni equation. On the other hand, the maximum band edge emission (D0XA) peak intensity of Zn-polar sample was approximately 2.5 times higher than of O-polar sample. At room temperature, where the free exciton transition (FXA) is dominant, this ratio drops to 1.8 times. The temperature behaviors of the band edge emission peak intensities were determined by fitting an empirical relation assuming two thermally activated nonradiative centers with different activation energy and trapping rates. It was observed that the excitation power density dependence of the PL densities follows a power law of, where k power factor was found close to one for both samples. The ratios of green luminescence peak intensities of the excitonic transition were also compared in the respective excitation intensity range.