Polarization-dependent XEOL: Comparison of peculiar near-band-edge emission of non-polar a-plane GaN and ZnO wafers

Polarization-dependent hard X-ray excited optical luminescence (XEOL) spectroscopy was used to study the properties of near-band-edge (NBE) emission of non-polar a-GaN and a-ZnO wafers. We found similar behaviors of a-GaN with a-ZnO that include the positive intensity jump and the blue shift of the NBE emission peak in the XEOL spectrum following the polarization-dependent X-ray absorption near-edge structure (XANES) as the X-ray energy is tuned across the Ga or Zn K-edge. Furthermore, as the X-ray energy is set above the K-edges, different oscillations of XEOL, also following the XANES, were observed depending upon the directions of the excitation X-ray electric field. However, the blue shift of a-ZnO is about two times larger than that of a-GaN that results from the larger polar field in a-ZnO than that in a-GaN. For both a-GaN and a-ZnO, the above K-edge excited oscillations in XANES and XEOL spectra, where the polarization is set parallel to the c-axis, are attributed to simultaneous excitations of the π-bond along the c-axis and in-plane σ-bonds, whereas only the in-plane σ-bonds are excited for the polarization perpendicular to the c-axis. Therefore, these polarization dependent oscillation features of XEOL yields that follow the fluorescence yields can be used as an alternative way to determine the crystallographic orientations.