Surface Polarity-Dependent Cathodoluminescence in Hydrothermally Grown ZnO Hexagonal Rods

Vertically oriented ZnO hexagonal rod arrays were produced by site-specific hydrothermal growth. These ZnO nanostructures possess well-defined surfaces composed of Zn-terminated (0001) planes (for the top surface) and nonpolar {10 (1) over bar0} planes (for the rectangular side faces), thereby providing an opportunity to investigate the correlation between surface structures and local optical properties. The cathodoluminescence (CL) spectra of various sized ZnO rods revealed that the ratio of the deep-level emission (DLE) to the near-band-edge emission (NBE) peak intensities increased continuously with increasing diameter, which is contradictory to the general trend that the DLE increases by enhancing the surface-to-volume ratio. From the CL spectral mapping, significant NBE quenching was observed at the Zn-terminated (0001) surface, whereas the DLE was bright for every surface. Based on these observations, the anomalous behavior observed in our ZnO rods can be attributed to the surface polarity-dependent NBE characteristics. We have also shown that the surface defects associated with the NBE quenching could be eliminated by appropriate thermal annealing.