Comparative study of the (0001) and (0001¯) surfaces of ZnO

The authors compare the surface and optical properties of the Zn-polar (0001) and O-polar (0001¯) surfaces of bulk ZnO samples. For optical characterization, steady-state photoluminescence using a He–Cd laser was measured at 15 and 300K. At room temperature, the (0001¯) surface demonstrates nearly double the near-band-edge emission intensity seen for the (0001) surface. Using scanning Kelvin probe microscopy, the authors have measured surface contact potentials of 0.39±0.05 and 0.50±0.05V for the (0001) and (0001¯) surfaces, respectively. The resulting small difference in band bending for these two surfaces indicates that charge transfer between the surfaces is not a dominant stabilizing mechanism. Conductive atomic force microscopy studies show enhanced reverse-bias conduction in localized regions on the (0001¯) vs (0001) surface. The differences in surface conduction and band bending between the two polar surfaces can be attributed to their chemical interactions with hydrogen and water in the ambient.