Electronic structures of ZnO(0001)-Zn and (000−1)-O polar surfaces

We investigated the electronic structures of ZnO(0001)-Zn and (000−1)-O surfaces by means of first-principles calculations. According to the results of the relaxation and the electrostatic criterion, the surface geometric relaxations and the charge transfer almost take place on the outer four double layers. For the (0001)-Zn surface, steep surface states appear in the band gap of bulk ZnO and follow the bottom of bulk conduction band. Moreover, Fermi level shifts up into the conduction band, which leads to the n-type conduction behaviour of (0001)-Zn surface. The dispersed partial charge densities derived from the Zn-4s state spread evenly on the surface and apparently lead to difficulty of getting an atom-resolved STM image. For the (000−1)-O surface, flat surface states emerge above the top of the valence band of bulk ZnO as Fermi level shifts down a little into the valence band. Based on that, the (000−1)-O surface can be predicted to have the p-type conductivity. The localized partial charge densities derived from the O-2p state makes it possible to get an atom-resolved STM image within the low bias voltage.