Effect of surface reconstruction on the electronic structure of ZnO(0001)

Abstract

Density functional calculations are presented for a (4$\ifmmode\times\else\texttimes\fi{}$4) reconstruction of the polar ZnO(0001) surface which is, in agreement with previous knowledge, characterized by one ZnO double-layer high triangular islands or pits. Its structure and electronic properties are compared to those of other reconstructions, the (1$\ifmmode\times\else\texttimes\fi{}$1) and the (2$\ifmmode\times\else\texttimes\fi{}$2) reconstructions with a Zn vacancy, and a (4$\ifmmode\times\else\texttimes\fi{}$4) reconstruction with triangular islands and pits that satisfies the electron counting rule. Among the four calculated reconstructions, the model surface is verified to be energetically preferable for a large range of oxygen chemical potential. This justifies its further exploration. Its layer and wave-vector-resolved projected density of states (PDOS) of the O atoms reveals highly surface-localized valence states well below the Fermi level. The PDOS around the Fermi level, however, is very small, which is compatible with the states not being observed by photoemission. Qualitatively experimental surface core-level shifts by Lahiri et al. [Phys. Rev. B 78, 155414 (2008)] compare favorably with the calculated electrostatic potential variations close to the ZnO(0001) surface. An ionic model with everywhere equally charged Zn and O ions turns out not to explain the potential variation obtained from density functional theory. This points towards an additional charge transfer at the surface not covered by the ionic model.