Electronic structure of the polar ZnO{0 0 0 1}-surfaces

The surface relaxation and the electronic structure of the two polar surfaces of ZnO have been investigated using ab initio supercell calculations. The relaxation of the slab, confined by the two different polar surfaces, compressed the Zn–O double layers, in particular for the (0 0 01)-surface, where the Zn–O double-layer separation decreased to 48% of the bulk value. The calculated bandstructure revealed an occupied pz-surface state on the (0 0 01)-surface, strongly localized to the surface O-atoms. The conduction band on the (0 0 0 1)-surface was split off from the bulk region to form a 2D metallic surface state. This enabled charge transfer from the (0 0 01)- to the (0 0 0 1)-surface to quench the intrinsic dipole by a similar process that has been observed previously on the unreconstructed MgO(1 1 1)-surface.