Density functional theory study of stoichiometric and nonstoichiometric ZnO grain boundaries

We present a density functional theory analysis of stoichiometric and nonstoichiometric ZnO tilt grain boundaries (GBs) that reveals under which conditions such extrinsically undoped GBs may become electrically active. In the case of ZnO the self-interaction correction (SIC) scheme used allows a more accurate description of the formation energies as well as the electronic levels than the local density approximation (LDA). The results obtained with the SIC scheme deviate in some crucial ways from the LDA results of recent years. First, stoichiometric and nonstoichiometric ZnO GBs can show occupied deep levels whenever oxygen atoms are undercoordinated. Second, ZnO GBs with an oxygen excess at the boundary plane can exhibit unoccupied deep levels which may account for an experimentally observed weak varistor effect found in undoped polycrystalline ZnO.