A density functional theory analysis was made, of stoichiometric and non-stoichiometric ZnO tilt grain boundaries, that revealed under which conditions such extrinsically undoped grain boundaries could become electrically active. In the case of ZnO the self-interaction correction scheme used allows a more accurate description of the formation energies as well as the electronic levels than the local density approximation. The results obtained with the self-interaction correction scheme deviate in some crucial ways from the local density approximation results of recent years. First, stoichiometric and non-stoichiometric ZnO grain boundaries could show occupied deep levels whenever oxygen atoms were under-coordinated. Second, ZnO grain boundaries with an oxygen excess at the boundary plane could exhibit unoccupied deep levels which might account for an experimentally observed weak varistor effect found in undoped polycrystalline ZnO.

Density Functional Theory Study of Stoichiometric and Nonstoichiometric ZnO Grain Boundaries. Körner, W., Bristowe, P.D., Elsässer, C.: Physical Review B, 2011, 84[4], 045305