First-Principles Calculations of Clean and Defected ZnO Surfaces

We report on a theoretical study of the nonpolar ZnO (101̅0) and (112̅0) surfaces carried out in the framework of density functional theory, aiming to elucidate the thermodynamic and kinetic stability of the clean surface against the formation and diffusion of oxygen vacancies. At variance with other oxide materials and ZnO surfaces with different orientation, we show that, under exposure to molecular oxygen in the gas phase, no significant amounts of oxygen vacancies can be sustained by the surface, in agreement with recent Scanning Tunnelling Microscope (STM) observations. However, our calculations show also that under ultrahigh vacuum and high-temperature conditions the observation of oxygen vacancies might be possible, as reported in earlier experiments of Göpel and Lampe.(1) We characterize the defected surfaces electronic and structural properties as a function of the position of the defect with respect to the surface and discuss the diffusion paths of such defects both parallel and across the surface.