A comprehensive first-principles investigation was made, of native point defects in ZnO, based upon density functional theory within the local density approximation as well as the LDA+U approach for overcoming the band-gap problem. An O deficiency, manifested in the form of O vacancies and Zn interstitials, had long been cited as the source of a commonly observed unintentional n-type conductivity in ZnO. However, contrary to conventional wisdom, it was found that native point defects were very unlikely to be the cause of unintentional n-type conductivity. Oxygen vacancies, which have most often been cited as the cause of unintentional doping, were deep rather than shallow donors and had high formation energies in n-type ZnO (hence unlikely to form). The Zn interstitials were shallow donors, but they also had high formation energies in n-type ZnO and were fast diffusers; with migration barriers as low as 0.57eV. They were therefore unlikely to be stable. The Zn antisites were also shallow donors, but their high formation energies (even under Zn-rich conditions) made them unlikely to be stable under equilibrium conditions. However, a different low-energy atomic configuration was identified, for Zn antisites, that might play a role under non-equilibrium conditions such as irradiation. The Zn vacancies were deep acceptors and were probably related to the frequently observed green luminescence. They acted as compensating centers in n-type ZnO. The O interstitials had high formation energies. They could occur as electrically neutral split interstitials in semi-insulating and p-type materials or as deep acceptors at octahedral interstitial sites in n-type ZnO. The O antisites had very high formation energies and were unlikely to exist in measurable concentrations under equilibrium conditions. Based upon results for migration energy barriers, activation energies for self-diffusion were calculated and defect-annealing temperatures were estimated. The results provided a guide to making more refined experimental studies of point defects in ZnO and of their influence upon the control of p-type doping.

Native Point Defects in ZnO. A.Janotti, C.G.Van de Walle: Physical Review B, 2007, 76[16], 165202