Recent first-principles studies of point defects in ZnO were reviewed with a focus on native defects. Key properties of defects, such as formation energies, donor and acceptor levels, optical transition energies, migration energies and atomic and electronic structure, were evaluated using various approaches including the local density approximation and generalized gradient approximation to DFT, LDA+U/GGA+U, hybrid Hartree-Fock density functionals, sX and GW approximation. Results significantly depend on the approximation to exchange correlation, the simulation models for defects and the post-processes to correct shortcomings of the approximation and models. The choice of a proper approach is, therefore, crucial for reliable theoretical predictions. First-principles studies have provided an insight into the energetics and atomic and electronic structures of native point defects and impurities and defect-induced properties of ZnO. Native defects that were relevant to the n-type conductivity and the non-stoichiometry toward the O-deficient side in reduced ZnO were debated. It was suggested that the O vacancy was responsible for the non-stoichiometry because of its low formation energy under O-poor chemical potential conditions. However, the O vacancy was a very deep donor and could not be a major source of carrier electrons. The Zn interstitial and anti-site were shallow donors, but these defects were unlikely to form at a high concentration in n-type ZnO under thermal equilibrium. Therefore, the n-type conductivity was attributed to other sources such as residual impurities including H impurities with several atomic configurations, a metastable shallow donor state of the O vacancy, and defect complexes involving the Zn interstitial. Among the native acceptor-type defects, the Zn vacancy was dominant. It was a deep acceptor and could not produce a high concentration of holes. The O interstitial and anti-site were high in formation energy and/or were electrically inactive and, hence, were unlikely to play essential roles in electrical properties. Overall defect energetics suggested a preference for the native donor-type defects over acceptor-type defects in ZnO. The O vacancy, Zn interstitial and Zn anti-site have very low formation energies when the Fermi level was low. Therefore, these defects were expected to be sources of a strong hole compensation in p-type ZnO. For the n-type doping, the compensation of carrier electrons by the native acceptor-type defects could be mostly suppressed when O-poor chemical potential conditions, i.e. low O partial pressure conditions, were chosen during crystal growth and/or doping.

Point Defects in ZnO - an Approach from First Principles. Oba, F., Choi, M., Togo, A., Tanaka, I.: Science and Technology of Advanced Materials, 2011, 12[3], 034302