Because of the crucial roles of point defects in the physical properties of pristine and doped oxide semiconductors, a fair amount of experimental research has been devoted to their characterization in previous decades. However, the understanding of the defects is limited, particularly at the atomistic and electronic level. A density functional approach is useful for the study of the defects and has provided various insights into their characteristics. In this talk, I will present our recent results on the defects in several oxide semiconductors, ZnO , SrTiO3 , BaTiO3 , and SnOx [4, 5], obtained using semilocal and hybrid density functional calculations. The defect that is responsible for the n-type conductivity of ZnO has been debated, in which the O vacancy, Zn interstitial, and residual H impurity are prime candidates. Our results indicate that the O vacancy induces an exceedingly deep electronic state, while the Zn interstitial and H impurity are shallow donors and can be sources of carriers . However, only the formation of the O vacancy and H impurity is likely under thermal equilibrium in view of their formation energies. We thus propose that the O vacancy is relevant to the nonstoichiometry of ZnO and a source other than native defects, such as the H impurity, should be considered for the n-type conductivity. For SrTiO3, we suggest important roles of Ti antisite defects as a new insight into the defect-induced properties. The antisite defects are energetically favorable as well as the O vacancy and can explain interesting electrical and optical properties of SrTiO3 . Concerning the O vacancy in BaTiO3, double shallow donor behavior is identified, indicating its contribution to the n-type conductivity . In addition, a metastable configuration of the O vacancy is found, which shows an off-symmetric atomic structure in conjunction with deep localized electronic states in the band gap. Other results to be presented include the energetics and electronic structure of native defects in p-type SnO  and their relation to the structures of a series of Sn-O compounds .
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