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 [1], SrTiO3 [2], BaTiO3 [3], 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 [1]. 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 [2]. Concerning the O vacancy in BaTiO3, double shallow donor behavior is identified, indicating its contribution to the n-type conductivity [3]. 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 [4] and their relation to the structures of a series of Sn-O compounds [5].

[1] F. Oba, A. Togo, I. Tanaka, J. Paier, and G. Kresse, Phys. Rev. B, 77, 245202 (2008).

[2] M. Choi, F. Oba, and I. Tanaka, Phys. Rev. Lett., 103, 185502 (2009).

[3] M. Choi, F. Oba, and I. Tanaka, Appl. Phys. Lett., 98, 172901 (2011).

[4] A. Togo, F. Oba, I. Tanaka, and K. Tatsumi, Phys. Rev. B, 74, 195128 (2006).

[5] A. Seko, A. Togo, F. Oba, and I. Tanaka, Phys. Rev. Lett., 100, 045702 (2008).