Rich variety of defects in ZnO via an attractive interaction between O vacancies and Zn interstitials: origin of n-type doping

Native defects in oxide semiconductors: a density functional approach

We report a semilocal and hybrid Hartree-Fock density functional study of native defects in three oxide semiconductors: ZnO, SrTiO(3), and SnO. The defect that is responsible for the n-type conductivity of ZnO has been debated, in which the O vacancy, Zn interstitial, their complexes, and residual H impurity are considered candidates. Our results indicate that the O vacancy induces a deep and localized in-gap state, whereas the Zn interstitial is a shallow donor and hence can be a source of the carriers. In view of the formation energies, the O vacancy is likely to form with a substantial concentration under O-poor conditions, but the Zn interstitial is unlikely. We thus propose that the O vacancy is relevant to the nonstoichiometry of ZnO and that a source other than the native defects, such as the H impurity, needs to be considered for the n-type conductivity. For SrTiO(3), the O vacancy and its complexes have been regarded as the origins of some of the remarkable electrical and optical properties. We suggest significant roles of the Ti antisite for a new insight into the defect-induced properties. Two types of Ti antisite, both of which are off-centered from the Sr site but toward different directions, exhibit low formation energies under Ti-rich conditions as does the O vacancy. They can explain optical properties such as visible-light emission, deep-level absorption, and the ferroelectricity observed in reduced SrTiO(3). As an example of p-type conductors, SnO has been investigated with a focus on the acceptor-like native defects. Under O-rich conditions, the Sn vacancy and O interstitial are found to be energetically favorable. The Sn vacancy induces shallow acceptor levels and can therefore be a source of carriers. The O interstitial shows no in-gap levels and hence it is inactive in terms of the carrier generation and compensation. However, this defect is a key to the understanding of the structures of intermediate compounds between SnO and SnO(2).

Enhancing the thermoelectric power factor with highly mismatched isoelectronic doping

We investigate the effect of O impurities on the thermoelectric properties of ZnSe from a combination of first-principles and analytic calculations. It is demonstrated that dilute amounts of O impurities introduce peaks in the density of states (DOS) above the conduction band minimum, and that the charge density near the DOS peaks is substantially attracted toward O atoms due to their high electronegativity. The impurity-induced peaks in the DOS result in a sharp increase of the room-temperature Seebeck coefficient and power factor from those of O-free ZnSe by a factor of 30 and 180, respectively. Furthermore, this effect is found to be absent when the impurity electronegativity well matches the host that it substitutes. The results suggest that highly electronegativity-mismatched alloys can be designed for high performance thermoelectric applications.