The Role of Metallic Impurities in Semiconductor Oxides: First-Principles Calculations and TDPAC Experiments

L.A. ERRICO; G. FABRICIUS; M. RENTERÍA

Pucón

Conferencia; XI Latin American Congress of Surface Science and its Applications; 2003

Universidad de Chile

We present an ab initio study of the electronic structure and the structural relaxations introduced by aceptor (Cd) and donor (Ta) impurities when they replace cations in a series of binary oxides. Calculations were performed with the Full-Potential Linearized Augmented Plane Waves (FLAPW) method that allows us to treat the electronic structure and the atomic relaxations in a fully self-consistent way. We considered different charge states for each impurity and study the dependence of the electronic properties and structural relaxations on these charge states. In the case of TiO2, for example, we have obtained that the electric field gradient (EFG) tensor at Cd impurities changes the orientation of its principal component with respect to the pure system, in agreement with a TDPAC experiment that we designed and performed to check this point [1]. We have proved that this change is due to the large anisotropic relaxations that Cd introduces in its nearest oxygen neighbors. On the other side, when a Ta impurity is introduced in TiO2 no change in the principal component orientation of the EFG tensor is predicted, also in agreement with experiment. While electronic properties are strongly dependent on the charge states of the impurity in the case of Cd, the opposite holds for the Ta impurity. We study compounds with a variable degree of ionicity and bondlenghts: TiO2, SnO2, and In2O3 and obtained that simple models (like the point charge model with antishielding factors) can not describe the measured EFG tensors even in systems with a high degree of ionicity. Our combined experimental-ab initio approach allows us to obtain a new insight on the role that metal impurities play in semiconductor oxides.   This research was partially supported by Fundación Antorchas, ANPCyT (PICT 03-03727), CONICET, Argentina, and TWAS, Italy.   References   [1] L. A. Errico et al.,   Phys. Rev. Lett. 89, 055503 (2002).