Structural and electronic properties of dilute oxide semiconductors: experimental and ab initio study of Ta-doped Al2O3 single crystals

M. RENTERÍA; G. N. DARRIBA; L. A. ERRICO; E. L. MUÑOZ; J. RUNCO; P. D. EVERSHEIM

La Plata, Argentina

Workshop; International Workshop 35th Anniversary of Hiperfine Interactions at La Plata; 2005

In the last 3 years, electronic structure ab initio methods in the framework of the Density Functional Theory (DFT) have been intensively applied to describe interesting properties in nonmagnetic [1] and magnetic [2] dilute impurities in oxide semiconductors, the last appealing for spintronics [3]. The Full-Potential Linearized-Augmented Plane Waves (FP-LAPW) method, focusing at the electric-field gradient tensor (EFG) in doped systems, was first applied succesfully to the (N,P,As,Sb)-doped CdTe semiconductor [4] and at an impurity site in Cd-doped TiO2 oxide [5]. Being the electric-field gradient (EFG) a magnitude strongly dependent of the asphericity of the electronic density near the probe-nucleus, its determination enables to check the approximations made beyond the DFT and to study interesting electronic and structural properties in these systems [5]. In this work we report Perturbed-Angular-Correlation (PAC) experiments on 181Hf/181Ta-implanted corundum a-Al2O3 single crystals. The magnitude, asymmetry, and orientation (with respect to the crystalline axes) of the EFG were determined measuring the spin-rotation curves as a function of different orientations of the single crystals relative to the laboratory system. The experiments were carried out as a  function of  temperature and of diverse thermal treatments, giving rise to two hypefine interactions instead of the single interaction expected for probe atoms localized at defect-free Al sites. These results are analyzed in terms of self-consistent electronic structure FP-LAPW calculations in the dilute (1:4) Al2O3:Ta system and compared with EFG results coming from PAC experiments using 111In/111Cd tracers and for both probes (181Ta and 111Cd) in isomorphous polycrystalline a-Fe2O3. This combined study enables the determination of lattice relaxations induced by the presence of the impurity and its displacement in the [001] axis and of the charge state of a deep impurity donor level (Ta 5d) located in the band gap of the semiconductor. [1] C.Verdozzi et al., Phys.Rev.Lett. 80, 5615 (1998); A. Settels et al.,  Phys.Rev.Lett. 83, 4369 (1999); C. Stampfl et al., Phys.Rev. B 61, R7846 (2000). [2] M. S. Park et al., Phys.Rev. B 65, 161201 (2002); Z. Yang et al.,  Phys.Rev. B 67, 060402(R) (2003); W. T. Geng and K. S. Kim, Phys.Rev. B 68, 125203 (2003; L.A. Errico, M. Rentería, and M. Weissmann, Phys.Rev. B, (2005), in press. [3] H. Ohno, Science 281, 951 (1998); T. Dietl et al., Science 287, 1019 (2000). [4] S. Lany, P. Blaha, J. Hamann, V. Ostheimer, H. Wolf, and T. Wichert, Phys.Rev. B 62, R2259 (2000). [5] L.A. Errico, G. Fabricius, M. Rentería, P. de la Presa, and M. Forker, Phys.Rev.Lett. 89, 55503 (2002);  L.A. Errico, G. Fabricius, and M. Rentería, Phys.Rev. B 67, 144104 (2003).