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 Y P. D. EVERSHEIM

La Plata, Bs As, Argentina

Workshop; 35th anniversary of Hyperfine Interactions at La Plata International Workshop; 2005

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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.