Electric-field gradient characterization at 181Ta impurities in sapphire single crystals

We report Perturbed-Angular-Correlation (PAC) experiments on corundum Al2O3 single crystals implanted with Hf-181/Ta-181 ions at the ISKP at Bonn and measured at La Plata with high efficiency and time-resolution. The magnitude, asymmetry, and orientation (with respect to the crystalline axes) of the electric-field gradient (EFG) tensor were determined measuring the spin-rotation curves as a function of different orientations of the single crystals relative to the detector system. These results are analyzed in the framework of point-charge model and ab initio Full-Potential Linearized-Augmented Plane Wave calculations, and compared with EFG results coming from PAC experiments with In-111/Cd-111 impurities. This combined study enables the determination of lattice relaxations induced by the presence of the impurity and the state of charge of a deep impurity donor level in the band gap of the semiconductor. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.