Site localization of donor and acceptor impurities in sapphire: an ab initio and PAC study

G.N. DARRIBA; M. RENTERÍA; L.A. ERRICO; E.L. MUÑOZ; H.M. PETRILLI; P.D. EVERSHEIM

Foz do Iguazu

Conferencia; XIV International Conference on Hyperfine Interactions & XVIII International Symposium on Nuclear Quadrupole Interaction; 2007

Departamento de Fisica de Materiales - Instituo de Física - USP

The semiconductor -Al2O3 (sapphire) have important technological applications due to its mechanical, thermal, and optical properties. Some of these applications are related with it use as substrates, radiation detector crystals, etc. An important issue to be addressed is the site characterization of impurities in the oxide host. An important tool to structural characterization on the atomic scale is the use of hyperfine techniques combined with ab initio electronic structure calculations. Single electric-field gradients (EFG) obtained from - Perturbed Angular Correlation (PAC) experiments on 111In/111Cd impurity sites in Al2O3 were reported and assigned to Cd probes localized at the substitutional Al site in the corundum structure [1-2]. On the other hand, in another study in the literature [3] the authors claimed that Cd could be localized at interstitial sites. Using another probe, we perform here a detailed PAC investigation on 181Hf/181Ta implanted in Al2O3 single crystals. Our results shows that a single EFG can be assigned to Ta probes at undisturbed cationic sites in this structure. To study the EFGs inferred from PAC measurements, we present a theoretical investigation of Ta and Cd impurities in -Al2O3 using ab initio calculations. The calculations of the EFG tensor are performed in a super-cell approach , for different charge states and site localizations of the impurity (both substitutional and interstitial), using the Full Potential Linearized Augmented Wave (FP-LAPW) and the Projector Augmented Wave (PAW) methods. Point charge model calculations are also discussed using (or not) the structural relaxations predicted by the ab initio calculations . In the Cd-doped system, the comparison between theory and experiment agrees with both (substitutional and interstitial) scenarios for Cd atoms. The Ta-doped case shows an interesting EFG dependence on ionization that is discussed in the light of the experimental results. [1] S. Habenicht, D. Lupascu, M. Neubauer, M. Uhrumacher, K.P. Lieb and the ISOLDE-Collaboration, Hyp. Int. 120/121, 445-448, (1999). [2] J. Penner and R. Vianden, Hyp. Int. 158, 389-394, (2005). [3] J.G. Marquez, A.A. Melo, J.C. Soares, E. Alves, M.F. da Silva, K. Freitag, Nucl. Instr. And Meth. In Phys. Res B 106, 602-605, (1995).