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 , AND P.D. EVERSHEIM

Foz de Iguazú, Brasil

Conferencia; 14th International Conference on Hyperfine Interactions & 18th International Symposium on Nuclear Quadrupole Interaction; 2007

UNLP, CBPF, USP, UNCor, UFES

The semiconductor alpha-Al2O3 (sapphire) have important technological applications dueto its mechanical, thermal, and optical properties. Some of these applications are relatedwith it use as substrates, radiation detector crystals, etc. An important issue to be ad-dressedis the site characterization of impurities in the oxide host . An important tool tostructural characterization on the atomic scale is the use of hyperfine techniques combi-nedwith ab initio electronic structure calculations. Single electric-field gradients (EFG)obtained from - Perturbed Angular Correlation (PAC) experiments on 111 In/111 Cdimpurity sites in Al2O3 were reported and assigned to Cd probes localized at the substi-tutionalAl site in the corundum structure [1-2]. On the other hand, in another study inthe literature [3] the authors claimed that Cd could be localized at interstitial sites. Usinganother probe, we perform here a detailed PAC investigation on 181 Hf/181 Ta implantedin Al2O3 single crystals. Our results shows that a single EFG can be assigned to Taprobes at undisturbed cationic sites in this structure. To study the EFGs inferred fromPAC measurements, we present a theoretical investigation of Ta and Cd impurities inalpha-Al2O3 using ab initio calculations. The calculations of the EFG tensor are performedin a super-cell approach , for different charge states and site localizations of the impu-rity(both substitutional and interstitial), using the Full Potential Linearized AugmentedWave (FP-LAPW) and the Projector Augmented Wave (PAW) methods . Point chargemodel calculations are also discussed using (or not) the structural relaxations predictedby the ab initio calculations . In the Cd-doped system, the comparison between theoryand experiment agrees with both ( substitutional and interstitial) scenarios for Cd atoms.The Ta-doped case shows an interesting EFG dependence on ionization that is discussedin the light of the experimental results.[1] S. Habenicht, D. Lupascu, M. Neubauer, M. Uhrumacher, K.P. Lieb and theISOLDE-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).