Site localization of Cd impurities in Sapphire

G. N. DARRIBA; M. RENTERíA; H. M. PETRILLI

La Plata, Argentina

Workshop; III Workshop on Novel Methods for Electronic Structure Calculations; 2009

The semiconductor a-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, diluted magnetic semiconductors (DMS), etc. For this last use, e.g., the nature of the impurity species in the oxide host must be well known and understood in order to govern the structural and electronic properties of the system. Single electric-field gradient (EFG) obtained from ã-ã Perturbed Angular Correlation (PAC) experiments at 111In/111Cd impurity sites in a-Al2O3 have been already reported in the literature. In these works the hyperfine interaction observed are assigned to Cd probes localized at substitutional cation  sites in the corundum structure [1-2]. Nevertheless, other authors  [3] claimed that Cd could be localized at interstitial sites In this work we present a theoretical study of  Cd impurities in a-Al2O3 using ab initio calculations for different impurity charge states and site localizations (substitutional and interstitial)  and compare the EFG results (in magnitude, orientation and symmetry) with the experimental ones. The structural relaxations and electronic properties induced by the presence of the Cd impurity in both sites (substitutional and interstitial) in the a-Al2O3 lattice are discussed too. The ab initio calculations were performed with the FP-APW+lo (Full-Potential Augmented Plane Wave plus Local Orbitals) and PAW (Projector Augmented Wave) methods, both in the framework of the Density Functional Theory,  using WIEN2K and CP-PAW codes, respectively. From the comparison between theory and experiment both substitutional and interstitial scenarios for Cd atoms are possible. Finally, future possible experiments are discussed that could  resolve these ambiguity.        [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).