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

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

FOz do Iguazú, Brasil

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

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 understand in order to govern the structural and electronic properties of the system. Single electric-field gradients (EFG) obtained from ã-ã Perturbed Angular Correlation (PAC) experiments on 111In/111Cd impurity sites in Al2O3 were reported and assigned to Cd probes localized at substitutional cation  sites in the corundum structure [1-2]. Nevertheless,  other work [3] claimed that Cd could be localized at interstitial sites. On the other hand, we have performed a detailed PAC investigation on 181Hf/181Ta implanted in Al2O3 singlecrystals, obtaining a single EFG assigned to Ta probes at undisturbed cationic sites in the structure.  In this work we present a theoretical study of Ta and 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 ab initio calculations were performed with the FP-LAPW  (Full Potential Linearized Augmented Wave) and PAW (Projector Augmented Wave) methods, both in the framework of the Density Functional Theory using WIEN2K and CP-PAW codes, respectively. Point charge model calculations were also performed for comparison with and without using 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. In the Ta-doped case, the partial ionization of the substitutional donor impurity is discussed in terms of the experimental EFG result.