Structural and electronic properties of Ta-doped sapphire semiconductor: new experiments and ab initio calculations

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

San Pablo, Brasil

Workshop; 13th Brazilian Workshop of Semiconductors Physics; 2007

USP, Brasil.

The semiconductor a-Al2O3 (zapphire) have important technological applications due to its mechanical, thermal, and optical properties. Some of these applications are related with its use as substrates, radiation detector crystals, diluted magnetic semiconductors (DMS), etc. In this work new γ-γ Perturbed Angular Correlation (PAC) experiments in radioactive 181Ta-implanted sapphire single crystals are presented. In previous experiments [Phys. Stat. Sol. (b) 242, 1928 (2005)], only one of the two observed hyperfine interactions, with a very small population, could be assigned to 181Ta probes located at substitutional cation sites, due to the irreversible process generated by a high temperature thermal annealing in air performed to eliminate the radiation damage. In the new one, the sample was carefully annealed at lower temperatures in many steps achieving to locate 100 % of the impurities at substitutional cation sites. Two hyperfine interactions were observed, one of them assigned to Ta impurities at free of defects cation sites and the other with distant damage. The PAC results are compared with PCM predictions and ab initio calculations. The ab initio calculations were performed with the FP-LAPW method in the framework of the Density Functional Theory, using WIEN2K code. The PCM calculations were performed with and without the structural relaxations predicted by the FP-LAPW calculations. The excellent agreement between the FP-LAPW predictions and the experimental results for the electric-field gradient (in magnitude, orientation and symmetry) enables to determine the structural relaxations introduced by the impurity in the host lattice and the charge state of the impurity level introduced in the band gap of the semiconductor.  This results in a double donor level, which is not ionized at room temperature.