Experimental and First-Principles Theoretical Study of Structural and Electronic Properties in Tantalum-Doped In2O3 Semiconductor: Finding a Definitive Hyperfine Interaction Assignment

摘要

In this work we present an experimental and theoretical study from first principles of the structural, electronic, and hyperfine properties of Ta-doped In2O3 semiconductor. The ab initio electronic structure calculations in the Ta-diluted In2O3 system enabled to obtain the structural lattice distortions and the hyperfine parameters when the Ta atom is placed at each cationic site of the bixbyite crystal structure. To this purpose we used the full-potential augmented plane wave plus local orbital (FP-APW+lo) method, within the density functional theory. The obtained results indicate that the substitutional Ta probe-impurity produces strong changes on the local structure. In addition, we performed accurate time-differential perturbed gamma-gamma angular correlations (TDPAC) key experiments in Hf-181(-> Ta-181)-implanted In2O3 samples with high crystallinity, in order to obtain high quality measurements of the electric-field gradient tensor (EFG) that unraveled the controversy settled in the literature and overcome dissimilar interpretations of previously reported TDPAC experiments. The experiments were performed at room temperature in air, after each step of a series of thermal annealing treatments in air at increasing temperatures in order to remove radiation damage and locate the Hf-181 probes at substitutional cationic sites. We succeeded to obtained two well-defined hyperfine interactions that were assigned to Ta-181 probes located at the two defect-free inequivalent cationic sites of the In2O3 crystal structure. The EFG calculations are in excellent agreement with the results of these TDPAC measurements, and show that the largest component of the diagonalized EFG, V-33, at the Ta site has mainly p character. The accuracy of the experiments together with the reliable and precise ab initio results allowed a definitive determination of the EFG at both cationic sites in this system. Formation energy calculations of defects were needed to determine the charge state of the Ta-181 impurity, which agrees with a semiconducting character for the In2O3:Ta doped system.