Electronic defects in semiconductors: using ab initio calculations and PAC experiments to follow impurity levels populations

L. A. ERRICO; M. RENTERÍA; A. G. BIBILONI; G.N. DARRIBA

Foz do Iguazu

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

Departamento de Fisica de Materiales - Instituo de Física - USP

The electronic charge density in a solid and its temperature dependence can be studied by measuring the electric-field-gradient tensor (EFG), which is very sensitive to small changes in (r). The Time-Differential Perturbed-Angular-Correlation technique (PAC) [1] is specially suited to study the temperature dependence of the EFG because its sensitivity is temperature independent. In metallic systems, the temperature dependence of the EFG is relatively well understood in terms of lattice vibrations. But, in semiconductors several behaviors have been observed, which have been explained (with more or less success) in terms of host properties or processes induced by the presence of the PAC probes (generally impurities in the host). In particular, the semiconductors that crystallize in the bixbyite structure have been subject of systematic studies with 111Cd probes. In these oxides, the presence of dynamic hyperfine interactions or a strong temperature dependence of the EFG appear selectively, depending on the fact that if the host cations present closed electronic shells (such as Sc, Y, and In) or if they have incomplete electronic shells, as is the case of the 4f-orbitals of the rare-earths [2]. Lutetium is the only rare-earth that presents a closed-shell electronic structure in its 3+ oxidation state, converting this oxide into an interesting ?laboratory? to check the models proposed to explain the phenomena mentioned above. For a long time, the EFG in solids was estimated performing calculations based in the point-charge model (PCM). These calculations depend on empirical parameters and often deviate significantly from the experimental values. To interpret the experimental EFG and to extract the electronic structure of the system under study, ab initio calculations are indispensable. About 15 years ago, Blaha et al. [3] developed such calculations using the Full-Potential Linearized Augmented Plane Wave (FLAPW) method [4]. In recent years, the FLAPW method was applied with success to complex systems like doped semiconducting oxides [5]. Recently, we have showed that an ab initio calculation can be used to explain the temperature dependence of the EFG at Cd impurities in TiO2 [6]. In this work we have applied the FLAPW method to the case of Lu2O3 doped with Cd.