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 iguazú, Brasil.

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

The electronic charge density r(r) and its temperature dependence can be studied by measuring the electric-field gradient tensor (EFG) that is very sensitive to small changes in r(r). In metallic systems, the temperature dependence of the EFG is relatively well understood in terms of lattice vibrations. 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 probes (generally impurities in the host). In particular, the semiconductors that crystallize in the bixbyite structure have been subject of systematic studies with 111Cd tracers [1]. In these compounds, the presence of dynamic hyperfine interactions, and a strong positive linear temperature dependence of the EFG appear selectively, depending on the fact that if the host cations present closed electronic shells or if it has incomplete electronic shells (as is the case of the 4f-orbitals of the rare-earths) [1]. 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 mention above. The experimentally observed anomalous “step-like” EFG temperature dependence at 111Cd sites in Lu2O3 was explained in the frame of a “two-state” model that considers an extremely fast fluctuation between two static EFG configurations [2], which enabled the experimental determination of an acceptor energy level introduced by the Cd impurity and the estimation of the oxygen vacancy density in the sample. 20 years ago, Blaha et al. have developed such first-principles calculations using the Full-Potential Linearized Augmented Plane Wave (FLAPW) method. Increasing computer power and progress in method developing have made it possible to applied the FLAPW method to fairly complicated systems like doped semiconductors [3] and to doped oxide semiconductors [4], enabling the description of structural and electronic properties on these systems. More 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 [5]. In the present work we have applied the FLAPW method to the case of Cd-doped Lu2O3. Our results show that we can reproduce not only the experimental results at RT but also the temperature dependence of the EFG. In this new insight, the EFG thermal dependence arises from the ionization of an impurity acceptor level, in good agreement with the previously proposed two-state model.  1 J Shitu et al., Mod.. Phys. Lett. B 12, 281, 1998. 2 L Errico et al., Hyp. Int. 120/121, 457, 1999. 3 S Lany et al., PRB 62, R2259, 2000. 4 L A. Errico et al., PRL 89, 55503, 2002. 5 L Errico, Hyp. Int., 158, 29, 2004.