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 de Iguazú, Brasil

Conferencia; 14th International Conference on Hyperfine Interactions & 18th International Symposium on Nuclear Quadrupole Interaction; 2007

UNLP, CBPF, USP, UNCor, UFES

The electronic charge density ˆ(r) and its temperature dependence can be studiedby measuring the electric-field gradient tensor (EFG) that is very sensitive to smallchanges in ˆ(r). In metallic systems, the temperature dependence of the EFG is relativelywell understood in terms of lattice vibrations. In semiconductors, several behaviors havebeen observed, which have been explained (with more or less success) in terms of hostproperties or processes induced by the presence of the probes (generally impurities inthe host). In particular, the semiconductors that crystallize in the bixbyite structurehave been subject of systematic studies with 111 Cd tracers [1]. In these compounds,the presence of dynamic hyperfine interactions, and a strong positive linear temperaturedependence of the EFG appear selectively, depending on the fact that if the host cationspresent closed electronic shells or if it has incomplete electronic shells (as is the caseof the 4f-orbitals of the rare-earths) [1]. Lutetium is the only rare-earth that presentsa closed-shell electronic structure in its 3+ oxidation state, converting this oxide intoan interesting “laboratory” to check the models proposed to explain the phenomenamention above. The experimentally observed anomalous “step-like” EFG temperaturedependence at 111 Cd sites in Lu2O3 was explained in the frame of a “two-state” modelthat considers an extremely fast uctuation between two static EFG configurations [2],which enabled the experimental determination of an acceptor energy level introduced bythe Cd impurity and the estimation of the oxygen vacancy density in the sample. 20years ago, Blaha et al. have developed such first-principles calculations using the Full-PotentialLinearized Augmented Plane Wave (FLAPW) method. Increasing computerpower and progress in method developing have made it possible to applied the FLAPWmethod to fairly complicated systems like doped semiconductors [3] and to doped oxidesemiconductors [4], enabling the description of structural and electronic properties onthese systems. More recently, we have showed that an ab initio calculation can be usedto explain the temperature dependence of the EFG at Cd impurities in TiO2 [5]. In thepresent work we have applied the FLAPW method to the case of Cd-doped Lu2O3. Ourresults show that we can reproduce not only the experimental results at RT but also thetemperature dependence of the EFG. In this new insight, the EFG thermal dependencearises from the ionization of an impurity acceptor level, in good agreement with thepreviously 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.A. Errico, Hyp. Int., 158, 29, 2004.