"After-effects" in (111In-->)111Cd-doped Al2O3 semiconductor: a modelization from first principles.

M. RENTERÍA; R. VIANDEN; G. N. DARRIBA

Leuven

Conferencia; International Conference on Hyperfine Interactions and their Applications; 2016

The phenomena called "electron-capture after-effects" (ECAE) in the pioneering works from La Plata group in the eighties is produced by the electronic relaxation process that occur in the probe-atom 111Cd subsequent to the electron-capture (EC) nuclear decay of its father nuclide 111In. The ECAE is reflected in time-differential perturbed γ-γ angular correlations (TDPAC) experiments by the presence of time-dependent hyperfine (dynamic) interactions, reversible with the measuring temperature. We propose here that the electronic relaxation of 111Cd generates different electronic configurations at the probe-atom, and therefore different electric-field gradients (EFG), which originates the observed dynamic hyperfine interaction. These electronic configurations may survive during the time window of the TDPAC measurement (τ 1/2=84.1 ns, in this case), from a highly ionized initial atomic charge state (generated after the EC nuclear decay) to a final stable state that is not necessarily the ground neutral state, and which depends on the temperature and nature of the host system. In order to validate the proposed model, we present in this work ab initio calculations of the electronic structure and hyperfine properties in the Cd-doped α-Al2O3 system, comparing with TDPAC results on (111In(EC)→)111Cd-doped α-Al2O3 semiconductor [1,2]. The calculations were performed using the full-potential augmented plane-wave plus local orbitals (FP-APW+lo) method, implemented in the WIEN2k code. We studied carefully the EFG as a function of the charge state of the impurity, taking into account the acceptor character of the Cd impurity when it replaces an Al atom. We showed that the different final stable electronic configurations reached after the relaxation process affects the measured EFG value. Small variations of the Fermi level near the top of the valence band (partially filling the acceptor impurity level introduced by the Cd impurity) generates EFGs that are in agreement with those experimentally observed that correspond to the equilibrium charge states obtained experimentally at each temperature after the probe-atom electronic relaxation.[1] S. Habenicht, D. Lupascu, M. Neubauer, M. Uhrmacher, and K. P. Lieb, Hyp. Int. 120/121, 445 (1999).[2] J. Penner and R. Vianden, Hyp. Int. 158, 389 (2005).