Revistting "after-effects" in oxides: an ab initio approach

G. N. DARRIBA; M. RENTERÍA

Brasov

Conferencia; International conference on HYPERFINE interactions and their applications; 2021

In semiconductor and insulator oxides, the presence of metallic impurities induces interesting features in the density of electronic states depending on the impurity`s real character (donor or acceptor) in each host system, besides the nominal character of the impurity (donor, acceptor or isovalent). In particular, certain binary oxides studied by time-differential γ−γ perturbed angular correlation (TDPAC) spectroscopy with the (111In→)111Cd probe (usually an impurity) present time-dependent hyperfine interactions (HFIs) (in opposition to the usual static interactions originated from static electronic configurations) attributed to the so-called ?after-effects?: electronic relaxation processes following the electron-capture (EC) decay of the 111In parent (ECAE). The EC creates many electronic holes at the 111Cd atom through Auger processes, diffusing almost all of them (from the probe site) in the valence band fast enough (around 10-12 s) in such a way that this dynamic effect cannot be observed during the time window of the TDPAC measurement. Nevertheless, the latest electronic holes to be ionized at the probe can produce a dynamic HFI. This interesting effect is characterized by a strong dampening of the spectra in approximately the first 10-50 ns (after this range the already reduced anisotropy remains constant in the spectra), dampening that increases, in general, as the measuring temperature decreases. This behavior is reversible with measuring temperature and the full anisotropy (i.e., the amplitude of the spectra) is fully recovered at high temperature. To ensure a correct interpretation of these time-dependent interactions experimentally observed when doping with 111In→111Cd, an ab initio study of the doped system must be performed. In this regard, an all-electron method such as the Full-Potential Augmented Plane Wave plus local orbitals (FP-APW+lo) in the framework of the Density Functional Theory (DFT) has been demonstrated to be the most accurate calculation of the EFGs in solids. We review here a combined experimental (by TDPAC) and ab initio study (using the WIEN2k code) of this general phenomenon investigating in two selected cases, sharing the same cation, the temperature dependence of the electric-quadrupole HFIs at diluted 111In→111Cd impurities doping both SnO2 and SnO. Particularly, we were interested to study the ECAE in two key cases: a) when a native cation of the host is replaced by a nominally acceptor impurity, generating in principle two electron holes (Cd doping SnO2) and b) when the cation is replaced by a nominally isovalent impurity (Cd doping SnO), which in principle should not introduce electron holes in the semiconductor´s valence band. The TDPAC spectra were successfully analyzed with a time-dependent on-off model for the perturbation factor. These results showed combined dynamic plus static interactions whose EFGs were associated in this model to different stable electronic configurations very close to the Cd nucleus. The dynamic regime is then originated in fast fluctuations between different electronic configurations of the Cd neighborhood. The combined study showed that different charge states of the Cd impurity near the Fermi level generates modifications in the electron density around the Cd nucleus, giving rise to different EFGs, in excellent agreement with the experimental results, supporting from first principles the proposed scenario of the electron-capture after-effects. Additionally, we show here that the presence of an acceptor impurity level (electron holes) is not a necessary condition to produce the ECAE effect.