Search for “After-Effects” in Cd-doped ZnO semiconductor: PAC experiments supported by ab initio results.

E. L. MUÑOZ, M. E. MERCURIO, L.F. PEREIRA, A. W. CARBONARI, J. MESTNIK-FILHO, AND M. RENTERÍA

Ginebra

Conferencia; HFI/NQI2010, 15th International Conference on Hyperfine Interactions & 19th International Symposium on Nuclear Quadrupole Interaction; 2010

Executive Committee del 3th HFI/NQI International Conference

During the last 25 years, several Time-Differential Perturbed-Angular-Correlation(PAC) experiments have observed dynamic hyperfine interactions when the probe isotope111Cd, obtained after the electron-capture (EC) decay of its parent 111In was used incertain semiconductor and insulating oxides. In the eighties of the previous centurythe group from La Plata proposed that these dynamic interactions were originated inthe electronic relaxation of the probe atom, usually called “after-effects” (AE)that follows the electron capture decay of the 111In isotope. This relaxation mustoccurs during the time-window of the intermediate sensitive nuclear state of thegamma-gamma cascade used to measure the hyperfine interaction at the probe nucleus.It was believed that the ECAE can only be detected if the probe atom was an impurityin the system under study. In order to check this necessary condition, the onlybinary oxide where 111Cd is not an impurity is CdO, and ECAE where also not reportedin this system and the observed electric-field gradient (EFG) is null due to the highsymmetry (a regular octahedron) of the coordination of the cation site with thenearest oxygen neighbors. Following these ideas, we present here results of PACexperiments performed in an oxide, ZnO, were the 111Cd probe atom is not an impurity,at least from the nominal valence point of view of the involved cations (Cd and Zn).But this time the observed non-null EFG behavior will be analyzed under the light ofab initio calculations of the (EFG) as a function of the charge state of the Cd atom.PAC experiments carried out on 111In-diffused polycrystalline ZnO have been performedin order to measure the EFG at (111In (EC)-->) 111Cd nuclei located at the cationsite of the ZnO crystal structure. The PAC experiments were performed in a largetemperature range. The absence of dynamic hyperfine interactions were verifiedfitting the spectra with a perturbation factor based in the Bäverstam and Othazmodel [2,3]. The experimental results were compared with ab initio calculationsperformed with the Full-Potential Augmented Plane Wave plus local orbital (FP-APW+lo)method. The FP-APW+lo calculations were performed in the framework of the DensityFunctional Theory (DFT), using the Wien2K code. The dependence of the EFG at the Cdsites as a function of the charge state of the calculated supercell (i.e. the chargestate of the impurity atom) was determined. From the ab initio-experimentalcomparison we can explained why we do not observe dynamic hyperfine interactions inthe ZnO:Cd system.