Defects in CeO2/SrTiO3 fabricated by automatic feeding epitaxy probed using positron annihilation

Abstract

Vacancy-type defects in the CeO2/SrTiO3 structure were studied by means of positron annihilation. Thin CeO2 films were grown on SrTiO3 substrates by molecular-beam epitaxy without using an oxidant; oxygen was supplied by diffusion from the substrate (this process is referred to as automatic feeding epitaxy). A preferential epitaxial growth of CeO2(001) on SrTiO3(001) was observed, but an increase in the film thickness or annealing in an O2 atmosphere caused a partial growth of CeO2(110). The introduction of this phase was found to correlate with the reduction in the concentration of vacancy-type defects. The species of defects introduced into the SrTiO3 substrate was found to be not only oxygen vacancies but also Sr vacancies or their complexes. We discuss the mechanism that introduced cation vacancies in terms of oxygen nonstoichiometry. We found the trapping rate of positrons by Sr vacancies to depend on the sample temperature. This was associated with the temperature dependence of the dielectric constant near the defects or the presence of shallow trapping centers.