The energy level of the Fe2+/3+-transition in BaTiO3 and SrTiO3 single crystals

Abstract

An approach to determine the defect energy levels of the Fe impurities in BaTiO₃ and SrTiO₃ single crystals using electrical conductance measurements is presented. The defect levels are obtained from the dependence of the activation energy of electrical transport on the oxygen vacancy concentration, which is varied by stepwise re-oxidation of a reduced sample. An energy level at 0.7–0.8 eV below the conduction band minimum E_CB is identified for BaTiO₃, which can be assigned to the Fe^2+/3+-transition in good agreement with literature. In contrast, the conductivity of Fe-doped SrTiO₃ does not show a defect energy level in the upper half of the band gap, indicating that the Fe^2+/3+-transition in SrTiO₃ is near the conduction band minimum. The often reported alignment of defect energy levels, which is fulfilled for the Fe^3+/4+-transition in BaTiO₃ and SrTiO₃, does not hold for the Fe^2+/3+-transition in these compounds. This limits the applicability of Fe-doped SrTiO₃ as a model system for studying resistance degradation in acceptor-doped high-permittivity dielectrics.