Influence of orbital contributions to the valence band alignment of Bi2O3, Fe2O3, BiFeO3, and Bi0.5Na0.5TiO3

Abstract

The formation of an interface between Bi${}_{2}$O${}_{3}$, Fe${}_{2}$O${}_{3}$, BiFeO${}_{3}$, Bi${}_{0.5}$Na${}_{0.5}$TiO${}_{3}$, and the high work function metallic RuO${}_{2}$ is studied using photoelectron spectroscopy with in situ RuO${}_{2}$ deposition. Schottky barrier heights are derived and the valence band maximum energies of the studied materials are aligned with respect to each other as well as to other functional oxides like SrTiO${}_{3}$ and PbTiO${}_{3}$. The energy band alignment follows systematic trends compared to a large number of oxides, and can be understood in terms of the contribution of Fe $3d$ and Bi $6s/6p$ (lone pair) orbitals to electronic states near the valence band maximum. The results indicate that the valence band maxima are largely determined by the local environment of the cations, which allows to estimate valence band maximum energies of oxides with multiple cations from those of their parent binary compounds. The high valence band maximum of BiFeO${}_{3}$ is consistent with reported $p$-type conduction of acceptor doped material, while the high conduction band minimum makes $n$-type conduction unlikely.