Band Alignment in Ultrathin Mixed Conducting Oxide Layers

At heterojunctions between mixed ionic and electronic conductors (MIECs), band alignment takes place in order to equilibrate electronic and ionic charge carriers. The bulk properties of such MIECs can be used to describe and understand the corresponding interfacial space charges. For ultrathin films in MIEC heterolayers, however, these interfacial effects may differ from those between bulk materials. In this work, the interfacial regions between SrTiO₃ (STO) and the two MIECs (La, Sr)FeO_3-δ (LSF) and (La, Sr)MnO_3-δ (LSM) are considered. Stacks of LSF|LSM and LSM|LSF were deposited on STO single crystals, and the resulting space-charge regions in STO are characterized by means of impedance spectroscopy at 500 °C in the p(O₂) range between 1 and 5 × 10^-4 bar. By extracting the STO bulk and space-charge resistances from the impedance data, space-charge potentials are derived. Interestingly, even extremely thin LSM interlayers (0.5 nm) cause bulk-like LSM band bending beneath LSF top layers. STO space charges at LSF interlayers of the same thickness, however, are strongly affected by LSM top layers. A model is introduced to interpret the measured difference in critical thicknesses of LSF and LSM in terms of different accumulation- and depletion-layer thicknesses in MIECs.