Barrier height at (Ba,Sr)TiO₃/Pt interfaces studied by photoemission

The interface formation of Nb-doped SrTiO₃ single crystals and (Ba,Sr)TiO₃ thin films with Pt has been studied by using photoelectron spectroscopy with in situ sample preparation. For the single crystal sample, a Schottky barrier height for electrons of 0.5–0.6 eV is determined after deposition of Pt in vacuum environment. After annealing in 0.05 Pa oxygen pressure, a strong increase in the barrier height to ≥1.2 eV is observed. X-ray induced photovoltages of up to 0.7 eV are observed in this case and have to be taken into account for a proper determination of the barrier height. A subsequent annealing in vacuum reduces the barrier again. Hence, the barrier height can be reversibly switched between an oxidized state with a large barrier height and a reduced state with a low barrier height. Quantitative analysis of the barrier heights indicates that the changes are related to the changes of interfacial defect concentration. Due to the occurrence of a Ti³⁺ related signal, the defects are identified as oxygen vacancies. The same effects are observed at interfaces between Pt and (Ba,Sr)TiO₃ thin films with a smaller absolute value of the barrier height in the oxidized state of ∼1 eV. Deposition of (Ba,Sr)TiO₃ onto a metallic Pt substrate also results in a barrier height of 1.0 eV.