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A determination of the Schottky barrier height at the interface between ferroelectric Pb(Zr,Ti)O3 thin films and Pt by photoelectron spectroscopy is presented. Stepwise Pt deposition was performed in situ onto a contamination-free Pb(Zr,Ti)O3 thin film surface. The substrate surface is reduced in the course of Pt deposition as evident from the observation of metallic Pb. The Fermi level is found at EF − EVB = 1.6 ± 0.1 eV above the valence band maximum of the as-prepared interface. Annealing of the sample in an oxygen pressure of 0.1 and 1 Pa strongly reduces the amount of metallic Pb and leads to a reduction in the Fermi level position at the interface to EF − EVB = 1.1 ± 0.1 eV. Storage in vacuum at room temperature strongly reduces the interface leading to a significantly higher Fermi level position (EF − EVB = 2.2 ± 0.1 eV). The reduction is attributed to the presence of hydrogen in the residual gas. The change in barrier height might be a severe issue for stable device operation with Pt contacts even at ambient temperatures.

The interface formation between Pb(Zr,Ti)O 3 (PZT) and RuO 2 and between PZT and In 2 O 3 :Sn (ITO), respectively was characterised using in-situ X-ray photoelectron spectroscopy (XPS). No interface reaction was observed for the interfaces studied. The Fermi level position at the interface (Schottky barrier height) is strongly different for the two electrode materials. A Fermi level position of 1.0± 0.1 eV above the valence band maximum (VBM) is observed for the contact between PZT and the high work function oxide RuO 2 . For the contact between PZT and the low work function oxide ITO a Fermi level position of 2.1± 0.2 eV above the VBM is found.

The interface formation between PbTiO 3 and SrTiO 3 has been studied by in situ photoelectron spectroscopy. A valence band offset of 1.1 ± 0.1 eV, corresponding to a conduction band offset of 1.3 ± 0.1 eV, is determined. These values are in good agreement with the band offsets estimated from measured ionization potentials of SrTiO 3 and PbTiO 3 surfaces. The observed band offsets are also in line with a ∼1.1 eV difference in barrier heights of PbTiO 3 in contact with different electrode materials as compared to barrier heights of SrTiO 3 with the same electrode materials. The results indicate that the band alignment is not strongly affected by Fermi level pinning and that the barrier heights are transitive. The limits of Fermi level variation observed from a number of thin films prepared on different substrates with different conditions are the same for both materials when these are aligned following the experimentally determined band offsets. By further comparing electrical conductivities reported for SrTiO 3 and PbTiO 3 , it is suggested that the range of Fermi level position in the bulk of these materials, which corresponds to the range of observed conductivities, is comparable to the range of Fermi level position at interfaces with different contact materials. In particular the possibly low barrier height for electron injection into SrTiO 3 is consistent with the metallic conduction of donor doped or reduced SrTiO 3 , while barrier heights 1 eV for PbTiO 3 are consistent with the high resistivity even at high doping concentrations. The variation of barrier heights at interfaces therefore provides access to the range of possible Fermi level positions in the interior of any, including insulating, materials, which is relevant for understanding defect properties.

The energy band alignment at interfaces between different materials is a key factor, which determines the function of electronic devices. While the energy band alignment of conventional semiconductors is quite well understood, systematic experimental studies on oxides are still missing. This work presents an extensive study on the intrinsic energy band alignment of a wide range of functional oxides using photoelectron spectroscopy with in‐situ sample preparation. The studied materials have particular technological importance in diverse fields as solar cells, piezotronics, multiferroics, photo‐electrochemistry and oxide electronics. Particular efforts have been made to verify the validity of transitivity, in order to confirm the intrinsic nature of the obtained band alignment and to understand the underlying principles. Valence band offsets up to 1.6 eV are observed. The large variation of valence band maximum energy can be explained by the different orbital contributions to the density of states in the valence band. The framework provided by this work enables the general understanding and prediction of energy band alignment at oxide interfaces, and furthermore the tailoring of energy level matching for charge transfer in functional oxides. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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