PbTiO₃/SrTiO₃ interface: Energy band alignment and its relation to the limits of Fermi level variation

The interface formation between PbTiO₃ and SrTiO₃ has been studied by in situ photoelectron spectroscopy. A valence band offset of 1.1±0.1eV, corresponding to a conduction band offset of 1.3±0.1eV, is determined. These values are in good agreement with the band offsets estimated from measured ionization potentials of SrTiO₃ and PbTiO₃ surfaces. The observed band offsets are also in line with a ~1.1eV difference in barrier heights of PbTiO₃ in contact with different electrode materials as compared to barrier heights of SrTiO₃ 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 SrTiO₃ and PbTiO₃, 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 SrTiO₃ is consistent with the metallic conduction of donor doped or reduced SrTiO₃, while barrier heights ≳1eV for PbTiO₃ 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.