We address the issue of accurate determination of the valence band maximum (VBM) for SrTiO3(001) single crystals and epitaxial ?lms, as well as TiO2(001) anatase and SrO epitaxial ?lms. These measurements are of critical importance in determining valence band offsets in heterojunctions of these oxides with Si. Three different methods are analyzed: (1) ?tting a Gaussian broadened theoretical density of states to the x-ray photoelectron valence band spectrum; (2) ?nding the intersection of a regression line that spans the linear portion of the x-ray photoelectron valence band leading edge with the background between the valence band maximum and the Fermi level; and (3) determining the energy at which high-resolution ultraviolet photoemission intensity at the leading edge goes to zero. We ?nd that method 1 yields physically unreasonable results when used in conjunction with density functional theory because the latter does not predict the detailed shape of the valence bands in these oxides with suf?cient accuracy. In contrast, methods 2 and 3 give physically reasonable results that are in good mutual agreement. The difference in VBM between method 1 and methods 2 and 3 is 0.4–0.6 eV, depending on the oxide. Methods 2 and 3 yield the most reliable VBM, provided the experiments are carried out with adequate energy resolution.
Revised: August 18, 2014 |
Published: August 20, 2004