X-ray photoelectron spectroscopy of epitaxial films and heterostructures

X-ray photoelectron spectroscopy is a powerful experimental technique that yields invaluable information on a range of phenomena that occur in solids, liquids, and gasses. The binding energy and shape of a photoemission peak is sensitive not only to the atomic number, valence and orbital from which the electron is ejected, but also to complex many-body effects that accompany photoemission. Provided the influences of these different drivers of spectral line shapes can be disentangled, a great deal can be learned about the electronic structure of materials of interest. In addition to these largely local effects, the long-range electrostatic environment and resulting electric potential at the emitting atom also have a direct effect on the measured binding energies. This fact opens the door to extracting information about the dependence of the valence and conduction band minima on depth below the surface, which in turn allows both vertical and lateral electrical transport data to be better understood. One purpose of this Report is to summarize how the different physical forces described above impact the spectral properties of complex oxide epitaxial films. This class of materials typically incorporates transition metal cations in different valences and such ions exhibit the most complex core-level spectra of any on the periodic chart. A second purpose is to show how a comprehensive understanding of local physical effects in x-ray photoemission allows one to model spectra and extract from core-level line shapes and binding energies detailed information on built-in potentials and band edge discontinuities in heterostructures involving complex oxides.