Band offsets in complex-oxide thin films and heterostructures of SrTiO₃/LaNiO₃ and SrTiO₃/GdTiO₃ by soft and hard X-ray photoelectron spectroscopy

The experimental determination of valence band offsets (VBOs) at interfaces in complex-oxide heterostructures using conventional soft x-ray photoelectron spectroscopy (SXPS, hν ≤ 1500 eV) and reference core-level binding energies can present challenges because of surface charging when photoelectrons are emitted and insufficient probing depth to clearly resolve the interfaces. In this paper, we compare VBOs measured with SXPS and its multi-keV hard x-ray analogue (HXPS, hν > 2000 eV). We demonstrate that the use of HXPS allows one to minimize charging effects and to probe more deeply buried interfaces in heterostructures such as SrTiO₃/LaNiO₃ and SrTiO₃/GdTiO₃. The VBO values obtained by HXPS for these interfaces are furthermore found to be close to those determined by first-principles calculations.