Tracking the amorphous to epitaxial transition in RF-sputtered cubic BFO-STO heterojunctions by means of X-ray photoelectron diffraction

The epitaxial growth of cubic BiFeO3 ultrathin films on SrTiO3 (001) substrates by off-axis RF sputtering is demonstrated, suitable to X-ray spectroscopies interface investigation. X-ray photoelectron diffraction is used as a tool to probe the long-range crystal order and to track the transition from amorphous to epitaxial growth as a function of deposition parameters. Further spectroscopic measurements, in particular, X-ray linear dichroism on the Fe L 3, 2 edge, confirm the heteroepitaxial growth of BiFeO3 and clearly indicate a 3+ valence state for the iron cation. Finally, XPS is used to reconstruct the band alignment diagram, which results in a staggered configuration with a remarkable energy shift of the SrTiO3 band edges which can ultimately favor the n-type doping of SrTiO3.