Surface characterization of clean SrTiO₃(100) substrates by x-ray photoelectron spectroscopy

The authors analyze the x-ray photoelectron spectra of SrTiO₃(100) (STO) substrates to clarify the electronic structure characteristics induced by a sputter-anneal procedure. The survey spectra, the valence band spectra, and the relevant core level spectra, i.e., Sr 3s, Sr 3p, Sr 3d, Sr 4s, Sr 4p, Ti 2p, Ti 3s, Ti 3p, O 1s, and O 2s along with their satellite structures, of both pristine and sputtered and annealed STO have been measured. A comparison of spectra reveals the appearance of new features in the Ti 2p spectra of sputtered and annealed STO, corresponding to Ti³⁺ states and gap states near the Fermi energy. The presence of Ti³⁺ states points to the formation of a two-dimensional electron system at the STO surface generated by Ar⁺ sputtering and annealing in vacuum. Furthermore, the sputter-annealed surface shows only minor carbon contamination as compared to that of the pristine substrate.