Sensitive Bandgap Reduction of SrTiO₃ through Incorporation of Sulfur Using Ion Implantation

SrTiO3 (STO) is a well-known perovskite oxide often used, among others, as a crystalline substrate for epitaxial deposition. However, its indirect bandgap of 3.25 eV is too high for solar applications. Tentative experiments to reduce its bandgap are very welcome. To this end, sulfurization of STO substrates using ion implantation is performed. The simulated profile is controlled by time-of-flight secondary-ion mass spectrometry. S is at least partially inserted into the STO lattice, as evidenced by X-ray diffraction and X-ray photoelectron spectroscopy. UV-vis spectroscopy indicates a drastic reduction of the bandgap of S:STO from 3.25 eV (indirect) down to 2.14 eV for 10% S:STO, a trend also observed by spectroscopic ellipsometry. The results are confronted to ab initio calculations. Besides bandgap, the valence band level, that is the ionization energy, is determined by ambient-pressure photoemission spectroscopy so that the complete energy diagrams could be plotted. This work paves the way for tuning the bandgap of perovskites in a highly controlled manner for solar energy applications.