Electronic Structure of the SrTiO₃(001) Surfaces: Effects of the Oxygen Vacancy and Hydrogen Adsorption

Abstract

The influence of electron irradiation and hydrogen adsorption on the electronic structure of the SrTiO 3 (001) surface was investigated by ultraviolet photoemission spectroscopy (UPS).Upon electron irradiation of the surface, UPS revealed an electronic state within the band gap (in-gap state: IGS) with the surface kept at 1×1. This is considered to originate from oxygen vacancies at the topmost surface formed by electron-stimulated desorption of oxygen.Electron irradiation also caused a downward shift of the valence band maximum indicating downward band-bending and formation of a conductive layer on the surface. With oxygen dosage on the electron-irradiated surface, on the other hand, the IGS intensity was decreased along with upward band-bending, which points to disappearance of the conductive layer. The results indicate that electron irradiation and oxygen dosage allow us to control the surface electronic structure between semiconducting (nearly-vacancy free: NVF) and metallic (oxygen de cient: OD) regimes by changing the density of the oxygen vacancy. When the NVF surface was exposed to atomic hydrogen, in-gap states were induced along with downward band bending. The hydrogen saturation coverage was evaluated to be 3.1±0.8×10 14 cm −2 with nuclear reaction analysis. From the IGS intensity and H coverage, we argue that H is positively charged as H~0 :3+ on the NVF surface. On the OD surface, on the other hand, the IGS intensity due to oxygen vacancies was found to decrease to half the initial value with molecular hydrogen dosage. H is expected to be negatively charged as H − on the OD surface by occupying the oxygen vacancy site.