Defect-induced electrical/optical properties of SrTiO(3-x) (001) by photo-assisted tunneling spectroscopy

The (001) surface of monocrystalline strontium titanate is used in a variety of commercial applications as an active substrate or electrode and therefore represents an important technological material. Its functionality is enabled by defect states energetically located in the forbidden gap which are introduced upon removal of oxygen from the lattice. Studies by conventional surface analysis techniques as well as first principles calculations have not yet resulted in agreement regarding the microscopic origin of the acceptor type surface states. The combined techniques of optical spectroscopy and scanning tunneling spectroscopy, however, afford a unique opportunity to probe the local origins of deep level surface states by direct modification of the surface charge density through optical excitation.; The technique of photo-assisted tunneling spectroscopy (PATS) using continuous illumination of mono-energetic light was applied to study the optical responsivity of a series of samples with increasing degrees of reduction. The surface structures were characterized by conventional scanning tunneling microscopy (STM), photo-assisted tunneling microscopy (PATM), and low energy electron diffraction (LEED). A theoretical model was developed to generate tunneling spectra and facilitate interpretation of the experimental results.; This work reports the first STM images and STS spectra obtained on undoped and transparent single crystalline SrTiO3-x. The surface structure and optical responsivity was found to strongly depend on the processing conditions, where the latter increased with increasing degree of reduction. The results were explained in terms of variations of the local surface potential induced by local charge transfer mechanisms, where evidence of both increasing and decreasing surface charge was observed depending on the incident photon energy.; It has been determined that oxygen vacancy association is necessary to introduce a deep level gap state centered at 1.77 eV below the conduction band edge and that this state is localized on surface terrace sites. This work represents the first successful demonstration of spectroscopic PATS, combined with theoretical modeling, as a strong metrological tool to study the local electrical/optical properties of wide band gap semiconducting oxide materials.