Electronic and optical properties of anodic oxide films on titanium.

摘要

The scope of this thesis is to study the effect of disorder (in absence of interactions) on the electronic and optical properties of metal oxides. In these systems Metal-Insulator (M-I) transition (Anderson-like) may occur and would have large effects on the corrosion properties. We apply the Equation-of-motion (EOM) method to a full tight-binding (3D) model of rutile {dollar}TiOsb2{dollar} developed by Vos. Comparing this method to the well-known Green's-function (GF) method, we show that the EOM can be used competitively to study defect electronic structure event down to very low impurity concentrations. We study in detail the electronic structure of the above model as a function of oxygen vacancy concentration (up to 10%) and film thickness. Comparing the bulk density of states (DOS) to photospectroscopy (PS) experiments, we show a close similarity of the functional form of the photospectra of the anodic oxide films to the bulk rutile (single crystal) DOS. A comparison of surface DOS to scanning tunneling microscopy (STM) experiments, on the other hand, suggests that there are about 5% oxygen vacancies at the surface of these films.; To resolve this apparent contradiction we evaluated the Kubo-Greenwood formula for the DC-conductivity (including localization effects) in a simplified 2D version of rutile {dollar}TiOsb2{dollar}. Our results of both conductivity and local DOS predict an impurity band formation at a defect density p of about {dollar}psb{lcub}c{rcub}{dollar} = 9%. For concentrations up to {dollar}psb{lcub}c{rcub}{dollar} the conductivity has a functional form much like the bulk DOS (invariant mobility edge), thus accounting semiquantitatively for the contradiction. The calculated conductivity function also accounts for the invariance of the indirect bandgap as function of film thickness seen in PS experiments. We discuss the nature of the conduction when {dollar}p < psb{lcub}c{rcub}{dollar}.; We conclude that the anodic {dollar}TiOsb2{dollar} films have a defect density less than {dollar}psb{lcub}c{rcub}{dollar} and as a consequence the gap states are localized and no impurity band exists. In other words, our work shows that these films have disordered electronic structures, but bulk-like transport properties.