The influence of stoichiometry on the properties of titanium oxide films for optical coatings

Abstract

This study investigates various properties of titanium oxide thin films. The samples are prepared by electron-beam evaporation of the Ti₂O₃ material in an oxygen environment. Papers about the vaporization study of the titanium-oxygen system are reviewed; special attention is paid to the congruent vaporization in the titanium-oxygen system. The occurrence of congruent vaporization in our coating system is discussed. The compositions of the films are identified by Rutherford Backscattering Spectrometry. The effect of water vapor and the substrate temperature on the oxygen contents in RBS measurements is discussed. The optical properties of the samples are measured. With the spectrophotometric measurements, the methods for deriving the optical constants of transparent and opaque films are developed. The absorption of the TiO₂ film is investigated, and the corresponding mechanisms are discussed. The envelope method is employed to find the optical band gap of the TiO₂ film. The electrical resistivity of the titanium oxide films are measured with the four-point probe method, and the phenomenon of metal to insulator transition is demonstrated. The tensile stresses in our titanium oxide films are examined with a Nomarski microscope. The grain boundary model is adopted to explain the influence of thickness and oxygen content on the stresses development in thin film. Molecular dynamics simulation is used to study the structure and the thermal expansion of titanium dioxide rutile.