Physical and electrochemichromic properties of rf sputtered tungsten oxide films

Abstract

The physical and electrochemichromic properties of rf sputtered tungsten oxide films from a compressed powder WO3 target have been investigated. This report mainly concerns the dependence of the properties on the total pressure of the sputtering atmosphere. The oxide films, 5000–5300 Å thick, were deposited on substrates maintained at 200 °C under total pressures of 5×10−3–8×10−2 Torr in an atmosphere of Ar gas or Ar–5% O2 mixture. The physical and electrochromic properties of the oxide films prepared are dependent on the total pressure during deposition. The electrical resistivity ranges from 4.75×107 to 3.33×1011 Ω cm depending on the pressure. From x-ray analysis, the films prepared under pressures of 5×10−3–1.5×10−2 Torr were found to be crystalline and assigned a composition of WO2.83. The films prepared under pressures of 1.8×10−2–8×10−2 Torr were found to be amorphous. Electrochemical coloration of these oxide films was observed using an asymmetric cell, and the coloration characteristics of the oxide films were found to be dependent on the electrical resistivity and structure of the films. The amorphous films with high resistivity have good electrochromic properties, but the crystalline films with low resistivity are hardly colored. The optical band gap, refractive index, and density of the oxide films are described.