Preparation and properties of the dc reactively sputtered tungsten oxide films

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Abstract
Preparation and properties of the dc reactively sputtered tungsten oxide films have been investigated in relation to the oxygen concentration in sputtering atmosphere. The films with 1500–20 700 Å thickness were deposited on the glass substrates maintained at 200 °C by dc reactive sputtering from a metallic tungsten target under a constant operating pressure of 6.5×10−2 Torr in Ar‐1%–30% O2 gas mixture. The films formed in an Ar‐3%–20% O2 gas mixture are crystalline WO3, and have an electrical resistivity of 107–1011 Ω cm which is dependent on the oxygen concentration of the sputtering atmosphere. These films have a spectral transmittance above 80% in the visible and near‐infrared regions, and optical band gap of the films ranges from 3.15 to 2.98 eV, depending on the oxygen concentration. Densities of the film deposited in Ar‐3% O2 and in Ar‐20% O2 gas mixtures are 5.85 g/cm3 and 6.65 g/cm3, respectively. Electrochemichromic properties of the transparent‐crystalline WO3 films were studied using asymmetric cells, and were found to be dependent on the crystal orientation of the films. The films with the orientation of WO3 (020) and WO3 (021) formed in an Ar‐3%–6% O2 gas mixture have a very good electrochromic property, and the cells composed of the crystalline WO3 films have a higher coloration rate than the cells composed of the vacuum‐evaporated amorphous film. The films with the orientation of WO3 (001) and WO3 (021) formed in an Ar‐8%–20% O2 gas mixture were found to have a poor electrochromic property.