Physical and electrochemichromic properties of rf sputtered tungsten oxide films

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Abstract
The physical and electrochemical coloration characteristics of tungsten oxide films rf sputtered from a compressed powder WO3 target have been investigated. Oxide films with 3600–9800 Å thickness were deposited on substrates maintained at 200 °C at a total pressure of 0.5–8×10−2 Torr in Ar gas or an Ar‐0.5‐50% O2 gas mixture. Physical properties of the oxide films depend on the oxygen concentration and total pressure of the sputtering atmosphere. The films prepared at 4×10−2 Torr in a mixture of Ar‐0.5‐20% O2 gas are transparent and amorphous, and their electrical resistivity ranges from 6.5×108 to 2.4×1011 Ω cm. The films prepared at pressures between 4 and 6×10−2 Torr in an Ar‐50% O2 gas mixture are transparent, and have crystallites with a composition of WO3. The films prepared at 1×10−2 Torr in a mixture of Ar‐0.5 and 5.0% O2 gas are blue colored and transparent, respectively, and these films are crystallites with a composition of WO2.83. Electrochemichromic properties of the rf sputtered tungsten oxide films depend on the film structure, or on the sputtering conditions under which the films are prepared. The amorphous and crystalline WO3 films formed at a high total pressure of 3–8×10−2 Torr have good electrochemichromic properties and are colored deep blue. However, the crystalline WO2.83 films formed at a low total pressure of 0.5–1.5×10−2 Torr have poor electrochemichromic properties and are hardly colored. The amorphous oxide films with resistivity of 108–109 Ω cm and low density of ∼6.0 g/cm3, formed at a deposition rate higher than ∼100 Å/min, have very good coloration characteristics. The optical properties and density of the oxide films are also described.