Oscillator strength of small-polaron absorption in WOx (x≤3) electrochromic thin films

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Abstract
The oscillator strength of small-polaron absorption is studied for various thin films of tungsten oxide, WOx (x≤3), prepared by rf reactive magnetron sputtering, reactive ion plating, and vacuum evaporation. The oscillator strength greatly varies depending on the procedure of film preparation and treatment. For WOx films prepared by vacuum evaporation, the effective oscillator strength decreases down to 0.04 by annealing in air at 200 °C and increases up to 0.24 by annealing in air at 400 °C. The film deposited by reactive ion plating at an rf power of 300 W exhibits a large effective oscillator strength of 0.31. Concerning the film prepared by rf reactive magnetron sputtering, the oscillator strength drastically increases with an increase in rf power. For the film deposited at an rf power of 1000 W, an effective oscillator strength of 0.51 is obtained, which is five times greater than that of the film prepared by vacuum evaporation. X-ray diffraction and XPS measurements reveal that the oscillator strength is affected by the two factors, that is, crystallinity and oxygen content of the film. It is concluded that the enhancement of oscillator strength occurs through crystallization of the film from its amorphous state and a lack of oxygen in the film. Using a configuration coordinate diagram, the mechanism of the oscillator strength enhancement is discussed in terms of a delocalization of polaron wave function.