Characterization of Amorphous Tungsten Oxide Films through Optical Absorption Spectra

Abstract

Absorption spectra of evaporated amorphous WO₃ films were measured under various conditions. Slope of the Urbach tail of fundamental absorption edge was found to decrease with increasing temperature of annealing. Decrease of the slope at the annealing temperature of 100°C is caused by the release of water from the films, while the decrease at 300 °C is originated from crystallization. Shift of optical gap was also observed with increase of annealing temperature. These effects are considered to be caused from the change in the disorder of atomic configurations.
Electrolytic coloration procedure induces a near-infrared absorption band. The absorption band produced by H⁺ injection was narrower than that by Li⁺ injection. This kind of difference in absorption spectra is believed to relate with the difference of frequency of atomic vibrations in the color centers. Optical gap was found to shift to higher energies during electrolytic coloration. This shft is brought about by incorporation of H⁺ or Li⁺ ions into WO₃ matrix.