Optical and photoelectric properties and colour centres in thin films of tungsten oxide

Abstract

Abstract Thin films of Wo3 deposited on quartz substrates at room temperature have been shown to be amorphous in structure. The optical absorption spectra of the amorphous and crystalline films have been measured in the temperature range 110° to 500°K. The fundamental absorption edge of an amorphous film occurs at 3800 Å which on crystallization moves to 4500 Å. On the high-energy side of the absorption edge several absorption peaks are resolvable in both types of film. The frequency dependence of the absorption coefficient below 104 cm−1 is described by an expression of the form K (v, T) = K 0 exp[− (β/kT) (E 0 − hv)] and above 104 cm−1 it follows a square law dependency. The temperature coefficient of the band edges was found to be − 5.0 × 10−4 eV/°K and the estimated band gaps at 0°K were found to be 3.65 and 3.27 eV for the amorphous and crystalline films, respectively. The electrical conductivity of a thin film has been measured in the temperature range 298–573°K and the activation energy was found to be 1.04 eV. Irradiation within the fundamental absorption edge gives rise to photoconductivity. Threshold wavelengths for photoconductivity were observed at 3250 and 5500 Å for the amorphous and crystalline films, respectively. A broad colour-centre band having a maximum at 9100 Å and a shoulder at 1.6 μ has been observed on irradiating the amorphous film with wavelengths shorter than 3500 Å and also on applying an electric field of ∼ 104 V./cm. The colour centre, thus formed, shows a slight bleaching with light. However, it bleaches thermally and in presence of oxidizing atmosphere. The formation of colour-centres is associated with increased electrical conductivity of the film. No colouration is observed in fully oxidized samples of WO3. An energy level diagram has been proposed to account for the optical and electrical properties as well as the colour-centre formation in WO3 films.