Structural and optical characterization of WO3 deposited on glass and ITO

Abstract

Electrochromic materials exhibit variable and reversible optical properties under the action of voltage pulses. The interest in these materials has increased in the last few years due to their potential application in a wide variety of optical modulation devices. Tungsten oxide (WO3) is typical in such devices. In this work, we present a study of the structural and optical properties of tungsten oxide films deposited on glass and indium tin oxide. The films were produced by reactive dc magnetron sputtering at different temperatures (room temperature and 200°C) and bias voltage (−60 to +60 V). The sputtering atmosphere was composed of an Ar+O2 mixture so that sample could be deposited with different oxygen partial pressure (0.2⩽ p(O2)⩽0.8). Spectral transmission in the visible and near infrared was measured using a double-beam spectrophotometer. X-ray diffraction was used in order to characterize the film structure. The surface microtopography was analyzed by atomic force microscopy (AFM). XRD results show that for low p(O2) the films present an amorphous WO3 phase, while for high oxygen partial pressure they present a mixture of a more crystalline WO3 phase and a W20O58 phase. The structure change is for p(O2)=0.5–0.6. A corresponding minimum was observed in the transmission and in the optical band gap, at these oxygen partial pressures. These results show that the optical properties of the films are dependent on the presence of oxygen deficient regions on the films. This was also observed in the samples deposited with an applied bias voltage. Microstructure (AFM) and X-ray measurements in these samples show that electron bombardment (positive bias) favors crystallinity and ion bombardment (negative bias) favors amorphization of the tungsten oxide phases present on the films.