Pulsed laser deposited WO3 thin films for gas sensor

WO3 has attracted attention because of its potential for NOx gas sensing. Thin films of WO3−x were prepared by KrF excimer pulsed laser deposition technique on quartz and Al2O3 with Pt electrode. The films were deposited at various substrate temperature, oxygen pressure and conditions of the post-annealing. The substrate temperature over 400 °C was needed for the crystallization of the WO3−x thin film. The crystal structure of the WO3−x thin film deposited at oxygen pressure of 100 mTorr and the substrate temperature of 400 °C was tetragonal phase, however, the films deposited at the oxygen pressure over 200 mTorr showed triclinic phase. The atomic force microscopy image of the WO3−x thin film post-annealed in oxygen atmosphere for 30 min showed an average grain size of 268 nm. It was approximately two times larger than that of the as-deposited film. The maximum sensitivity (RNO/RN2) of the as-deposited triclinic WO3−x thin film gas sensor in the NO gas (60 ppm in N2) was 4.2 at an operating temperature of 200 °C.