Characterization and electrical properties of WO3 sensing thin films

The microstructure and the electrical properties of thermally evaporated WO3 thin films have been investigated by glancing angle XRD, atomic force microscopy AFM, X-ray photoelectron spectroscopy XPS and de techniques. Thin films of 1500 Angstrom thickness have been obtained by evaporating high purity WO3 powders by an electrically heated crucible at 5 x 10(-4) Pa on sapphire substrates. The as-deposited films have resulted to be amorphous. After annealing at 500 degrees C in dry air for 6, 12 and 24 hours the films have shown well crystallized structures with preferential orientations of WO3 in the (200) direction. The increase of the annealing time has shown marked influence on the microstructural features of the films surface, as highlighted by AFM investigations. The binding energies of W 4f(7/2) have been close to that of WO3, the 24 h annealed yielded an O/W ratio close to 2.9 which is in good agreement with the theoretical one. The gas sensitivity, selectivity and stability of the annealed films in presence of NO2 gas (between 0.7 and 5 ppm) have been evaluated by measuring the electrical change of the film resistance in dry air and in gas atmosphere conditions. The influence of NO and Humidity interfering gases to the NO2 electrical response has been also evaluated. The 500 degrees C annealed at 24 h has shown better electrical properties in terms of NO2 sensitivity, stability and cross sensitivity effects.