Study of the surface morphology and gas sensing properties of WO3 thin films deposited by vacuum thermal evaporation

WO3 thin films have been obtained by evaporating high-purity WO3 powder at 5 x 10(-4) Pa on SiO2, Si3N4 and annealed at 500 degrees C for 6, 12, 24, 9 6, and 200 h. The film morphology, crystalline phase and chemical compositi on have been characterized through atomic force microscopy, grazing-angle x -ray diffraction, and x-ray photoelectron spectroscopy (XPS) techniques. Th e as-deposited film is amorphous. After annealing at 500 degrees C the film s are well crystallized with preferential growth along the [200] plane of m onoclinic WO3 (JCPDS 43-1035). The mean roughness and the surface area of t he films are influenced by the nature of the substrate and the annealing ti me. The binding energies of the annealed films are close to those of WO3. H igh-resolution XPS valence-band spectra have shown a well-defined W metalli c peak near the Fermi edge, confirming the occurrence of oxygen vacancies o n the surface. The electrical response of the films in dry and NO2-rich atm ospheres (0.7-7.2 ppm concentration in dry air) has been evaluated by de cu rrent mode. (C) 1999 American Vacuum Society. [S0734-2101(99)02102-8].