Characterization of sol-gel prepared WO3 thin films as a gas sensor

Tungsten trioxide (WO3) thin films have been prepared by the sol-gel proces s and annealed at different temperatures of 400, 500, 600, and 700 degrees C for 1 h. The morphology, microstructure crystalline structure, and compos ition of the films have been analyzed using scanning electron microscopy (S EM), x-ray diffraction, Rutherford backscattering spectroscopy (RBS), and x -ray photoelectron spectroscopy (XPS) techniques. The SEM analysis showed t hat the films annealed at 400 degrees C are smooth and uniform. However, th ese evolved as granular at an annealing temperature of 500 degrees C. The f ilms annealed at still higher temperatures have two distinct grains of diff erent shapes and sizes. The films annealed below 400 degrees C are amorphou s. Annealing at 500 degrees C resulted in the films having polycrystalline structure. RES and XPS characterization have revealed that the films anneal ed at 400 degrees C are stoichiometric. Annealing above this temperature re sulted in the films becoming off-stoichiometric. The electrical resistance of the films annealed at 500 degrees C increased 18 times when exposed to 1 75 ppb O-3 gas compared to the air exposure, with the response time being a s short as 1-2 min. (C) 1999 American Vacuum Society. [S0734-2101(99)08804- 1].