Optical properties and surface morphology of evaporated (WO3)1-x-(Fe2O3) x thin films

Thin films of (WO3)1-x-(Fe2O3) x composition were deposited by thermal evaporation on glass substrates and then all samples were annealed at 400 deg. C in air. Optical properties such as transmittance, reflectance, and optical bandgap energy of the 'as deposited' and the annealed films were studied using ultraviolet-visible spectrophotometry. It was shown that the annealing process did not substantially change the optical transmittance and reflectance of all the films except the films having x = 0.75. By increasing Fe2O3 content in the films from x = 0 to x = 0.75, optical bandgap energy decreased from 3.4 to about 1.3 eV and from 3.1 to 2.1 eV for the 'as deposited' and the annealed samples, respectively. Moreover, using X-ray photoelectron spectroscopy, we have shown that the 'as deposited' (WO3)0.25-(Fe2O3)0.75 films contained Fe3O4, and it converted to Fe2O3 after the annealing process. Using atomic force microscopy, it was found that the grain size and surface roughness of the annealed films were greater than the 'as deposited' ones. In addition, for the annealed films with intermediate x's, we have observed a smoother surface with smaller grain size as compared to films with the other x's