Study of optical dispersion parameters of WO3 polycrystalline thin films

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Abstract
The optical properties of WO3 polycrystalline thin films prepared by oxidation of ‘‘reflective’’ tungsten films in three different conditions of temperature and ambient have been studied by standard reflection-transmission measurements. The resulting dispersion parameters E0 and Ed [S. H. Wemple and M. DiDomenico, Phys. Rev. B 3, 1338 (1971)] have been analyzed using the effective-medium approximation. The experimental values of E0 and Ed can be reproduced if the films are considered to be composed of a crystalline phase, disordered phase, and voids, the concentration of each phase depending on the annealing mode and the thickness. The arguments proposed have been supported by x-ray diffraction, scanning electron microscope, and density measurements. Films prepared at 600 °C in air exhibit a high concentration of crystalline phase (≳85%) while the void content remains at low levels and unaffected by thickness variations (<10%). In films obtained by annealing at 500 °C in air or in a stream of 90% argon-10% oxygen, the void content increases while the concentration of disordered phase decreases with thickness.