STUDY OF OPTICAL DISPERSION PARAMETERS OF WO3 POLYCRYSTALLINE THIN-FILMS

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 E(d) [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 E(d) 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-degrees-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-degrees-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.