Research and development on thin films has led to the conclusion that different classes of materials are of particular interest for different applications. Tungsten trioxide thin films are extensively studied because of their potential applications in electrochromic devices. Tungsten trioxide, W03, is a soft lemon yellow powder, which turns orange when heated. It has a molecular weight of 231.85, and becomes distinctly crystalline when it is strongly ignited. Tungsten trioxide is obtained from the minerals scheelite, wolframite, and ferberite. W03 crystals have perovskite-like atomic configurations based on corner-sharing W06 octahedra . Literature reports indicate that thin films of tungsten oxide have been prepared by a number of techniques by many researchers. These include the reactive dc magnetron sputtering, spray pyrolysis , reactive rf sputtering . In this paper, we present and discuss temperature coefficient of resistance (TCR), activation energy and Hall effect study of tungsten trioxide (W03) films produced by electron beam deposition technique. Although there are reports of metallic behavior of tungsten oxide films but we have found that our films exhibit a semi conducting behavior.
Tungsten trioxide thin films have been prepared by electron beam technique onto glass substrates of dimension 3 cm x2.5 cmx I cm using tungsten trioxide powder (99.9 % purity).The materials were evaporated in an E306A Edwards vacuum coating system at a pressure of about 6X 10-6 Torr. Before depositing the films, the glass substrates were first cleaned in chromic acid for several days and then washed in distilled water. After washing and drying in hot air, the substrates were again cleaned in acetone and dried in hot air and then used for deposition. Films of different thickness have been deposited. The film thickness was measured by Tolansky's interference method with an accuracy of ±5 nm. The thickness of the films varied from 110 to 300 nm with a deposition rate of about 5.7 nms-1. The films were annealed in air for about 90 minutes at temperature 373 K. The conductivity was obtained by using a four-probe Van-der-Pauw technique.
