Vacuum temperature-dependent ellipsometric studies on WO_3 thin films

Abstract

Vacuum temperature-dependent ellipsometric studies on WO(3) thin films are reported at a single wavelength, lambda = 0.633 microm, and across a temperature range of 100 < T <or= 453 K. All the measurements were made in an optical cryostat fixed in the sample compartment of the ellipsometer. Experimental results involving reduction and oxidation of WO(3) are discussed in terms of electrochromic characteristics and structural changes, which can be helpful for many and various technological applications. Temperature-dependent drifts in the real part of the refractive index n and extinction coefficient k have been explained by use of a variety of chemical relations and have also been utilized to evaluate their temperature coefficients. Moreover, polaronic excitations between localized states around the Fermi level are put forward to explain the ellipsometric results at or above room temperature, and both polaronic and bipolaronic transitions are proposed for interpreting low-temperature ellipsometric measurements.