Vacuum temperature-dependent ellipsometric studies on WO3 thin films

Vacuum temperature-dependent ellipsometric studies on WO3 thin films are reported at a single wavelength, lambda = 0.633 mu m, and across a temperature range of 100 < T less than or equal to 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 WO3 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. (C) 1999 Optical Society of America. OCIS codes. 310.6860, 240.0310, 230.0250, 160.2100, 130.5990, 260.2130.