Spectroscopic ellipsometry study of effect of annealing on optical properties of sol-gel deposited WO 3 films

Abstract

The dispersion relation of optical constants of sol-gel deposited hydrated tungsten oxide films have been measured using spectroscopic ellipsometry. The films were deposited on glass substrates from a tungsten butoxide precursor solution using a dip-coating technique and fired a t three different temperatures, 200 degree(s)C, 300 degree(s)C, and 400 degree(s)C. Ellipsometric measurements at 7 different angles of incidence and over the range 1.5-5 eV have been made for each of the films. The amorphous semiconductor dispersion relation proposed by Forouhi and Bloomer was used to model the tungsten oxide optical constants and to fit the measured ellipsometric angles to determine the parameters in the model. the refractive index and extinction coefficient have been found using this procedure. Computer simulations of transmittance of the films, based on the optical constants obtained from the ellipsometric measurements and the fitting procedure, have been compared to the measured spectra. This comparison confirms that the optical constants found from ellipsometry describe the system accurately. The effect of increasing the annealing temperature of the films on the optical parameters is also discussed. In particular, changes are observed in the optical energy gap and film thickness. The reasons for these variations is discussed in terms of the microstructure of the films.