Correlation between structural and optical properties of WO3 thin films sputter deposited by glancing angle deposition

Tungsten oxide WO3 thin films are prepared by DC reactive sputtering. The GLancing Angle Deposition method (GLAD) is implemented to produce inclined columnar structures. The incident angle alpha between the particle flux and the normal to the substrate is systematically changed from 0 to 80 degrees. For incident angles higher than 50 degrees, a typical inclined columnar architecture is clearly produced with column angles beta well correlated with the incident angle alpha according to conventional relationships determined from geometrical models. For each film, the refractive index and extinction coefficient are calculated from optical transmittance spectra of the films measured in the visible region. The refractive index at 589 nm drops from n(589) = 2.18 down to 1.90 as alpha rises from 0 to 80 degrees, whereas the extinction coefficient reaches k(589) = 4.27 x 10(-3) for an incident angle alpha = 80 degrees, which indicates that the films produced at a grazing incident angle become more absorbent. Such changes of the optical behaviors are correlated with changes of the microstructure, especially a porous architecture, which is favored for incident angles higher than 50 degrees. Optical band gap E-g, Urbach energy E-u and birefringence Delta n(617), determined from optical transmittance measurements, are also influenced by the orientation of the columns and their trend is discussed taking into account the disorder produced by the inclined particle flux. (C) 2013 Elsevier B. V. All rights reserved.