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

• Oriented columnar WO3 thin films were grown by glancing angle deposition.
• Structural and optical properties depend on the incident angle of the particle flux.
• Refractive index correlates with the resulting porous architecture.
• Birefringence is connected to the structural anisotropy of the columnar growth.

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 α between the particle flux and the normal to the substrate is systematically changed from 0 to 80°. For incident angles higher than 50°, a typical inclined columnar architecture is clearly produced with column angles β well correlated with the incident angle α 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 n589 = 2.18 down to 1.90 as α rises from 0 to 80°, whereas the extinction coefficient reaches k589 = 4.27 × 10− 3 for an incident angle α = 80°, 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°. Optical band gap Eg, Urbach energy Eu and birefringence Δn617, 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.