Tuning the Optical Properties of WO3 Films Exhibiting a Zigzag Columnar Microstructure

Charalampos Sakkas,Jean-Marc Cote,Vincent Tissot,Jean-Yves Rauch,Nicolas Martin,Joseph Gavoille

doi:10.3390/coatings11040438

Abstract

Tungsten oxide WO3 thin films are deposited by DC reactive magnetron sputtering. The Reactive Gas Pulsing Process (RGPP) associated with the GLancing Angle Deposition method (GLAD) are implemented to produce zigzag columnar structures. The oxygen injection time (tON time) and the pulsing period are kept constant. Three tilt angles α are used: 75, 80, and 85° and the number of zigzags N is progressively changed from N = 0.5, 1, 2, 4, 8 to 16. For each film, refractive index, extinction coefficient, and absorption coefficient are calculated from optical transmission spectra of the films measured in the visible region from wavelength values only. Absorption and extinction coefficients monotonously drop as the number of zigzags increases. Refractive indices are the lowest for the most grazing tilt angle α = 85°. The highest refractive index is nevertheless obtained for a number of zigzags close to four. This optimized optical property is directly correlated to changes of the microstructure, especially a porous architecture, which is favored for high tilt angles, and tunable as a function of the number of zigzags.

Highlights

Structuring of solid materials at the micro- and nanoscale appeared as a key strategy to generate novel materials properties
Two degrees of freedom characterizes the GLancing Angle Deposition method (GLAD) experimental setup: the deposition angle α defined as the angle between the substrate normal and the direction of incident vapor flux, and the substrate rotation φ about its normal defining the azimuthal position of the substrate [19]
We report on WO3 thin films exhibiting a zigzag columnar architecture (WO3 material was chosen since it exhibits a high refractive index, which allows a significant tuneability of optical properties by playing on the film microstructure)

Summary

Introduction

Structuring of solid materials at the micro- and nanoscale appeared as a key strategy to generate novel materials properties. Two degrees of freedom characterizes the GLAD experimental setup: the deposition angle α defined as the angle between the substrate normal and the direction of incident vapor flux ( called tilt angle), and the substrate rotation φ about its normal defining the azimuthal position of the substrate [19] These two key parameters modify in an indirect way, the position of the particle source. Adjusting precisely α and φ parameters and combining some recent improvements such as a phisweep motion [21] or a co-deposition process [22], more original structures can be obtained Such developments exploit the effects of shadowing created by a tilted substrate relative to normal incidence, and by a change of the direction of the particle flux through a rotating substrate during the film growth, which extend even more the panel of engineered nanostructures [23]. This optimized index is discussed, considering the evolution of the films’ morphology and structure at the micro- and nanoscale

Operating Conditions for WO3 Films Growth

Characterization

Optical Properties

Findings

Conclusions