Optimized infrared switching properties in thermochromic vanadium dioxide thin films: role of deposition process and microstructure

This work deals with high efficient optical switching properties at 68 degreesC of thermochromic vanadium dioxide (VO2) thin films deposited on amorphous silica substrates. VO2 thin films were deposited by radio frequency reactive sputtering process. Conditions of deposition were optimized making use of parameters such as film thickness, gas ratio and substrate temperature. Process was optimized adjusting the distance between target and substrate, and dimensions of target and substrates, to obtain a good uniformity and reproducibility of the layers. X-Ray diffraction patterns and scanning electron microscopy convincingly illustrated that VO2 thin films could grow on amorphous silica substrates with a specific preferential crystal orientation: the [001](M) crystallographic direction of oxygen octahedral chains is parallel to the substrate plane and corresponds with vanadium-vanadium links (insulating state) or with a maximum of electron delocalization (metal state). Optical switching properties in the mid-infrared range are discussed: transmittance, reflectance and emissivity values are strongly modified at the thermochromic transition temperature (Tc=68 degreesC). A maximum of optical transmittance contrast is observed for a thickness of 120-nm, then interpreted in terms of absorption law. Using a specific software, the n and k optical indices are determined and used to simulate the variation of transmittance vs. film thickness. (C) 2003 Elsevier B.V. All rights reserved.