Thin Solid Films 446 (2004) 287–295 0040-6090/04/$ - see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.09.062 Optimized infrared switching properties in thermochromic vanadium dioxide thin films: role of deposition process and microstructure Frederic Guinneton , Laurent Sauques , Jean-Christophe Valmalette , Frederic Cros , Jean- a,b a b a ´´ ´´ Raymond Gavarri * b, Centre Technique d’Arcueil, LOTyOPL, 16 bis, Avenue Prieur de la Cote d’Or, Arcueil, Cedex F-94 114, France a ˆ Laboratoire Materiaux et Microelectronique de Provence (L2MP), Universite de Toulon et du Var–BP 132, La Garde, Cedex F-83 957, b ´ ´ ´ France Received 18 July 2003; received in revised form 25 August 2003; accepted 22 September 2003 Abstract This work deals with high efficient optical switching properties at 68 8C of thermochromic vanadium dioxide (VO ) thin films 2 deposited on amorphous silica substrates. VO thin films were deposited by radio frequency reactive sputtering process. Conditions 2 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 VO 2 thin films could grow on amorphous silica substrates with a specific preferential crystal orientation: the w001x crystallographic M 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 (Tcs68 8C). 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.  2003 Elsevier B.V. All rights reserved. Keywords: Vanadium oxide; Coatings; Optical properties; Infrared spectroscopy 1. Introduction The general aim of this study was to better control the infrared switching properties of thermochromic thin films of VO vanadium dioxides. The doped 2 V MO vanadium dioxides, in which M is an elec- 1yx x 2 tron-doping element, are well known for their interesting insulator–metal transition associated with strong varia- tions of infrared optical properties w1x. The doping by M cations has two types of consequences: a modifi- nq cation of the transition temperature (Tcs68 8C for pure VO ) and a modification of the optical contrast. In 2 addition, when thin films are deposited on specific substrates, the optical transmittance, reflectance and emissivity are generally conditioned by texture effects, *Corresponding author. Tel.yfax: q33-494-142-311. E-mail address: gavarri.jr@univ-tln.fr (J.-R. Gavarri). film continuity, density, porosity, grain boundaries, defects and finally film thickness. Presently, we will focus our attention on two main parameters: the film thickness and the final microstructural orientation of deposited films. To establish correlations between these parameters, thermochromic vanadium dioxide thin films were elab- orated by radio frequency sputtering deposition on amor- phous silica substrate. Pure vanadium dioxide VO 2 exhibits a reversible metal–insulator phase transition (MIPT) at 68 8C. Using specific chemical substitutions for vanadium cation, this transition temperature can be lowered: for instance, in the case of V WO w2x,W stable 6q 1yx x 2 cations are formed through the local reaction W ™ 4q W q2e ; 2e q2V ™2V in which the W 6q y y 4q 3q 4q cations should behave as electron donors for vanadium