Thin Solid Films 449 (2004) 166–172 0040-6090/04/$ - see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.10.013 New thermochromic bilayers for optical or electronic switching systems Sebastien Saitzek , Guillaume Guirleo , Frederic Guinneton , Laurent Sauques , Sylvie Villain , a a a b a ´ ´´ Khalifa Aguir , Christine Leroux , Jean-Raymond Gavarri * c a a, Laboratoire Materiaux et Microelectronique de Provence (L2MP), Universite de Toulon et du Var – BP 132, F-83 957 La Garde Cedex, a ´ ´ ´ France Centre Technique d’Arcueil, LOTyOPL, 16 bis, Avenue Prieur de la Cote d’Or, F-94 114 Arcueil Cedex, France b ˆ Laboratoire Materiaux et Microelectronique de Provence (L2MP), Universite d’Aix- Marseille 3, Faculte des Sciences de Saint Jerome, c ´ ´ ´ ´ ´ˆ Case 142, F-13 397 Marseille Cedex 20, France Received 29 August 2003; received in revised form 22 September 2003; accepted 16 October 2003 Abstract Thin films of thermochromic vanadium dioxide (with phase transition at T s68 8C) were covered by a cerium dioxide layer. c Cerium dioxide was chosen for its chemical stability, infrared optical transparency and strong optical absorption in the ultraviolet range. The optical properties of these bilayers are comparable to optical properties of pure vanadium dioxide films. Vanadium dioxide was first deposited on a silica substrate by radio-frequency reactive sputtering. Later, a ceria thin film was deposited on the vanadium dioxide film by the same technique. The thin films were characterized by X-ray diffraction measurements, atomic force microscopy, scanning electron microscopy, transmission electron microscopy and ellipsometry techniques. Optical transmit- tance and optical switching efficiency were studied by Fourier transform infrared spectroscopy.  2003 Elsevier B.V. All rights reserved. Keywords: Vanadium oxide; Cerium; Multilayers; Optical properties 1. Introduction The general aim of this study is to elaborate and characterize new optical or electronic switching material made of VO –CeO bilayers. The thermochromic rutile- 2 2 type vanadium dioxide (VO ) has been extensively 2 studied for its optical switching properties. The ther- mochromic VO (monoclinic: Myrutile: R) variety 2 undergoes a well-known semiconductor to metal phase transition (metal–insulator phase transition) at approxi- mately T s68 8C w1–4x, with a monoclinic to tetragonal c lattice modification of the rutile-type packing. The reversible transition is associated with strong changes of electrical w5,6x, magnetic and optical properties w7–13x. Unfortunately, due to strong chemical reactivity with oxygen, VO is not stable in air, and hydration or 2 oxidation might occur. To preserve the thermochromic properties of a VO layer, a lot of solutions involving 2 polymer coating or paints were envisaged: however, any additional coating is generally strongly absorbing for IR *Corresponding author. Tel.yfax: q33-494-142-311. E-mail address: gavarri.jr@univ-tln.fr (J.-R. Gavarri). optical applications. Using VO nanosized pigments 2 inserted in paints w14x, we have shown that these VO 2 pigments could be protected from environmental degra- dation, the polymer medium acting as an insulating envelop. In the case of VO films, the use of polymer 2 paints in specific applications generally involves strong optical contrast attenuation. So, for thin films, one interesting solution could be the deposition of one additional inorganic thin film over the active layer, making use of a material sufficiently resisting to envi- ronmental aggressions and transparent in the interesting optical domain (IR). Cerium dioxide CeO (ceria) has 2 excellent operating properties such as high thermal, chemical endurance and adequate infrared transparency w15,16x. Therefore, it could be one ideal candidate as a transparent protective window for a thermochromic VO layer. As the thermochromic transition also affects 2 the electrical conduction of VO , such bilayer systems 2 might also be involved in electronic switching applications. Various deposition techniques are available for the elaboration of CeO thin films, mainly the RF reactive 2