Microstructure and electrical properties of RuO 2 –CeO 2 composite thin films P. Nowakowski a , S. Villain a , K. Aguir c , J. Guérin c , A. Kopia b , J. Kusinski b , F. Guinneton a , J.-R. Gavarri a, ⁎ a Université du Sud Toulon Var, IM2NP, UMR CNRS 6242, BP 20132, 83957 La Garde Cedex, France b University of Science and Technology, AGH, Al. Mickiewicza 30, 30059 Cracow, Poland c Université Aix Marseille III - Paul Cézanne, IM2NP, UMR CNRS 6242, 13397 Marseille Cedex 20, France abstract article info Article history: Received 2 February 2009 Received in revised form 17 August 2009 Accepted 20 August 2009 Available online 27 August 2009 Keywords: Ruthenium cerium dioxides electrical properties thin films composite thin films electron microscopy electrical percolation RuO 2 –CeO 2 composite thin films are deposited on various Si substrates by a radiofrequency magnetron sputtering technique. Compacted polycrystalline pellets of the nanostructured CeO 2 –RuO 2 composite system are used as standard samples for comparative electrical analyses. All films and composite samples are analyzed by X-ray diffraction and transmission electron microscopy. Electrical measurements of radio- frequency sputtering of thin films are performed as a function of the RuO 2 fraction and of the temperature (between 25 and 400 °C). A nonlinear variation in the electrical conductivity of the RuO 2 –CeO 2 composite thin films as a function of the RuO 2 volume fraction (Φ) is observed and discussed. It is interpreted in terms of a power law (in (Φ -Φc) m ), where m and Φc are parameters characteristic of the distribution of the conducting phase in a composite medium. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The general aim of this work is to elaborate thin films based on CeO 2 –RuO 2 composites. These thin films could exhibit metallic or semiconducting properties [1]. In the present study, we focus our attention on variable electrical behaviors of [xRuO 2 –(1 -x)CeO 2 ] composite systems, deposited by an radiofrequency sputtering process. The general interest in these systems is based firstly on their appli- cations as electrodes that are stable at high temperatures, and secondly, on catalytic interactions with specific gases (methane, carbon monox- ide, etc.). The rutile phase RuO 2 presents metallic conduction [2–4], whereas the fluorite phase CeO 2 behaves as a semiconducting (insulating) material [5]. Ruthenium dioxide has a tetragonal rutile structure and a large variety of interesting properties. It is a good catalyst for both reduc- tion and oxidation of specific gases [6–8]. This dioxide offers (i) good thermal and chemical stabilities [9,10], (ii) strong resistance to chemical corrosion [11], and (iii) excellent properties as a chemical diffusion barrier [12]. In the microelectronics field, RuO 2 was pro- posed as a complementary metal-oxide semiconductor component and as an electrode for a random access memory capacitor [13,14]. It was also investigated as a gas conversion catalyst for CO [15,16] or C 2 H 4 [17,18]. In our previous works, we developed studies on the catalytic efficiency of nanostructured ceria CeO 2 [19,20] subjected to air methane flows. We also studied ceria-based thin films [21] obtained by pulsed laser deposition. From infrared spectroscopy analyses of emitted gases, we recently showed [22,23] that nanostructured RuO 2 powders exhibit clear catalytic properties in flowing air/methane in the temperature range 200–450 °C. Acoustic sensor devices based on piezoresistive composite thick films have recently been tested [24]: the composite system is based on piezoresistive pastes charged in RuO 2 nanoparticles. Such thick-film composite resistors are used in the field of pressure and force sensors owing to their low cost, good stability, and high piezoresistive response. The typical microstructure of these composite systems is characterized by an insulating phase that embeds randomly dis- persed, conducting submicron RuO 2 grains. The dominant transport mechanism is assumed to be quantum tunneling through the nanostructured thin film of the insulating phase between two neighboring conducting particles [25]. Another interpretation has been proposed based on an anisotropic distribution of the conducting particles [26]. The macroscopic transport properties strongly depend on the metallic volume fractions of the metal phase [27,28]. 2. Experimental details 2.1. Elaborations 2.1.1. Powders and composites Each CeO 2 or RuO 2 powder was synthesized via the classical sol– gel route described in our previous work on the CeO 2 phase [20] and the RuO 2 phase [22,23]. Ruthenium and cerium dioxides were Thin Solid Films 518 (2010) 2801–2807 ⁎ Corresponding author. Tel./fax: +33 494 142 311. E-mail address: jr.gavarri@univ-tln.fr (J.-R. Gavarri). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.08.034 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf