SURFACE AND INTERFACE ANALYSIS Surf. Interface Anal. 2006; 38: 736–739 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/sia.2266 XPS investigation of CoO x -based MRISiC structures for hydrocarbon gas sensing E. Comini, 2 A. Cusm ` a, 1 S. Kaciulis, 1 S. Kandasamy, 3 G. Padeletti, 1 L. Pandolfi, 1* G. Sberveglieri, 2 A. Trinchi 3 and W. Wlodarski 3 1 ISMN-CNR, P.O. Box 10, 00016 Monterotondo Stazione, Roma, Italy 2 Department of Materials Chemistry and Physics, University of Brescia, Via Valotti 9, 25133 Brescia, Italy 3 RMIT University, GPO Box 2476V, Melbourne, Victoria 3001, Australia Received 1 July 2005; Revised 22 November 2005; Accepted 29 November 2005 Sensors based on metal–reactive insulator–silicon carbide (MRISiC) devices are widely employed for monitoring industrial processes because of their resistance to harsh and high-temperature environments. MRISiC structures composed of the films of Pt and cobalt oxide (CoO x ) were fabricated and investigated. The films of Pt and CoO x were deposited on SiC substrates by using r.f. magnetron sputtering. The depth profiles of their chemical composition were studied by XPS combined with cyclic Ar + sputtering. The surface morphology was investigated by AFM. XPS analysis of the Co 2p peaks and shake-up satellites revealed the presence of the mixed-valency oxide Co 3 O 4 on the sample surface, while deeper in the film only stoichiometric CoO was present. The platinum film deposited on the top of cobalt oxide was found to be metallic and uniform enough to inhibit further oxidation of Co 2+ . The hydrocarbon (propene) gas sensing performance of the Pt/CoO x /SiC devices was studied. Copyright  2006 John Wiley & Sons, Ltd. KEYWORDS: XPS; AFM; depth profiling; metal oxide; gas sensors; Schottky diodes INTRODUCTION The monitoring of the produced gases is of great impor- tance in many industrial processes because of the necessity to maximize the efficiency and control of these processes. The interest in the nature and amount of the emitted gases has become even more important owing to the laws concerning the prevention of ambient pollution. In the past years, the attention of many researchers has been concentrated in monitoring hydrogen and hydrocarbons, which are the main gases emitted by industry. The direct monitoring of these gases requires the use of sensors in extreme conditions (high temperature, corrosive environ- ments, etc.). This requirement has generated the search for materials capable of working reliably in such severe con- ditions. Metal–oxide–semiconductor (MOS) 1 and, partic- ularly, metal–reactive insulator–siliconcarbide (MRISiC) 2,3 devices have attracted much interest for their applications in extreme industrial conditions. Moreover, hydrogen sensing is becoming very important because of the possible use of hydrogen as a clean source of energy. The MRISiC device consists of a metal oxide layer (reactive insulator) selected on the basis of its reactivity to target gas and is deposited between a SiC substrate and a catalytic metal (Pt). It can be operated in two modes: Schottky diode or capacitor, biased at constant current or capacitance, respectively, and registering the response to gases as a change L Correspondence to: L. Pandolfi, ISMN-CNR, P.O. Box 10, I-00016 Monterotondo Stazione, Roma, Italy. E-mail: pandolfi@mlib.cnr.it in the output voltage. The selectivity, sensitivity and the stability of these devices depend on the surface chemical properties of the reactive film and on the interface between the different layers. In this work, the chemical and structural properties of an MRISiC device composed of Pt/CoO x on SiC substrate were investigated. The microchemical characterization of the samples was carried out by using XPS depth profiling, whereas the surface morphology was analyzed by AFM. The fabricated Pt/CoO x /SiC was tested for different concentrations of propene ⊲C 3 H 6 ⊳ in the presence of synthetic air. EXPERIMENTAL CoO x thin films were prepared by reactive r.f. magnetron sputtering. The metallic target was Co of 99.99% certified purity and the oxidizing atmosphere composed of Ar C O 2 (1 : 1) with a working pressure of 10 2 mbar and a sputtering power of 50 W. During the deposition, the substrate was kept at 300 ° C in order to favor the formation of a stable layer. For electrical measurements, the films were deposited on 3 ð 3 mm 2 SiC substrates of 250 μm thickness. Pt contact ⊲thickness D 500 nm⊳ was deposited on the top of the cobalt oxide layer and a double layer of Ti and Pt (thicknesses of 50 and 500 nm, respectively) was deposited on the bottom of the SiC transducer. The geometry of the samples was maintained identical to the MRISiC devices with a different oxide described previously. 4 The following samples were prepared and subsequently investigated: Copyright  2006 John Wiley & Sons, Ltd.