Nanosized Ti-doped MoO 3 thin ®lms for gas-sensing application V. Guidi a,d,* , D. Boscarino b , L. Casarotto a , E. Comini c , M. Ferroni a , G. Martinelli a,d , G. Sberveglieri c a Dipartimento di Fisica and INFM, Universita Á di Ferrara, Via Paradiso 12, I-44100 Ferrara, Italy b Laboratori Nazionali Legnaro, Via Romea 4, 35020 Legnaro, Padova, Italy c Dipartimento di Chimica e Fisica dei Materiali and INFM, Universita Á di Brescia, Via Valotti 9, I-25133 Brescia, Italy d Istituto Nazionale di Fisica Nucleare Ð Sezione di Ferrara, Via Paradiso 12, I-44100 Ferrara, Italy Abstract Development of MoO 3 as a novel material for gas sensing was addressed. Thin ®lms were produced by reactive rf sputtering assisted by annealing. Doping with Ti was performed to enhance the conductance of the MoO 3 ®lm. It came out that the layers were two orders of magnitude more conductive than undoped material. Good and reversible response to CO was achieved at 3008C, which fell off at higher temperatures. An interesting feature of the ®lms was a considerably fast response for both CO and NO 2 despite low operating temperature. Doped®lmswerefoundtooperateatbestabout1008CbelowthanforpureMoO 3 layers. # 2001ElsevierScienceB.V.Allrightsreserved. Keywords: Ti:MoO 3 ; Sputtering; Gas sensor 1. Introduction Molybdenum trioxide is a long-time known catalyst [1] and possesses photochromic [2] and electrochromic [3] effects. Electrochromism and photochromism, i.e. the changeinlightadsorptionandemissionunderelectrolithical or luminous stimulus, are evidence of material electronic activity, while catalysing proprieties involves surface adsorption of molecules and interaction with them. Due to these particular proprieties, molybdenum trioxide is a promising material for gas-sensing application. Those pro- prieties, in fact, are bound up to the features that make a materialsensibletogases:indeedgasdetectioncomesfrom modi®cationinsurfaceconductionbandsduetoachangeof free electrons number when a gas molecule interacts with surface layer. Molybdenum trioxide, therefore, exhibits just the appropriate requirements that a material would possess to act as a good gas detector. Thereby,researchonmolybdenumtrioxide-basedsensors was performed. Thin ®lms were produced and investigated towardsCOandNO 2 ,but,inspiteofeffectivesensitivityand good characteristics exhibited, the material proved to be rather high resistive for a practical implementation with standard electronics [4±6]. In order to overcome such limit, titanium-doped molyb- denum oxide has been produced and investigated. Through this experimental work, complete electrical and structural characterization has been performed in order to explore doped material and to compare it with pure molybdenum trioxide ®lm. 2. Experimental Deposition was performed by radio frequency sputtering RFS) operated at 200 W rf power in an Alcatel plant. In order to achieve a metal oxide thin ®lm, sputtering was performed in reactive atmosphere, consisting of argon and oxygen at 2  10 3 mbar partial pressure for each gas. A ®rst set of deposition was executed in order to inves- tigate the material features. In this case, a composite target was used: a Ti target 4 in. in diameter, certi®ed at 99.9% purity) was covered on the center by a Mo foil 36 cm 2 )so that the larger part of material was removed from the molybdenum area. The layers were deposited over either alumina or silicon substrate. Films over alumina were employed in sensors production, while ®lms over silicon were dedicated to structuralcharacterization.Duringdeposition,thesubstrates were kept at 3508C. After deposition, some of the samples underwent annealing up to 5008C for 12 h in synthetic air atmosphere. Sensors and Actuators B 77 2001) 555±560 * Corresponding author. Tel.: 39-532-781-884; fax: 39-532-781-810. E-mail address: guidi@fe.infn.it V. Guidi). 0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0925-400501)00703-1