Interaction of reducing gases with tin oxide films prepared by reactive evaporation techniques Rajesh Kumar a , Atul Khanna b, * , V.S. Sastry c a Department of Physics, Block-28, Lovely Professional University, Phagwara 144024, Punjab, India b Department of Physics, Guru Nanak Dev University, Amritsar 143005, Punjab, India c Condensed Matter Physics Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603012, Tamil Nadu, India article info Article history: Received 17 November 2011 Received in revised form 2 January 2012 Accepted 9 January 2012 Keywords: SnO 2x films H 2 S H 2 Ethanol sensitivity X-ray diffraction abstract SnO 2x films were prepared by reactive thermal and e-beam evaporation of Sn on alumina substrates and by post deposition thermal treatment. X-ray diffraction measurements found that films are tin dioxide (SnO 2 ) phase with small amounts of SnO phase. The surface conductivity of films was measured in air and in presence of H 2 S, H 2 and C 2 H 5 OH vapors at four sensor operating temperatures of 433e493 K. The resistance of SnO 2x films decreases on exposure to H 2 S but shows no change with hydrogen and ethanol. H 2 S response decreases with rise in sensor temperature while both response and recovery times improve. H 2 S signal enhances with increase in resistivity of SnO 2x coatings. Our experiments conclude that increase in film conductance is due to chemical reaction between H 2 S and SnO 2x surface and there is little or no role of interaction of gas molecules with surface adsorbed charged oxygen species. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Elemental semiconductors like Ge and metals oxides such as ZnO, SnO 2 , WO 3 and TiO 2 have an intriguing and useful property that they exhibit significant changes in their electrical conductance on exposure to gases [1,2]. Exposure of n-type semiconductors like SnO 2x to oxidizing gases like O 2 decrease its conductivity while reducing gases like CO, H 2 S, H 2 and C 2 H 5 OH increase it; this property has been fruitfully exploited for the detection of several toxic and inflammable gases like H 2 ,H 2 S, CO, liquid petroleum gas (LPG), NO, NO 2 and ethanol by solid state gas sensors [3e8]. Gas sensors based on thick and thin films of tin oxide (IV) find application in ecological monitoring of the environment, as indi- cators for gas discovery in mines and for the detection and control of poisonous and inflammable gases in industry. Semiconducting tin oxide films have also been widely used as transparent con- ducting electrodes in devices like solar cells, Li-ion batteries, transparent conductive electrodes, electrochromic displays and LCD’s. Tin oxide coatings have been prepared by variety of techniques such as thermal and e-beam evaporation [9e12], RF sputtering [13e16], spray pyrolysis [17e19], chemical vapor deposition [20e22], pulse laser deposition [23,24] and thick film technology [25]. Stoichiometric tin dioxide (SnO 2 ) is an insulator while non- stoichiometric tin oxide (SnO 2x ) is a direct band gap (3.6 eV at 300 K), n-type electron degenerate semiconductor, whose proper- ties largely depend upon deviation from its stoichiometry (Sn/O ratio) [26,27]. The electrical conductance of SnO 2x can be altered by changes in its stoichiometry. Greater the concentration of oxygen vacancies, more is the conductivity of SnO 2x . The surface resistivity of non-stoichiometric tin oxide and other semi- conducting metal oxides moreover depends on the gases present in the ambient. There is a direct correlation between gas response and film stoichiometery although other properties like crystallite size, porosity, concentration of dislocations, thickness and temperature are equally important parameters [28e31]. It is reported that thin films of tin oxide show more sensitivity at higher temperature than their thick films [30]. There is a wide scatter in the reported gas sensing properties of tin oxide films and the mechanisms respon- sible for changes in electrical conductance of these materials are still not well understood despite last four decades of vigorous studies. It has been conventionally believed that the mechanism of conductivity change in tin oxide films on exposure to reducing and oxidizing gases is the interaction between the gas molecules and the surface adsorbed charged oxygen species like O 2  and O  . However this interaction is significant only under special condi- tions like very small size of crystallites in tin oxide. * Corresponding author. Tel.: þ91 183 225 8802x3168; fax: þ91 183 225 8820. E-mail addresses: ak.ap@gndu.ac.in, akphysics@yahoo.com (A. Khanna). Contents lists available at SciVerse ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum 0042-207X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2012.01.007 Vacuum 86 (2012) 1380e1386