Vacuum 82 (2008) 966–970 Response to oxygen and chemical properties of SnO 2 thin-film gas sensors Bogus"awa Adamowicz b,Ã , Weronika Izydorczyk a , Jacek Izydorczyk a , Andrzej Klimasek b , Wies"aw Jakubik b , Janusz Z ˙ ywicki c a Institute of Electronics, Silesian University of Technology, Akademicka 16, 44-100 Gliwice, Poland b Department of Applied Physics and Department of Optoelectronics, Institute of Physics, Silesian University of Technology, Krzywoustego 2, 44-100 Gliwice, Poland c High-Tech International Services, Rome, Italy Abstract The measurements of the response—in terms of the conductance changes—to oxygen adsorption of tin dioxide (SnO 2 ) thin-film-based gas sensors were performed. The sensing SnO 2 layers were obtained by means of the rheotaxial growth and thermal oxidation (RGTO) method. The sensor responses were measured under a dry gas flow containing oxygen in nitrogen, within the range of temperature from 25 to 540 1C. For comparison, similar studies were performed for a commercial SnO 2 thick-film (TGS 812) gas sensor. The in-depth profiles of the chemical composition of the RGTO SnO 2 layers were determined from the scanning Auger microprobe experiment. The changes in concentration ratios [O]/[Sn] and [C]/[Sn] from the near-surface region towards the grain bulk were shown. r 2008 Elsevier Ltd. All rights reserved. Keywords: Tin dioxide; RGTO technique; Oxygen adsorption; Surface electronic properties; Solid-state gas sensors; Auger electron spectroscopy 1. Introduction Tin dioxide (SnO 2 ) is a basic material for fabrication of thick/thin-film gas sensor structures, due to high sensitivity of the film conductance to both oxidizing and reducing gases as well as stability to heat treatment [1,2]. Recently, we have reported on the sensor devices for nitrogen dioxide (NO 2 ) and oxygen (O 2 ) detection, which were based on thin SnO 2 films (thickness in the submicrometer range) obtained by means of the rheotaxial growth and thermal oxidation (RGTO) method [3,4]. This method, developed by Sberveglieri [5], consists of a two-step process: namely, deposition of a thin Sn film with almost spherical droplet morphology on the substrate maintained at a temperature slightly exceeding the Sn melting point (231.8 1C), and then tin oxidation at high temperature (up to 700 1C), leading to the formation of a granular SnO 2 layer with high surface-to-volume ratio. Such layers seem to be particularly interesting for the gas sensor technology because of lower power consumption compared to thick film devices. Our studies showed that the RGTO thin-film sensor sensitivity, in terms of the relative changes of the electrical conductance, depends markedly on the control of technological process parameters (i.e. substrate tempera- ture during tin layer deposition, crystallite diameters and SnO 2 layer thickness), as well as on working conditions of the sensor (e.g. temperature, humidity and gas concentration). Furthermore, we proved theoretically the quantitative dependencies among the surface and bulk stoichiometric defects (donor-type oxygen vacancies) and the electrical properties (in-depth carrier profiles in the surface space charge region and electron conductivity behaviour versus temperature) of n-type SnO 2 films [6]. However, the sensing mechanism of RGTO SnO 2 films and its relationship with film chemical composition is not clear yet. Particularly important is the understanding of the interplay between oxygen species and SnO 2 film surfaces. This is related to the use of gas sensors in real-world conditions—at atmospheric pressure and at a high back- ground oxygen concentration (20.5 vol.%) [7]. ARTICLE IN PRESS www.elsevier.com/locate/vacuum 0042-207X/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.01.003 Ã Corresponding author. E-mail address: Boguslawa.Adamowicz@polsl.pl (B. Adamowicz).