Formation and structure of tin-iron oxide thin film CO sensors
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C. Perego and G. Sberveglieri Dipartimento Ingegneria Elettronica, Universita di Brescia, Via Branze 38, 25123 Brescia, Italy G. Quattroni Centro Ricerche ENEL di Brindisi, Via Dalmazia 211c, Brindisi, Italy (Received 5 March 1993; accepted 22 December 1993)
Rheotaxial growth and thermal oxidation (RGTO) for depositing thin films is a recognized technique in preparing gas sensitive semiconducting oxides. This paper presents a study performed by x-ray diffraction and scanning Auger microscopy of the mechanisms of growth and formation of the thin films of the new ternary compound Sn^Fe^Oy with an iron content in the range 0 < x < 25 at. %. A structural model of this compound, which is found to be stable over a very large range of Sn/Fe ratios, can be derived by partially substituting Fe3+ ions in Sn4+ sites. This is an easy substitution in view of the similar values shown by the ionic radii (Fe3+ = 0.64 A, Sn4+ = 0.71 A) and the Pauling electronegativity (Fe3+ = 1.8, Sn4+ = 1.8) of these two ions. Experimental data, showing that this material is an excellent CO sensor, are reported.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
Since the discovery of sintered SnO 2 as gas sensors,1 now commercially available as Taguchi Gas Sensor (TGS), many articles have been published on this subject. Thick film and thin film techniques are most commonly used to prepare these solid-state gas sensors.2'3 The higher sensitivity is obtained with thick-film materials since they have a larger surface versus volume ratio and porosity. Nevertheless, the thin film technique seems to offer some technological advantages and now it is widely studied. One of the authors (G. S.) recently developed a new method for synthesizing thin film gas sensors.4'5 This process, called Rheotaxial Growth and Thermal Oxidation (RGTO), is based on the sputtering of metal thin films on a substrate, maintained at a temperature higher than the metal melting point, followed by annealing in oxygen to obtain the metal oxide.4'5 This technique has been successfully used to prepared SnO2-based materials, suitable as gas sensors. However, some bulk and surface properties of these materials are still obscure. The aim of this work is to present the structural properties of Sn^Fe^Oy thin films prepared by the RGTO technique that are fundamental to the understanding of the mechanisms of metal-semiconductor phase transformation and growth of this material. Finally, we present the electrical responses of this material to a mixture of CO (10-1000 ppm) in standard air.
Polished A12O3 supports were used to deposit the metal thin films. During the thermal evaporation the pressure in the deposition chamber was maintained by means of a cryogenic pump at about 7.5 X 10"7 Torr. Sn was first deposited with a deposition rate of 6—7 A/s. During the Sn deposition, the substrate temperature was kept at T = 533-543 K, i.e., at a temperature higher than the Sn melting point (T = 505 K). The Sn thin film is formed by spherical agglomerates due to the surface tension of the melted metal; t
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