Chemical composition and crystalline structure of SnO 2 thin films used as gas sensors

Abstract

SnO 2 films are commonly used as gas sensors. An important environmental application is the use and development of SnO 2 thin films in order to detect low concentrations of toxic gases (i.e., CO, NO 2, O 3, …) in urban environments. The gas response of the SnO 2 sensors strongly depends on the preparation process and especially on deposition parameters. SnO 2 thin films deposited by reactive sputtering are, in general, highly sensitive and selective above all towards CO and NO 2, even at very low concentrations. These sensors also present short response times to gases, as well as good reproducibility and repeatability. The gas detection capability of these sensors is based on the variations of the sensor resistance caused by the adsorption of gas on the sensor surface. This gas adsorption involves, above all, the first layers of the SnO 2 films. The aim of this work was to correlate the chemical composition focusing on the first layers and structural properties of SnO 2 films with their capability to detect gas. SnO 2 thin films were prepared with varying sputtering deposition parameters (i.e., the percentage of O 2 in the Ar sputtering plasma and the substrate temperature). The chemical composition of the first layers of SnO 2 films was examined by X-ray Photoelectron Spectroscopy (XPS). The morphology and the long and short range orders of SnO 2 thin films were analysed by Glancing Angle X-Ray Diffraction (GAXRD) and by Extended X-ray Absorption Fine Structure (EXAFS) at the Sn K edge. The results showed that SnO 2 films deposited at room temperature (RT) and with 6–8% of O 2 in the Ar sputtering plasma were the better gas sensors displaying high sensitivity to the gases, good reproducibility and reliability. These samples had an average SnO 2 grain size of 30–50 Å and a high percentage of adsorbed oxygen in the first two layers of SnO 2 film.