Electrode-sample capacitance effect on Ethanol sensitivity of nano-grained SnO2 thin films

Tin oxide thin films were deposited on float glass, Coming 7059 glass and alumina substrates by sol-gel technique using SnCl2. 2H(2)O as a precursor. The average crystallite size, determined by X-ray diffraction (XRD) was similar to 4.5 nm for films on float glass and similar to 9.0 nm for films on Coming and alumina substrates. Atomic force microscope (AFM) studies confirmed the nano-grained structure of the films. It was observed that the samples retain some charge after switching off the 9V de bias applied to them for sensitivity measurements. This charge was found to decay with time. The amount of charging was considerably low if the samples were kept in a vacuum similar to 10(-2) Pa or at temperatures < 573 K. Exposure of the samples to ethanol vapour eliminates the charging effect. The effect was found to be prominent in films on float glass substrates and very small in films on Coming and alumina substrates. The films on float glass showed an extremely high sensitivity (similar to 240 for 700 ppm of ethanol) to ethanol vapour. A lesser sensitivity was obtained for films on Coming and alumina substrates (similar to 70 for films on both the substrates for 700 ppm of ethanol). The occurrence of high sensitivity has been correlated with the grain size as well as the charging effect which is attributed to the accumulation of the chemisorbed oxygen ions at the electrode-sample contact region on the application of the de bias. These accumulated ions form a capacitor at the electrode-sample contact region and increase the depletion layer width near this region. Reduction of this capacitance and depletion layer width in the presence of ethanol increases the current in the circuit, thereby increasing the sensitivity. Due to the larger grain size and negligible charging effect for films on coming and alumina substrates, the sensitivity was less for films on these substrates. (C) 1998 Elsevier Science S.A. All rights reserved.