Electrode-sample capacitance effect on Ethanol sensitivity of nano-grainedSnO2 thin films

Tin oxide thin films were deposited on float glass, Coming 7059 glass and a lumina substrates by sol-gel technique using SnCl2. 2H(2)O as a precursor. The average crystallite size, determined by X-ray diffraction (XRD) was sim ilar 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 confir med the nano-grained structure of the films. It was observed that the sampl es retain some charge after switching off the 9V de bias applied to them fo r sensitivity measurements. This charge was found to decay with time. The a mount 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 t o 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 Co ming and alumina substrates. The films on float glass showed an extremely h igh sensitivity (similar to 240 for 700 ppm of ethanol) to ethanol vapour. A lesser sensitivity was obtained for films on Coming and alumina substrate s (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 t he chemisorbed oxygen ions at the electrode-sample contact region on the ap plication of the de bias. These accumulated ions form a capacitor at the el ectrode-sample contact region and increase the depletion layer width near t his region. Reduction of this capacitance and depletion layer width in the presence of ethanol increases the current in the circuit, thereby increasin g the sensitivity. Due to the larger grain size and negligible charging eff ect for films on coming and alumina substrates, the sensitivity was less fo r films on these substrates. (C) 1998 Elsevier Science S.A. All rights rese rved.