Electrical behavior of SnO2 thin films in humid atmosphere

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Abstract

This paper presents results of research of the temperature dependence of resistance (R) of thin film gas sensors (TFGS) in a humid atmosphere. It was shown that the R(T) dependencies, measured in the usual atmospheric conditions (relative humidity is in the range of 40–100% and ambient temperature ∼20°C), had some maximums and significant hysteresis, the form and area of which depends on the maximum temperature reached in the measurement. A critical temperature (TCR=410–430°C) was observed for the maximum temperature. Upon exceeding TCR, the second loop of hysteresis appears. It was established that such behavior of R(T) is connected with the influence of water on the SnO2 surface state. These anomalies disappear in a dry atmosphere. The explanation of the observed effects is given and the mechanism of interaction of the SnO2 surface with H2O, explaining such behavior of R(T), is proposed.

Introduction

It was noted in many investigations [1], [2], [3], [4], devoted to analysis of operation of thick film and ceramic gas sensors, that conductivity of SnO2 gas sensors depends on the concentration of H2O in atmosphere. In studying of the characteristics of high sensitive SnO2 thin film gas sensors, we have observed also a considerable influence of the humidity of surrounding atmosphere on their parameters. However, analyzing the literature, dedicated to this question, we have concluded that detailed investigations directed to the studying of a humidity influence on the electrical conductivity of tin dioxide thin films were not carried out yet. And there is significant lack in understanding the humidity influence on a gas sensing mechanism..

Analysis of results of research [1], [5], which studied the ceramic tin oxide’s behavior in humid atmosphere, has shown that the basic processes responsible for the observed alterations of R with temperature, are the transition O2⇒O (T>150°C), O2 desorption (T>150°C), water loss (T=280–440°C) and O desorption (T>520°C) from SnO2. For confirmation or refutation of above conclusions in application to SnO2 thin films we have carried out the present research.

Section snippets

Experimental

For the research of humidity influence on the conductivity of SnO2 thin films we used TFGS, manufactured by technology described in Dmitriev et al. [6]. The SnO2 thin films were deposited by spray pyrolysis from a starting 0.2 M solution of tin chloride pentahydrate (SnCl4·5H2O) in ethanol or deionized water on an alumina ceramic substrate [6], [7]. The time of spraying was 10–15 s and the volume of sprayed solution was in the range 5×10−4 to 2×10−3 l. The substrate temperature was kept at

Results

The influence of relative humidity of air on the temperature dependence of resistance of SnO2 thin films is presented in Fig. 1, Fig. 2, Fig. 3. The R(T) dependencies measured in usual atmosphere (Tambient=20°C and relative humidity (RH)≥50–100%) had nonmonotonous character with particularly pronounced maximums and minimums in the range of: 40–70, 200–260, and 330–350°C. The presence and magnitude of these extremes are dependent not only on a relative humidity but also on the nature of solvent,

Discussion

It was established in previous literatures [9], [10], [11], [12] that H2O is adsorbed on the oxide surface in the molecular and hydroxyl forms. In addition, it was established by Boyle and Jones [9] that water demonstrates donor properties at adsorbtion in the molecular form while the acceptor properties in the hydroxyl form. It is also known [13], [14] that sheet resistance of dense, solid and continuos thin films is determined by the surface potential, controlled by surface charge. Proceeding

Conclusion

As a way for thin film gas sensors development, influence of humidity on electric properties of SnO2 films, prepared by spray pyrolysis method, have been studied. The main conclusions are summarized as follows:

  • 1.

    The appearance of some maximums and complicated form hysteresis in the R(T) dependencies of SnO2 films in a humid atmosphere, is the consequence of the influence of water, existing in the atmosphere, on the charge state of SnO2 surface. The specifics of such R(T) behavior may be explained

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    /Gdry (where Gh.a. is the conductivity in humid air, and Gdry is the conductivity in dry or synthetic air) is approximately equal to magnitudes of film’s conductivity response to water vapors observed in the experimental studies [46–49]. This corresponds to the changes in the surface charge ΔNs ∼ 1011÷1013cm−2.

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