Comparison study of SnO2 thin- and thick-film gas sensors

doi:10.1016/S0925-4005(00)00390-7

Sensors and Actuators B: Chemical

Volume 67, Issues 1–2, 10 August 2000, Pages 122-127

https://doi.org/10.1016/S0925-4005(00)00390-7 Get rights and content

Abstract

Polycrystalline SnO₂ thin films were prepared at 600°C by metal organic chemical vapor deposition (MOCVD) technique using tetraethyltin as an organometallic (OM) source and UHP O₂ as oxidant. The films were analyzed by means of XRD, SEM, and AES for their microstructure characterization and subjected to H₂ and CO gas detection. The results were compared to SnO₂ thick-films derived from metal organic decomposition (MOD) in order to study differences in gas sensing characteristics in relation to the microstructure. The microstructure of the MOCVD-derived thin films was fully dense columnar structure with rough surface morphology while that of the MOD-derived thick films was porous structure resulting from loosely interconnected small crystallites. Both types of sensors showed good reproducibility and stability toward 1% H₂ gas with an enhancement of the sensitivity and the time response in the thick-film sensor. The sensing characteristics were degraded under 1% CO gas for both types of films.

Introduction

Since the first proposed use of oxide semiconductors to detect flammable gases by Seiyama et al. [1] and the subsequent manufacturing of the first commercially sintered SnO₂ Taguchi Gas Sensor (TGS) [2], SnO₂ has been subjected to extensive research as a solid state gas sensor [3], [4], [5]. Nowadays, SnO₂ thin-film gas sensors have become commercially available.

It is well known that the resistance of SnO₂ changes according to various reducing gas environments [6], [7], [8]. In atmosphere, oxygens are adsorbed onto SnO₂ surface by capturing electrons from the conduction band and remain as O₂⁻, O⁻, or O²⁻ ions until they desorb at higher temperature [9]. These adsorbed oxygen species induce a subsequent potential barrier at a grain contact and a resistive depletion layer is formed which determines most of the sensor resistance. When a flammable gas is introduced, the adsorbed oxygen is removed by oxidation of the gas and the captured electrons are injected into the conduction band. This results in a reduction of the potential barrier height and a decrease in resistance of the sensor.

However, the correlation of the sensing mechanism and the microstructure of the sensing materials has not been clearly established yet. The lack of understanding becomes severe in case of film type sensors.

Of many technical difficulties involved in SnO₂ gas sensors, the poor reproducibility and stability is of the greatest concern. In this study, metal organic chemical vapor deposition (MOCVD) technique was employed to prepare SnO₂ thin films to study their microstructure and gas sensing reproducibility and baseline stability as compared to metal organic decomposition (MOD)-derived thick films.

Section snippets

Sample preparation

Thin films were prepared on polished alumina substrates using a cold-wall, horizontal, low-pressure MOCVD system. Tetraethyltin, Sn(C₂H₅)₄, and UHP oxygen were used as an organometallic (OM) source and as an oxidant, respectively. The carrier gas for both OM source and oxygen was UHP N₂. The description and detailed growth parameters of the MOCVD system can be found in our earlier publication [10].

Thick films were prepared by spin coating of metal organics on alumina substrates, followed by MOD

Microstructures of the thin and thick films

Fig. 2 shows the XRD pattern of both films. Both films have the cassiterite phase without any measurable impurity phases. There was no particular preferred orientation observed in both types of sensors whereas Sberveglieri et al. [12] reported strong (200) orientation in their CVD-derived thin films. The broadening of the SnO₂ peaks in the thick film indicates a smaller grain size compared to the thin film.

SEM micrographs in Fig. 3, Fig. 4 show detailed microstructure of both films. In case of

Conclusion

SnO₂ thin-films have been prepared by MOCVD technique and their sensing properties were characterized. MOD-derived thick films were also characterized for the comparison study of the sensing characteristics. Both sensors showed a good sensing reproducibility and stability under 1% H₂ gas with an enhancement of the sensitivity in the thick film. The enhancement seems originate from its microstructure. The greater surface area as well as the exposed grain boundary area caused by the pore

Acknowledgements

This work was supported by the US Department of Energy, with a grant (DEFG02-96ER45439) through the University of Illinois at Urbana-Champaign, Frederick Seitz Materials Research Laboratory. Partial financial aid from the Materials Research Laboratories, Industrial Technology Research Institute, Taiwan is appreciated. Thanks are also due to Mr. Chun-Hsiung Lin for helping us deposit gold electrodes.

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Comparison study of SnO2 thin- and thick-film gas sensors

Abstract

Introduction

Section snippets

Sample preparation

Microstructures of the thin and thick films

Conclusion

Acknowledgements

Sens. Actuators

Sens. Actuators

Sens. Actuators

Sens. Actuators, B

Sens. Actuators, B

Sens. Actuators

A new detector for gaseous components using semiconductive thin films

Anal. Chem.

Comparison study of SnO₂ thin- and thick-film gas sensors