Preparation and Characterization of Thin Films by Pulsed Laser Deposition for NO x gas Sensor
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Preparation and Characterization of Thin Films by Pulsed Laser Deposition for NOx gas Sensor M. M. H. Bhuiyan1, T. Ueda1, H. Shingu1, T. Ikegami and K. Ebihara 1 Graduate School of Science and Technology1, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan Department of Electrical and Computer Engineering, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan ABSTRACT Gas sensors based on WO3 thin films doped with platinum (Pt) or palladium (Pd) were prepared by KrF excimer pulsed laser deposition method combined with dc sputtering. The films were deposited on silicon, quartz and Al2O3 sensor substrate with Pt interdigital electrodes at various substrate temperatures from 300°C to 500 °C, and oxygen pressures from 100 mTorr to 300 m Torr, respectively during the deposition. The morphology and structure of the films were examined by AFM and XRD. The sensor property of the WO3−x thin films was measured by the two terminal resistance method at operating temperatures of 25 °C to 400 °C. The sensitivity of the WO3 thin film gas sensors with doping (platinum or palladium) was found to be higher than that of undoped WO3 thin films gas sensors. The sensitivity of the pt doped WO3 films to different concentration of NO gas was examined and the sensitivity was found to be increased with increasing NO gas concentration. INTRODUCTION Tungsten trioxide (WO3) based materials as sensors for monitoring of environmental gases such as NOx have been developed for various potential applications. WO3 is a wide band-gap ntype semiconductor that has attracted much recent interest as a promising sensor because of its excellent sensitivity and selectivity [1]. WO3 gas sensors have been produced in forms of thick films, ceramics and thin films. Among these, thin film sensor seems to be more promising to improve the sensor performance as well as they are compatible with semiconductor technology for making small intregrated gas sensors [2]. The sensing mechanism lies in the change of film resistance resulting from physisorption, chemisorption and catalytic reactions of gas phase species with the film surface [3,4]. The mechanisms to detect gases were based on the interactions between the surface of the semiconductor crystallatices and the gases, which built up schottky barriers between the adjacent grains. The gases modulated the height of the barriers, which changes the sensor conductance accordingly [5]. It has been acknowledged that the thin film microstructure plays a major role in this behavior. This is mainly because grains and grain boundary regions are expected to have significantly different electronic response after the interaction with gases. It is known that the sensitivity of NO2 is many times higher than NO over the WO3 based sensors and most of the reported promoters are also known to be good oxidation catalyst. As a result, one of the roles of promoter is to provide a surface for the catalytic conversion of NO to NO2 that are responsible for the high sensitivity associated with the promotion.
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