Effect of Thickness of Ion Beam Deposited Molybdenum Trioxide Thin Films on Gas Sensing Properties
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Effect of Thickness of Ion Beam Deposited Molybdenum Trioxide Thin Films on Gas Sensing Properties Arun K. Prasad and Pelagia I. Gouma Department of Materials Science and Engineering, 314, Old Engineering Building, SUNY-Stony Brook, Stony Brook, NY 11794-2275, U.S.A. ABSTRACT This paper documents the study of thickness effects of ion beam deposited molybdenum trioxide thin films on their gas sensing properties. For processing the films, the primary plasma beam source was used to sputter material from the target, while the secondary beam was used as an ion assist source. The deposited material was annealed at 500°C for 8 hours. The microstructural characterization of the films was done using Scanning Electron Microscopy and X-ray diffraction. Sensing tests were performed in the temperature range of 350-500°C under varying and constant concentration levels of ammonia. The recorded data have been qualitatively and quantitatively analyzed and significant trends according to different parameters (thickness, sensing temperature, gas concentration) have been identified. These suggest a relation existing between processing conditions, the microstructure of the material and its response to gases at different concentration, and the optimum working temperature regime. INTRODUCTION MoO3 has been used as gas sensing material for almost a decade and there are few reports indicating its suitability to sensing different gases such as NH3, H2, CO and NO2 [1-13]. Molybdenum trioxide has a high resistivity which has limited its applications in gas sensing due to electrical circuitry difficulties. The high resistivity is due to wide band gap of 3.2 eV. MoO3 exhibits good response characteristics, but it is very unstable and irreversible which degrades as the operating temperature increases. This is due to its low melting point of 795ºC. These two drawbacks have led to very little research on MoO3 in gas sensing applications. However, in the lower temperature range, it shows excellent sensing response and high selectivity in certain cases. The sensing response can be controlled by processing conditions and altering the microstructure. In this study, the effect of thickness on sensing response is discussed.
EXPERIMENTAL Thin films of MoO3 were deposited on alumina substrates using ion beam deposition technique comprising a radio frequency inductively coupled plasma primary source and a secondary assist source. The chamber for deposition was maintained at a pressure of 1.6 × 10-4 Torr with argon being the background gas. Oxygen was used in assist source to facilitate reactive sputter deposition and the ratio of oxygen to argon from the assist
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source was maintained at 1:1. A more detailed description of the ion beam deposition system used for this study is described elsewhere [10]. The sensor substrates were alumina pre-patterned with inter-digitated gold electrodes. Three sets of samples of different thicknesses were prepared. The film thicknesses were controlled by the time of deposition. A 4 min deposition time yielde
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