Elevated Temperature Silicon Carbide Chemical Sensors
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D. ILA** and D. J. LARKIN*** Departmentof Chemistry, University of Alabama in Huntsville Centerfor Irradiationof Materials,Alabama A &M University ***NASA Lewis Research Center
ABSTRACT In this study, the (I-V) properties of the sensors were measured as a function of hydrogen, propylene and methane exposure at temperatures up to 4000 C and sensor responses were observed for each gas. The response to hydrogen and propylene had a rapid increase and leveling off of the current followed by the subsequent decrease to the baseline when the gas was switched off. However, exposure to methane resulted in a rapid spike in the current followed by a gradual increase with continued exposure. X-ray photoelectron (XPS) studies of methane exposed SiC sensors revealed that this behavior is attributed to the oxidation of methane at the Pd surface. INTRODUCTION The Study of SiC has focused on methods to grow high quality SiC for a wide range of applications including high temperature, high power devices [1] as well as optoelectronic devices [2]. Recently, it has also been shown that SiC can be employed as both an oxygen and a hydrogen sensor that operates in a temperature regime considerably higher than conventional sensors such as tin oxide (SnO 2) or silicon. Because of its outstanding thermal stability, silicon carbide can be employed as a hydrogen and hydrocarbon sensor that can potentially operate at temperatures up to 1000 'C [3-7]. Potential uses of elevated temperature SiC sensors include automotive applications, process gas monitoring, aeronautics, and aerospace applications. The deposition of a catalytic metal such as Palladium (Pd) onto silicon carbide (SiC) results in Schottky diode behavior. The adsorbing gas changes the space charge region under the metal clusters which in turn affect the conductivity of the crystal. This change in conductivity is measured and can be correlated to surface concentrations and to the levels of the sampled gas in the ambient. The high sensitivity for hydrogen containing combustible gases is enhanced by the presence of catalytic metals. Hydrogen containing species dissociatively adsorb to the metal and hydrogen atoms migrate to the Pd/SiC interface where they affect the current-voltage (I-V) properties of the SiC [7]. EXPERIMENTAL For
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response of silicon carbide z sensors to various gases, 5 x 7 mm samples of epitaxial silicon " carbide films deposited on bulk 2 silicon carbide substrates were examined. Both silicon face and carbon face samples were produced, however the preliminary studies were performed on silicon faced silicon
carbide films only. The sensors were prepared by depositing 100 nm thick aluminum films to the backside of the sample, while palladium films were deposited to
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Time (sec) Figure 1 Current response at 0.7 V to H2 and N2 exposure. 123
Mat. Res. Soc. Symp. Proc. Vol. 572 ©1999 Materials Research Society
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the epitaxial silicon carbide film side. The Pd deposits ranged from 0.1 mm in diamet
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