Saturation and Flow Rate Effects on the Response of a Pd/AlN/SiC Hydrogen Sensor
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Saturation and Flow Rate Effects on the Response of a Pd/AlN/SiC Hydrogen Sensor M.H. Rahman a) and K.Y.S. Ng Department of Chemical Engineering and Materials Science, Wayne State University, Detroit, MI-48202 E.F. McCullen and R. Naik Department of Physics and Astronomy, Wayne State University, Detroit, MI-48202 G. Newaz Department of Mechanical Engineering, Wayne State University, Detroit, MI-48202 Y. Danylyuk, L. Rimai, R.J. Baird and G.W. Auner Department of Electrical and Computer Engineering, Wayne State University, Detroit, MI-48202
ABSTRACT A Pd/AlN/SiC device was fabricated with a 550 Å thick AlN layer in between the SiC substrate and Pd gate. At fixed flow rate, the response to Hydrogen initially increases roughly linearly with concentration, but eventually becomes saturated. The magnitude of the response as well as the saturating concentration increase with temperature. At 120° C the response saturates around 60 ppm of H2 whereas at 250° C, the onset of saturation occurs at higher ppm. The magnitude of the response also increases with the total flow rate, up to a flow rate of 250 sccm beyond which it remains constant. In the linear region, the magnitude of the response increases by a factor of 4 between 120° and 250° C, at a flow rate of 250 sccm.
INTRODUCTION A semiconductor based gas sensing device with catalytic metal gates was described more than 25 years ago by Lundstrom et al [1]. It consisted of a field effect structure with a palladium gate. Since then, a number of studies have been made on different types of device structures. Growing interest in developing semiconductor sensors operating over a wide range of temperatures has stimulated the study of devices fabricated on SiC substrate, as it can be effectively operated at high temperature due to its rather wide band gap [2,3]. The electrical behavior of Pd/AlN/SiC structure is that of a rectifying diode, the forward bias corresponding to positive Pd contact. The sensor response is obtained by monitoring the change in forward current as the concentration of hydrogen in the surrounding flow is being changed. In this report the detailed results will be presented describing the dependence of this forward current on hydrogen concentration and on flow rate as well as temperature effects.
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DEVICE FABRICATION Pd/AlN/SiC thin film devices were fabricated by a combination of plasma source molecular beam epitaxy (PSMBE) and magnetron sputtering techniques. A typical device consisted of 550 Å thick AlN film deposited by PSMBE on the top of n-type 6H-SiC wafer. A circular 1 mm diameter, 1900 Å thick Pd film was deposited on AlN as a catalytic electrical contact and 1800 Å of Pt was deposited on the back side of the wafer to form an ohmic contact. The metal depositions were carried out by magnetron sputtering. Details of the fabrication, as well as preliminary data on the response to Hydrogen have been presented [4,5].
RESULTS AND DISCUSSIONS Figs.1 show the current, at a constant bias (2.25 volts) vs.
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