Stabilized Pd-alloy/AlN/Si Hydrogen Sensors
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J16.5.1
Stabilized Pd-alloy/AlN/Si Hydrogen Sensors Linfeng Zhang1, Ibrahim Al-Homoudi2, Md H. Rahman3, Erik F. McCullen1, Lajos Rimai1, Ronald J. Baird1, Ratna Naik4, Golam Newaz2, Gregory W. Auner1, K.Y. Simon Ng3 1 Department of Electrical and Computer Engineering 2 Department of Mechanical Engineering 3 Department of Chemical Engineering and Material Science 4 Department of Physics and Astronomy Wayne State University, Detroit, MI 48202 ABSTRACT In order to extend the dynamic range of a hydrogen sensor with a metal-insulatorsemiconductor (MIS) type structure, Pd-Cr and Pd-Ni alloy gated samples were studied. The PdCr gated sample shows quite stable and reproducible response, and could measure hydrogen concentrations from 100 to 50,000 ppm. While the Pd-Ni gated sample shows turn-on/off response drift, this drift is probably due to the presence of oxygen on the surface of the insulator and the formation of metal hydride. Furthermore, the effect of oxygen on the sensors response was investigated; oxygen may deplete protons from the metal/insulator interface and could reduce the sensor’s response. INTRODUCTION Hydrogen attracts more and more attention as a variable clean fuel and thus there are extensive research activities in hydrogen storage, generation, and fuel cells. Since hydrogen is explosive when its concentration in air is more than 4 vol %, sensors with high sensitivity and selectivity are needed to detect the H2 leakage. Also, a sensor with a wide dynamic range and stable response would be necessary to measure the concentration of H2 in many reforming processes and quality control applications. Our research of MIS type hydrogen sensors differs from other research in one important aspect: uniform aluminum nitride (AlN) is used as insulator layer and is deposited using plasma source molecule beam epitaxy (PSMBE). Because the bulk aluminum nitride has a bandgap of 6.2 eV, and it is a good thermal conductor with thermal conductivity comparable to that of pure Al [1, 2]. Owning to its bandgap, AlN is an excellent candidate as an insulator in the MIS type hydrogen sensor. We focused on the sensor with a metal/AlN/n-Si(111)/Al structure, which behaves as a voltage dependent capacitor [3-5]. The hydrogen molecules adsorb on the metal outer surface and are dissociated as atoms, these atoms would diffuse from the metal outer surface to the metal/insulator interface and form a layer of protons on the insulator side. Due to this charged layer, the width of the depletion region in the silicon side would change, thus there is a shift in the sensor’s capacitance-voltage (C-V) curve. During testing, the sensor is operated in the depletion region with constant capacitance; the voltage shift would be a function of hydrogen concentration. In order to improve the performance of the sensor, Pd-Cr and Pd-Ni gated samples were studied. The result of this study would provide a better understanding of the sensing mechanism and shed some light on optimal sensor design.
J16.5.2
EXPERIMENT RESULTS AND DISCUSSIONS
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