Pd/AlN/Si or SiC Structure for Hydrogen Sensing Device
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Pd/AlN/ Si or SiC Structure for Hydrogen Sensing Device Flaminia Serina, ChE Dept., C. Huang, ECE Dept., G. W. Auner, ECE Dept., R. Naik, Physics and Astronomy Dept., S. Ng, ChE Dept., and L. Rimai, ECE Dept., Wayne State University, Detroit MI
ABSTRACT An AlN (insulator) MIS Hydrogen Sensor was created using plasma source molecular beam epitaxy (PSMBE) deposition on Si (111) and 6H-SiC. A Pd layer was deposited on top of the AlN film via magnetron sputtering technique utilizing a hard mask. Pd was chosen since H2 readily diffuses within its bulk, thus Pd acts not only as a metal electrode of the MIS structure, but also as a catalyst for hydrogen dissociation. To optimize the design structure several sensors with different AlN and Pd thickness have been developed. RHEED and XRD measurements show that AlN film is epitaxial on both Si (111) and 6H-SiC substrates. The sensors were characterized using capacitance versus voltage C(V) and I(V) measurements, at different frequencies ranging from 1kHz to 1 MHz. Shifts in the C-V and I-V curves occurred with the introduction of hydrogen in the chamber. The temperature, hydrogen partial pressure, effects of oxygen and hydrocarbon gases, insulator and metal thicknesses on sensor response were analyzed. INTRODUCTION In various transportation applications there is demand for a miniature device capable of detecting the concentration of hydrogen in the presence of other gases. These applications include but are not limited to the following: ground and exo-atmospheric based space system, such as the space shuttle and space station, automotive Hydrogen Internal Combustion Engine, Fuel Cells used for industrial and domestic applications. These technologies require the use of Hydrogen gas sensors which could function in a wide range of temperatures, pressures and gas compositions. In this paper, we report on new device structures that enabled the fabrication of gas sensors which, under a wide range of conditions, exhibit marked specificity for hydrogen. These devices with proper adjustment of flow conditions, exhibit rather wide dynamic range in sensitivity from sub ppm to percent concentration of hydrogen in the surrounding gas flow. These are thin film structures analogous to the MOS device first described by Lundstr|m for use as a Hydrogen sensor [1], but with the oxide insulating layer replaced by a film of AlN epitaxially grown on the semiconductor substrate. Two kinds of devices were made with Si and SiC semiconductor substrates. The gate electrodes for both consisted of thin palladium films. Both types of devices behaved as Hydrogen sensors. However, they exhibited different electrical characteristics. For example, the Si device responded to the presence of hydrogen by a shift in its ac capacitance versus bias voltage (C(V)) characteristic, In contrast, the SiC based structure responded by a shift in its forward (I(V)) characteristic, as its electrical properties were those of a rectifying diode.
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DEVICE FABRICATION AND CHARACTERIZATION The structure of the device res
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