Effects of Substrate Materials on the Electrical Behavior of Pd/AlN/Semiconductor Based Hydrogen Sensors
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J18.12.1
Effects of Substrate Materials on the Electrical Behavior of Pd/AlN/Semiconductor Based Hydrogen Sensors Md. H. Rahman1*, L. Zhang2, L. Rimai2, R.J. Baird2, R. Naik3, K.Y.S. Ng1, G. Auner2, G. Newaz4 1
Department of Chemical Engineering and Materials Science, 2Department of Electrical and Computer Engineering, 3Department of Physics and Astronomy, 4Department of Mechanical Engineering. Wayne State University, Detroit, MI-48202 ABSTRACT C-plane textured wurtzite AlN films deposited on both, 3C and 6H SiC as well as Si, have been employed in Pd/AlN/Semiconductor diode Hydrogen sensor structures. The SiC based devices behave as rectifiers and do not show the bias dependent capacitance expected for an MIS structure when reverse biased. The Si based devices, albeit exhibiting rectifying characteristics do show the expected dependence of capacitance on reverse bias, with typical depletion and inversion regions. The SiC based devices show the sensor response as change in bias voltage at constant forward current but the Si based device showed no response in this mode of operation albeit it did respond well in terms of the Hydrogen induces shift of the depletion capacitance vs. bias characteristic. As the AlN itself has the same structure in all cases, the differences in both, electrical characteristics and sensing response mechanism are related to differences in the AlN/Substrate heterojunction resulting from differences in the electronic band structure of the substrates. INTRODUCTION A gas sensor using a metal/oxide/silicon capacitor with a catalytic palladium gate was first described 30 years ago by Lundstrom at el. [1]. It was observed that the presence of hydrogen in the ambient caused a change in the electrical characteristics of these structures. Specifically it caused a shift along the voltage axis, of the dependence of the capacitance on the bias when the device was being operated in inversion or depletion. An analogous device but with Si replaced by SiC as the semiconductor substrate, and the same insulator (SiO2) behaved in the same manner [2]. However when the insulator on the SiC was replaced by AlN, the electrical behavior of the device became that of a rectifier rather than that of a MIS capacitor, and the sensor response was exhibited as a shift of the Current vs. Bias characteristic again along the bias axis [2-4].This report presents the results on MIS devices with wurtzite AlN as the “insulating” layer, fabricated on three different substrates with different band gaps (Si, 3C-SiC and 6H-SiC).
*Corresponding author. Phone:+1-313-577-5507. Fax:+1-313-577-1101. Email: [email protected]
J18.12.2
EXPERIMENTAL DETAILS Pd/AlN/Semiconductor based devices were fabricated by a combination of Plasma Source Molecular Beam Epitaxy (PSMBE) and magnetron sputtering techniques. Three different n-type semiconductor substrates were employed: on-axis 6H-SiC, on-axis 3C-SiC, and Si(111) with carrier concentrations of 2.14x1018, 2.71 x1018, and 2x1015 cm-3 respectively, at room temperature. As all these substrates
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