Interdiffusion and Reaction of Pd on Atomically Stepped 6H-SIC Surfaces: Progress Toward Thermally Stable High Temperatu
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Interdiffusion and Reaction of Pd on Atomically Stepped 6H-SiC Surfaces: Progress Toward Thermally Stable High Temperature Gas Sensors C.D. Stinespring,1 C.Y. Peng,1 A.A. Woodworth,2 K. Meehan,3 M.J. Murdoch-Kitt,3 and C.L. Anderson3 1
Department of Chemical Engineering, West Virginia University, Morgantown, WV 26506-6102 Department of Physics, West Virginia University, Morgantown, WV 26506-6315 3 Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24060 2
ABSTRACT High temperature Pd-SiC Schottky diode gas sensors are known to thermally degrade due to interdiffusion and reaction at the metal-semiconductor interface. To understand and possibly eliminate this problem, detailed surface studies of thermally induced Pd-SiC surface interactions have been performed. These experiments compare standard 6H-SiC (0001) surfaces typical of those used in device fabrication with periodically stepped surfaces prepared by high temperature hydrogen etching. The Pd films range in thickness from the monolayer level (~0.4 nm) to actual device dimensions (~46.5 nm) and are deposited under ultrahigh vacuum conditions at ~50 oC. These films are characterized in-situ using Auger electron spectroscopy both before and after annealing at 670 oC. The Auger lineshapes provide quantitative information on the chemistry of the reaction products. Ex-situ atomic force microscopy is used to characterize changes in surface morphology.
INTRODUCTION SiC is a robust, wide band gap semiconductor suitable for fabricating high temperature gas sensors [1]. One of the simplest sensor structures studied to date has been the Pd-SiC Schottky diode. These devices change their electrical characteristics when gases such as H2, O2, CO, NO, and hydrocarbons are adsorbed on the metal contact [2]. Sensor lifetime studies indicate that thermally induced compositional and structural changes at the Pd-SiC interface adversely impact sensor performance [3]. One factor that may contribute to this thermal instability is the quality of the SiC substrate surfaces on which the devices are fabricated. The vendor supplied 6H-SiC substrates have a varying number of surface and sub-surface defects associated with the polishing process. High temperature hydrogen etching can be used to remove several microns of this damaged material and yields periodically stepped surfaces with unit cell high steps and atomically flat terraces [4]. In this paper, we report studies that compare the thermally induced changes in Pd thin films deposited on stepped SiC surfaces with those observed on the standard surfaces commonly used in SiC sensor device fabrication. These results provide a basis for understanding the degradation mechanism and will ultimately be extended to more advanced sensor designs.
EXPERIMENTAL PROCEDURE Commercially available n-type, 6H-SiC wafers were purchased from CREE Research Inc. and Sterling Semiconductor and diced into 1 cm x 1 cm squares. All wafers had vendor
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polished (0001)
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