High Harmonic Surface Acoustic Wave Devices for Harsh Environment Sensor Applications
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High Harmonic Surface Acoustic Wave Devices for Harsh Environment Sensor Applications J. Justice, M. Elbaz, L. E. Rodak* and D. Korakakis Lane Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, WV 26506 U.S.A. ABSTRACT Surface acoustic wave (SAW) devices are ideal candidates for gas sensors due to their small size, low cost of production and high sensitivity. Increasing restrictions on pollution and emissions create the necessity for sensors that can operate in the harsh environments found in vehicle exhaust systems and industrial production. Gallium nitride (GaN) is a robust, chemically inert, piezoelectric semiconductor, making it an attractive material for SAW devices designed to detect and monitor gases in harsh environments. In this work, SAW devices designed to operate at the 5th and 7th harmonics are fabricated on GaN thin films and their performance is measured through insertion loss, signal to noise ratio, operating frequency and quality factor. Devices are directly exposed to the exhaust gas of a common diesel engine. Device performance is then remeasured and compared. SAW devices fabricated in this work have measured operating frequencies above 1 GHz, and quality factors up to and higher than 2000, depending on the harmonic mode. SAW devices on GaN showed good chemical stability and measured changes in device performance after exhaust exposure was negligible. INTRODUCTION Worldwide legislation for limiting environmental pollution has been the driving force for the exhaust gas monitoring industry over the last several decades. Extensive research has been conducted to develop sensor technology for exhaust gas monitoring applications. The most commonly used technology is based on electrochemical sensors [1-5]. These generally resemble a fuel cell in their operation. However, the sensing device is not directly exposed to the exhaust flow. Gas detection is a two-stage process where the exhaust gas passes through a porous membrane to reach the sensing parts, and their lifetime will be shortened in very hot areas. Fiber optic technology has also been used for exhaust gas monitoring [6-7]. Here, operation is based on optical absorption of light by gas species at characteristic wavelengths. Optical fiber sensors have some advantages such as freedom from electromagnetic and radio frequency interference. However, they are costly and the detection systems may be complex. Other available gas sensors used by automotive industries include catalytic combustion, field effect and IR devices. However, poor gas selectivity is evident in these technologies and their performance can degrade significantly due to dust and dirt. Surface acoustic wave (SAW) sensors are superior in regards to their sensitivity when compared to other conventional sensor technologies [8]. Commonly used piezoelectric crystals for SAW sensors are quartz, lithium niobate (LiNbO3) and lithium tantalate (LiTaO3). These however, are not suitable for high temperature and harsh environments as they do not survive
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