Characterization of Group III-Nitride Based Surface Acoustic Wave Devices for High Temperature Applications
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Characterization of Group III-Nitride Based Surface Acoustic Wave Devices for High Temperature Applications J. Justice*, L.E. Rodak*, V. Narang†, K. Lee*, L.A. Hornak* and D. Korakakis* * Lane Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, WV 26506 U.S.A. † Department of Physics, West Virginia University, Morgantown, WV 26506 U.S.A. ABSTRACT In this study, aluminum nitride (AlN) and gallium nitride (GaN) thin films have been grown via metal organic vapor phase epitaxy (MOVPE) on silicon and sapphire substrates. Samples were annealed at temperatures ranging from 450 to 1000 °C in atmosphere. AlN and GaN thin film quality has been characterized before and after annealing using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and atomic force microscopy (AFM). Surface acoustic wave (SAW) devices with titanium/platinum interdigital transducers (IDTs) designed to operate at the characteristic frequency and fifth harmonic have been realized using traditional optical photolithographic processes. SAW devices on GaN were thermally cycled from 450 to 850 °C. The S21 scattering parameter of SAW devices was measured before and after thermal cycling by a vector network analyzer (VNA). An approach for the suppression of electromagnetic feedthrough (EF) to improve device performance is discussed. Feasibility of 5th harmonic excitation for GHz operation without sub-micron fabrication is also investigated. SAW devices have also been fabricated on the more traditional SAW substrate, lithium niobate (LiNbO3), and device response was compared with those on AlN and GaN at room temperature. INTRODUCTION SAW devices are suitable for many different operations such as signal processing, bandpass filtering, pulse compression and sensing. However, most SAW operation and sensing occurs at room temperature. The fundamental operation of SAW devices is due to the piezoelectric effect of the material that the SAW devices are fabricated on. Common piezoelectric materials for SAW devices lose their piezoelectric properties above their Curie temperature, which is typically < 300 °C. AlN and GaN exhibit weaker piezoelectric responses than traditional SAW materials, but retain their piezoelectric properties at higher temperatures. There has been recent research in the development of SAW devices fabricated on AlN that can operate in high temperature environments up to 950 °C [1]. This study focuses on characterizing high temperature SAW response and explores methods for improving the SAW response of devices fabricated on GaN thin films. Those methods include the reduction of EF, which degrades device performance due to the direct coupling of electromagnetic energy between input and output IDTs, and improving the operating frequency by designing SAW devices to operate at the 5th harmonic. 5th harmonic device operation has previously been investigated to increase operating frequency into the gigahertz range without the use of sub-micron fabrication on LiNbO3 [2]. In this study
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