Harmonic Excitation of Surface Acoustic Waves on Gallium Nitride Thin Films for Biological and Chemical Sensor Applicati
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Harmonic Excitation of Surface Acoustic Waves on Gallium Nitride Thin Films for Biological and Chemical Sensor Applications J. Justice, L. E. Rodak, 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. ABSTRACT Gallium nitride (GaN) is a robust piezoelectric semiconductor with excellent thermal and chemical stability, making it an attractive material for surface acoustic wave (SAW) sensors operating in high temperature and harsh environments. The sensitivity of SAW devices is proportional to the square of the operating frequency. Therefore, high operating frequencies into the GHz regime are desirable for SAW sensors. For GaN, this requires sub-micron interdigital transducers (IDTs) when devices are designed to operate at the fundamental Rayleigh mode frequency. The necessity for sub-micron IDTs can increase fabrication costs and complexity. By designing SAW devices to operate at harmonic frequencies, GHz operation can be realized with relatively large IDTs, resulting in simpler and more cost effective solutions for GaN based SAW sensors. Devices have previously been designed to operate at the 5th and higher harmonics on lithium niobate, but there are no reports of using this technique on GaN in the literature. In this study, GaN thin films have been grown via metal organic vapor phase epitaxy on sapphire substrates. SAW devices designed to operate at the fundamental frequency and higher harmonics have been fabricated and measured. Operating frequencies greater than 2 GHz have been achieved using IDTs with 5 μm fingers. In addition, reduction of electromagnetic feedthrough around the 5th and 7th harmonic is demonstrated through varying ground electrode geometries. INTRODUCTION Over the last few decades, there has been considerable research into surface acoustic wave (SAW) devices for sensor applications [1]. Surface acoustic waves are generated on a piezoelectric material via surface electrodes. These surface electrodes, known as interdigital transducers (IDTs), have an overlapping comb-like pattern with a metallization ration, η = a/p, as illustrated in Figure 1A. In a traditional delay line SAW device, there are two adjacent IDTs with one serving as the input, and one the output [2]. Because the energy of a SAW is confined to the surface of the material, the frequency response of a SAW device will change in response to any change in the surface density [3]. This makes SAW devices very sensitive to mass loading. Traditionally, an active layer is deposited between the input and output IDTs of a SAW device that can absorb or bind to the desired target species, trapping it on the surface [4]. As the surface becomes loaded, the resonant frequency of the SAW device will shift to lower frequencies and the insertion loss will increase as illustrated in Figure 1B. The sensitivity of a SAW device to mass loading is directly proportional to the square of the operating frequency [1]. The operating frequency, f0, is defined as
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