High-frequency Sezawa guided mode of GaN/sapphire using high aspect ratio electrode
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The propagation of surface acoustic wave (SAW) on the piezoelectric substrate requires conventionally an interdigitated electrode structure to excite the mechanical displacement at resonance frequency. The control of the electrode thickness could be useful to manipulate the energy confinement and the band dispersion of the surface guided mode. It has been demonstrated recently that high aspect ratio (HAR) electrode could produce a dispersive shear horizontal and vertically polarized surface modes in the bulk piezoelectric substrate. In this theoretical study, we propose to employ a high aspect ratio electrode on top of the GaN/sapphire-layered substrate enabling the presence of Sezawa surface mode. Based from the dispersion band, we obtained a higher frequency of surface guided mode in the non-radiative zone in the GaN/sapphire heterostructure configuration compared to the bulk GaN substrate. Indeed, these guided modes are induced by the hybridization between Sezawa surface mode and the mechanical resonance of the HAR electrode producing nearly a flat band at the limit of First Brillouin Zone. Furthermore, the displacement of each guided modes indicates the confinement of energy mostly in the electrode with a slight amount of energy in the top layer of the substrate. We demonstrated also the frequency tuning of guided mode using diverse materials for the electrode but also the thickness of GaN layer. The obtained results could be useful for the development of high-frequency telecommunication and sensing device based on Sezawa surface acoustic wave.
We acknowledge financial support from Universiti Kebangsaan Malaysia and Ministry of Education of Malaysia through with the code of GGPM-2018-015 and by the partially supported by the “Center for mmWave Smart Radar Systems and Technologies” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. This project is also supported in part by the Ministry of Science and Technology, Taiwan, under Grant MOST 107-3017-F-009-001. The first author would also like to further extend our gratitude to Skim Zamalah Penyelidik Tersohor from Pusat Pengurusan Penyelidikan dan Instrumentasi (CRIM), Universiti Kebangsaan Malaysia.
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