Design And Fabrication Of Surface Acoustic Wave Resonators And Their Hybrid Devices

Surface acoustic wave(SAW)is an elastic wave that propagates on the surface of solids.It has low loss,high information density,and is strongly resistive to electromagnetic interference.Nowadays,various devices based on SAW,such as resonators and sensors,which are small in size and light in weight,show diverse complex functions.In recent years,hybrid devices based on SAW have attracted considerable research interests.During progragration,SAW can easily generate controllable displacement and electric fields on the surface of the substrates.In such sense,SAW can interact with other systems through mechanical and electrical mechanisms.The hybrid systems based on these mechanisms have great potentials in applications.For example,in the hybrid devices of SAW and nanomechanical resonators,SAW can be used to manipulate local mechanical vibration modes;In the hybrid devices of SAW and field-effect transistors,SAW can be used to realize long-distance transport of charge carriers.At present,the research of these two types of devices is still in the initial stage.The nanomechanical resonators in the first type of devices have naive structures and cannot be controlled electrically;In the second type of devices,especially the hybrid devices of SAW and graphene-based field-effect transistors,the graphene used in previous studies is mostly monolayer graphite produced by chemical vapor deposition.Such device has low mobility and no bandgap,which greatly limits the tunability and applications.This thesis aims at optimizing the design and performance of SAW devices and SAW hybrid devices.The specific content includes:1.The concepts and background of SAW devices are briefly introduced.The development status and the challenge of SAW hybrid devices,especially the hybrid devices of SAW and nanomechanical resonators as well as the hybrid devices of SAW and charge transport,are also presented.2.The fabrication of SAW devices is introduced.The quality factors,the reflection of interdigital transducer(IDT),and the structure of the mirrors of SAW resonators are optimized and improved.Meanwhile,the behaviors of SAW devices are simulated using COMSOL Multiphysics.A single-port SAW resonator with a high quality factor up to 1.6×104 is achieved.3.Based on the optimized single port SAW resonators,a hybrid device of SAW and graphene nanomechanical resonators is designed.The feasibility of the device is verified in simulation,and a prototype device is fabricated and characterized.The interaction between SAW and graphene nanomechanical resonators is expected to be achieved.4.A hybrid device of SAW and graphene field-effect transistors is fabricated using mechanically exfoliated bilayer graphene encapsulated in hexagonal boron nitride(h-BN).The relationship between the magnitude of the SAW-driven acoustoelectric current and gate voltage as well as SAW power indicates that SAW can interact with bilayer graphene encapsulated in h-BN.5.Summary of the results and prospect of future directions on SAW hybrid devices.The innovation points of this thesis:1.We use inverse Fourier transform to convert the S-parameters into time-domain signals.The optimization of the SAW devices is instructed through analyzing the time-domain data.2.Utilizing the frequency tunablity of graphene nanomechanical resonators,the tunability of hybrid devices of SAW and nanomechanical resonators is improved.3.For hybrid devices of SAW and bilayer graphene field-effect transistors,high-mobility bilayer graphene encapsulated by h-BN is obtained,and SAW-driven acoustoelectric current is measured simutanously.This lays the foundation for further research on the relationship between tunable bandgap and acoustoelectric current.