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Title: Fabrication of electronically switchable and tunable bulk acoustic wave resonators with graphene electrodes
Author: Liu, Wei
ISNI:       0000 0004 7657 7164
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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This work is devoted to the fabrication of multilayer hetero-structure for switchable and tunable bulk acoustic wave resonators. This structure uses a ferroelectric material in the paraelectric phase as the resonator layer. The switchable and tunable abilities are achieved by varying the external DC electric field applied. The intermediate electrode chosen is single layer graphene with an ultra-thin Ti/Au capping layer. The device uses all metal Bragg reflectors to confine the acoustic wave within the resonator layer. The device structure is silicon with 90nm oxide layer/Bragg reflectors/graphene/metal/BST/graphene/top electrode. Chemical Vapour Deposition (CVD) has been used to grow the graphene, PLD has been used to deposit the BST layer, a Mantis sputtering system has been used to grow the Bragg reflector layer and the electrode metal layers, and photolithography, ion milling and reactive ion etching have been used to pattern the device. It has been found that for the graphene/metal bilayer, graphene with Ti (5nm) + Au (5nm), is the most suitable combination for this project. The measured resistance is 6.5Ω, and the graphene underneath is preserved after the sputtering of Ti. High temperature annealing of graphene is carried out to examine the mechanisms that cause the elimination of graphene during the growth of TiN. It is suspected that the difference in thermal expansion coefficient between the graphene and the silicon substrate creates cracks on the graphene layer, and the subsequent sputtering process destroys the graphene. For the growth of Bragg reflectors, Ti/Ru has been chosen for the growth of switchable and tunable TFBARs due to durability for the high temperature growth process. For the BST based full device, resonance has been observed under an applied external ACbias at various frequencies, but the crystallinity of the BST layer can be further optimised to achieve higher quality factor.
Supervisor: Petrov, Peter K. ; Alford, Neil M. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral