| Summary: | 博士 === 國立中興大學 === 物理學系所 === 107 === In this study, we investigate the properties of ultimately short vertical graphene Josephson junctions. A suspended Si3N4 membrane with holes was made on a silicon substrate. Vertical Josephson junctions were made by transferring 2D material on top of the hole, followed by evaporation of superconductor electrodes from the top and bottom.
For the suspended substrate fabrication, we make a silicon nitride layer on the top of the substrate and immerse the pattered wafer in 20% KOH etch solution until the bottom Si was fully etched away to reveal a 80μm×80μm suspended Si3N4 membrane. The holes were sculptured by transmission electron microscope (TEM) on Si3N4 membrane with a 100 nm diameter. This method is clean without photoresist. We also applied a full wafer sized process by patterning the holes using electron beam (e-Beam) writer on Si3N4 membrane before etching away the bottom silicon. Diverse vertical hybrid superconducting devices were made from the structure, including Al, Ti/Al, V/Al, Pb/Al and Pb/Au as the superconductor electrode, and CVD graphene exfoliation graphene as the barrier layer between the electrodes.
We discover that the samples have very largely scattered resistance at room temperature. For samples with a resistance higher than 100kΩ, it shows an insulating behavior and gains huge resistance at low temperature. On the other hand, lower resistance devices present metallic behavior and its resistance does not change much at low temperature. In the V-junction, we successfully observed the supercurrent below the superconducting transition temperature Tc of 3.6K. We also observe the supercurrent suppression by an external magnetic field due to the quench of superconductivity of the V electrodes. The large resistance of the junction is due to the structure fault in the contact region. We tried to reconstruct the Al/graphene/Al and Ti/Al/graphene/Ti/Al junction by annealing and large current driving. We are able to switch the junction resistance state of T4 sample by application of forward and reversed currents.
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