Towards the fabrication of suspended superconductor-graphene-superconductor Josephson junctions

Abstract

Graphene, a newly discovered material. has been the subject of much experimental and theoretical study due to its unique electronic behavior. In this thesis, I present, my work with graduate student Joel Wang to design and fabricate Josephson junctions on high quality graphene samples. Using niobium, with its high critical magnetic field., as the superconducting contact metal, along with high mobility suspended graphene samples, we hope to measure the behavior of graphene Josephson junctions in the quantum Hall regime. However, difficulties involved in the suspension and annealing processes of fabricating suspended graphene Josephson junctions have made it necessary for us to to develop a new fabrication process for making suspended Josephson devices. Over the course of this project, we have developed a fabrication process which uses a flip-chip bonder and graphene deposited on PMMA films to make suspended graphene samples. With these suspended flakes, we have produced niobium-graphene-niobiun Josephson junctions which exhibit multiple Andreev reflection at temperatures of 4 K. and a Dirac peak near -0.4 V, all without any annealing of the graphene. We have also developed an annealing and lithography process which can clean graphene and preserve its cleaness during the rest of the fabrication process. This annealing process has yielded suspended graphene samples with mobility upwards of 200, 000 cm 2 V-i s-1 comparable with high mobility flakes presented in literature.