Epitaxy of Hybrid Nanowires, Shadow Junctions and Networks

Research output: Book/ReportPh.D. thesisResearch

This thesis explores the growth of hybrid semiconductor-superconductor nanowire (NW), which is considered as a potential material platform to host topologically protected Majorana fermions. Considering the requirements of strong spin-orbit interaction and large Landé-g factor, first, InAs and InSb semiconductor NWs are investigated using molecular beam epitaxy. Au catalyst assisted InAs NWs are directly grown from the InAs substrate, whereas InSb NWs are grown with a stem. Post growth structural analysis exhibits a wurtzite structure for InAs NWs and a pure zinc-blende structure for InSb NWs. Hybridization of these NWs are performed with Al and Sn superconducting films. To confirm the high interface quality, thin film deposition is executed in situ, right after the NW growth. With optimized growth conditions, uniform Al film is exhibited on both InAs and InSb NWs. Furthermore, a homogeneous Sn film is achieved on InSb NWs. However, under the same growth conditions, Sn film remains dewetted islands on InAs NWs due to the high interface energy density. Besides, structural analysis of Sn film displays 훼 − Sn phase domination in the presented growth conditions. On the other hand, the electrical measurements of the Sn film manifest superconducting transitions around 3.9K or higher. It is presumed that the superconducting 훽 − Sn phases proximitized the neighboring 훼 − Sn phases during the measurements.
Epitaxially grown superconductors with in situ shadowed junctions on InAs, InSb and InAs1−xSbx NWs are shown next. The junction formation on the NW is studied as a function of interwire distances. It is found that, the morphology of the junction is directly dependent on the flux distribution, shadow geometry and material kinetics of the deposited film. With three deposited metal films (Al, Sn and Pb) on the NWs, the kinetic effect on the junction edge sharpness is investigated. Furthermore, the correlation between junction morphology and their electrical performance is examined. It is shown that the sharp-edge junctions exhibit high transparency with quantized conductance for all three NWs. In addition, a statistical comparison of shadowed and etched junction devices is presented, where the shadowed ones reveal better electrical performance.
In the last stage, the growth of quantum nanowire networks is presented with InAs, InSb, and InAs1−xSbx NWs. The emergence of a network is highly dependent on the Au catalyst position. With optimal growth conditions, the formation of InSb and InAs1−xSbxnanocrosses are shown. The growth dynamics of the nanocrosses are explained as a function of time-dependent axial and radial growth. Structural analysis of the nanocross demonstrates the single-crystalline zinc blende phase. By tuning the catalyst position, growth of InSb nanoplate and nanobridge structures are also shown, where the merging position plays a crucial role. In contrast to the Sb-based networks, reduced radial growth is observed in InAs networks. Thus, the final structure is driven by the axial growth. Varying the distance of the Au positions, V-shaped, T-shaped and y-shaped junctions are created with InAs NWs. Unlike Sb-based networks, structural analysis of the InAs networks demonstrates a crystal phase change in the merging point. Finally, anin situ shadowing process for patterning nanocrosses is developed, exhibiting a material platform for NW based multi-terminal Josephson junction.
Original languageEnglish
PublisherNiels Bohr Institute, Faculty of Science, University of Copenhagen
Number of pages142
Publication statusPublished - 2020

ID: 257742915