Montemurro, Domenico (2016) Hybrid superconducting InAs-nanowire based nanojunctions. [Tesi di dottorato]

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Semiconductor-superconductor hybrid devices have been investigated for many years. In these devices the macroscopic quantum correlations typical of superconductors can be induced via proximity effect in a wide class of low-dimensional materials such as, topological insulator , ferromagnetic barriers ], graphene and in particular semiconducting nanowires (NWs) which o�er a unique freedom in the design and control of quantum states. Precise control of the geometry, composition and chemical properties enabled the NWs to be a potential building block in felds, like nanoelectronics, photonics, mechanical and biological systems or sensors [32]. The most intriguing consequence of such a material combination is the possibility to give raise to novel excitations and properties that none of the single device components originally possesses. Recently, intense activity was aimed at arti�cially creating topologically protected Majorana fermion states. Many proposals make use quasi-onedimensional (quasi-1D) low critical temperature superconductors (LTS) in contact with topological insulators or quasi-one-dimensional materials with strong spin-orbit interactions (InAs & InSb NWs). Efforts have been made to use unconventional barriers with LTS, which o�er a much better compatibility and therefore nominal better interfaces, thus limiting, by low critical magnetic felds and low gap energy, for instance, the range parameters necessary to observe MBSs. HTS are much harder to integrate with other systems, but present the advantage as higher critical temperature, gap energy and huge stability to the magnetic field. Device that combine semiconducting nanowires (InAs-NWs) with HTS exploit both superconducting properties available with ceramic HTS and the high-purity of InAs-NWs together with xviii Hybrid superconducting InAs-nanowire based nanojunctions xix the tunability of their transport. The NWs represent the conductive channel in a superconductor-semiconductor-superconductor �eld-e�ect transistor (SUFET) device architecture and provide an ideal mesoscopic system to study both quantum con�nement and interference e�ects at low temperature, which is a promising platform to develop novel quantum devices, and the limits of quantum suppression of superconductivity. Nanoscale ordering and phase transition in complex oxides, where the electrons self-organize in ways qualitatively di�erent from those of conventional metals and insulators, is one of the most outstanding problems in physics today, and studies of nanoscale devices may have a formidable impact on that. Mixing bottom-up and top-down nanofabrication approaches have allowed building Josephson junctions in non-suspended and suspended design. Sophisticate EBL techniques have been developed and aim to align the InAs-NWs. Design device on which the InAs-NWs are positioned in random and guided (Dielectrophoresis (DEP) technique) way are fabricated. The InAs-NWs show external native oxide shell that hampers the current ux trough the interface, reason that procedures that allow to remove the oxide and get a good transparency between InAs-NWs and superconductor has been developed. Currentvoltage measurements as function of the temperature show, for both designs, a critical current amplitude that increases when the temperature decrease. Novel insights on macroscopic superconducting coupling in extreme conditions imposed by the nanostructures have been derived. The nanotechnology platform developed for suspended LTS junctions has represented the starting point for the integration between YBCO, the most used HTS, and NWS. This is an extremely challenging, high risk but extremely rewarding activity. Many designs have been developed for this type of junction. Many nanofabrication problems due to the complex structure of YBCO unit cell have been solved. In order to minimize the YBCO damaging e�ects due to out-di�usion oxygen from unit cell the nanofabrication recipe developed includes two sessions of electron beam lithography (EBL), two e-resists (PMMA) baking steps at low temperature and for few minutes, one dry etching session performed at very low temperature and one fast wet etching Hybrid superconducting InAs-nanowire based nanojunctions xx step. Special design of devices that allows verifying electric shorts before the InAs-NWs deposition have been developed. Innovative superconducting hybrid devices InAs-NWs/YBCO have been characterized with electric transport measurements as function of temperature and show as a current injected into YBCO banks can through the InAs-NWs that works as a weak link. I have demonstrated the feasibility of the fabrication procedure of YBCO/suspended InAs-NW/YBCO junctions. The �rst achievement of my work is that junctions YBCO/ suspended InAs-NW/YBCO of length � 200nm are not insulating. Current passes through the InAs-NWS, demonstrating the feasibility of the whole fabrication process. The other encouraging result is that di�erently from InAs-NWs/Aluminium junctions resistance behavior observed for InAs- NW/YBCO devices depends on the normal length (L) of junction [43]. The work is structured in seven chapters. The �rst chapter is dedicated to a review of the general mechanisms of the electric transport in the normal state. In the second chapter an introduction to superconductivity and to the concept of Josephson coupling have been discussed. The third chapter is dedicated to the description of the properties of InAs material (bulk and nanostructutered). The fourth chapter summarizes all the nanofabrication procedures developed for the fabrication of non-suspended junction between InAsnanowire and aluminium, suspended junction between InAs-nanowire and aluminium and in the last sections,the e�orts accomplished for the fabrication of the novel type of hybrid systems between InAsnanowire/ YBCO will be shown. In the �fth chapter the cryogenic system and the measurement setup used for the characterization of the devices builded are described. In the sixth chapter the transport measures, and their elaboration, of Hybrid superconducting InAs-nanowire based nanojunctions xxi InAs-nanowires and aluminium junctions will be shown. In the seven chapter the transport measurements of InAs-nanowires/YBCO junctions and their comparative analysis performed with InAs-nanowires and aluminium junctions have been performed

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