Experimental Study Of Quantum Transport Properties Of Ge/Si Core/Shell Nanowire Devices

In this thesis, the low temperature quantum transport properties of holes in Ge/Si core/shell nanowire devices were investigated experimentally. By measuring the weak antilocalization of the holes in Ge/Si core/shell nanowires and analyzing the data, sev-eral kinds of relaxation lengths of the holes and the spin-orbit coupling strength of this system were estimated. By measuring the quantum and magnetic transport properties of superconductor contacted Ge/Si core/shell nanowire quantum dot devices, the inter-actions between superconducting states in leads and energy levels in quantum dots were studied.The main content of this thesis includes:1. Introduced the physical concepts and principles, which were related to the work mentioned in this thesis based on Ge/Si core/shell nanowires.2. Briefly introduced the micro/nano-fabrication processes, which were used to fab-ricate the samples in this thesis. Presented the detailed recipes we used during the sample preparation and fabrication.3. Studied the low temperature weak antilocalization of holes in an individual Ge/Si core/shell nanowire. The fitted relaxation parameters and spin-orbit coupling strength in this system were found to be effectively gate tunable.4. In the weak coupling limit, investigated the transport properties of the Ge/Si core/shell nanowire quantum dots coupled to superconducting leads. The sin-gle hole tunneling was found to be blocked by superconducting gaps in source and drain leads of the quantum dot.5. In the strong coupling limit, investigated the transport properties of the Ge/Si core/shell nanowire quantum dots coupled to superconducting leads. The single hole tunneling, which should be blocked by the superconducting gaps in source and drain leads, was enhanced by Andreev reflections between the quantum dot and contacts. Thus an Andreev reflection enhanced phase coherent single hole tunneling transport became available.6. In superconductor contacted Ge/Si core/shell nanowire quantum dots, experi-mentally observed high order cotunneling spectroscopy and negative differential conductance, which were induced by Andreev reflections and disappeared after source and drain leads lost superconductivity.7. Summarized the above experimental work, and emphasized the importance of our work. Gave some suggested improvements and perspectives for future ex-plorations on Ge/Si core/shell nanowires.The main innovations of this thesis are:1. For the first time, systematically investigated the quantum and/or magnetic trans-port properties of Ge/Si core/shell nanowire devices at low temperatures.2. Deduced the spin-orbit coupling strength in Ge/Si core/shell nanowires and showed the spin-orbit coupling strength could be effectively modulated by an external electrical field. Suggested Ge/Si core/shell nanowire as a candidate platform for designing future spintronics applications.3. Investigated the superconductor contacted quantum dots based on Ge/Si core/shell nanowires in different coupling regimes. Gave a further understanding of inter-actions between superconducting states in source and drain leads and the energy levels in quantum dots.4. Based on experimental results, proposed a transport mechanism of Andreev re-flection enhanced phase coherent single hole tunneling through quantum dot, which was not reported before either in experiment or in theory.