Deterministic tunnel barriers in 1D quantum electronic systems

In this thesis, we investigate the formation of tunnel barriers in one-dimensional semiconductors by means of low temperature transport experiments. The aim of this thesis is to form large deterministic tunnel barriers in InAs nanowires or carbon nanotubes that result in spatially and energetically well-defined quantum dots for systematic tunnel spectroscopy experiments. Two types of tunnel barriers are studied in this thesis: electrostatic gate-defined tunnel barriers in carbon nanotubes and InAs nanowires, and in-situ grown tunnel barriers in InAs/InP heterostructure nanowires.
Chapter 2 begins with an introduction to relevant theoretical concepts for the experiments presented in this thesis. In Chapter 3, carbon nanotubes and InAs nanowires are introduced as two realizations of one-dimensional semiconductors used in this thesis. The fabrication processes and measurement setup of electrostatic gate-defined and integrated tunnel barrier devices is presented in Chapter 4. In Chapter 5, the results of locally tunable electrostatic gate-defined tunnel barrier devices in carbon nanotubes and InAs nanowires is presented. We find that electrostatic gates result in tunnel barriers that are energetically and spatially ill-defined making them difficult for tunnel spectroscopy measurements. Chapter 6 takes the other approach to forming deterministic tunnel barriers by inducing band offsets in the nanowire by forming a InAs/InP heterostructure nanowire. An in-depth analysis of InAs/InP heterostructure nanowire demonstrating their broad electrical tunability, as well as a comprehensive characterization of the InP tunnel barriers, is presented. Next, we use the quantum dot formed by the integrated tunnel barriers in an InAs/InP heterostructure nanowire as a tunnel probe to investigate the local density of states in the nanowire leads when coupled to normal metal (Chapter 7) and superconducting (Chapter 8) reservoirs, respectively. To conclude, a short summary of the thesis and outlook of possible future experiments is presented in Chapter 9.