Transport in nanowire-based quantum dot systems: Heating electrons and confining holes

Abstract: Quantum dots embedded in an electronic circuit allow precise control over the charge transport behaviour of the system: Charge carriers can be individually trapped or precisely shuffled between a series of quantum dots in a strictly sequential manner. This introduces ideal conditions to study fundamental quantum physics and such devices are in the focus of extensive efforts to develop quantum information related applications. This thesis contributes to the development of model systems enabling control of, and abiding by quantum mechanical effects. The aim of the model systems is to search and use advantages compared to devices governed purely by the laws of classical physics.In this thesis, transport phenomena in n- and p-type III-V semiconductor nanowire quantum dot systems are explored. First, the concepts necessary to build an understanding of charge transport across quantum dot systems, namely quantum confinement in nanostructures and Coulomb blockade, are introduced. Next, the principles of transport across single and double quantum dot devices are discussed and various experimental device designs are presented. The experimental work falls into two separate research directions and the thesis includes three published papers, which are put into context and supplemented with additional experimental results.Paper I characterizes the properties of p-type GaSb nanowires to assess the material's applicability for the realization of spin-orbit qubits as fundamental building blocks of solid state quantum computers. Experimentally, g-factors and the spin-orbit energy are determined and fabricational challenges for the realization of serial double quantum dot devices are discussed and overcome.Papers II and III study thermally driven currents in InAs nanowire double quantum dots, where heat is essentially converted to electrical power. Such nanoscale energy harvesters operate in a regime where fluctuations are highly relevant and give insights into fundamental nanothermodynamic concepts. Thermally induced currents in double quantum dot devices are the result of three-terminal phonon-assisted transport or the two-terminal thermoelectric effect. Paper II studies the interplay of the two effects, the relevance of the interdot coupling and the impact of excited states. Paper III develops a versatile device architecture which combines bottom-gating and heating and enables the localized application of heat along the nanowire axis. Such devices provide ideal, controlled conditions for future studies of fundamental nanothermodynamics.