Transport Studies of Local-Gate Defined Quantum Dots in Nanowires

Sammanfattning: This thesis focuses on electrical transport in semiconductor InAs nanowires grown by chemical beam epitaxy. Initially, transport length scales of homogeneous InAs n-type nanowires are characterized by low-temperature magnetoconductance measurements. The measurements show phase-coherent conductivity corrections. By fitting the data, we find the nanowire mean-free path to be 50 nm and the phase-coherence and spin scattering lengths to be ~200nm. The spin scattering is attributed to spin-orbit coupling which is strong in narrow-bandgap semi-conductors such as InAs. The major part of the thesis work concerns quantum dots in InAs nanowires. We demonstrate a technique where quantum dots are electrostatically induced along the nanowire by depletion of the electronic gas from nano-scale local gate electrodes. The major advantage of this method is the tunability of the devices as the charge states of induced quantum dots and the couplings between them are controlled by external voltages. Using low temperature transport spectroscopy on a single dot in the few-electron Coulomb blockade regime we determine an effective g^'-factor in these dots of 8pm 1. In a two-electron dot we observe a Zeeman-driven ground state transition from singlet S to triplet T^+, which is characterized by an anti-crossing. This is attributed to S-T^+ mixing by spin-orbit coupling and a spin-orbit scattering length of ~130 nm for confined electrons can be extracted from the magnitude of the anti-crossing. In gate-defined few-electron double quantum dots, we observe a leakage current at the spin-blockaded (1,1) to (0,2) charge transition, which also originates in singlet-triplet mixing. The leakage current is strongly dependent of external magnetic field and of level detuning, however, this dependence changes drastically when the interdot coupling is varied. From a device point of view, the controllable charging of a (many-electron) double dot is utilized to demonstrate the operation of a single electron pump. Within measurement accuracy the pumping current equals one electron per cycle for frequencies up to 2 MHz. We also consider Ge/Si core-shell heterostructure nanowires wherein the band offsets result in a hole channel in the wire core. Using local top-gates, we demonstrate fully tunable gate-induced single and double quantum dots. Low-temperature transport measurements were used to extract the hole $g$-factor. The data indicate a strongly anisotropic g-factor with |g_parallel|=0.6 and |g_perp|<0.12.