Charge and Spin Transport in Parallel-Coupled Quantum Dots in Nanowires

This thesis explores crystal-phase engineering of nanowires to fabricate advanced quantum structures for charge and spin transport studies. Quantum dots formed by crystal-phase tuning during epitaxial growth of InAs nanowires were used as a starting point to realize and electrically characterize two different types of parallel-coupled quantum dots; electron-hole quantum dots and electron-electron quantum dots. In the InAs nanowire, two thin segments of wurtzite in an otherwise zinc blende crystal structure acted as tunnel barriers for electron transport and defined the quantum dot in the axial dimension. We estimated the offset in the conduction-band alignment at the wurtzite-zinc blende interface to be ~100 meV. The axial extension of the quantum dot could be tuned to less than 10 nm, which led to a strong quantum confinement and enabled the quantum dot to become fully depleted of electrons.In few-electron InAs quantum dots, pairs of local side gates and a global back gate were used to reproducibly tune the system from one quantum dot into parallel double quantum dots, for which we can control the populations down to the last electrons. Here, the interdot tunnel coupling of the two first orbitals could be tuned by one order of magnitude, owing to the combination of hard-wall barriers to the source and drain, shallow interdot tunnel barriers, and very high single-particle excitation energies (up to ~30 meV). In addition, the large |g*|-factors (~10) facilitated detailed studies of the magnetic-field dependency of the one- and two-electron states. In particular, we investigated the magnetic field-induced transition between singlet and triplet two-electron ground states. Here, the strong spin-orbit coupling in the system hybridized the single and triplet states. By controlling the interdot tunneling coupling we demonstrated a widely tunable anticrossing of the ground and excited states. Parallel electron-hole core-shell quantum dots were realized by using the InAs nanowire quantum dot as a template for selective radial growth of GaSb on the zinc blende crystal phase. As a heterostructure in bulk, InAs and GaSb form a broken band-gap alignment with spatially separated electrons and holes. In quantum dots, the overlap of the InAs conduction band and GaSb valence band can be tuned, which is of interest in studies of electron-hole interactions and transport via hybridized states. The electrical measurements of devices in the many-electron/hole regime showed clear evidence of transport via parallel quantum dots in the form of a beating pattern of small and much larger diamonds. We attributed the small-diamond pattern to electron transport in the core and the larger-diamond pattern to hole transport via the shell. From shifts in the conduction lines at the degeneracy point, we extracted an upper estimation of the electron-hole interaction strength of 4.5 meV.The work presented in this thesis demonstrate the great potential of using atomically precise crystal-phase design of nanowires to access and probe fundamental quantum physics