QDev Seminar: Malin Nilsson
PhD Student, Lund University
Charge and spin transport in parallel quantum dots in nanowires
Malin Nilsson‡, Sebastian Lehmann‡, Luna Namazi‡, I-Ju Chen1, Martin Leijnse‡, and Kimberly A. Dick1‡§, Claes Thelander‡,
We use quantum dots (QDs) formed by crystal-phase tuning during epitaxial growth of InAs nanowires as a starting point to realize and electrically characterize two different types of parallel-coupled quantum dots; electron-hole QDs and electron-electron QDs. In the InAs nanowire, two thin segments of wurtzite (WZ) in otherwise zinc blende (ZB) crystal structure act as tunnel barriers (~100 meV) for electron transport and define the QD in the axial dimension . The axial extension of the QD can be tuned to less than 10 nm, which leads to a strong quantum confinement and enables the QD to be fully depleted of electrons, see Fig 1(a).
In few-electron InAs QDs, pairs of local side gates and a global back gate are used to reproducibly tune system from one QD into parallel double QDs, for which we can control the populations down to the last electrons. The combination of hard-wall barriers to source and drain, shallow inter-dot tunnel barriers, and very high single-particle excitation energies (up to 27 meV), allow an order of magnitude tuning of the strength for the first intramolecular bond. In addition, the large |g*|-factor (~9) facilitate detailed studies of the magnetic-field dependency of the one- and two-electron states.
Parallel electron-hole core-shell QDs can be realized by using the InAs nanowire QD as a template for selective radial growth of GaSb on the ZB crystal phase. [2,3] As a heterostructure in bulk, InAs and GaSb form a broken band-gap alignment with spatially separated electrons and holes. In QDs, 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 show clear evidence of transport via parallel QDs in the form of a beating pattern of small and much larger diamonds, see Fig1(b).  We attribute 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 extract an upper estimation of the electron-hole interaction strength of 4.5 meV.
Owing to the large spin-orbit coupling in InAs and GaSb, combining the strongly confined InAs QDs with GaSb shell growth could potentially provide a way for separate electrostatic manipulation of the spin configuration in such core-shell QDs.
 M. Nilsson et al., Single-electron transport in InAs nanowire quantum dots formed by crystal phase engineering, Phys. Rev. B 93, (2016).
 L. Namazi et al., Selective GaSb radial growth on crystal phase engineered InAs nanowires, Nanoscale 7, (2015).
 M. Nilsson et al., Electron-hole interactions in coupled quantum dots based on nanowire crystal phase templates, Phys. Rev. B 94, (2016).