  We report growth and characterization of a coupled quantum dot structure that
utilizes nanowire templates for selective epitaxy of radial heterostructures.
The starting point is a zinc blende InAs nanowire with thin segments of
wurtzite structure. These segments have dual roles: they act as tunnel barriers
for electron transport in the InAs core, and they also locally suppress growth
of a GaSb shell, resulting in coaxial InAs-GaSb quantum dots with integrated
electrical probes. The parallel quantum dot structure hosts spatially separated
electrons and holes that interact due to the type-II broken gap of InAs-GaSb
heterojunctions. The Coulomb blockade in the electron and hole transport is
studied, and periodic interactions of electrons and holes are observed and can
be reproduced by modeling. Distorted Coulomb diamonds indicate voltage-induced
ground-state transitions, possibly a result of changes in the spatial
distribution of holes in the thin GaSb shell.
