  Embedding quantum dots (QDs) on nanowire (NW) sidewalls allows the
integration of multi-layers of QDs into the active region of radial p-i-n
junctions to greatly enhance light emission/absorption. However, the surface
curvature makes the growth much more challenging compared with growths on
thin-films, particularly on NWs with small diameters ({\O} <100 nm). Moreover,
the {110} sidewall facets of self-catalyzed NWs favor two-dimensional growth
(2D), with the realization of three-dimensional (3D) Stranski-Krastanow growth
becoming extremely challenging. Here, we demonstrate thermally-driven formation
of Ge dots on the {110} sidewalls facets of thin self-catalyzed NWs without
using any surfactant or surface treatment. The 2D-3D transition of the
pseudomorphic Ge layer grown on GaAs NWs is driven by energy minimization under
high-temperature annealing. This method opens a new avenue to integrate QDs on
NWs without any restriction on NW diameter or elastic strain, which can allow
the formation of QDs in a wider range of materials systems where the growth of
islands by traditional mechanisms is not possible, with benefits for novel
NWQD-based optoelectronic devices.
