Thermally-driven formation of Ge quantum dots on self-catalysed thin GaAs nanowires

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.