Atomic-scale mapping of quantum dots formed by droplet epitaxy

Quantum dots (QDs) have applications in optoelectronic devices, quantum information processing and energy harvesting. Although the droplet epitaxy fabrication method allows for a wide range of material combinations to be used, little is known about the growth mechanisms involved. Here we apply direct X-ray methods to derive sub-ngström resolution maps of QDs crystallized from indium droplets exposed to antimony, as well as their interface with a GaAs (100) substrate. We find that the QDs form coherently and extend a few unit cells below the substrate surface. This facilitates a droplet-substrate exchange of atoms, resulting in core-shell structures that contain a surprisingly small amount of In. The work provides the first atomic-scale mapping of the interface between epitaxial QDs and a substrate, and establishes the usefulness of X-ray phasing techniques for this and similar systems.