Atomic-scale mapping of quantum dots formed by droplet epitaxy

Quantum dots (QDs) have applications in optoelectronic devices(1,2), quantum information processing(3,4) and energy harvesting(5,6). Although the droplet epitaxy fabrication method(7-9) allows for a wide range of material combinations to be used, little is known about the growth mechanisms involved(10,11). Here we apply direct X-ray methods(12-14) to derive sub-angstrom 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(15) 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.