Multistage droplet epitaxy for the fabrication of InAs/GaAs quantum dots with ultra-low density

In this paper, we demonstrate a novel technique enabling fabrication of small sized InAs/GaAs quantum dots with a very low surface density during droplet epitaxy. In contrast to the traditional two-stage approach, we introduce an additional stage of exposure to the ultra-low arsenic flux which enables partial diffusion decay of droplets with a large initial size. While exposure of droplets to large arsenic fluxes leads to their transformation into rings, disks and holes, exposure to the ultra-low flux makes it possible to reduce the volume of droplets maintaining their initial surface density. At the following stages of crystallization and annealing, In droplets are converted into InAs quantum dots with an average diameter below 30 nm and a surface density below 108 cm-2. The standard deviation of quantum dot diameters is found to be less than 5%. Furthermore, we demonstrate that the growth procedure is well-reproducible, which makes it a promising method of quantum dot fabrication for advanced nanophotonic devices.