We have proposed a novel self-assembling growth method, termed Droplet Epitaxy, for the direct formation of QDs without using any lithography in 1990. Compared with the island formation based on the Stranski-Krastanow growth mode, the Droplet Epitaxy is applicable to the formation of quantum dots not only in lattice-mismatched but also in lattice-matched systems such as GaAs/AlGaAs. The process of the Droplet Epitaxy in MBE chamber consists of forming numerous III-column element droplets such as Ga or InGa with homogeneous size of around 10nm on the substrate surface first by supplying their molecular beams, and then reacting the droplets with As molecular beam to produce GaAs or InGaAs epitaxial microcrystals. Another advantage of the Droplet Epitaxy is the possibility of the fabrication of QDs structures without wetting layer by corralling the stoichiometry of the substrate surface just before the deposition of III-column element droplets. Also we can control the shape of the QDs structure self-organizingly such as pyramidal shape, single-ring shape and concentric double-ring shape. These ring structures will provide excellent possibilities for the investigation of quantum topological phenomena.
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