Self-Assembly of Semiconductor Quantum Dots by Droplet Epitaxy
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Self-Assembly of Semiconductor Quantum Dots by Droplet Epitaxy Nobuyuki Koguchi Quantum Dot Research Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, 305-0047, Japan
ABSTRACT 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 10 nm 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 cotrolling 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. INTRODUCTION Self-assembling quantum dots (QDs) has been attracted much attention to form structures with dimensions on the order of a few tens of nm that are necessary for the realization of advanced quantum devices such as lasers or infrared detectors or quantum information processing. There are several pioneering research works on the self-assembling formation of quantum dots (QDs). In the beginning of 1980s, two kinds of nucleation have been identified when InAs is grown on a GaAs substrate, depending on growth conditions [1-3]. For ultra-thin InAs films a two-dimensional (2D) growth is observed. When the layer thickness is increased, the strain in the epitaxial film induces a transition from 2D to a three-dimensional growth (3D) with island formation, which is known as Stranski-Krastanow (SK) growth mode. This has been evidenced by reflection high-energy electron diffraction (RHEED) observations [1]. Goldstein et al [4] observed photoluminescence (PL) originated from InAs islands formed in InAs/GaAs strainedlayer superlattices. In 1993, Petroff`s group reported the direct formation of InGaAs QDs [5] based on the SK growth mode.
In 1990, we have proposed a novel self-assembling growth method, termed Droplet Epitaxy, for the direct formation of QDs [6] without using any lithography. The SK type growth occurs only in the lattice-mismatched system. Compared with the island formation based on the SK growth mode, the Droplet Epitaxy is applicable to the formation of QDs not only in latticemismatched system but also in lattice-matched system. Another advantage of the Droplet Epitaxy is the possibility of controll
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