The formation of defect-free 3-dimensional nanoscale islands as a pathway for strain relief beyond a critical deposition amount in highly strained overgrowth on planar substrates, discovered in our group [46], provides another approach to realizing quantum dots. When buried by an appropriately overgrown capping layer that does not induce unacceptable density of defects, the epitaxical islands of one material surrounded in volume by larger band gap capping and substrate materials, confines the electrons in the island material and turns the islands into quantum dots [66], the so-called self-assembled quantum dots (SAQDs).

     We have introduced innovative approaches to manipulating the density, size, shape, and uniformity of these SAQDs [73,75]. We employ extremely low temperature (<350°C) capping of the InAs islands by InGaAlAs using MEE instead of MBE [64,66]. This should reduce the inter-mixing of atoms via inter diffusion during capping layer growth and thus distinguishes our SAQDs with those produced by all other groups. We are the first group to have investigated the impact of the stress induced by the islands on the evolution of the profile of the capping layer growth and shown that, from the evolving profile, one can extract spatially dependent Ga diffusion coefficient as a function of deposition thickness [64].

     Once the islands are capped, and atomically smooth cap layer surface realized, there remains a spatially varying stress at the cap layer surface due to the buried islands. We postulated that this surface stress would cause preferentially directed migration of the atoms of the subsequent layer of island material leading to spatially preferential formation of this layer of islands. Our group provided the first demonstration and dynamical model and theory for such vertically self-organized growth of the SAQDs [69].

Surface stress engineered spatially-selective and directed assembly of strain-driven epitaxical island quantum dots

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