AVS 46th International Symposium
    Nanometer-scale Science and Technology Division Monday Sessions
       Session NS2-MoA

Paper NS2-MoA6
Coherently Strained Sn Quantum Dot Formation in Si via Phase Separation

Monday, October 25, 1999, 3:40 pm, Room 6C

Session: Quantum Dots and Wires
Presenter: K.S. Min, California Institute of Technology
Authors: K.S. Min, California Institute of Technology
H.A. Atwater, California Institute of Technology
N.J. Choly, California Institute of Technology
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Diamond cubic @alpha@-Sn is a zero band gap semiconductor and band structure calculations predict a direct and tunable energy gap for Sn-rich Sn@sub x@Si@sub 1-x@ alloy system. One approach for realization of a direct band gap material based on coherently strained Sn-rich Sn@sub x@Si@sub 1-x@/Si system is to synthesize coherently strained Sn-rich quantum dots. For high Sn concentration Sn@sub x@Si@sub 1-x@/Si quantum dot structures, one might potentially take advantage of quantum carrier confinement to further tune the energy gap over a wide range in the infrared frequency range. The biggest difficulty in growing Sn@sub x@Si@sub 1-x@ quantum dots via conventional epitaxial growth techniques, however, is the strong tendency for Sn atoms to segregate to the surface during growth at ordinary Si epitaxy temperatures. We report a novel two-step process for synthesizing coherent Sn-rich quantum dots contained within Si, where the enthalpy of mixing is highly positive. First, an ultrathin homogeneous Sn@sub x@Si@sub 1-x@ metastable solid solution sandwiched between Si is grown by temperature-modulated molecular beam epitaxy. The as-grown epitaxially stabilized ultrathin homogeneous film is then thermally annealed in high vacuum, whereupon the quantum dots precipitate as the ultrathin alloy film phase separates. The quantum dots appear in planar-view transmission electron micrographs as square-shaped with facets along the elastically soft <100> direction. The mean size ranges from 2 nm to 3 nm for annealing temperature between 500°C and 800°C. Cross-sectional high-resolution transmission electron microscopy reveals that the dots are completely coherent with the Si matrix. The early stage of phase separation proceeds via spinodal decomposition, followed by diffusion-limited coarsening in the late stage. The optical properties of the quantum dots will also be presented.