(366d) Modeling of Mass Transport in Semiconductor Nanocrystals and Single-Layer Epitaxial Islands On Crystalline Substrates

Species transport underlies a number of important assembly processes in electronic materials used in the fabrication of microelectronic, optoelectronic, and photovoltaic devices.  In this presentation, we focus on the analysis of such species transport and its impact on the synthesis of semiconductor quantum dots with optimal electronic function and on the external-field-enabled nanopatterning of crystalline substrate surfaces.

First, we address the optimization of the function of quantum dots (QDs) of ternary compound semiconductors, such as In_xGa_1-xAs, ZnSe_1-xS_x, and ZnSe_1-xTe_x, by precise tuning of their electronic band gap through control of the QD composition (x).  We report results on compositional distributions in ternary QDs and how they affect the QDs’ electronic band gap.  We follow a hierarchical modeling approach that combines first-principles density functional theory (DFT) calculations and classical Monte Carlo simulations with a continuum model of species transport in spherical nanocrystals.  In certain cases, the model predicts the formation of a concentration boundary layer near the nanocrystal surface, i.e., the formation of core/shell-like structures with shell regions rich in the surface segregating species.  A systematic parametric analysis generates a database of transport properties, which can be used to design post-growth thermal annealing processes that enable the development of thermodynamically stable QDs with optimal electronic properties grown through simple one-step colloidal synthesis techniques.

Next, we analyze the current driven morphological evolution of single-layer epitaxial islands on crystalline solid substrates.  We develop and validate a continuum model for the island current-driven dynamics.  Mass transport is dominated by adatom diffusion on the island edge; electromigration, misfit-strain-driven diffusion, and edge diffusional anisotropy are accounted for by the model.  Simulations based on the model show that the dependence of the stable steady island migration speed v_m on the inverse of the island size is not linear for larger-than-critical island sizes.  In this nonlinear regime, we report morphological transitions, Hopf bifurcations, and instabilities for various surface crystallographic orientations and island misfit strains.  Proper rescaling of v_m gives a universal linear relationship for its dependence on island size.  Our findings set the stage for designing numerical simulations and experiments of the current-driven evolution of epitaxial island populations and for exploring the collective island dynamical phenomena that govern pattern formation on the crystalline surface.