(514h) Current-Driven Evolution of Single-Layer Epitaxial Islands on Crystalline Solid Conductors

The driven assembly of confined quantum structures is of special importance to nanoelectronics and nanofabrication technologies. In this context, a particularly interesting problem is the current-driven dynamical response of single-layer adatom and vacancy clusters, i.e., islands and voids of single-layer thickness/depth, on surfaces of crystalline conducting or semiconducting substrates.  In this presentation, we report theoretical and computational results on the current-driven morphological response of single-layer epitaxial islands on crystalline elastic substrates with periphery or edge diffusion being the dominant mode of mass transport.

We develop and validate a transport model for the current-driven dynamics of such single-layer epitaxial islands on crystalline substrates. 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 substrate surface crystallographic orientations and island misfit strains; this strain is due to the lattice mismatch with the substrate for heteroepitaxial islands. A systematic parametric study is conducted to understand the effect on the individual island dynamics of edge diffusional anisotropy parameters, including the angle between a fast edge diffusion direction and the direction of the externally applied electric field. Proper rescaling of v_m accounting for the island morphology gives a universal linear relationship for its dependence on island size.

We also report numerical simulation results on an approach to surface nanopatterning based on the current-driven assembly of single-layer epitaxial islands on crystalline substrates. As a first step in obtaining a fundamental understanding of collective island dynamics, we focus on the evolution of pairs of different-size islands driven to coalescence and explore the effects of three key geometrical parameters: the sizes of the two islands of the pair and their center-to-center line misalignment with respect to the electric-field direction. Based on the understanding developed from the dynamics of island pairs, we extend this study to entire populations (distributions) of epitaxial islands on the substrates of interest. We discover various patterns ranging from equal- and different-size stable steady island-pair configurations to many-island patterns that can be tailored by controlling geometrical parameters of the initial island population and the duration of application of the electric field.