(160g) Surface Nanopatterning By Electric-Field-Driven Assembly of Single-Layer Epitaxial Islands

Authors: 
Kumar, A., University of Massachusetts, Amherst
Dasgupta, D., University of Massachusetts, Amherst
Dimitrakopoulos, C., University of Massachusetts Amherst
Maroudas, D., University of Massachusetts Amherst

The driven assembly of nanostructures under the action of external fields is of special importance to nanoelectronic and nanofabrication technologies.  Toward this end, in this presentation, we report a systematic and comprehensive theoretical and simulation study of an approach to surface nanopatterning based on electric-field-driven assembly of single-layer epitaxial islands on face-centered cubic (FCC) crystalline substrates.  We have developed and validated a continuum-scale, fully nonlinear driven island evolution model with diffusional mass transport limited to the island edges and accounting for edge diffusional anisotropy and island coalescence and breakup.  The model predicts island edge fingering and necking morphological instabilities are triggered when the island size exceeds critical values that depend on the substrate surface orientation.  We place special emphasis on the dynamics of single-layer epitaxial islands under the action of an electric field which is directed along the slowest edge diffusion direction.  These conditions lead to formation of morphologically stable elongated, nanowire-shaped island configurations due to the occurrence of a fingering instability along the island edge.  We develop a linear theory of edge morphological stability, which predicts the occurrence of the fingering instability that leads to formation of nanowire-shaped morphology.  Depending upon the island size and the substrate surface orientation, the theory predicts the number of fingers that form on the island edge and the critical island sizes for the onset of the respective fingering instabilities.

We assess the validity of the linear stability theory by comparing its predictions with the results of direct dynamical simulations of electric-field-driven single-layer island dynamics on {110}, {100}, and {111} FCC substrate surfaces and over a range of island sizes.  The simulations also predict that on {100} and {111} substrate surfaces the elongated nanowire-shaped configurations that emerge from the fingering instability evolve along directions parallel to each other and, following breakup due to subsequent necking instabilities, form nanowires parallel to each other on the substrate.  These nanowire configurations can be used in the fabrication of consistently shaped and sized, perfectly aligned nanostructures by controlling the direction of the electric field.  Such necking instabilities do not occur on {110} surfaces, where a single stable nanowire-like feature is formed for each single-layer island.

Finally, using direct dynamical simulations, we have predicted very intriguing complex patterns of island distributions on {110} and {100} FCC substrate surfaces, starting from islands with sizes much larger than the critical size for island breakup for externally applied electric fields along directions of fast edge diffusion.  Following a sequence of breakup and coalescence events, the patterns forming upon turning off the electric field consist of distributions of islands that are symmetric with respect to the field direction, dispersed far from this symmetry axis, characterized by a broad size distribution, and even containing symmetric distributions of stable voids (not simply connected domains).  This rich variety of patterns and length scales can be exploited for systematic physical, field-driven patterning of solid material surfaces.  In general, our study advances our fundamental understanding of the current-driven island dynamics on crystalline substrate surfaces as a means of directed assembly for physical nanopatterning.