Sunburst pattern by kinetic segregation in core-shell nanowires: A phase-field study

Compositional segregation is commonly observed during the growth of III-V alloys in core-shell nanowires. Nanometer-thin stripes, enriched in one of the alloy components, are observed along the six <1 1 2> directions perpendicular to the [1 1 1]-wire axis, departing from the core to the outer shell edges in between the {1 1 0} faceted sidewalls. While it has been well established that the phenomenon occurs because of the different mobility of the alloy components, the actual mechanisms by which it happens are yet unclear. A phase-field model, coupling deposition and surface diffusion dynamics, is here developed to inspect the simultaneous evolution of the shell morphology and composition during growth. Both surface energy anisotropy and orientation-dependent growth rates are taken into account to identify their different role. Simulations reveal that the observed segregation is mainly triggered by the enhanced growth rate at the facet edges, while surface anisotropy keeps the stripes thin. Polarity effects are also included to differentiate the behavior between <1 1 2>A and <1 1 2>B orientations, so to reproduce the experimental observation of a 3-fold symmetry.