Anisotropic edge retraction and hole growth during solid-state dewetting of single crystal nickel thin films

At elevated temperatures, single crystal thin films sometimes undergo solid-state dewetting to form holes and islands with anisotropic morphologies and regular features. We have measured the rates of edge retraction for single crystal Ni(1 0 0) and Ni(1 1 0) films with edges patterned along different in-plane crystallographic directions. The polar plots of the edge retraction velocities have anisotropic shapes with prominent cusps in particular directions. The characteristic anisotropy in the edge retraction velocity can be understood in terms of in-plane orientation-dependent edge faceting, with single faceting of specific edges and multiple micro-faceting in other edges. Faceting occurs due to surface energy minimization, and is modified by changes in the annealing ambient. Changes in facet morphologies affect the relative edge retraction rates in different crystallographic orientations and also the initial shapes of holes growing in the film. The mechanistic understanding of edge retraction provided through these studies will provide a basis for understanding the evolution of complex morphologies that result from solid-state dewetting of unpatterned single crystal films.