Michel Jabbour (Laboratoire de Mécanique des Solides, École Polytechnique). Revisiting the morphological stability of atomic steps on crystal surfaces.

Crystal growth is an old problem, yet it continues to generate new questions. Although it is of a fundamental nature, examples of its technological relevance are legion. The advent of methods for thin-film deposition and of microscopy techniques that probe surfaces at the atomic scale has provided the theory of crystal growth with opportunities it can seize and challenges it must face. Of interest are instabilities affecting the shape and distribution of the atomic steps that make up crystal surfaces, the view being that control of the underlying mechanisms paves the way to the nanoscale patterning of these surfaces. In this talk, I will present a model for the epitaxial growth of thin solid films, one that couples atomic transport (by surface diffusion, evaporation-condensation, etc.) with elastic deformation. Its derivation is based on the balance laws of chemomechanics and the principles governing the thermodynamics of nonequilibrium processes as applied to evolving interfaces. The resulting free-boundary problem for the motion of atomic steps on crystal surfaces is a departure from the formulations one finds in the literature. I will then revisit step instabilities, with the aim of quantifying how they are influenced by the interplay between diffusion, chemical kinetics, and elasticity. Finally, I will conclude by discussing how the proposed theory provides a possible resolution of existing discrepancies between the stability predictions of standard step-flow models and experimental observations of semiconductor and metallic surfaces.