Mechanisms of mound coarsening in unstable epitaxial growth

Corner diffusion is shown to play a crucial role in determining the asymptotic mound coarsening exponent n in the case of unstable epitaxial growth on $(001)$ and $(111)$ surfaces. For the case of island-relaxation without corner diffusion the asymptotic exponent is found to satisfy n1/4. However, when rapid corner-diffusion is allowed, the coarsening exponent is found to approach $1/3.$ An explanation for these results is presented in terms of the effects of corner diffusion on the surface current and mound morphology.