Surface nanopatterning mechanisms by keV ions: Linear instability models and beyond

Sputtering solid surfaces with low energy ions is well-known to induce a wide array of nanoscale pattern forming behavior (sputter ripples). A simple continuum model originally developed by Bradley and Harper (BH) provides a useful framework that can qualitatively explain multiple types of patterning behavior seen experimentally and their relationship to the ion beam and material parameters. The basis of the model is a dynamic competition between roughening by ion bombardment and smoothing by surface transport that leads to the growth of roughness with a preferred periodicity on the surface. However, there are many experimental results that cannot be accounted for within this framework and additional physical mechanisms are discussed that may close the gap in our understanding.