Formation of parallel and perpendicular ripples on solid amorphous surfaces by ion beam-driven atomic flow on and under the surface

The off-normal ion irradiation of semiconductor materials is seen to induce nanopatterning effects. Different theories are proposed to explain the mechanisms that drive self-reorganization of amor-phisable surfaces. One of the prominent hypothesis associates formation of nanopatterning with the changes of sputtering characteristics caused by changes in surface morphology. At ultra-low energy, when sputtering is negligible, the Si surface has still been seen to re-organize forming surface ripples with the wave vector either aligned with the ion beam direction or perpendicular to it. In this work, we investigate the formation of ripples using molecular dynamics in all the three regimes of ripple formation: low angles where no ripples form, intermediate regime where the ripple wave vectors are parallel to the beam, and high angles where they are perpendicular to it. We obtain atom-level insight on how the ion-beam driven atomic dynamics at the surface contributes to organization, or lack of it, in all the different regimes. Results of our simulations agree well with experimental observations in the same range of ultra-low energy of ion irradiation.