  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 amorphisable 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.
