  We show that the ion beam induced incompressible amorphous solid flow in
terms of advection transport mechanism leads to the erosion and deposition of
atoms at the amorphous/crystalline (a/c) interface resulting in the formation
of pattern at the a/c interface as well as at the free surface. The ion beam
impact generated erosion and mass redistribution at the free surface are found
to have insignificant effect in patterning of surface. By varying the
thicknesses of amorphous layer, it has been established that a/c interface
plays the dominant role in surface patterning. The morphological variation of
Si surface after 50 keV Ar+ ion bombardment has been investigated by atomic
force microscopy (AFM) and cross-sectional transmission electron microscopy
(X-TEM) as a function of ion fluence. Navier-Stokes flow inside the amorphous
layer coupled with the Exner equation successfully explains the growth
mechanism of surface patterning.
