  Ion sputtering induced nanoscale pattern formation on Ge (001) surface by 500
eV Ar+ bombardment has been investigated for a wide range of ion incidence
angles at temperature of 300 deg.C. A fourfold symmetric topography forms in
the angular regime 0 - 65 deg. Above 65 deg, they show a remarkable transition
into highly regular one-dimensional (1D) asymmetric pattern, known as
perpendicular mode ripples. In order to understand growth dynamics of both kind
of patterns, we have investigated their temporal evolution as a function of ion
fluence in a wide range from 1*10^{17} to 1*10^{20} ions cm-2. In addition, we
study the effect of substrate rotation on Ge surface morphology in whole
angular range. The four-fold symmetric patterns effect does not found to alter
their symmetry, while the ripples degenerate into hole structure with a weak
fourfold symmetric pattern. The origin of square topographies and their
symmetry independency on ion incident angle in the range 0 to 65 deg can be
attributed to the growth process due to biased diffusion of vacancies arising
from Ehrlich-Schwoebel barrier. Whereas, the ripple formation at grazing
incidence angles indicates the dominance of curvature dependent surface
instability induced by the incident ion direction.
