  Ion irradiation during film growth has a strong impact on structural
properties. Linear stability analysis is employed to study surface
instabilities during ion-assisted growth of binary alloys. An interplay between
curvature-dependent ion-driven and deposition-driven instabilities is
investigated. We demonstrate that ion irradiation of growing binary alloys
leads to the formation of composition-modulated surface patterns. It is shown
that the ion-to-atom arrival ratio R is the pattern control parameter. Close to
the instability threshold we identify different regimes of instabilities driven
by ion- or deposition-induced surface roughness processes, or
roughness-composition feedback interactions. In particular, the synergistic
effects of the curvature-dependent displacement and deposition coupling to the
preferential sputtering or to the preferential diffusivity are found to induce
instabilities and pattern formation. Depending on the film growth and
ion-irradiation conditions, the instabilities show stationary or oscillating
behavior. The latter one is exclusively connected with ion irradiation. The
corresponding phase diagrams are presented in terms of experimentally
accessible parameters. This shows an alternative way to control surface
patterning and to grow three-dimensional laterally or vertically ordered
nanostructures.
