Unusual pattern formation on Si(1Â 0Â 0) due to low energy ion bombardment

► An angular window of 65-82.5°, in terms of ion incident angle, was chosen for this study. ► Parallel-mode ripple formation is observed in the angular window of 65-67°. whereas unusual patterns evolve over the angular window of 70-77.5°. ► Perpendicular mode ripples are formed at an incident angle of 80°. ► We showed that the transition from parallel- to perpendicular-mode ripples is not a sharp one. ► The observed patterns are attributed to surface confined viscous flow and sputter erosion under ion bombardment.