The sputter cross section of a surface-vacancy island

Using molecular-dynamics simulation we investigate the effect of surface-vacancy islands on ion-induced sputtering. As an exemplary case, the sputtering of a Pt(1 1 1) surface by 5 keV Ar+ ions incident at 83° towards the surface normal is investigated. We find that only the ascending step of the island induces sputtering. Wide vacancy islands exhibit the direct-hit, indirect-hit and channeling zones previously identified for surface steps and adatom islands. A special role is played by the descending step edge. Even though it is not sputtered itself, it deflects ion trajectories and may direct them to the ascending step edge thus enhancing sputtering. We derive a simple criterion based on the shadow cone of the descending step to decide whether a vacancy island contributes to sputtering or not.