Surface topography effects in C₆₀ bombardment of Si

Molecular dynamics simulations of multi-impact bombardment of Si with 20-keV C₆₀ projectiles at normal incidence are performed for a total of 400 impacts, which corresponds to a fluence of of 7 × 10¹³ C₆₀/cm². The surface is roughened by successive bombardment and achieves a steady-state root mean square roughness of 2.0 nm after about 100 impacts. There is a direct correlation between the local topography of the region around the impact point and the sputtered yield. The greatest yields of sputtered atoms are produced when the projectile impacts a mound, which is characterized by the height of the surface relative to the average surface height. When the projectile hits a local region corresponding to a crater with a height much less than the average surface height, the sputtered yield is very small. However, it is these trajectories that deposit carbon atoms at depths beneath the region from which atoms are sputtered, and are responsible for the buildup of carbon at the bottom of craters.