A model for the hillock formation on graphite surfaces by 246 MeV Kr+ ions

Scanning tunnelling microscopy has been used to study the effect of ion bombardment at different angles of incidence on a graphite surface. The 246 MeV energy of Kr+ ions was selected in the medium energy range, where the electronic and nuclear stopping is nearly balanced. The low dose (1 x 10(12)/cm2) of ions allows the characterization of single features caused by bombardment in perpendicular, at 30 degrees and at 60 degrees incidence. The density of hillocks caused by the ion bombardment is significantly lower than the ion dose and this density depends on the angle of incidence. The hillocks are attributed to knocked-on atoms leaving the sample surface. A simple model for the scattering process is presented to enlighten the hillock density differences. Other features produced by the ion bombardment, such as elongated traces and (square root of 3 x square root of 3) R30 superstructures are also reported.