Surface and sub-surface defects on graphite after single ion impact studied with STM

Utilizing scanning tunneling microscopy we have investigated the defects induced by single ion impacts on the (0001) graphite surface. The ion species ranged from Ne-20(+) to Pb-208(+) and their energy was fixed at 50 keV (also 150 keV in case of Xe-132(+)). For normal incidence we found two types of well-separated hillock-like defects which we attribute to isolated interstitials and interstitial clusters underneath the surface. Each defect originates from a single ion impact. For 75 degrees tilted incidence we found a strongly increased number of hillocks per incident ion which were aligned in chains along the projection of the incident beam direction. The typical length of these chains is close to the projected ion range, and the number of defects can be explained by the recoil distribution in a single displacement cascade.