BEIJING UNIV,INST HEAVY ION PHYS,BEIJING 100871,PEOPLES R CHINA
; ACAD SCI CHINA,INST HIGH ENERGY PHYS,BEIJING 100080,PEOPLES R CHINA
Surface damage on highly oriented pyrolytic graphite induced by bombardment of 530 keV Au+ or 4.5 MeV Au++ at a low dose of 1 X 10(12) cm(-2) was studied by scanning tunneling microscopy. The most prominent surface features are hillocks with a diameter of about 1.8 nm. The average spatial density of the hillocks is in good agreement with the ion dose used, indicating that they are induced by impact of individual incident gold ions. There also exists another type of hillock (which is called bump in this paper) with a smaller diameter and a frequency of about twice the ion dose. These small hillocks have never been reported before and are likely to be caused by the outward sputtering of secondary particles as products of collision cascades. Beside the various root 3 X root 3 R 30 degrees superstructures of graphite, Friedel oscillations are very often seen surrounding most of the hillocks. The physical mechanism of the formation of both types of the hillocks is also discussed.