Latent track formation in silicon irradiated by 30 MeV fullerenes

It is now well accepted that electronic excitation and ionisation arising from the slowing-down of swift heavy ions can lead to structural modifications in most targets. It is shown here that new effects take place during irradiations with high energy fullerene beams. Electron microscopy observations were performed at room temperature on prethinned monocrystalline silicon samples after irradiation with 30 MeV fullerene ions. The observed damage is continuous and confined around the projectile paths. The tracks consist of amorphous material as shown by high resolution transmission electron microscopy observations. These tracks recrystallize very rapidly in the electron microscope during the observations in high resolution conditions. Furthermore, a decrease of the track diameter is observed as the cluster ions penetrate deeper inside the target, which is related to an angular scattering of the cluster constituents. At large penetration depths, before disappearing completely, the tracks end as aligned damaged regions of decreasing diameters. Finally, strong sputtering effects occur on the target surfaces, so that craters are generated at the impacts of the projectiles. The fact that amorphous tracks are generated in crystalline silicon following heavy cluster bombardment can be attributed to the strong localization of the deposited energy during the slowing-down process of rather slow projectiles. (C) 1998 Elsevier Science B.V. All rights reserved.