Cluster beams can be an efficient tool for manipulating agglomerates of atoms providing control over the synthesis of structures as well as modification of surfaces on the nm-scale. The application of cluster beams requires knowledge of the physics of cluster-surface impact that is only beginning to be studied.
A series of experiments on keV-energy implantation of inert gas cluster ions into silicon, sapphire and graphite has been carried out. The presence of nanostructures called complex craters (craters with centrally positioned hillocks and outer rims) was observed on the surface of the implanted silicon and sapphire using AFM. Detailed experimental studies of implantation depending on the cluster species, their size and energy as well as on the type of substrate material have provided data for a qualitative model of the complex crater formation. This model is based on the effect of multiple collisions causing high density of energy deposition via the nuclear stopping mechanism resulting in a local melting of material around the impact spot. Local tensions and elastic rebound lead to viscous flow and expulsion of the molten material forming the hillock. In the case of graphite bombarded by argon cluster ions, the STM images show nm-size bump formation. AFM, SEM and Raman studies are in progress to obtain more detailed information about the structure of these surface features.
MD simulations of argon cluster implantation into silicon show the possibility of both simple crater and hillock formation. The experimentally found sizes of the inner hillocks are comparable with those obtained from the MD and binary collision approximation calculations. Based on recent simulations, we expect that the outer rim is probably formed by a viscous flow of material during the relaxation phase following the initial melting phase of the collision. For the graphite, the simulations predict large amplitude oscillations of the lattice planes on cluster impact but they lead only to small damaged regions resulting in formation of small craters.
|