Computer simulation of cluster ion impacts on a solid surface

The sputtering probabilities for normal and oblique cluster ion impacts were calculated by the use of two-dimensional molecular dynamics (MD) calculations. These simulations have revealed the angular dependence of ejecting surface atoms on the cluster incidence angle. The ejecting flux has a symmetrical form with an essential lateral component in the case of normal cluster incidence. For an oblique cluster indidence we found a sharp asymmetry of sputtering orientation. We obtained that the ejecting flux consists of three components: a) fast flying atoms with the velocities higher than the cluster velocity ν approx. 2.3ν₀, b) approximately self-similar component with ν ≈ ν₀, and, finally, c) slowly moving tail with ν approx. 0.2ν₀, where ν₀ is the cluster velocity. According to our MD results we developed a new model of surface modification phenomena which consists of the Langevin Dynamics based on the Kardar-Parisi-Zhang equation, combined with a Monte-Carlo procedure for crater formation at normal and oblique cluster impacts. We obtained that for a symmetrical crater shape with a size in the order of 20 angstrom, a significant smoothing occurs after irradiation by approx. 10³ cluster impacts which has been supported by experiment at dose of approx. 10¹⁴ ion/cm². The rate of the smoothing process can be significantly accelerated if the lateral sputtering phenomenon is taken into account. Simulation of oblique cluster impact on a surface at a grazing angle of 30° by constructing of asymmetric crater shape gives an opposite result: the surface roughness increases. The latter obtaining qualitatively agrees with the experiment.