Np85 Computer Simulation of Crystal Surface Modification by Accelerated Cluster Ion Impacts

Gas cluster ion impacts on a solid suface lead to a modification of the surface microscopic flatness in the case of normal ion impacts. In this work we have studied the surface smoothing effect under irradiation with cluster beams. Langevin Dynamics, based on a KPZ-type equation for discrete surface heights, allows us to confirm the experimental findings of surface modifications by gas cluster irradiation. We supposed that a normal cluster impact creates a hemi-spherical crater, with a diameter defined by cluster energy, and used the Monte-Carlo method for the crater formation process. Different sputtering angles could easily be incorporated into the model. The probabilities of sputtering were taken from our new hybrid MD method, which has an advantage over the conventional MD method for later impact stages. We have obtained better agreement for the angular distribution of sputtered target material for normal cluster impact, calculated by the new MD method, and experiment. In the case of oblique cluster impacts the shape of the crater has been chosen to be a shallower and a wider hollow. Compared with normal impact, an essential part of the cluster energy is reflected back into the vacuum at oblique impact. The surface temperature can be lower, and this effect will reduce the intensity of surface diffusion. We obtained that significant smoothing occurs after irradiation by normal cluster impacts on a surface area which data has been supported by experiment. The rate of the smoothing process depends on the value of the surface diffusion coefficient, and can be significantly accelerated if the lateral sputtering phenomenon is taken into account.