COMPUTER-SIMULATION OF CRYSTAL-SURFACE MODIFICATION BY ACCELERATED CLUSTER ION IMPACTS

Gas cluster ion impacts on a solid suface lead to a modification of th e surface microscopic flatness in the case of normal ion impacts. In t his work we have studied the surface smoothing effect under irradiatio n with cluster beams. Langevin Dynamics, based on a KPZ-type equation for discrete surface heights, allows us to confirm the experimental fi ndings of surface modifications by gas cluster irradiation. We suppose d that a normal cluster impact creates a hemi-spherical crater, with a diameter defined by cluster energy, and use of the Monte Carlo method for the crater formation process, Different sputtering angles could e asily 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 b etter agreement for the angular distribution of sputtered target mater ial for normal cluster impact, calculated by the new MD method and exp eriment. In the case of oblique cluster impacts the shape of the crate r has been chosen to be a shallower and a wider hollow. Compared with normal impact, an essential part of the cluster energy is reflected ba ck into the vacuum at oblique impact. The surface temperature can be l ower, and this effect will reduce the intensity of surface diffusion, We obtained that significant smoothing occurs after irradiation by nor mal 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 accelerat ed if the lateral sputtering phenomenon is taken into account.