Computer simulation of crystal surface smoothing by accelerated cluster ion impacts

Gas cluster ions accelerated to high kinetic energies can be used successfully for surface modification. The most impressive result of cluster ion irradiation appears to be a surface smoothing effect: a rough surface containing valleys and hills of average heights of the order of 100 Å becomes almost flat with the residual roughness reduced by a factor of 10. The Langevin dynamics based on the Kardar-Parisi-Zhang equation, combined with a Monte-Carlo procedure for crater formation, was used for simulation of the surface modification at cluster ion impacts. We found that for a crater size of the order of 20 Å, significant smoothing occurs after irradiation by approximately 10³ cluster impacts; this was confirmed by experiment at dose of 10¹³ ions cm^-2. The rate of the smoothing process depends on the value of surface diffusion coefficients, and can be accelerated significantly if the lateral sputtering phenomenon is taken into account.