Study of density effect of large gas cluster impact by molecular dynamics simulations

Large gas cluster impacts cause unique surface modification effects because a large number of target atoms are moved simultaneously due to high-density particle collisions between cluster and surface atoms. Molecular dynamics (MD) simulations of large gas cluster impacts on solid targets were carried out in order to investigate the effect of high-density irradiation with a cluster ion beam from the viewpoint of crater formation and sputtering. An Ar cluster with the size of 2000 was accelerated with 20 keV (10 eV for each constituent atom) and irradiated on a Si(1 0 0) solid target consisting of 2 000 000 atoms. The radius of the Ar cluster was scaled by ranging from 2.3 nm (corresponding to the solid state of Ar) to 9.2 nm (64× lower density than solid state). When the Ar cluster was as dense as solid state, the incident cluster penetrated the target surface and generated crater-like damage. On the other hand, as the cluster radius increased and the irradiation particle density decreased, the depth of crater caused by cluster impact was reduced. MD results also revealed that crater depth was mainly dominated by the horizontal scaling rather than vertical scaling. A high sputtering yield of more than several tens of Si atoms per impact was observed with clusters of 4-20× lower volume density than solid state.