Molecular dynamics simulation of low energy ion beam mixing

Abstract

Molecular dynamics (MD) simulations have been performed of ion beam mixing for 100 eV Ar and Xe ion bombardment of a Cu (100) crystal. Different potentials have been used: the Gibson II potential was splined to a) the Morse potential or b) to a tight-binding many-body potential. Calculations were performed at 0 K for up to 4 ps. A comparison of the results obtained for the different potentials are given. From the results of the MD simulation we have calculated the relocation function, the mean drift velocity of recoils and the mixing coefficient as well as the mean square displacement (MSD). The problem of marker degradation has been solved numerically using the integro-differential mixing equation with the MD simulated function of atomic jumps. The kinetics of defect distributions (ad-atoms, vacancies, interstitials), mean square displacements of atoms in the cascade and the number of atomic jumps of an atom from one Wigner-Seitz cell to another are discussed. Ar ions create roughly twice as many vacancies in the first layer and ad-atoms on the surface as the Xe ions. On the other hand, the number of interstitials (which occur only in the internal part of the crystal) is slightly larger for Xe bombardment. The negative value of the mean drift velocity in the first layer for Ar bombardment is connected with a large number of atomic jumps from the first layer to ad-atom positions. The slower decrease of the mixing coefficient with depth for Xe ion bombardment is connected with the larger penetration depth of Xe ions into the crystal and the smaller backscattering coefficient as compared to Ar. The necessity of taking into account the thermal stage of the cascade for low energy ion beam mixing problem is shown in this work.