Spike models for sputtering: Effect of the surface and the material stiffness

Bombarding a solid with fast ions can give atoms within a localized region enough kinetic energy to escape from the material. Thermal spike models have been used to explain this process for metals, insulators and condensed gas solids. Here we use molecular dynamics (MD) simulations of a cylindrical spike to examine the effect of material stiffness and surface boundary conditions on the spike calculations of the yield. When loss from the surface of the material is suppressed, the dependence of the yield on the effective stopping power, (dE/dx)eff, is roughly quadratic at high (dE/dx)eff as in most spike calculations. If escape is allowed, rather than reducing the surface temperature and the yield, both the temperature and the yield increase dramatically and the yield exhibits the roughly linear dependence reported earlier for full MD calculations at high (dE/dx)eff and constant track radius. This change in dependence is determined by the radial pressure pulse and the energy flow to the surface. By changing a parameter in the interaction potential the stiffness of the material in the MD simulations is varied which changes the effect of the pressure pulse. Not surprisingly, for very stiff materials the yield cannot be related to the spike model but more closely resembles spallation