Molecular-dynamics simulations of electronic sputtering

Abstract

Following electronic or collisional excitation of a solid by a fast ion, an energized cylindrical region is produced which can lead to sputtering. Here ejection from such a region is studied via molecular-dynamics simulations using Lennard-Jones and Morse potentials. Over the full range of excitations studied the yield vs the energy release per unit path length in the solid, which we call $dE/dx,$ is shown to scale with the binding energy and with the density of the material for all materials studied and at all $dE/dx.$ This allows the simulation results to be applied to low-temperature, condensed-gas solids and to more refractory solids over a broad range of $dE/dx.$ The effect of a distribution of energies for the initial energizing events, and the effect of a spatial distribution of such events for a given $dE/dx$ are examined. Three regimes have been identified. When the energy release per excitation event is greater than the escape energy, sputtering is linear in $dE/dx$ at low $dE/dx.$ With increasing $dE/dx$ a spikelike regime occurs in which the yield again becomes nonlinear with $dE/dx.$ For fixed cylindrical radius ejection then saturates so that at very high $dE/dx$ the yield again becomes nearly linear with $dE/dx.$ In this regime the size of the yield increases with the initial radial extent of the track and is determined by the removal of energy radially by the pressure pulse and by the transport of energy from depth to the surface. Therefore, the clear nonlinearities observed in the knock-on sputtering yields by heavy ions require consideration of the radial extent of the cascades. For electronic sputtering yields of condensed-gas solids, the observed nonlinearity in the sputtering yield suggests that the radial extent of the excited region varies in a manner different from that predicted or that the energy release to the lattice is nonlinear in the stopping power.