Stress evolution due to medium-energy ion bombardment of silicon

The evolution of stress in silicon, induced by argon ion bombardment up to fluences of 4.5 * 10^14 ions/cm^2, is studied using molecular dynamics simulations with empirical interatomic potentials. A periodically replicated 5.43 nm cube with an exposed (0 0 1) surface models the sample of silicon. An interatomic force balance method computes stresses directly across planes in the cube. After every impact, the target material is cooled to 77 K, a temperature that inhibits structural changes in the material until the next impact. This procedure makes it unnecessary to simulate explicitly any long-timescale relaxation process. For low fluences (up to about 7 * 10^13 ions/cm^2) the mean induced stress is tensile, but it becomes compressive with further bombardment and appears to saturate at 1.36 and 1.62 GPa when calculated from forces acting on a 5.43 nm * 5.43 nm cross-section, for the 500 and 700 eV cases, respectively. The evolution of compressive stress is observed to be directly proportional to the number of implanted argons. The results are statistically converged by ensemble averaging multiple randomized simulations.