The contributions of defect kinetics and sputtering to pattern formation

The search for the dominating driving force for surface pattern formation under ion irradiation is performed by atomistic 3D computer simulations taking into account the full ion-induced collision cascades as well as the defect relaxation kinetics. For this aim a novel program package called TRIDER (TRansport of Ions in matter with DEfect Relaxation), which unifies full collision cascade simulation with the calculation of atomistic relaxation processes, has been developed. The collision cascades are provided by simulations based on the Binary Collision Approximation, and the relaxation processes are simulated with the 3D lattice kinetic Monte-Carlo method. This allows, without any phenomenological model, a full 3D atomistic description on experimental spatiotemporal scales. The dependence of the local morphology on sputtering yield and recently discussed new mechanism contributing to surface patterning like ballistic mass drift are inherently included in our atomistic approach.
Compared to 2D simulations based on the Solid-On-Solid (SOS) approach or continuum theories, the presented 3D simulations include the contribution of ion-induced subsurface processes to pattern formation, e.g. bulk interstitial and vacancy recombination at the surface. In order to reveal the dominating driving force for pattern formation, collision cascade simulations have been performed for two artificial cases: (i) simulations where sputtering has been suppresses artificially, (ii) simulations where only sputter-induced vacancy creation is taken into account (the original idea for the formulation of the Bradley-Harper equation). In this connection, for checking continuum theories, the 3D computer experiments can be considered as ‘cleanest’ possible experiments.
The computer simulations predict that sputtering is not the dominant driving force of pattern formation. Processes like bulk and surface defect kinetics dominate the surface morphology evolution. Only at grazing incidence the sputtering has been found to dominate the ripple formation. Surface mass currents originating from bulk defect recombination at the surface cause smoothing at ion impact angles <45°.