Molecular dynamics studies of ion beam implantation and patterning of silicon: Effect of noble gas cluster formation

The use of energetic ion beams to induce nanopattern formation at surfaces has been well studied both experimentally and theoretically. However, the influence on morphological evolution of the implanted species themselves remains little understood, particularly in the case when the incident ion species does not interact chemically with the target material. In this work, MD simulation results are presented for cumulative ion bombardment of Si to a fluence of 3×1015cm^-2 or more for a range of incident ion energies (20-1000 eV), angles (0-85), and species (Ne, Ar, Kr, Xe). For most cases, the implanted ions are observed to form gas clusters or bubbles beneath the surface as the fluence increases. The implantation and cluster formation decrease in magnitude with increasing ion incidence angle, and remain fairly similar for the heavier-than-Si species (Ar, Kr, and Xe). However, the implantation and cluster formation are much more prominent for Ne irradiation. As the fluence continues to increase beyond ∼1015cm-2, the gas clusters begin to become exposed to the vacuum as the Si layers trapping the gas atoms are eroded by the incident ions. The exposed gas clusters then degas very rapidly, leading to disruption at the surface and to viscous material flow of Si into the void left behind. Comparison to dynamic binary collision approximation (BCA) simulations indicates that cluster formation and degassing contributes to a wide distribution of single-impact emission yields of implanted ions, contrary to intuitive expectations based on BCA simulations. Notably, the increased size and frequency of many-atom implanted ion emission events contributes to a much lower concentration of the implanted species than is otherwise expected from BCA simulations. Additionally, this cluster degassing phenomenon is conjectured to provide a potential antipatterning mechanism by disrupting or destroying nanopattern seeds at the surface. This could provide an additional mechanism to improve model predictions of critical angles for patterning transitions, and may also provide at least a partial explanation for the difficulty of obtaining patterns on Ne-bombarded Si surfaces.