From groove to hillocks - Atomic-scale simulations of swift heavy ion grazing impacts on CaF₂

Surface nanopatterning of CaF2 by swift heavy ions irradiation under oblique angles is studied with a combination of the event-by-event Monte Carlo particle transport model and molecular dynamics simulations. The model describes the electronic system excitation and energy transfer to the lattice followed by the atomic response. The approach allowed us to simulate the kinetics of the electronic ensemble excited by a grazing ion demonstrating that the presence of the surface does not reduce the energy of the lattice as expected. On the contrary, the track core temperature near the surface is slightly higher than in the bulk, because electrons reflected from the surface bring a part of the energy back to the core. The formation kinetics of entire grazing ion tracks is studied revealing the mechanisms of various surface nanostructures formation. Depending on the penetration depth, the ion produces a groove bordered by hillocks, a single chain of nanohillocks, a huge hillock at the end of the rift, and a single continuous structure afterward. The critical depth of the material expulsion from the surface equals approximately to the transient molten zone radius (similar to 4-4.5 nm). The simulated structures are in reasonable agreement with the available experimental data.