Ion beam induced surface patterns due to mass redistribution and curvature-dependent sputtering

Recently it was reported that ion-induced mass redistribution would solely determine nano pattern formation on ion-irradiated surfaces. We investigate the pattern formation on amorphous carbon thin films irradiated with Xe ions of energies between 200 eV and 10 keV. Sputter yield as well as number of displacements within the collision cascade vary strongly as function of ion energy and allow us to investigate the contributions of curvature-dependent erosion according to the Bradley-Harper model as well as mass redistribution according to the Carter-Vishnyakov model. We find parallel ripple orientations for an ion incidence angle of 60 degrees and for all energies. A transition to perpendicular pattern orientation or a rather flat surface occurs around 80 degrees for energies between 1 keV and 10 keV. Our results are compared with calculations based on both models. For the calculations we extract the shape and size of Sigmund's energy ellipsoid (parameters a, sigma, mu), the angle-dependent sputter yield, and the mean mass redistribution distance from the Monte Carlo simulations with program SDTrimSP. The calculated curvature coefficients S-x and S-y describing the height evolution of the surface show that mass redistribution is dominant for parallel pattern formation in the whole energy regime. Furthermore, the angle where the parallel pattern orientation starts to disappear is related to curvature-dependent sputtering. In addition, we investigate the case of Pt erosion with 200 eV Ne ions, where mass redistribution vanishes. In this case, we observe perpendicular ripple orientation in accordance with curvature-dependent sputtering and the predictions of the Bradley-Harper model.