The effect of low energy irradiation, where the sputtering is imperceptible, has not been deeply studied in the pattern formation. In this work, we want to address this question by analyzing the nanoscale topography formation on Si surface, which is irradiated at room temperature by Ar + ions near the displacement threshold energy, for incidence angles ranging from 0 to 85 •. The transition from smooth to ripple patterned surface, i.e. the stability/instability bifurcation angle is observed at 55 • , whereas the ripples with their wave-vector is parallel to the ion beam projection in the angular window of 60-70 • , and with 90 • rotation with respect to the ion beam projection at the grazing angles of incidence. A similar irradiation setup has been simulated by means of molecular dynamics, which made it possible, firstly, to quantify the effect of the irradiation in terms of erosion and redistribution using sequential irradiation and, secondly, to evaluate the ripple wavelength using the crater function formalism. The ripple formation results can be solely attributed to the mass redistribution based mechanism, as erosion due to ion sputtering near or above the threshold energy is practically negligible.

Patterns on sand generated by blowing winds are one of the most commonly seen phenomena driven by such a self-organization process, as has been observed at the nanoscale after ion irradiation. The origins of this effect have been under debate for decades. Now, a new methodology allows to simulate directly the ripple formation by high-fluence ion-irradiation. Since this approach does not pre-assume a mechanism to trigger self-organization, it can provide answers to the origin of the ripple formation mechanism. The surface atom displacement and a pile-up effect are the driving force of ripple formation, analogously to the macroscopic one. IMPACT STATEMENTThe presented model allows to follow the ripple formation and propagation in different steps, at the atomic level, for the first time under low irradiation energies. ARTICLE HISTORY

Ion beams are frequently used in industry for composition control of semiconducting materials as well as for surface processing and thin films deposition. Under certain conditions, low-and medium energy ions at high fluences can produce nanoripples and quantum dots on the irradiated surfaces. In the present work, we focus our attention on the study of irradiation of amorphous silicon (a-Si) target with 250 eV and 1 keV Ar + ions under different angles, taking into special consideration angles close to the grazing incidence. We use the molecular dynamics (MD) method to investigate how much the cumulative displacement of atoms due to the simulated ion bombardment contribute to the patterning effect. The MD results are subsequently analysed using a numerical module Pycraters that allows the prediction of the rippling effect. Ripple wavelengths estimated with Pycraters are then compared with the experimental observations, as well as with the results obtained by using the binary collisions approximation (BCA) method. The wavelength estimation based on the MD results demonstrates a better agreement with the experimental values. In the framework of the utilized analytical model, it can be mainly attributed to the fact that the BCA ignores low energy atomic interactions, which, however, provide an important contribution to the displacement of atoms following an ion impact.