Pattern formation on ion-irradiated Si surface at energies where sputtering is negligible

Abstract

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.

Direct observation of ion-induced self-organization and ripple propagation processes in atomistic simulations

“…Notice that, eventually, on average the crest-trough distance is about 1.5 nm, similar to the one measured in Ref. [15].…”

“…We estimate the average sputtering yield S y = 0.03. However, when we compare this value to the one obtained in the previous study [15] for the single impact calculations (S y = 0.035 ± 0.013), we see that the increased curvature of the increasingly roughened surface does not affect significantly the sputtering yield and it remains within the error bars. We measure the role of erosion by analyzing the amount of missing atoms in the trough of the ripple, excluding the volume of its crest.…”

“…More detail can be found in Refs. [15,31]. The size of the flat-surface cell used as a starting point 16.56 × 16.56 × 5.15 nm 3 ; it contained 73,584 atoms.…”

“…A few studies have taken into account this effect as a non-linear extension term related to the erosion rate [9][10][11][12][13][14]. However, it was recently shown the erosive component alone is not able to capture the complex phenomenon [15,16]. Norris et al [17,18] proposed a mathematical model predicting patterns based on the crater function formalism, which allows also describing atom redistribution as an additional term contributing to ripple formation.…”

“…[23,24] even directly state that MD simulations are not suited for studying the surface rippling due to time and size limitations. However, in recent experiments [15,[25][26][27], it was demonstrated that the 30 eV Ar ion irradiation caused the ripple formation on the surface of amorphous Si as well. In the current work, we gain new insights on the process of ripple formation due to a unique combination of a recently developed speedup scheme for MD simulation [28] to simulate tens of thousands of ion impacts on the same surface, with consideration of the new experimental condition of very low ion energy-induced ripples.…”