Energetic particle irradiation of solids can cause surface ultra-smoothening, self-organized nanoscale pattern formation in surface topography or bulk composition, or degradation of the structural integrity of nuclear reactor components. Topographic pattern formation has previously been attributed to the effects of the removal of target atoms by sputter erosion. Here we show that surface stability or instability is determined by the effects of ion impact-induced atomic redistribution, and that the effect of sputter erosion is essentially irrelevant. We study the transition, with increasing angular deviation from normal incidence, from stability to instability of a flat silicon surface under 1 keV argon ion beam irradiation.  We use Grazing Incidence Small Angle X-Ray Scattering to measure in situ, in real time, the damping of noise or its amplification into patterns via the linear dispersion relation. The predicted behavior of the sputtering effect is quantitatively negligible and of the wrong sign compared to the observed effect. A model based on the effects of impact-induced redistribution of those atoms that are not sputtered away both explains the observed ultra-smoothening at low angles, and also drives its own instability at higher angles. Our results imply that the negligibility of impact-induced erosion rates is not sufficient to ensure morphological stability of plasma-facing fusion reactor walls, and that considering ion-induced target atom redistribution may lead to improved design criteria.

Charbel Said Madi*, Harvard Universtiy, United States
Eitan Anzenberg, Boston University, United States
Karl Ludwig Jr., Boston University, United States
Michael John Aziz, Harvard University, United States

*presenting author