Surface Patterning by heavy-ion induced melt pools

The driving forces for surface patterning by ion bombardment have been under discussion for many years. Bradley and Harper developed a continuum theory based on the competition between the surface instability due to curvature dependent sputtering and surface smoothing by Mullins-Herring diffusion [1]. Later, a continuum theory with a surface destabilizing term based on ion impact induced mass drift was published [2]. Recently, this momentum transfer to target atoms by ion impacts has been proven to be the dominating driving force for pattern formation in many cases [3], it can be treated by a neat crater function formalism. In case that the collision-induced defects cannot reach the surface to form a crater function, nonlinear diffusion induced pattern like holes and sponges can form. However, it should be noted that the manifold of beautiful patterns on Si and Ge published recently are dominated by metal impurities [4].
Thus, currently the community arrived at the consensus that at normal ion incidence on elemental, amorphous targets no surface pattern should evolve. However, we recently found well-ordered dot patterns at normal irradiation of Ge with polyatomic Bi ions of ~10…20 keV kinetic energy per atom [5]. Similar patterns (Figure 1) were found with monoatomic Bi ions at elevated Ge substrate temperatures [6], where the energy per Ge atom exceeds a critical value within a larger volume (Figure 2).
To identify the driving force for this unexpected dot pattern formation, focused ion beam and broad beam studies have been combined with modeling based on molecular dynamics and kinetic Monte-Carlo simulations. The studies prove that these patterns appear only, if nanomelt pools form at the Ge surface.
It will be shown that melt pools induce a surface smoothing process like in the well-known laser polishing technology, which evolves as . The competing surface destabilizing term results from the missing material due to intense sputtering by Bi ions. This leads to a depression of the melt pool surface, a meniscus, which is visualized in Figure 3 due to amorphous resolidification after Bi3+ ion impact into c-Ge. For off-normal incidence, the meniscus is shifted with respect to the ion impact point in dependence on the surface slope, which leads to a surface destabilizing up-hill mass drift.