Surface Nanopatterning by Irradiation with Heavy Polyatomic Ions

The driving forces for surface patterning by ion bombardment have been under discussion for many years. Bradley and Harper proposed a competition between the surface instability due to curvature dependent sputtering and the surface smoothing by Mullins-Herring diffusion. Later, Carter and Vishnyakov proposed another surface destabilizing term based on ion impact induced mass drift. The groups of Aziz (Harvard) and Nordlund (Helsinki) have recently proven that this momentum transfer to target atoms by ion impacts is the dominating driving force for pattern formation in many cases. However, in cases where collision-induced bulk defects cannot reach the surface forming a “crater”, defect diffusion induced patterns like pits and sponges can form. Another complicating fact is that the manifold of beautiful patterns on Si and Ge published recently are dominated by metal impurities. Thus, it is now commonly accepted that at normal ion incidence on elemental, amorphous targets no surface pattern should evolve.
However, we found recently [1,4] well-ordered dot patterns at normal irradiation on Ge and Si with polyatomic Bi ions of ~10…20 keV kinetic energy per atom (see 3D SEM and XTEM image). Similar patterns were found with monoatomic Bi ions at elevated Ge substrate temperatures, when the energy per Ge atom exceeds a critical value [2].
To identify the driving force for this unexpected dot pattern formation, focused ion beam and broad beam studies have been performed in parallel with molecular dynamics [3] and kinetic Monte-Carlo simulations [4]. This investigation proves that these patterns appear only, if nanomelt pools form at the surface of irradiated Ge or Si. It will be shown that melt pools induce a surface smoothing process like in the well-known laser polishing technology. The surface destabilizing term results from the shift of the center of the melt pool meniscus with respect to the ion impact point, where the meniscus arises from the missing material due to sputtering.
1. L. Bischoff, K.-H. Heinig, B. Schmidt, S. Facsko, W. Pilz; NIM B272 198 (2012).
2. R. Böttger, L. Bischoff, K.-H. Heinig, W. Pilz, B. Schmidt; JVST B30, 06FF12 (2012).
3. C. Anders, K.-H. Heinig, H. Urbassek; to be submitted to Phys. Rev. B (2013).
4. R. Böttger, K.-H. Heinig, L. Bischoff, B. Liedke, R.Hübner, W. Pilz; submitted to Adv. Materials (2013).