Surface patterning by heavy-ion induced melting

The driving forces for surface patterning by ion bombardment have been under discus-sion for many years. At first, a continuum theory based on competition between the sur-face instability due to curvature dependent sputtering and surface smoothing by Mul-lins-Herring diffusion was proposed [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]. In case that collision-induced defects cannot reach the surface to form a crater, defect diffusion induced pat-terns like pits and sponges can form. It should be noted that the manifold of beautiful patterns on Si and Ge published recently are caused by metal impurities [4]. Thus, it is now commonly accepted that at normal ion incidence on elemental, amorphous targets no surface patterns 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 were found with monatomic Bi ion irradiation of heated Ge substrates, when the deposited energy per Ge atom exceeds a critical value within a larger volume [6].
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 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, which evolves as . The competing surface roughening term results from the missing material due to intense sputtering by Bi ions. This leads to a depression of the melt pool surface. 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.
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[2] G. Carter and V. Vishnyakov, Phys. Rev. B 54 (1996) 17647.
[3] C. S. Madi, H. B. George, and M. J. Aziz, J. Phys. Condens. Matter 21 (2009) 224010.
[4] B. Ziberi, M. Cornejo, F. Frost, and B. Rauschenbach, J. Phys. Condens. Matter 21 (2009) 224003.
[5] L. Bischoff, K.-H. Heinig, B. Schmidt, S. Facsko, and W. Pilz, Nucl. Instr. Meth. Phys Res. B 272 (2012) 198.
[6] R. Böttger, L. Bischoff, K.-H. Heinig, W. Pilz, and B. Schmidt, J. Vac. Sci. Technol. B 30 (2012) 06FF12.