Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Pattern formation on Si(001) through 2 keV Kr+ ion beam erosion under ultra high vacuum conditions is investigated by in situ scanning tunneling
microscopy, ex situ atomic force microscopy and electron microscopy. Under these conditions, at room temperature for fluences up to ≈ 2×10^22 ions m^-2 no ion beam patterns develop for angles ϑ <= 55° with respect to the global surface normal. Only for shallow incidence with 60° <= ϑ <= 81° pronounced patterns form. These patterns expose facets for which the ion beam angle q with respect to the local surface normal of each facet is in the stable range, i.e. outside 60° <= q <= 81°. Analysis of the fluence dependence of pattern formation was conducted in the unstable range with ϑ = 63° and ϑ = 75°. While the speed of pattern evolution depends strongly on the angle of incidence, the sequence does not. The flat surface evolves via small amplitude, regular ripple patterns to large amplitude, irregular facet patterns. The regular ripple pattern transforms through a coarsening mechanism to a facetted pattern.
Co-deposition of stainless steel during ion beam erosion results in well developed hole, dot and ripple patterns already for small ion fluences. Codeposition induced pattern formation does not depend on the deposition method as it takes place for both, co-sputter deposition and co-evaporation. The key factor selecting the type of pattern realized is the ion-to-impurity arrival ratio. While in a broad range from 150 K to 440 K pattern formation is rather temperature independent, dramatic changes take place above a threshold temperature ≈ 600 K, when structures of crystalline iron silicide are shaped on the surface. For these high temperatures needles and sponge patterns with amplitudes of the order of 100 nm and directed towards the ion beam evolve. Variation of the angle between ion beam and impurity source has a significant effect on pattern formation. The larger this angle, the more efficient is pattern formation. This observation points to the relevance of shadowing.