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978-3-8439-0248-9, Reihe Physik
Mechanisms and Manipulation of Ion Beam Pattern Formation on Si(001)
172 Seiten, Dissertation Universität Köln (2011), Hardcover, B5
Ion beam pattern formation is a versatile and cost-efficient tool for the fabrication of well-ordered nanostructures. Furthermore, silicon is known to be a prime material in microelectronics. The thesis at hand deals with pattern formation on Si(001) through 2 keV Kr^+ ion beam erosion under ultra high vacuum conditions investigated by in situ scanning tunneling microscopy, ex situ atomic force microscopy, scanning electron microscopy, and transmission electron microscopy. Under highly pure conditions, at room temperature, and for fluences of F≈1 × 10^22 ions m^(-2), no ion beam induced patterns develop for ion incidence angles ϑ ≤ 55° with respect to the global surface normal. In fact, the ion beam induces a smoothing of preformed patterns. Only for grazing incidence angles 60° ≤ ϑ < 81° pronounced ripple and tiled roof patterns develop. Analysis of the fluence dependence of pattern formation was conducted at ϑ = 75° in the unstable ion incidence angular range. The initially flat surface develops small amplitude, regular ripple patterns which then evolve to large amplitude, irregular facet patterns.
Experiments were conducted to rule out or determine the processes of relevance in ion beam pattern formation on Si(001) with impurities. Co-deposition of stainless steel during ion beam erosion results in well developed hole, dot and ripple patterns for even small ion fluences of F≈ 5 × 10^(21) ions m^(-2). The key factor determining the type of pattern realized is the ion-to-impurity arrival ratio. While in a broad range from 140 K to 440 K pattern formation tends to be temperature independent, dramatic changes take place above a threshold temperature of about 600 K, when structures of crystalline iron silicide are shaped upon the surface. For these high temperatures nanopillars and spongelike patterns with amplitudes in the order of 100 nm and directed towards the ion beam evolve. Furthermore, variation of the angle between ion beam and impurity source has a significant effect on pattern formation. The larger this angle is, the more efficient the pattern formation. This observation highlights the relevance of shadowing. Our investigations on the phenomenology of metal assisted ion beam pattern formation identify height fluctuations, local flux variations, induced chemical inhomogeneities, silicide formation and ensuing composition-dependent sputtering to be of relevance.