We present in situ scanning tunneling microscopy investigations of pattern formation on Si(001) caused by a low concentration of co-deposited Fe metal. Erosion was performed in ultra high vacuum using fluences of ~ 5 x 10^22 ions/m^2 of 2 keV Kr ions incident at an angle of 30° with respect to the surface normal. Co-sputter-deposition and co-evaporation of Fe at room temperature lead to similar patterns including dot and ripple patterns. We infere that the type of pattern formed
is largely impurity concentration dependent. Moreover, due to the large difference in Fe atom energy
in co-sputter-deposition and co-evaporation, we conclude that the Fe atom energy is not of primary importance in pattern formation. An analysis of the fluence dependent morphological evolution of ripple formation demontrates that upon emergence the ripples display facets in orientation close to parallel and perpendicular directions with respect to the incoming ion beam. Temperature
dependent Fe co-sputter-deposition measurements below (140 K) and above room temperature (440 K) generate equal classes of patterns. Therefore, thermally activated diffusion is irrelevant for impurity assisted ion beam pattern formation around room temperature. Above a threshold temperature around 600 K co-sputter-deposition experiments lead to new classes of high amplitude patterns e.g. a spongelike structure and nano pillars. We attribute the onset of these patterns to the onset of thermal diffusion and silicide formation.

Sven Macko*, University of Cologne, 2nd Institute of Physics, Germany
Frank Frost, Leibniz-Institut f. Oberflaechenmodifizierung e.V., Leipzig, Germany
Martin Engler, University of Cologne, 2nd Institute of Physics, Germany
Thomas Michely, University of Cologne, 2nd Institute of Physics, Germany

*presenting author