We report on in situ scanning tunneling microscopy investigations of the surface
morphological evolution of Si(001) through ion beam erosion.
The experiments are performed under ultra high vacuum conditions with a differentially pumped ion source hitting the sample only. We use 2 keV Kr ions and fluences up to ~10^22
ions/m^2. Under these conditions on Si ion beam patterns form only for angles in the range of 60°-83° with respect to the global surface normal. In fact, at less grazing angles the ion beam induces a smoothening of preformed patterns. To uncover the mechanism of surface destabilization  the fluence dependence of pattern formation at 75° ion incidence is investigated. A facetted ripple pattern evolves with a wavelength increasing with ion fluence.
The ripple facets are oriented close to perpendicular or parallel with respect to the incoming ion beam. Based on the facetted nature of the ripples already in their infant state it appears that pattern formation is tightly linked to the angular dependence of the sputtering yield.

Sven Macko*, University of Cologne, 2nd Institute of Physics, Germany
Frank Frost, Leibniz-Institut f. Oberflaechenmodifizierung e.V., Leipzig, Germany
Bashkim Ziberi, Leibniz-Institut f. Oberflaechenmodifizierung e.V., Leipzig, Germany
Daniel F. Foerster, University of Cologne, 2nd Institute of Physics, Germany
Thomas Michely, University of Cologne, 2nd Institute of Physics, Germany

*presenting author