Fe impurity induced ion-nanopatterning: atomistic simulations using a   new force field for FeSi
  The ion-bombardment induced nanopatterning of Si(001) has been simulated by atomistic simulations with and without Fe impurity. The surface contamination has been simulated by using a new force field developed for FeSi. This is a fitted bond order potential (BOP) given for Si and for Fe by Albe et al. This BOP formula has been optimized simultaneously for FeSi, Si and for Fe. Using this new force field we are able to follow the ion-beam assisted deposition for Si in the presence of Fe contamination in the surface region. As an overall result, we get an unexpectedly rich variety of nanopatterns formed by the reorganization of the crater rims of the individual ion impacts. The previously thought simple atomistically roughened surfaces show unprecedented landscapes and topography with nanoscale features. The characteristic size of the units of the pattern is in the range of a few nms. Typical of the occurred pattern is the network of interconnected elongated adatom islands. We also see the self-organization of this pattern upon ion-bombardments. At $50^{\circ}$ impact angle we get a nanoporous surface (sponge-like) both for Fe-contaminated and Fe-free simulations. At $70^{\circ}$ of impact (grazing angle of incidence) the pattern resembles to that of elongated atomic chains (adatom islands) along the ion impact direction. This latter pattern could be understood as a prepattern state towards rippling. At lower angles ($30^{\circ}$) nanoholes rule the landsdcape. The obtained pattern corresponds to low fluence experiments which are used to consider as simple roughening without showing any sing of patterning. 
P. S\"ule
cond-mat.mtrl-sci