Evolution of ion beam induced patterns on Si(001)

In the range of incidence angles between 58 degrees and 79 degrees, Si develops erosion patterns through room-temperature exposure to 2 keV Kr+, while for other incidence angles it remains flat. We investigate the formation of these patterns through all in situ sample preparation and investigation under ultrahigh vacuum conditions. The ion fluence is varied by a factor of 1000 for the incidence angles of 63 degrees and 75 degrees. The resulting morphologies are imaged by scanning tunneling microscopy and quantitatively analyzed in view of roughness, wavelength, disorder, and surface slopes. We find it necessary to distinguish between low-fluence and high-fluence regimes of pattern formation. While in both low-fluence regimes a similar parallel mode ripple pattern evolves, the high-fluence regimes are distinctly different and evidence either the evolution of disordered perpendicular ripples or a roof-tile structure for 63 degrees or 75 degrees, respectively. The data are compared to other experimental data for ion beam erosion of Si and Ge enabling us to assess the universality of our observations. Comparison to existing models for the surface evolution under ion exposure allows us to draw conclusions on the applicability of these models for pattern formation on Si(001)