  The morphology evolution of Si (100) surfaces under 1200 eV Ar+ ion
bombardment at normal incidence with and without metal incorporation is
presented. The formation of nanodot patterns is observed only when the
stationary Fe concentration in the surface is above 8x10^14 cm^-2. This is
interpreted in terms of an additional surface instability due to non-uniform
sputtering in connection with metal enrichment at the nanodots. At low metal
concentration smoothing dominates and pattern formation is thus inhibited. The
transition from a k^-2 to a k^-4 behavior in the asymptotic power spectral
density function supports the conclusion that ballistic smoothing and
ion-enhanced viscous flow are the two dominant mechanisms of surface
relaxation.
