  We report on the production of ordered assemblies of silicon nanostructures
by means of irradiation of a Si(100) substrate with 1.2 keV Ar ions at normal
incidence. Atomic Force and High-Resolution Transmission Electron microscopies
show that the silicon structures are crystalline, display homogeneous height,
and spontaneously arrange into short-range hexagonal ordering. Under prolonged
irradiation (up to 16 hours) all dot characteristics remain largely unchanged
and a small corrugation develops at long wavelengths. We interpret the
formation of the dots as a result of an instability due to the sputtering yield
dependence on the local surface curvature
