Nanoscale self-affine surface smoothing by ion bombardment

Topography of silicon surfaces irradiated by a 2 MeV Si$^+$ ion beam at normal incidence and ion fluences in the range $10^{15}-10^{16}$ ions/cm$^{2}$ has been investigated using scanning tunneling microscopy. At length scales below $\sim$~50 nm, surface smoothing is observed; the smoothing is more prominent at smaller length scales. The smoothed surface is self-affine with a scaling exponent $\alpha=0.53\pm0.02$.