Roughening instability and evolution of the Ge(001) surface during ion sputtering

We have investigated the temperature-dependent roughening kinetics of Ge surfaces during low energy ion sputtering using energy dispersive x-ray reflectivity. At 150 ^∘{}C and below, the surface is amorphized by ion impact and roughens to a steady state small value. At 250 ^∘{}C the surface remains crystalline, roughens exponentially with time, and develops a pronounced ripple topography. At higher temperature this exponential roughening is slower, with an initial sublinear time dependence. A model that contains a balance between smoothing by surface diffusion and viscous flow and roughening by atom removal explains the kinetics. Ripple formation is a result of a curvature-dependent sputter yield.