Simultaneous Formation of Two Ripple Modes on Ion Sputtered Silicon

It is well known that oblique low and medium energy (typically 0.1 – 100 keV) ion erosion of solid surfaces can lead to the formation of periodic ripple patterns with wavelength ranging from 10 to 1000 nm. The ripples produced in this way are oriented either parallel or normal to the projection of the ion beam and their wavelength scales with ion energy. These structures were found on a large variety of materials, such as semiconductors, metals, and insulating surfaces. The formation and early evolution of the ripple patterns can be qualitatively reproduced by a linear continuum equation derived by Bradley and Harper. However, at longer times nonlinear terms have to be taken into account, leading to nonlinear models based on the Kuramoto-Sivashinsky equation.
In this work we report on the simultaneous formation of two perpendicular ripple modes on amorphized Si(100) during high fluence sputtering at sub-keV energies. The evolution of both modes was studied for two different energies and over a wide range of fluence. Coarsening of both ripple modes was found. The wavelengths of the two modes are of the order of a few ten and several hundred nanometers, respectively, and show similar time dependence. To gain better understanding of the evolution of the surface morphology, the results are compared to simulations of the damped Kuramoto-Sivashinsky equation, finding good qualitative agreement.