Dynamics of surfactant induced ripple patterns on Si generated by ion beam erosion

Some metallic surfactants induce pronounced dot and ripple patterns on Si substrates during normal and near normal ion incidence sputter erosion. In the absence of metal co-deposition uniform flat surfaces are obtained. It is known that surface ripples generated by ion beam erosion at glancing ion incidence propagate across the surface. In this work we investigate the propagation of ripple patterns triggered by surfactant atoms and a possible correlation between ripple propagation and directed deposition of surfactant atoms. Si substrates were irradiated with 5 keV Xe ions at normal ion incidence under continuous deposition of Fe or Mo surfactant atoms. With surfactants pronounced patterns like dots, combinations of dots and ripples as well as ripples with about 100 nm wavelength and up to 12 nm in height are generated. The dynamics of patterns, in particular the propagation of ripples across the surface with increasing ion fluence was investigated using sequential ion irradiation and scanning electron microscopy analysis. Specific regions on the irradiated samples were identified using thin marker grooves prepared by focused ion beam milling. Using pattern recognition methods we are able to determine the fluence dependent shift of the ripple patterns with respect to the marker grooves. We find dynamic patterns with a propagation velocity of about 0.7 nm per 1015 ions/cm2. A qualitative model for the ripple propagation is presented.