The molecular dynamics simulation of ion-induced ripple growth.

The wavelength-dependence of ion-sputtering induced growth of repetitive nanostructures, such as ripples has been studied by molecular dynamics (MD) simulations in Si. The early stage of the ion erosion driven development of ripples has been simulated on prepatterned Si stripes with a wavy surface. The time evolution of the height function and amplitude of the sinusoidal surface profile has been followed by simulated ion-sputtering. According to Bradley-Harper (BH) theory, we expect correlation between the wavelength of ripples and the stability of them. However, we find that in the small ripple wavelength (lambda) regime BH theory fails to reproduce the results obtained by molecular dynamics. We find that at short wavelengths (lambda<35 nm) the adatom yield drops hence no surface diffusion takes place which is sufficient for ripple growth. The MD simulations predict that the growth of ripples with lambda>35 nm is stabilized in accordance with the available experimental results. According to the simulations, few hundreds of ion impacts in lambda long and few nanometers wide Si ripples are sufficient for reaching saturation in surface growth for for lambda>35 nm ripples. In another words, ripples in the long wavelength limit seems to be stable against ion-sputtering. A qualitative comparison of our simulation results with recent experimental data on nanopatterning under irradiation is attempted.