Linear vs. Non-linear Behaviour in Ion Irradiation Nanostructuring of Nickel and Silicon Surfaces

Spontaneous nanometre-scale quasi-periodic ripple-like structures are formed at the surface of polycrystalline Ni films and Si(111) single crystal wafers by irradiation with a broad Ar+ ion beam at room temperature and studied with Atomic Force Microscopy as a function of fluence. The development of these structures can be reproduced by numerical solution of a continuum equation describing the evolution of surface morphology under ion irradiation, using realistic coefficients derived from material properties. In particular, we demonstrate that differences observed in pattern formation on the two surfaces under the conditions studied, such as wavelength stability and exponential growth of interface width for the Ni surfaces compared with wavelength coarsening and interface width saturation on Si(111), can be understood in terms of a cross-over between linear and non-linear behaviours.