Compositionally modulated ripples during composite film growth: Three-dimensional pattern formation at the nanoscale

Abstract

Three-dimensional, ion-induced nanoscale pattern formation in the growth mode is studied for a bicomponent thin film. C:Ni films were grown by dual ion beam cosputtering applying an assisting oblique-incidence low-energy ${\text{Ar}}^{+}$ ion beam. Their microstructure was determined by scanning electron, atomic force, and transmission electron microscopy, as well as by grazing-incidence small-angle x-ray scattering. The role of ion-induced collisional effects was investigated by binary collision computer simulations. The formation of compositionally modulated ripples on the C:Ni film surface is demonstrated. They consist of metal-enriched topographic crests and carbon-enriched valleys. Since the surface is constantly covered by incoming species, this pattern is transferred into the bulk as a periodic array of ${\text{Ni}}_{3}\text{C}$ nanoparticles or of Ni-enriched regions in a carbon matrix. Lateral ripple propagation is shown to be one of the crucial phenomena for the film morphology. The essential experimental features are reproduced by the computer simulations. The results reveal the importance of ion-induced preferential displacements as the driving factor for a surface instability, which gives rise to the observed pattern formation.