Early stages in the self-organization of Si nanopatterns induced by ion bombardment

We reveal early stages of self-organization of nanopatterns created by 2 keV Cs⁺ ion-beam irradiation of a Si surface coated with Au and a Ti adhesion layer. After ion-beam etching of the metallic layers, at normal incidence, we first observe distinct transient stages: (I) a dewetting-like pattern of grooves in the Si amorphized layer, sparsely populated with holes, followed by (II) the coexistence of rounded mounds and faceted holes distributed on a flat surface, the latter being an indication of the decisive role played by the crystalline/amorphous interface. Subsequently, the system evolves to stage III, a nanopattern of densely packed nanodots convoluted with a long-wavelength surface corrugation. A momentum-space analysis shows that stages (I) and (II) are identified, respectively, with channel-type and sphere-type quasi order.