Periodic Nanoscale Patterns Induced by Ion Irradiation: Ripples, Dots, and Holes

The morphology of surfaces after irradiation with low en- ergy ions (E < 50 keV) exhibits a variety of character- istics depending on the ion beam parameters and the ma- terial properties. Surfaces exposed to the ion beam can turn atomically smooth, stochastically or self-affine rough, or can evolve towards regular self-organised patterns. The structure size of these patterns is in the range of 10 to 100 nm and occasionally a high degree of ordering is achieved. Therefore, they have attracted interest recently as templates for nanostructured thin films or for structuring films by an erosive process [1].
On materials which turn amorphous during ion irradiation the formation of periodic patterns relies on at least two inter- playing processes: surface roughening due to local variation of erosion rate and smoothing via diffusional processes. In addition, atomic relocations on the surface and in the bulk resulting from the collision cascade have been identified as equally important. Therefore, the surface morphology de- pends on the details of the energy deposition by the incom- ing ion beam and on the details of surface and bulk diffu- sion. At the atomic level sputtering, the creation of surface and bulk defects, and the influence of the ion beam on sur- face diffusion processes play a decisive role for the mor- phology evolution.
surface normal and at room temperature produces ripple patterns oriented perpendicular to the ion beam direction. Higher incidence angles can lead to ripple patterns oriented parallel to the ion beam direction. Normally, coarsening of the ripple pattern with ion fluence is observed. In addition, the order increases with fluence up to 1 × 1018 cm−2 .
At normal incidence or for incidence angles smaller than 50◦ smoothing dominates on elemental materials. However, additional surface instabilities can exist due to the presence of a second atomic species on the surface. Hexagonally or- dered dot or hole patterns are thus observed at normal ion incidence on compound materials, like III-V semiconduc- tors, or on Si and Ge surfaces with concurrent deposition or implantation of foreign atoms [2, 3].