Self-organized Nanoscale Patterning under Ion Beam Irradiation.

Abstract

Energetic ion bombardment can lead to the development of complex and diverse nanostructures on or near the material surface. One of interests in these structures is the formation of highly ordered patterns whose optical, electronic and magnetic properties are different from bulk materials and might have important technological applications. In this work, the ordered pattern formation in a broad range of materials was fabricated and investigated, including one-dimension periodic arrays of nanoparticles and ripples, two-dimensional patterns of highly ordered Ga droplet on the surface of GaAs, ordered nanocrystals on argon ion sputtered polymer film, hexagonally ordered nanoholes on the surface of Ge, and three-dimensional void/bubble formation inside materials. In addition, angular and curvature dependence of sputtering yield was also discussed. Special emphasis was placed on the fundamental understanding of ordered pattern formation under irradiation. Sputtering, redeposition, viscous flow, and surface diffusion which are responsible for ordered pattern formation are investigated through a combination of modeling and in situ and ex-situ observations. It was proposed that a common feature of mechanism responsible for pattern formation during ion irradiation is the anisotropic movement of species. This anisotropy movement can take place either on the surface of materials driven by the off-normal angle ion bombardment, for example ripple and ordered droplet formation, or inside materials driven by crystalline structure, such as void/bubble lattice formation. Other mechanisms, such as redeposition, viscous flow and surface diffusion can enhance or weaken the tendency of the pattern formation and give rise to the change of initial patterns with a long time limit, such as ordered hole formation on Ge surface. It was suggested that dynamic balance between anisotropic mass loss and gain can lead to the formation of highly ordered Ga droplet on the ion irradiated GaAs surface. With computer modeling, it was found that nonlinear effects for long time limit determine the hexagonally ordered, honeycomb-like structure of nano-scale holes induced by the ion beam bombardment on the Ge surface.