The role of surface anisotropy for ion beam pattern formation

Ion beam irradiation can produce different patterns on semiconductor surface. While the surface is amorphized at low temperatures, the surface remains crystalline above the dynamic recrystallization temperature. Reverse epitaxy leads to pattern formation, which is driven by diffusion of vacancies and adatoms on the surface, above this temperaure.

In contrast to amorphous surfaces, diffusion on terraces, the energy for step formation, the height of the Ehrlich-Schwoebel barrier, and the attachment of adatoms and vacancies are anisotropic and depend on the step orientation. These anisotropies in diffusion lead to anisotropic patterns.

We studied the patterns formation by low energy ion irradiation of different surfaces with atomic force and scanning tunneling microscopy. The patterns are aligned to the surface crystal structure and reflect the surface symmetry. We present a model for reverse epitaxy based on atomic processes on the surface, which includes the surface anisotropies and ballistic atom redistribution by the ion beam. To test the model we studied the pattern formation on GaAs(0 0 1) and GaAs(0 0 -1). On both surfaces, a regular array of faceted nanoripples forms. The direction of the ripples is rotated by 90° between both surfaces, which can be explained with our model. The symmetry of the GaAs lattice leads to rotation of 90° of the Ga rows on GaAs(0 0 -1) compared to GaAs(0 0 1).