Formation of self-organized nanostructures on Ge during focused ion beam sputtering

This chapter describes the formation and self-organization of nanoscale structures during normal-incidence focused ion beam (FIB) sputtering of Ge. Ridge-shaped ripples with large height-to-width ratios form. The chapter provides evidence that the ripples form spontaneously from nanostructured networks at doses below 10¹⁷ ions.cm^-2. Highly oriented structures running in the fast scan direction evolve at higher doses. The chapter explains the ripple formation to spatio-temporal overlap of FIB induced micro-explosions. The orientation of the ripples is usually determined by the incidence angle of the ion beam relative to the surface normal, and sometimes by the surface crystallography. The Bradley-Harper mechanism is generally accepted as the origin of ripple formation in most situations. However, ripples may still form when surface crystallography introduces azimuthal anisotropy in either the roughening or smoothing mechanisms. It is of both theoretical and practical interest to understand surface morphological evolution during ion beam bombardment.