Ion beam flux dependence of nanoporous structure formation induced by FIB in germanium

Abstract

Nanostructures fabricated on germanium substrate through ion beam irradiation are porous and formed by the self-organization of point defects. This study investigates the beam flux dependence of the shape and size of nanostructures fabricated by focused ion beam (FIB) with fluxes of 1.3 × 1018–8.9 × 1020 ions/m2s, which correspond to fluences of 5 × 1019–1 × 1021 ions/m2. The ions accelerated for irradiation were 30 keV-Ga+. The scanning and transmission electron microscope images show that the morphology or size of the nanostructures depends on the fluence and the flux. Abundant porosity appears on the surface at a fluence of 5 × 1019 or 1 × 1020 ions/m2. By contrast, the structure sizes decrease with the increasing flux at a fluence of 5 × 1019 ions/m2, but remain approximately constant at a fluence of 1 × 1020 ions/m2. At a fluence of 1 × 1021 ions/m2, the nanostructure porosity is covered with elongated sidewalls. Moreover, the sidewall morphology depends on the flux. At a higher flux, the sidewalls become elongated straight toward a surface forming nanostructures taller than those at a lower flux. In conclusion, the beam flux probably affects the sidewall morphology.