Amorphous solid foam structures on germanium by heavy ion irradiation

Ge (100) surfaces were irradiated by heavy Bi⁺ and Bi⁺⁺ ions extracted from a Bi-liquid metal ion source in a mass separated focused ion beam system with energy of 30 and 60 keV, respectively. Networks of different nanoporous (or sponge like) structures were found depending on ion energy, fluence, angle of incidence and irradiation temperature. The porous and amorphous surface structures are explained in terms of high concentration vacancies close to the surface. The surface modification was investigated using SEM and AFM imaging and FIB for cross section preparation.