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Nanoporous structure formations on germanium surfaces by focused ion beam irradiations

The formation of porous structures of nanometre size (nanoporous structures) on germanium (Ge) surfaces by focused ion beam (FIB) irradiations was investigated using various FIB conditions such as ion species, irradiation energies, total fluences, fluence rates, and incident angles. FIB-irradiated regions were observed using a scanning electron microscope and an atomic force microscope. It is found that, using a focused Ga ion beam (Ga FIB) at an energy of 100 keV, the irradiated Ge surface swelled up to ion fluence of 2 × 1017 cm−2 with nanoporous structures and then was etched for larger fluences. The shape of swollen nanoporous structures depended on the fluence rate and the incident angle of the Ga FIB. However, such porous structures were observed neither for low-energy (15–30 keV) FIB irradiations using Si and Au ions nor for high-energy (200 keV), heavy ion (Au) irradiation. These observations might be helpful in discussing the formation mechanisms of the nanoporous structures on Ge surfaces by ion beam irradiations. Fabrication of patterned structures at selected regions on the Ge surface was demonstrated without using any masks.