Morphology of nanoporous germanium layers formed by implantation of Cu$^+$, Ag$^+$ and Bi$^+$ ions of various energies

The formation of thin surface amorphous layers of nanoporous Ge of various morphologies due to the high-dose ion implantation of flat single-crystalline $c$-Ge substrates in the irradiation energy range 10–40 keV was investigated. The implantation was carried out with metal ions of different masses at the current density 5 $\mu$A/cm$^2$ and doses of 1.0 $\cdot$ 10$^{17}$ ($^{63}$Cu$^+$) and 5.0 $\cdot$ 10$^{16}$ ($^{108}$Ag$^+$, $^{209}$Bi$^+$) ion/cm$^2$. The morphology analysis of nanoporous structures was performed using high-resolution scanning electron microscopy. It was found that at low irradiation energies of 10–15 keV in the case of low mass ions such as $^{63}$Cu$^+$ and $^{108}$Ag$^+$ misoriented thin needle-shaped nanowires were created on the $c$-Ge surface, and in the case of $^{209}$Bi$^+$ ions a porous layer consisting of densely packed intertwined nanowires was formed. At high energies of 30–40 keV, the morphology of nanoporous Ge changed its shape with increasing mass of the implanted ion sequentially from a three-dimensional network structure to a spongy one, consisting of single spatially separated thin intertwined nanowires.