Change in the surface level of germanium with the formation of a thin porous layer during irradiation with bismuth ions

In this work, the nature of changes in the surface level of a single-crystal $c$-Ge substrate irradiated with $^{209}$Bi$^{++}$ ions with an energy of $E$ = 36 keV during the formation of thin surface layers of nanoporous Ge with increasing ion dose was analyzed. Dose values varied from 2.0 $\cdot$ 10$^{14}$ to 4.0 $\cdot$ 10$^{16}$ ion/cm$^2$. Observation of the surface morphology of the samples was carried out using high-resolution scanning electron and probe microscopy. It was found that at low doses of up to 1.0 $\cdot$ 10$^{15}$ ion/cm$^2$, ion sputtering of the sample surface occurs, forming a layer consisting of open surface pores in the form of pits. With increasing dose, this layer is replaced by swelling of the surface with the formation of a porous spongy nanowires structure.