Formation of Nanoporous Germanium Layers by Irradiation with Indium Ions

Currently, thin nanoporous Ge layers are applied in various technological devices, for example, in anode designs of lithium-ion batteries, IR-absorbing gas sensors, etc. A separate interesting application of such layers is their use as highly effective antireflection optical coatings for various photodetectors and solar cells. This study is devoted to the problem of creating an antireflection coating on a c-Ge surface using low-energy high-dose implantation of 115In+ ions in a vacuum, as opposed to the generally accepted chemical method, which leads to the accumulation of chemical reaction residues in the created nanoporous structures. The study results of the surface modification of polished monocrystal c-Ge substrate irradiated with 115In+ ions with an energy of 30 keV at a current density of 5 μA/cm2 and a wide range of high doses of 1.0 × 1014–7.2 × 1016 ion/cm2 are presented. Morphological analysis of surface topography was carried out using high-resolution scanning electron microscopy. The appearance and change in the morphology of porous layers with increasing ion dose were determined. At the lowest dose value of 1.8 × 1015 ion/cm2, a porous structure in the form of a honeycomb with nm-sized round holes is formed. When the critical dose value of 1.9 × 1016 ion/cm2 is exceeded, a spongy porous structure is formed by intertwining nanowires, geometric parameters of which do not further change with increasing dose. By measuring the optical reflection spectra of the implanted layers, it was shown that the formed material is characterized by a low reflectance in the spectral region of 220–1050 nm and can serve as an effective antireflection coating.