  Nanostructured surfaces are known to provide excellent optical properties for
various photonics devices. Fabrication of such nanoscale structures to
germanium (Ge) surfaces by metal assisted chemical etching (MACE) is, however,
challenging as Ge surface is highly reactive resulting often in micron-level
rather than nanoscale structures. Here we show that by properly controlling the
process, it is possible to confine the chemical reaction only to the vicinity
of the metal nanoparticles and obtain nanostructures also in Ge. Furthermore,
it is shown that controlling the density of the nanoparticles, concentration of
oxidizing and dissolving agents as well as the etching time plays a crucial
role in successful nanostructure formation. We also discuss the impact of high
mobility of charge carriers on the chemical reactions taking place on Ge
surfaces. As a result we propose a simple one-step MACE process that results in
nanoscale structures with less than 10% surface reflectance in the wavelength
region between 400 nm and 1600 nm. The method consumes only a small amount of
Ge and is thus industrially viable and also applicable to thin Ge layers.
