This paper reports about the mechanism of porous formation in bulk crystalline Ge wafers, poly-crystalline and amorphous Ge thin films as a consequence of Ge+ ion implantation at 300 keV, Ge+ fluence 5×1015 cm−2 or 1×1016 cm−2. Cross-section scanning electron microscopy and Rutherford backscattering spectrometry were used to characterize the nucleation mechanism of the nanoporous structure by revealing a uniform mechanism in the amorphous Ge thin layers, and a heterogeneous growth mechanism in bulk crystalline and poly-crystalline Ge substrates. The uniform growth is due to the presence of voids distributed over all the as-deposited amorphous films, which provide nucleation sites for the formation of the porous structure. Instead, the heterogeneous growth is catalyzed by the free surface and the film/substrate interface.