The thermal crystallization of heavily hydrogenated, amorphous Ge (a-Ge:H) and ultra-pure amorphous Ge (a-
Ge) thin films was investigated, on a comparative basis, by X-ray diffraction and high-resolution transmission
electron microscopy (HRTEM) performed during in situ heating. It was found that the crystallization process in
a-Ge:H is pronouncedly retarded, due to the initial presence of hydrogen-induced nanovoids. The crystallization
of a-Ge:H initiates only after considerable reduction of the amount of nanovoids in the amorphousmatrix by thermally
induced coarsening of the nanovoids leading to nanopores in the a-Ge:H. Such retarded crystallization leads
to the formation of nanocrystalline, porous Ge with nanocrystal sizes below 20 nm, in contrast with the fast formation
of very large-grained (grain sizes of hundreds of nanometer) polycrystalline Ge upon thermal crystallization
of pure a-Ge.