Highly retarded crystallization in hydrogenated amorphous germanium; emergence of a porous nanocrystalline structure

• In situ HRTEM unveils the crystallization mechanism of highly hydrogenated a-Ge.
• Diffusion of H induces coarsening of nanovoids to nanopores before crystallization.
• Nanocrystallized porous Ge results after full crystallization of hydrogenated a-Ge.
• Conversely, crystallization of pure a-Ge produces large grained polycrystalline Ge.

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 amorphous matrix 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.