Amorphous Ge under pressure

Pressure effects in sputtered a-Ge and a-Ge:H films have been investigated at room temperature up to 100 kbar. X-ray-diffraction patterns, the bulk compression characteristics, and the optical-absorption spectra for a-Ge show the amorphous-to-crystalline transition at around 60 kbar. The phase transition can be accounted for thermodynamically using a free-energy diagram. The optical-absorption edge in the amorphous films shifts to higher energies with a rate of 1 meV/kbar, which is smaller than the coefficient in the crystalline phase. The origin of the difference can be ascribed to distinct structural modifications in ordered and disordered tetrahedral networks under hydrostatic compression.