Hexagonal germanium formed via a pressure-induced phase transformation of amorphous germanium under controlled nanoindentation

We have studied the stable end phase formed in amorphous germanium (a-Ge) films that have been subjected to a pressure-induced phase transformation under indentation loading using a large (20 μm) spherical indenter. After indentation the samples have been annealed at room temperature to remove any residual unstable R8 and BC8 phases. Raman spectroscopy indicates a single broad peak centred around 292 cm^-1 and we have used first principles density functional perturbation theory calculations and simulated Raman spectra for nano-crystalline diamond cubic germanium (DC-Ge) to help identification of the final phase as hexagonal diamond germanium (HEX-Ge). Transmission electron microscopy and selected area diffraction analysis confirmed the presence of a dominant HEX-Ge end phase. These results help explain significant inconsistencies in the literature relating to indentation-induced phase transitions in DC- and a-Ge.