High-resolution transmission electron microscopy analysis of nanograined germanium produced by high-pressure torsion

The nanostructure of bulk nanograined germanium (Ge) processed by high-pressure torsion (HPT) has been analyzed by high-resolution electron microscopy. Crystalline Ge disks were subjected to HPT under the nominal pressure of 24 GPa. The samples processed at room temperature consisted of diamond-cubic Ge-I and simple-tetragonal Ge-III nanograins in addition to amorphous regions. The samples contained lattice defects such as dislocations, nanotwins, and stacking faults. Subsequent annealing at 573 K led to the phase transformation from Ge-III to Ge-I, but residual Ge-III nanograins and lattice defects remained due to the low annealing temperature. It was found that Ge-I as well as residual Ge-III nanograins and some amorphous phase were present after processing by HPT at cryogenic temperature. No other metastable phases such as body-centered-cubic Ge-IV or hexagonal-diamond Ge-V were observed in the cryogenic HPT-processed sample.