Changes in crystal structure and resistivity of deformed germanium by high-pressure torsion

Single-crystal Ge(100) disks were deformed by high-pressure torsion (HPT). The sample after HPT processed at a nominal pressure of 6 GPa without anvil rotation (N = 0) consisted of diamond-cubic Ge-I, while metastable Ge-III appeared after rotating the anvils and following pressure release. The volume fraction of Ge-III increased as the number of anvil rotations (N) increased. The crystallite sizes of Ge-I and Ge-III decreased to ∼10 nm for N ≥ 50. The Ge-III phase disappeared and only Ge-I existed after annealing at 573 K. The resistivity of Ge samples decreased from 64 Ω cm to 3.5 Ω cm after compression (N = 0). The resistivity increased to ∼10⁷ Ω cm when increasing N to 100. The decrease in resistivity for N = 0 indicated the introduction of carriers generated by lattice defects. The increase in resistivity with increasing N was due to the refinement of grains as well as to the formation of semiconducting Ge-III. No significant change in resistivity was observed after annealing. These resistivity changes observed in HPT-processed Ge were found to be different from those observed in HPT-processed Si.