Severe plastic deformation of semiconductor materials using high-pressure torsion

Severe plastic deformation (SPD) has been widely studied in order to enhance the strength and ductility of metallic materials. Among various SPD processing techniques, high-pressure torsion (HPT) can be applied to various brittle materials including semiconductors. In this overview, we report on the HPT processing of Si, Ge, and compound semiconductor GaAs. When crystalline Si was subjected to HPT, metastable body-centered-cubic (bcc) Si-III and rhombohedral Si-XII as well as amorphous regions were formed. After annealing, Si-III and Si-XII reversely transformed to diamond-cubic Si-I. No appreciable photoluminescence (PL) peak was observed from the as-HPT processed samples while a broad PL peak originating from Si-I nanograins appeared after annealing. The electrical resistivity was increased just after compression without anvil rotation, but it decreased after HPT-processing because of the formation of semimetallic Si-III. In the case of Ge, metastable tetragonal Ge-III was formed by room-temperature HPT processing. A broad PL peak originating from diamond-cubic Ge-I nanograins was observed after annealing. The metastable bcc Ge-IV was observed in the cryogenic-HPT-processed samples. In the case of GaAs, no metastable phase was observed in the HPT-processed samples. A strong PL peak associated with the bandgap disappeared after HPT processing. An additional PL peak in the visible light region appeared after annealing. These results suggested that noble properties such as optical and electrical properties can be obtained by applying HPT processing to semiconductor materials.