Metallization of silicon in a shock wave: the metallization threshold and ultrahigh defect densities

Time-resolved electrical conductivity measurements on monocrystalline doped silicon are performed under shock compression up to 23 GPa followed by release. With increasing normal stress, the electrical conductivity of silicon increases monotonically by five orders of magnitude and reaches that of 'poor' metals. The stress dependence of the conductivity comprises two parts: a steep rise and a 'plateau'. The 'plateau' conductivity corresponds to the metallic state of silicon; it does not depend on the compression regime or the doping type or amount of impurity. The onset of the metallic phase corresponds to a shock stress of about 10 GPa; most of the specimen is metallic at 12 GPa. The state of shock-compressed silicon proves to be extremely defective. The defect concentration in shocked silicon exceeds the equilibrium concentration by five orders of magnitude and exceeds the defect concentration in classic metals by an order of magnitude. This indicates distinctive features of brittle solid deformation. Experimentally, the metallic phase proves to be metastable. Releasing stress causes a temporary delay of the reverse transition.