No abstract

The conductivity has been measured between 55 mK and 4.2 K in zero field and in magnetic fields up to 7.5 T of a series of uncompensated p-type Si:B samples with dopant concentrations near the critical concentration for the metal-insulator transition. Acceptor wave functions, which are derived in silicon from the degenerate lightand heavy-hole J =valence-band maxima at k =0 and a spin-orbit-split J = 2 band, are quite different from donor wave functions associated with the six degenerate conduction-band minima at different equivalent points in the Brillouin zone. Despite this, the conductivity of Si:8 is found to be quite similar in many ways to that of Si:P. The critical conductivity exponent for Si:8 is close to 2 as in Si:P and Si:As, rather than having the expected value of 1. The correction to the zero-temperature conductivity arising from electron-electron interactions is comparable in size, and the temperature dependence of the conductivity in various fixed magnetic fields is also found to be quite similar. For the range of dopant concentrations and experimental parameters of these investigations, the only important experimental difference between the two materials is the sign and size of the magnetoresistance. In contrast with Si:P, which has both positive and negative components, the magnetoresistance of Si:8 is positive for all temperatures and magnetic fields studied. We attribute this to the strong spin-orbit scattering in p-type silicon associated with the degenerate valence bands.

We have determined the critical exponent which characterizes the approach of the zero-temperature conductivity to the insulating phase from measurements down to 60 mK of the resistivity of a series of just-metallic uncompensated p-type Si:B samples with dopant concentrations near the critical concentration for the metal-insulator transition. Our results indicate a critical exponent for Si:B of 0.65 -8:?i which is close to the "anomalous" values near y found for the uncompensated «-type silicon-based semiconductors Si:P, Si:As, and Si:Sb. This implies that, despite strong spin-orbit scattering, Si:B belongs to the same universality class as other silicon-based systems.