Insulator-to-Metal Transition in Sulfur-Doped Silicon

We observe an insulator-to-metal transition in crystalline silicon doped with sulfur to nonequilibrium concentrations using ion implantation followed by pulsed-laser melting and rapid resolidification. This insulator-to-metal transition is due to a dopant known to produce only deep levels at equilibrium concentrations. Temperature-dependent conductivity and Hall effect measurements for temperatures $T>1.7\mathrm{K}$ both indicate that a transition from insulating to metallic conduction occurs at a sulfur concentration between 1.8 and $4.3\times {10}^{20}{\mathrm{cm}}^{-3}$. Conduction in insulating samples is consistent with variable-range hopping with a Coulomb gap. The capacity for deep states to effect metallic conduction by delocalization is the only known route to bulk intermediate band photovoltaics in silicon.