Metal-insulator transition in boron-ion-implanted diamond

We have observed the metal-insulator transition in single crystal, high-purity type-IIa diamond which has been implanted at 77 K with boron ions in multiple steps and annealed at high temperatures between implants. Electrical conductivity measurements made at temperatures in the range 1.5-300 K have shown that, for boron concentrations below the critical concentration n(c) which we estimate as 3.9x10(21) cm(-3), Efros-Shklovskii hopping conduction occurs at sufficiently low temperatures. At the highest concentrations, just-metallic behavior is found, with the low-temperature conductivity governed by the relation sigma(T)=sigma(0)+bT(m). The conductivity critical exponent mu is estimated to be 1.7, with a fairly large uncertainty because of the limited number of concentrations on the metallic side of the transition. The high mu value found for this wide bandgap, uncompensated p-type system contrasts with the value muapproximate to1/2 reported for p-type Si and other uncompensated semiconductors.