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 770.3em0exK with boron ions in multiple steps and annealed at high temperatures between implants. Electrical conductivity measurements made at temperatures in the range 1.5--3000.3em0exK have shown that, for boron concentrations below the critical concentration nc which we estimate as 3.9×10²¹0.3em0excm^-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 σ(T)=σ(0)+bTᵐ. The conductivity critical exponent μ 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 μ value found for this wide bandgap, uncompensated p-type system contrasts with the value μ≈1/2 reported for p-type Si and other uncompensated semiconductors.