Metal-insulator transition in boron-ion-implanted diamond

Abstract

We have observed the metal-insulator transition in single crystal, high-purity type-IIa diamond which has been implanted at $77\mathrm{K}$ with boron ions in multiple steps and annealed at high temperatures between implants. Electrical conductivity measurements made at temperatures in the range $1.5–300\mathrm{K}$ have shown that, for boron concentrations below the critical concentration $n_c$ which we estimate as $3.9\times {10}^{21}{\mathrm{cm}}^{-3}$, 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 $\sigma \left(\mathrm{T}\right)=\sigma \left(0\right)+b{T}^m$. 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 $\mu \approx \frac{1}{2}$ reported for $p$-type $\mathrm{Si}$ and other uncompensated semiconductors.