Metal-insulator transition and superconductivity in boron-doped diamond

We report on a detailed analysis of the transport properties and superconducting critical temperatures of boron-doped diamond films grown along the {100} direction. The system presents a metal-insulator transition (MIT) for a boron concentration (nB) on the order of nc ∼4.5× 1020 cm-3, in excellent agreement with numerical calculations. The temperature dependence of the conductivity and Hall effect can be well described by variable range hopping for nB < nc with a characteristic hopping temperature T0 strongly reduced due to the proximity of the MIT. All metallic samples (i.e., for nB > nc) present a superconducting transition at low temperature. The zero-temperature conductivity σ0 deduced from fits to the data above the critical temperature (Tc) using a classical quantum interference formula scales as σ0 (nB nc -1)ν with ν∼1. Large Tc values (≥0.4 K) have been obtained for boron concentration down to nB nc ∼1.1 and Tc surprisingly mimics a (nB nc -1)1 2 law. Those high Tc values can be explained by a slow decrease of the electron-phonon coupling parameter λ and a corresponding drop of the Coulomb pseudopotential μ* as nB → nc.