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~4.5×10²⁰0.3em0excm^-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 (0.40.3em0exK) 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.