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 (n(B)) on the order of n(c)similar to 4.5x10(20) 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 n(B)< n(c) with a characteristic hopping temperature T-0 strongly reduced due to the proximity of the MIT. All metallic samples (i.e., for n(B)>n(c)) present a superconducting transition at low temperature. The zero-temperature conductivity sigma(0) deduced from fits to the data above the critical temperature (T-c) using a classical quantum interference formula scales as sigma(0)proportional to(n(B)/n(c)-1)(nu) with nu similar to 1. Large T-c values (>= 0.4 K) have been obtained for boron concentration down to n(B)/n(c)similar to 1.1 and T-c surprisingly mimics a (n(B)/n(c)-1)(1/2) law. Those high T-c values can be explained by a slow decrease of the electron-phonon coupling parameter lambda and a corresponding drop of the Coulomb pseudopotential mu(*) as n(B)-> n(c).