Phase diagram of boron-doped diamond revisited by thickness-dependent transport studies

We report on a detailed study of the electronic properties of a series of boron-doped diamond epilayers with dopant concentration ranging from 1×1020 to 3×1021cm-3 and thicknesses (d) ranging from 2μm to 8 nm. By using well-defined mesa patterns that minimize the parasitic currents induced by doping inhomogeneities, we have been able to unveil a new phase diagram differing from all previous reports. We first show that the boron concentration corresponding to the onset of superconductivity (above 50 mK) does not coincide with that of the metal-insulator transition; the latter one corresponding to the vanishing of the residual conductivity σ0 (deduced from σ(T)=σ(0)+AT fits to the low temperature data). Moreover, a dimensional crossover from 3D to 2D transport properties could be induced by reducing d in both (metallic) nonsuperconducting and superconducting epilayers but without any reduction of Tc in the latter.