Role of grain size in superconducting boron-doped nanocrystalline diamond thin films grown by CVD

The grain size dependence of the superconducting transition, the normal state resistivity, and the insulating behavior at high magnetic fields are studied on a series of boron-doped nanocrystalline diamond (B:NCD) thin films with different grain sizes. The systematic change of the grain size is achieved by varying the methane-to-hydrogen ratio (C/H ratio) for the growth of different B:NCD films. Even though a fixed trimethylboron- (TMB) to-methane gas ratio is supposed to induce the identical boron-doping level in all the B:NCD films, the boron concentration and the carrier density are found to be a decreasing function of the grain size. Another consequence of the increase in grain size is the decreasing grain boundary density. These two concurrent consequences of the chemical vapor deposition mode of B:NCD are responsible for the grain size dependence of the critical temperature T _C, the localization radius a _H at the boron site, the normal state resistivity ρ _norm, the Hall mobility μ _H, the Ioffe-Regel product k _Fl, the H _C2-T phase boundary, and the coherence length ξ _GL.