Scanning tunneling microscopy/spectroscopy on superconducting diamond films

We report on scanning tunneling microscopy/spectroscopy (STM/STS) experiments on (111)-oriented epitaxial films of heavily boron-doped diamond grown by microwave plasma-assisted chemical vapor deposition. STM/STS measurements were performed by He-3-refrigerator-based STM under ultrahigh vacuum. The STM topography on the film surface shows microstructures with a size of similar to 5-20 nmn and two types of atomic structures: a hydrogenated 1 x 1 structure, C(1 11) 1 x 1:H, and an amorphous structure. The tunneling spectra are analyzed by a modified Bardeen, Cooper, and Schrieffer (BCS) expression, and the superconducting energy gap is estimated to be A = 0.83 0.87 meV at T-c = 0.47 K. The obtained gap ratio 2 Delta/k(B)T(c) = 3.57-3.7 is consistent with the weak-coupling BCS theory. The relatively large value of the broadening parameter Gamma similar to 0.38 meV is discussed in terms of the inelastic electron- scattering processes. In the low-temperature region (T = 0.47 K), the tunneling conductance spectra do not show strong spatial dependence, and superconductivity is observed independent of the surface structures. In the high-temperature region (T = 4.2 K), on the other hand, the tunneling conductance spectra show significant spatial dependence, indicating the inhomogeneous distribution of the superconducting property due to the distribution of boron atoms.