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 ³He-refrigerator-based STM under ultrahigh vacuum. The STM topography on the film surface shows microstructures with a size of ∼ 5 - 20 nm and two types of atomic structures: a hydrogenated 1×1 structure, C(111)1×1: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 Δ = 0.83 - 0.87 meV at T = 0.47 K. The obtained gap ratio 2Δ/k_BT_c = 3.57 - 3.7 is consistent with the weak-coupling BCS theory. The relatively large value of the broadening parameter Γ ∼ 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.