Scanning tunneling microscopy and spectroscopy studies of superconducting boron-doped diamond films

We report on scanning tunneling microscopy/spectroscopy (STM/STS) experiments on (1 1 1)-oriented epitaxial films of heavily boron-doped diamond grown by using the microwave plasma-assisted chemical vapor deposition method. STM/STS measurements were performed by 3He-refrigerator based STM under ultra-high vacuum. The STM topography on the film surface shows a corrugation (with a typical size of ∼1 μm) and grain-like microstructures (∼5-20 nm). The tunneling conductance spectra do not show large spatial dependence and superconductivity is observed independent of the surface structures. The tunneling spectra are analyzed by the Dynes function and the superconducting energy gap is estimated to be Δ=0.87meV at T=0.47 K, corresponding to 2Δ/kBTc=3.7. The relatively large value of the broadening parameter Γ=0.38meV is discussed in terms of the inelastic electron scattering processes.