Weak superconductivity in the surface layer of a bulk single-crystal boron-doped diamond

We have grown and investigated bulk single-crystal heavily boron-doped diamonds possessing superconductivity with $T_{C}^{\mathrm{onset}} =1.7\text{--}3.5\ \text{K}$ . Only the surface layer with the thickness less than $1\ \mu \text{m}$ showed the degenerate semiconductor behavior with transition to the superconducting state, while the bulk of the crystal was a typical doped semiconductor. The morphology of the surface layer is dendritic polycrystalline with an average boron content of 2.5–2.9 at.%. The typical Josephson junction current-voltage characteristic was observed. The degenerate semiconductor-superconductor transition as in single-crystal high-temperature superconductors and the structural data analysis of the surface layer indicate the two-dimensional character of superconductivity, and the actual superconducting structure is a set of few-nanometer thick boron carbide layers embedded in a diamond structure