Valence band electronic structures of heavily boron-doped superconducting diamond studied by synchrotron photoemission spectroscopy

The valence band electronic structures of heavily boron-doped superconducting diamond films made by microwave plasma-assisted chemical vapor deposition (MPCVD) were investigated by hard X-ray photoemission spectroscopy (HXPES) and soft X-ray angle-resolved photoemission spectroscopy (SXARPES). The HXPES core-level spectrum of heavily boron-doped diamond shows a new feature at the lower binding energy side of the C 1s main peak. The HXPES valence band spectrum of a heavily boron-doped superconducting diamond film shows a broader spectral shape than that of a lightly doped non-superconducting sample. The SXARPES results of homoepitaxial CVD films show clear valence band dispersions with a bandwidth of ∼23 eV and the top of the valence band at the Γ point in the Brillouin zone, which are well explained by the calculated valence band dispersions of pure diamond. Boron concentration-dependent band dispersions near the Fermi level (E_F) by SXARPES exhibit a systematic shift in E_F, indicating electron depopulation due to hole doping, and an increase in the line shape corresponding to the broader density of states observed by HXPES. These results indicate that holes in the top of the valence band are responsible for the metallic states leading to superconductivity at low temperatures. The HXPES C 1s core-level spectra of lightly boron-doped non-superconducting and heavily boron-doped superconducting films are also shown.