Holes in the valence band of superconducting boron-doped diamond film studied by soft x-ray absorption and emission spectroscopy

Carbon- and boron-2p states of superconducting and non-superconducting boron-doped diamond (BDD) samples are measured using soft X-ray emission and absorption spectroscopy (XES and XAS) near C- and B-K edges. The electronic structure of B 2p does not show a marked boron-doping dependence, except that a considerable amount of in-gap state in empty states is observed. In C-K XAS spectra, two peaks H and I are observed around the Fermi level. The H peak is attributed to 2p state of carbon first-nearest neighboring to the dopant boron atoms (1NN-C), and the I peak to 2p state of carbon further than 1NN-C from an impurity boron. Incoherent (normal-excitation) XES spectra do not show a large chemical shift by B-doping, but its intensity just below the valence band maximum (VBM) decreases with B-doping. It cannot be interpreted within a simple rigid band model. An elastic peak of I-excited C-K XES spectrum and the width of I-peak of C-K XAS spectrum suggest that impurity state is localized in non-superconducting BDD but is not in superconducting BDD. Namely impurity state of superconducting BDD is merged with the valence band, and holes in the merged state play an important role in the occurrence of superconductivity of BDD.