Influence of nitrogen impurities on the electronic structure of niobium

We present x-ray photoemission results for a series of NbNx films with 0.0 x 0.2. Core and valence-band spectra show that charge is transferred from the pure metallic band to hybrid N-Nb states centered 6 eV below the Fermi edge. The x dependencies of the Fermi-level density of states nexpt(EF) and core-level-asymmetry parameters are compared to those of the superconducting transition temperature Tc and lattice constants obtained for similar samples by other workers. In particular, Tc decreases with nexpt(EF) for x<xc 0.1 after which both Tc and nexpt(EF) remain constant within experimental uncertainty. We suggest that for x>xc the metal valence electrons are not able to screen additional N impurities. The behavior of the Nb core-level widths, which measures static valence charge-density fluctuations, supports this conclusion. © 1981 The American Physical Society.