Electronic states and physical properties of hexagonal β-Nb2N and δ′-NbN nitrides

The electronic structure of hexagonal β-Nb2N and δ′-NbN, and cubic fcc δ-NbN, grown by magnetron sputtering, have been investigated by X-ray photoemission spectroscopy and ellipsometric measurements. The valence band (VB) energy distribution curves (EDC) of these nitrides significantly differ each from each other. The hybridized N2p–Nb4d bands of β-Nb2N originate a featureless peak centered at 6 eV below the Fermi level, those of the δ′-NbN are characterized by two narrow peaks centered at 5 eV and 6.5 eV. Striking changes are also observed in the EDCs near the Fermi level, these features are associated with the nearly metallic Nb4d states. The dielectrical function of these nitrides reveals structures near the screened plasma edge which correlate well with their associated electronic structure. The dielectric function spectra can be used in the phase identification of the hexagonal and fcc phases. Comparing hexagonal and fcc electronic structures, both β-Nb2N and δ′-NbN are more covalent that the cubic δ-NbN. The prominent covalent bonding in these hexagonal nitrides can be related to their higher hardness values compared to that of the cubic phase.