Crystal and Electronic Structure of β-Nb2N Thin Films Grown by Molecular Beam Epitaxy

Niobium nitrides have garnered significant research interest since their discovery due to their exceptional properties and broad applications. In this study, NbNx thin films are successfully grown on 4H(6H)-SiC(001) substrates using nitrogen-assisted molecular beam epitaxy. Through scanning transmission electron microscopy and X-ray diffraction, it is confirmed that the crystal structure of the thin films corresponds to the beta-Nb2N. Different from the previously reported structure of the beta A phase (P63/mmc) of beta-Nb2N, the results clearly match with the beta B phase (P3<overline>m1${\mathrm{P}}\bar{3}{\mathrm{m}}1$). Resistivity measurements reveal that beta-Nb2N exhibits superconductivity at approximate to 10 K with an upper critical field of approximate to 5 T. Its 3D electronic structure is further elucidated using angle-resolved photoemission spectroscopy combined with theoretical calculations. The observed superconductivity in beta-Nb2N is attributed to its relatively high electron-phonon coupling strength and density of states at Fermi level. Interestingly, it is found that the sample is close to a Lifshitz transition, suggesting potential for tunable physical properties. The results provide a comprehensive understanding of the crystal and electronic structures of beta-Nb2N, facilitating its future applications.