Density functional theory studies of Nb_xN and interfaces with AlN and GaN

Nb_xN is a superconductor which exists in a variety of crystal phases depending on stoichiometry. Recently heterostructures of Nb_xN with GaN and AlN have been proposed for creating Josephson Junctions for realizing superconducting qubits. We begin by studying the possible bulk structure of the 𝛽 phase (which has stoichiometry Nb₂N) via introducing vacancies in NbN. We find that the N vacancies prefer to be ordered in the P-31m structure, without any propensity for disorder. Then we calculate the surface energies and work functions of 𝛽-Nb₂N and 𝛿-NbN surfaces. We see that relaxation of surface atoms causes considerable changes to the surface energies and work functions. The latter decreases as the concentration of N close to the surface decreases, while overall Nb terminated surfaces have lower work functions. Finally, we calculate the Schottky barrier height of 𝛽-Nb₂N/𝛿-NbN - AlN/GaN interfaces, showing that the barrier potentials are positive for all interfaces. However, there is a large difference between the barrier heights for different AlN/GaN polarities.