Tunnel investigations of superconducting films of niobium nitride and niobium gallium

Systematic tunnel investigations are performed on films of niobium nitride with B1 structure obtained by getter, reactive cathode sputtering of a niobium target in a glow discharge in an atmosphere of nitrogen and argon. The maximum values of superconducting parameters of the films obtained are: T-C = 16.6 K (superconducting transition temperature), Delta (0)=2.84 MeV (superconducting energy gap at 0 K). The variation of adjusted gap Delta (T)/Delta (0) with adjusted temperature T/T-C was measured. In the area of voltages V beyond the gap (up to 30 MeV), a complex structure is observed for the first time in graphs of-dV/dI and d(2)I/dV(2) as a function of V (I is the current trough the tunnel junction). Films of Nb-Ga compounds are prepared with A15 structure with a gradient of composition and tunnel junctions are made from the films. It is found that under certain conditions when the composition of the film is near stoichiometric, instead of a smooth change in superconducting gap Delta and superconducting transition temperature Te along the concentration gradient, the superconducting phase is divided into areas ("domains") differing in superconducting parameters T-C and Delta. An empirical correlation is observed between Delta and T-C which in the range T-C (8.0-16 K) is near linear, not passing through 0. For a specimen with T-C approximate to 16.5 K a superconducting gap Delta = 2.79 MeV and ratio 2 Delta /kT(C) approximate to 3.92 are obtained.