Superconducting and structure properties of niobium nitride prepared by rf magnetron sputtering

The properties of rf-magnetron-sputtered NbN have been investigated for substrate temperatures from 200°C to 750°C as a function of the partial pressure of methane introduced into the Ar–N2 sputtering gas mixture. The best films were prepared at elevated substrate temperatures with approximately 2% methane added to the sputter gas. These films also had resistivities approximately 70 μΩ cm. At lower substrate temperature, T∼200°C, the best attained Tc was approximately 13.3 K with a resistivity of 140 μΩ cm. The crystal structure of these materials was investigated using x-ray diffraction and Read x-ray photographs. Those films prepared with no added methane were found to be two-phase δ and ε NbN. The addition of a small amount of methane produced single δ phase NbN with a predominant (200) preferrential orientation. The δ phase lattice parameter was determined to vary approximately from 4.39 Å to 4.45 Å as the amount of methane was increased. The effect of hydrogen doping on NbN was to produce multiphase NbN with the ε, δ phases and some evidence of the β phase. These material properties are of importance for the design and fabrication of NbN electrode Josephson tunnel junctions.