Abstract
NbN films with B1 structure have been prepared on sapphire and quartz substrates by rf sputtering, and investigated on their upper critical field Hc2 and its anisotropy. The superconducting transition temperature Tc of these films with thicknesses of 1500-6000 Å ranges from ∼12 to ∼16 K. Films deposited on quartz substrates have fine grains of B1 phase with the (111) plane parallel to the film surface and show higher values of Hc2. Films deposited on sapphire substrates contain an initial growth layer ∼1500 Å thick on which coarse columnar grains are formed. Both kinds of films show similar relationships between Hc2 and normal-state resistivity ρn, Hc2(4.2K) increases from ∼15 T with ρn in the range below ∼500 µΩ·cm and tends to saturate at above 500 µΩ·cm. The maximum Hc2 is obtained for a film with a ρn higher than 500 µΩ·cm and reaches ∼30 T. The Hc2 and its anisotropy are appreciably affected by the film microstructure and grain size.
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