Dual Ion-Beam Deposition of Superconducting NbN Films
Superconducting NbN films have been deposited onto unheated substrates using a dual ion-beam technique. The NbN films produced have Tc up to 12 K and resistivities of ≳150µΩ-cm. The substrate temperature does not exceed 100 °C. TEM analysis of these films shows a random in-plane orientation of <100 Å-size grains. Electron diffraction indicates fine-grain polycrystalline material, and a Read x-ray camera verifies this to be the high-Tc δ-phase with the B1 crystal structure. Using a native oxide barrier on these films and a Pb.71Bi.29 counterelectrode, tunnel junctions are produced with current densities of 30 A/cm2 and Vm of 50 mV at 4.2 K. Artificial barriers have been successfully produced, using oxidized Al or Ta overlayers or AlN. The quality of the resulting I-V curves is comparable to that of junctions with native oxide barriers. The junction resistance with the artificial barriers is higher, however, and the conductance at large voltages reflects the different properties of the artificial barriers.
KeywordsTunnel Junction Tunnel Barrier Current Rise Artificial Barrier Cryogenic Engineer
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