Characterization of low frequency noise in epitaxial NbN/AlN/NbN tunnel junctions

We measured the low frequency noise characteristics and subharmonic gap structures of epitaxial NbN/AlN/NbN tunnel junctions with different current densities. For all of the junctions, the voltage noise power spectrum S_v(f) showed a frequency dependence that is well described by 1/f behavior. Subharmonic gap structures were clearly observed in the dV/dI-V curves at voltage 2Δ/ne, and the values of n increased with the current density J_c in the junctions. We estimated the 1/f noise parameter η using the empirical theory of Rogers and Buhrman for the S_v(f), and investigated the correlation between the η and the inverse junction quality 1/Q. We found that the η-1/Q relationship for our epitaxial tunnel junctions gave a different behavior for nonepitaxial tunnel junctions. The tunnel barrier properties were investigated by applying a pulse voltage to the junctions at 4.2 K, and measuring the applied voltage dependence of the I-V characteristics and low frequency noise amplitude.