Correlation between the 1/f noise parameter and the Andreev conductance in epitaxial NbN/AlN/NbN junctions

Low-frequency noise properties of epitaxial NbN/AlN/NbN tunnel junctions have been investigated as a function of the bias voltage. The subharmonic gap structures (SGSs) were clearly observed in the differential resistance dV/dI-V curves. The SGSs were explained with Octavio, Tinkham, Blonder, Klapwijk (OTBK) theory based on the multiple-Andreev reflection (MAR) phenomena. To estimate the amplitude of the voltage noise power spectra S_V(f) in the range 1-10⁵ Hz, we extended Rogers and Buhrman empirical theory. The 1/f noise parameter η in this theory is given as η=S_V(f)I_b²R_d²/fA, where A is the junction area, I_b is the bias current and R_d is the dynamic resistance. The η values (η_sg) biased at the sub-gap voltage were larger than that (η_ag) of the above-gap voltage. In addition, the η_sg-V curves in the sub-gap region have dip and peak structures related to the SGSs in the dI/dV-V curves. In contrast, for the above-gap bias voltage, the η_ag values did not depend on the bias voltage. We concluded from these results that the excess 1/f noise in the sub-gap bias voltage was caused by the fluctuation of the MAR conductance.