Abstract
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/sub V/(f) in the range 1-10/sup 5/ Hz, we extended Rogers and Buhrman empirical theory. The 1/f noise parameter /spl eta/ in this theory is given as /spl eta/=S/sub V/(f)I/sub b//sup 2/R/sub d//sup 2//fA, where A is the junction area, I/sub b/ is the bias current and R/sub d/ is the dynamic resistance. The /spl eta/ values (/spl eta//sub sg/) biased at the sub-gap voltage were larger than that (/spl eta//sub ag/) of the above-gap voltage. In addition, the /spl eta//sub 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 /spl eta//sub 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.
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