Abstract
We have studied low-temperature electrical transport properties of nanoscale Nb∕(Al–)Al2O3∕Nb and NbN∕AlN∕NbN Josephson junctions (JJs) fabricated by focused-ion-beam (FIB) milling. This FIB fabrication process yields high-quality JJs whose superconducting gap energy agrees with the bulk value. In this paper, we report the improvement in the precision of the fabrication technology by employing a weaker ion beam current and by introducing a step of XeF2-gas-assisted milling, which allowed us to reduce the anodization voltage. For Nb JJs, we measured the current-voltage (I-V) characteristics of single-electron transistors (SETs) and examined the current observed within the superconducting energy gap. At finite voltages, we observed current peaks due to the Josephson-quasiparticle (JQP) cycle. Based on the JQP-peak positions, we estimated the charging energy Ec of the SETs. For all SETs, Ec∕kB is larger than 1K, which is consistent with 0.1×0.1μm2 junctions. We also analyzed the superconducting critical current at zero voltage. The ratio Ic∕Ic0 depended on the junction size, where Ic is the measured critical current and Ic0 is the theoretical one. The I-V curves of NbN single Josephson junctions also depended on the junction size, and they were qualitatively different, especially around the energy gap. We will discuss the possible origins for the junction-size dependence.
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