Measurement of Epitaxial NbN/AlN/NbN Tunnel Junctions With a Low Critical Current Density at Low Temperature

摘要

Superconducting qubit circuits require high quality low critical current density Josephson tunnel junctions. We have developed high quality epitaxial NbN/AlN/NbN tunnel junctions with critical current density, ${rm J}_{rm c}$, ranging from as low as a few ${rm A/cm}^{2}$ to a few tens of ${rm kA/cm}^{2}$. In this work, we measured current–voltage characteristics for junctions with a ${rm J}_{rm c}sim 32 {rm A/cm}^{2}$ in the temperature range from 17 mK to above 6.7 K. All the junctions are in good quality and have a high gap voltage ($>$ 5.5 mV). We found that the junctions' sub-gap resistances were temperature dependent above 2.5 K and saturated at temperatures below 2.5 K. A junction sub-gap leakage current ${rm I}_{rm r}$ of about 23 pA was measured. The voltage noise power spectral density for 3 junctions with different sizes has been measured at the base temperature of 17 mK with the junctions biased above the gap voltage. The spectral densities clearly show $1/f$ behavior at low frequency and are proportional to the junction's linear dimension.