Transport and Noise Properties of sub-100-nm Planar Nb Josephson Junctions with Metallic Hf - Ti Barriers for nano-SQUID Applications

Abstract

We analyze electric transport and noise properties at 4.2 K of self-shunted superconductor-normal metal-superconductor (SNS) sandwich-type Josephson junctions, comprising $\mathrm{Nb}$ as the superconductor and $\mathrm{Hf}$-$\mathrm{Ti}$ as the normal conducting material, with lateral dimensions down to approximately $80$ nm. The junctions are fabricated with an optimized multilayer $\mathrm{Nb}$ technology based on nanopatterning by electron-beam lithography and chemical-mechanical polishing. The dependence of transport properties on the junction geometry (lateral size and barrier thickness ${d}_{\mathrm{Hf}\mathrm{\ensuremath{-}}\mathrm{Ti}}$) is studied, yielding a characteristic voltage ${V}_{c}$ up to approximately $100\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{V}$ for the smallest ${d}_{\mathrm{Hf}\mathrm{\ensuremath{-}}\mathrm{Ti}}=17$ nm. The observed small hysteresis in the current-voltage curves of devices with high ${V}_{c}$ and large size can be attributed to self-heating of the junctions and fitted with an extended version of the resistively shunted junction model. Measurements of voltage noise of single junctions are consistent with the model including self-heating effects. The potential of our technology for further miniaturization of nanoscale superconducting quantum interference devices and for the improvement of their performance is discussed.