TiN nanobridge Josephson junctions and nanoSQUIDs on SiN-buffered Si

Abstract

We report the fabrication and properties of titanium nitride (TiN) nanobridge Josephson junctions (nJJs) and nanoscale superconducting quantum interference devices (nanoSQUIDs) on SiN-buffered Si substrates. The superior corrosion resistance, large coherence length, suitable superconducting transition temperature and highly selective reactive ion etching (RIE) of TiN compared to e-beam resists and the SiN buffer layer allow for reproducible preparation and result in long-term stability of the TiN nJJs. High-resolution transmission electron microscopy reveals a columnar structure of the TiN film on an amorphous SiN buffer layer. High-resolution scanning electron microscopy reveals the variable thickness shape of the nJJs. A combination of wet etching in 20% potassium hydroxide and RIE is used for bulk nanomachining of nanoSQUID cantilevers. More than 20 oscillations of the V(B) dependence of the nanoSQUIDs with a period of ∼6 mT and hysteresis-free I(V) characteristics (CVCs) of the all-TiN nJJs are observed at 4.2 K. CVCs of the low-I c all-TiN nJJs follow theoretical predictions for dirty superconductors down to ∼10 mK, with the critical current saturated below ∼0.6 K. These results pave the way for superconducting electronics based on nJJs operating non-hysteretically at 4.2 K, as well as for all-TiN qubits operating at sub-100 mK temperatures.