Characterization of Superconducting Josephson Junctions on Electro-Optic LiNbO3 Substrates at mK Temperatures

We report on the fabrication and millikelvin characterization of Nb/Al-AlOx/Nb Josephson junctions directly deposited on X-cut LiNbO3 substrates. Motivated by the strong electro-optic and piezoelectric properties of LiNbO3 , such devices are of interest for future hybrid superconducting-electro-optic architectures. The junctions were fabricated using standard processes involving DC magnetron sputtering followed by controlled thermal oxidation of the aluminum layer to form an AlOx tunnel barrier with reproducible junction areas. Insulation of the junctions was achieved by anodization and SiOx deposition. Electrical measurements performed at 30 mK reveal high-quality Josephson tunnel junction behavior with a critical current density of Jc=70A/cm2 and a characteristic voltage reaching values up to Vm=286mV. Differential conductance measurements exhibit pronounced quasiparticle peaks corresponding to a sum-gap energy of 2 Delta=(2.73 +/- 0.05)meV, confirming the good quality of the Nb electrodes. Magnetic-field-dependent measurements yield a Josephson penetration depth of lambda J=45 mu m, confirming operation in the small-junction regime, with deviations from the ideal Josephson interference pattern attributed to non-uniform current distribution across the junction area. Compared to our previous characterization at 4.2 K, this work demonstrates reliable Josephson junction operation on LiNbO3 at dilution-refrigerator temperatures relevant for quantum circuits, supporting the suitability of lithium niobate as a promising substrate for future hybrid superconducting-electro-optic device implementations.