Development of niobium-based superconducting junctions

Abstract

This thesis presents a review of publications which mainly focuses on the fabrication and performance of Nb-based normal metal-insulator-superconductor (NIS) tunnel junctions and superconductor-normal metal-superconductor (SNS) Josephson junctions at low temperatures. The Cu/AlOx-Al/Nb based NIS double tunnel junctions were successfully fabricated using conventional electron beam lithography combined with multi-angle thermal evaporation of metals in ultra high vacuum. The subgap characteristics of these junctions showed expected temperature dependence from 0.2 K to 5 K with a good thermometry sensitivity 0.2-0.3 mV/K. Signatures of small electronic cooling effects were observed near the Nb superconducting gap edge. Thin films of NbN were successfully deposited on (100)-oriented MgO using laser pulses from excimer and Nd:YAG lasers. The NbN films deposited using the 1064 nm laser pulses from Nd:YAG laser were studied further. An excellent quality of the NbN films was reflected as smooth surfaces of the films and critical temperature (TC) as high as 16 K. A strong correlation between TC, the lattice parameter of deposited NbN and ambient nitrogen gas pressure during the film ablation was observed. In addition, thin films of NbN were successfully employed in studies of suppression of superconductivity in the influence of an applied magnetic field. Pulsed laser deposited NbN films were used to fabricate Cu/AlOx-Al/NbN based NIS tunnel junction devices where the fabrication included an overlay electron beam lithography and multi-angle evaporation of Al and Cu layers. The subgap conductance of these junctions also showed appreciable dependence on the temperature. The broadening of the density of the states of the NbN was found small enough for potential cooling applications, but the native niobium oxide on the surface of NbN gave a high value of the tunneling resistance, which would limit the cooling power of the devices. Trilayers of pulsed laser deposited NbN/TaN/NbN were fabricated into SNS based Josephson junctions, using an unconventional method with electron-beam lithography and lift-off of chemical vapor deposited insulating layers. The quality of the junctions was tested by investigating the temperature dependence of the product of the critical current and normal state resistance. The obtained values are of the same order of magnitude as previous studies and are high enough values for practical applications.