Influence of the transparency of tunnel barriers in Nb/Al2O3/Al/Al2O3/Nb junctions on transport properties

The transparency of the tunnel barriers in double-barrier junctions influences the critical current density and the form of the current–voltage characteristics (IVC). Moreover, the barrier asymmetry is an important parameter, which has to be controlled in the technological process. We have performed a systematic study of the influence of the barrier transparency on critical current, IC, and normal resistance, RN, by preparing SIS and SINIS junctions under identical technological conditions and comparing their transport properties. We have fabricated Nb/Al2O3/Nb and Nb/Al2O3/Al/Al2O3/Nb devices with different current densities using a conventional fabrication process, varying pressure and oxidation time. The thickness of the Al middle electrode in all Nb/Al2O3/Al/Al2O3/Nb junctions was 6 nm. Patterning of the multilayers was done using conventional photolithography and the selective niobium etching process. The current density of SIS junctions was changed in the range from 0.5 to 10 kA/cm2. At the same conditions the current density of SINIS devices revealed 1–100 A/cm2 with non-hysteretic IVC and characteristic voltages, ICRN, of up to 200 μV. By comparing the experimental and theoretical temperature dependence of the ICRN product we estimated the barrier transparency and its asymmetry. The comparison shows a good agreement of experimental data with the theoretical model of tunneling through double-barrier structures in the dirty limit and provides the effective barrier transparency parameter γeff≈300. A theoretical framework is developed to study the influence of the barrier asymmetry on the current–phase relationship and it is proposed to determine the asymmetry parameter by measuring the critical current suppression as function of applied microwave power. The theoretical approach to determine the non-stationary properties of double-barrier junctions in the adiabatic regime is formulated and the results of calculations of the I–V characteristics are given in relevant limits. The existence and the magnitude of a current deficit are predicted as function of the barrier asymmetry.