Electrical stress effect on Josephson tunneling through ultrathin AlOx barrier in Nb/Al/AlOx/Nb junctions

Abstract

The effect of dc electrical stress and breakdown on Josephson and quasiparticle tunneling in Nb/Al/AlOx/Nb junctions with ultrathin AlOx barriers typical for applications in superconductor digital electronics has been investigated. The junctions' conductance at room temperature and current-voltage (I-V ) characteristics at 4.2 K have been measured after the consecutive stressing of the tunnel barrier at room temperature. Electrical stress was applied using current ramps with increasing amplitude ranging from 0 to ∼ 1000Ic corresponding to voltages across the barrier up to ∼ 0.65 V, where Ic is the Josephson critical current. A very soft breakdown has been observed with polarity-dependent breakdown current (voltage). As the stressing progresses, a dramatic increase in subgap conductance of the junctions, the appearance of subharmonic current steps, and gradual increase in both the critical and the excess currents as well as a decrease in the normal-state resistance have been observed. The observed changes in superconducting tunneling suggest a model in which a progressively increasing number of defects and associated additional conduction channels (superconducting quantum point contacts (SQPCs)) are induced by electric field in the tunnel barrier. By comparing the I-V characteristics of these conduction channels with the nonstationary theory of current transport in SQPCs based on multiple Andreev reflections by Averin and Bardas, the typical transparency D of the induced SQPCs was estimated as D ∼ 0.7. The number of induced SQPCs was found to grow with voltage across the barrier as sinh(V /V0) with V0 = 0.045 V, in good agreement with the proposed model of defect formation by ion electromigration. The observed polarity dependence of the breakdown current (voltage) is also consistent with the model. Based on the observed magnitude of breakdown currents, electric breakdown of AlOxbarrier during plasma processing was considered to be an unlikely cause of fabrication-induced, circuit pattern-dependent nonuniformities of Josephson junctions' critical currents in superconductor integrated circuits.

Diffusion Stop-Layers for Superconducting Integrated Circuits and Qubits With Nb-Based Josephson Junctions

“…The barrier height on the BE side, φ BE , is determined by the work function of Al, and the barrier height on CE side, φ CE , is determined by the work function of Nb, resulting in φ CE > φ BE. [12]- [14]. Therefore, the presence of hydrogen in Nb base electrode has no effect on the tunnel barrier height and hence has much less effect on the critical current density of Josephson junctions than hydrogen dissolved in Nb counter electrode, as was explained in our work [4] (see also [6] and our work [15] in the present issue).…”

Superconductor digital electronics: Scalability and energy efficiency issues (Review Article)

“…Therefore, self-shunting can be induced only by increasing the density of states in the gap or in the barrier allowing for a substantial ohmic conduction (junction "leakage") at V < V g . This can be achieved by introducing defects in the tunnel barrier, e.g., pin-holes and oxygen vacancies [120]- [124], or by using junction barriers with direct conduction such as normal metals, doped semiconductors, etc. However, high-quality tunnel junctions, e.g., using Nb 2 O 5 and AlN x barriers instead of AlO x , remain highly hysteretic at J c values much larger than 100 kA/cm 2 [125]- [127].…”

Anomalous Inner-Gap Structure in Transport Characteristics of Superconducting Junctions with Degraded Interfaces

“…When properly aligned, we get the tunneling current peak and a negative differential resistance just above the resonant bias. Our calculations were done using a standard formula for tunneling current in a symmetric S-n-I-n-S configuration [1] where the tunneling density of states should be replaced by the electron penetration probability (5). The results are shown in where n is an integer.…”

“…The first finding of such kind was revealed for tunnel junctions with some transition metals, especially, for Nb-based devices and intensively discussed in 70s of the previous century long before the discovery of high-temperature superconductivity. We mean the so-called 'knee', a sudden current decrease in low-temperature I-V characteristics of Nb-based S-I-S junctions at voltages slightly just above a strong increase of the current that was expected to occur at the sum of V  values for the two superconducting electrodes [4] (see also [5]). Moreover, the current values at very low temperatures and at voltages lower than the sum of V  were considerable larger than theoretically anticipated for the BCS densities of states [6].…”