Niobium trilayer Josephson tunnel junctions with ultrahigh critical current densities

Abstract

We describe the fabrication and properties of Nb-AlOx-Nb Josephson tunnel junctions having critical current densities as high as 400 kA/cm2, roughly an order of magnitude larger than any previously reported for this materials system. The quality of the junction characteristics, as indicated by the current level at subgap voltages, is considerably lower in high-Jc than in low-Jc junctions. However, over the entire high-Jc range of 20–400 kA/cm2, the quality of our junctions remains the same (i.e., subgap current scales with critical current).

Epitaxial aluminum nitride tunnel barriers grown by nitridation with a plasma source

“…In the inset of Fig. 2, Q has been plotted for two different batches of AlN based junctions, together with data on AlO x from Miller et al 14 In contrast to these AlO x devices, it is evident that Q is higher than 10 for all AlN devices. The lower subgap currents prove that the AlN barriers have a lower density of areas with T n Ϸ 1, in other words a better uniformity.…”

“…Therefore, the critical current density of amorphous aluminum oxide barriers is limited to 20 kA/ cm 2 . 14 We will demonstrate that aluminum nitride barriers are superior to aluminum oxide barriers with respect to barrier uniformity. In the work reported here a very good reproducibility is realized by using the afterglow region of a nitrogen plasma from an inductively coupled plasma source ͑COPRA͒ ͑see, for example, Ref.…”

What happens in Josephson junctions at high critical current densities

“…Therefore, hysteresis in I-V curves [15,53] does not necessarily indicate canonical Josephson phase dynamics, even in the presence of a Fraunhofer magnetic field pattern [8]. It may rather arise as a result of local heating processes, possibly induced by intrinsic inhomogeneous composition unavoidable for high-J c junctions.…”

“…The absence of a set of selfconsistent electrodynamics parameters to describe high-J c JJ is a strong indication of the failure of the standard Josephson dynamics. This failure is of general relevance, applying both to conventional low-T c superconductor (LTS) JJs [7,8] and to the emergent class of hybrid nanoscale junctions [15,33,34].…”

“…Unconventional junctions renew some inconsistencies, that were clearly detected in the past in traditional trilayer Nb technology [5,6], in a more subtle manner, probably due to the layout of the devices and their intrinsic complexity. We speculate on how higher J c in extended nonuniform barriers or in filaments embedded in an insulating matrix may even promote the generation of a vortex flow, of phase slips events or of heating modes [7,8]. When possible, this will be done through analysis of the switching distribution of the critical current in hysteretic I-V curves.…”