Universality of transport properties of ultrathin oxide films

Abstract

We report low-temperature measurements of current-voltage characteristics for highly conductive Nb/Al-AlO x -Nb junctions with thicknesses of the Al interlayer ranging from 40 to 150 nm and ultrathin barriers formed by diffusive oxidation of the Al surface. In a superconducting state these devices have revealed a strong subgap current leakage. Analyzing Cooper-pair and quasiparticle currents across the devices, we conclude that the strong suppression of the subgap resistance compared with conventional tunnel junctions is not related to technologically derived pinholes in the barrier but rather has more fundamental grounds. We argue that it originates from a universal bimodal distribution of transparencies across the aluminum oxide barrier proposed earlier by Schep and Bauer (1997 Phys. Rev. Lett. 78 3015). We suggest a simple physical explanation of its source in the nanometer-thick oxide films relating it to strong local barrier-height fluctuations in the nearest to conducting electrode layers of the insulator which are generated by oxygen vacancies in thin aluminum oxide tunnel barriers formed by thermal oxidation.

Dependence of the scatter of the electrical properties on local non-uniformities of the tunnel barrier in Nb/Al-AlOx/Nb junctions

“…The existence of pinholes, defects, and local thickness non-uniformities in the tunnel barrier creates quantum channels with very high transparencies, which can lead to higher order tunneling processes [6][7][8][9][10][11] and result in scatter in J c and R n A values. However, if the distribution of defects is uniform, [12][13][14] then the local scatter in R n and J c values will average out over the junction size, and hence, R n and J c values would not change over the wafer area. If the defects are not uniformly dispersed over the wafer area, the junctions' R n and J c values would experience scatter.…”

Controlling the Interface Properties of Submicrometric Nb/A– $\hbox{AlO}_{\rm x}$–Nb Josephson Junctions

“…In our previous papers [4]- [7] we developed a type of overdamped Josephson junctions, namely, Nb/A-AlO x -Nb four-layered structures with comparatively thick (30-120 nm) aluminum layers and comparatively thin Al-oxide barriers that have shown a number of significant advantages compared to conventional tri-layered devices. The planar dimensions of our previous devices were micrometer size and the challenging task has been to reduce the transverse dimensions further to submicron range to fabricate devices for applications ranging from small arrays for AC voltage standards, RSFQ building blocks, and nano-SQUIDs [8].…”

Tunneling through Al/AlOx/Al junction: Analytical models and first-principles simulations

“…Clearly, having a large set of parameters, it is no surprise that the simple barrier model can be fitted to the experimental current-voltage characteristics well [6][7][8][9] , but at the same time, it rises questions about the relevance of the model itself. 10,11 For example, the inclusion of the image potential can have a significant effect on the effective barrier width, but its presence depends on the time scales of the tunneling electrons and the interface plasmons in the metal 12,13 .…”

“…Nevertheless, in many experiments 8,9,11,15,[20][21][22] the studied interfaces have widths within the reach of ab initio simulations so that the accuracy of the potential barrier model to the interpretation of tunneling data can be tested. Specifically, Jung et al 15 presented such a study comparing the character of the equilibrium projected density of states of the Al/AlO x /Al interface obtained by a first principles simulation with the potential barrier model.…”