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IEEE Transactions on Applied Superconductivity

Double barrier long Josephson junctions with a contact to the intermediate superconducting layer

H. Kohlstedt, Alexey Vladimirovich Ustinov, F. Peter

Published Jun 1, 1995• 5 citations
Publisher

Abstract

We demonstrate a double barrier long Josephson tunnel junction device of overlap geometry with an external electric contact to the middle electrode. The junctions were fabricated with a standard Nb/Al-AlO/sub x//Nb process. To exhibit fluxon coupling effects in stacked junctions, the intermediate Nb layer thickness between two barriers has to be of the order of the London penetration depth, i.e. 90 nm for sputtered Nb films. By using a planarization process, we were able to avoid steep steps on the wafer and so to reduce the intermediate layer thickness to 90 nm. Double-junction stacks with areas from 10/spl times/10 /spl mu/m/sup 2/ to 20/spl times/600 /spl mu/m/sup 2/ were fabricated. In the best stacks, the difference in the critical currents of two junctions was less than 3%. Clear voltage locking effects were observed in current-voltage (I-V) characteristics.>

DOI
10.1109/77.403208

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We demonstrate a double barrier long Josephson tunnel junction device of overlap geometry with an external electric contact to the middle electrode. The junctions were fabricated with a standard Nb/Al-AlO/sub x//Nb process. To exhibit fluxon coupling effects in stacked junctions, the intermediate Nb layer thickness between two barriers has to be of the order of the London penetration depth, i.e. 90 nm for sputtered Nb films. By using a planarization process, we were able to avoid steep steps on the wafer and so to reduce the intermediate layer thickness to 90 nm. Double-junction stacks with areas from 10/spl times/10 /spl mu/m/sup 2/ to 20/spl times/600 /spl mu/m/sup 2/ were fabricated. In the best stacks, the difference in the critical currents of two junctions was less than 3%. Clear voltage locking effects were observed in current-voltage (I-V) characteristics.>
We have fabricated overdamped NbN Josephson junctions based on Nb/AlO x /Nb trilayer technology. The junctions have the structure of NbN/Nb/AlO x /Nb/NbN, where Nb/AlO x /Nb acts as a junction barrier. The overdamped junctions showed a critical current density J c and characteristic voltage as high as 0.44 kA/cm2 and 160 µV at 10 K, respectively. The junction characteristics at 4.2 K were investigated from the viewpoint of their dependence on the film thickness of the Nb layers. The junction uniformity was also evaluated. The standard deviation, 1-σ, was 2.1% for a series of 64 5×5 µm2 junctions at 4.2 K.
Vertical stacks of two Nb/Al-AlO/sub x//Nb long Josephson junctions with nearly identical parameters have been studied experimentally. The magnetic coupling between the junctions was provided by their common electrode of thickness d/spl sime//spl lambda//sub L/. The I-V characteristics displayed flux-flow resonances of the two junctions. Two different mutually phase-locked regimes were found: the lower velocity anti-phase mode, and the high velocity in-phase mode. These modes, while staying locked, could be tuned by the magnetic field in a broad voltage range which corresponds to the Josephson frequencies of 150-600 GHz for the lower mode and 280-400 GHz for the upper mode. Our observations suggest applications of the stacked long Josephson junctions as local oscillators for millimeter and sub-millimeter wave circuits.>
Nb/Al–AlOx–Nb/Al–AlOx–Nb double-barrier three-terminal Josephson devices with a thin middle Nb/Al bilayer, which have potential for use as switching and amplifying elements in superconducting electronics, have been investigated in the stationary state by means of low-temperature scanning electron microscopy. For the devices with lateral sizes comparable with the effective Josephson penetration depth, we observed nearly homogeneous current distribution over the region common to the top and bottom junctions when the devices were biased across both the barriers in the absence of an external magnetic field. When the two junctions are biased separately, the current is concentrated at the junction edges in accordance with the behavior characteristic of distributed junctions. In an applied magnetic field, the vortex structure is confined to the area shared by both junctions when the device is biased as a whole. The experiment gives an indication that in the stationary state the spatial phase difference distribution of the two junctions coincides in the shared region at least in some interval of the external magnetic field.
Abstract In this work, we present a detailed study of the electrical properties of stacked NbN/(TaN/NbN) N Josephson junctions. Cross-sectional scanning transmission electron microscopy analysis of the 5-stacked junction shows that the multilayer interface is very flat, each barrier has the same thickness, and the sidewalls of the junctions are nearly perpendicular to the substrate. Stacked junctions of different sizes and stacking numbers all have only one transition in their current–voltage curves. This indicates that the critical currents of the junctions in the stacked junctions are almost the same, showing the stability and repeatability of the multilayer fabrication and etching process. At 4.2 K, the 4-stacked junction shows excellent Josephson properties with characteristic voltage V c of 3.54 mV, which is about four times the 0.88 mV of the single junction. The temperature dependence of critical current density J c and V c of the stacked junction with N = 1, 2, 4 were measured, all of which can be fitted with dirty-limit theory. Stacked junctions with larger V c or more stacked layers can be achieved by optimizing electrode and barrier thickness, barrier resistivity, and thermal relaxation rate, etc.
Niobium/aluminum-oxide/niobium Josephson tunnel junctions have been fabricated using electron beam evaporation to form all-metal films in a high vacuum system. A quality factor, V m (the product of the maximum dc Josephson current and the subgap resistance defined at 2 mV) between 36–37 mV, has been obtained for the best junctions of a 6 nm-thick Al layer by cooling down the surface of the base electrode for 100 minutes under a high background pressure, before the deposition of aluminum, after the formation of the base niobium electrode. An array of 80 series-connected junctions has also been fabricated using the selective niobium anodization process and the planarization process. The current-voltage characteristics of the array showed a standard deviation of the maximum dc Josephson current of 3.3%.
A new structure of the base-electrode for Josephson devices have been developed in which niobium (Nb) and niobium nitride (NbN) films were sequentially sputtered to form a double-layered structure. Owing to this structure, it is possible not only to reduce the kinetic inductance of the base-electrode due to the small penetration depth of Nb film, but also to obtain good tunneling characteristics with a small subgap conductance due to the surface stability of NbN film. Single-flux-quantum (SFQ) memory cells with this structure have been fabricated and it has been observed that the kinetic inductance is much decreased comparing with that of the SFQ cell with a base-electrode of NbN single layer.