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IEEE Transactions on Magnetics

Properties of stacked NbN tunnel junctions

H.J. Hedbabny, Horst Rogalla

Published Mar 1, 1989• 6 citations
Publisher

Abstract

Stacks of up to three tunnel junctions were fabricated using the NbN-MgO technique. Different preparation methods were tested, and two gave favorable results. In the first one, the whole stack is prepared in situ and structured afterwards by lift-off and reactive and argon ion etching. The authors investigated the resulting I-V (current-voltage) characteristics of stacks of 8.3 mV. Since it was not possible to establish electrical connections to the intermediate electrodes by this method, a second one was applied in which each NbN/MgO layer is prepared in a separate step and structured by lift-off. Here the I-V characteristics, the interaction between the tunnel junctions, and their RF properties were investigated. Shapiro steps and photon-assisted tunneling were observed in the I-V characteristics of a receiver junction, while the bottom tunnel junctions were used as microwave generators.>

DOI
10.1109/20.92518

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The authors report on the fabrication and testing of all-refractory NbN/MgO/NbN SIS (superconductor-insulator-superconductor) tunnel junctions for use as high-frequency mixers. Progress in the development of techniques for the fabrication of submicron-area tunnel junctions is described. Junction structures which have been investigated include mesa, crossline, and edge geometries. Using reactive sputtering techniques, NbN tunnel junctions with critical currents in excess of 10/sup 4/ A/cm/sup 2/ have been fabricated with V/sub m/ values as high as 65 mV and areas down to 0.1 mu m/sup 2/. Specific capacitance measurements on NbN/MgO/NbN mesa-type tunnel junctions give values in the range 60-90 fF/ mu m/sup 2/. These SIS tunnel junctions have been integrated with antennas and coupling structures for mixer tests in a waveguide receiver at 207 GHz. Preliminary mixer results are reported.>

IEEE Transactions on Magnetics•Mar 1, 1989•17 citations
Stacks of up to three tunnel junctions were fabricated using the NbN-MgO technique. Different preparation methods were tested, and two gave favorable results. In the first one, the whole stack is prepared in situ and structured afterwards by lift-off and reactive and argon ion etching. The authors investigated the resulting I-V (current-voltage) characteristics of stacks of 8.3 mV. Since it was not possible to establish electrical connections to the intermediate electrodes by this method, a second one was applied in which each NbN/MgO layer is prepared in a separate step and structured by lift-off. Here the I-V characteristics, the interaction between the tunnel junctions, and their RF properties were investigated. Shapiro steps and photon-assisted tunneling were observed in the I-V characteristics of a receiver junction, while the bottom tunnel junctions were used as microwave generators.>
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.
We have evaluated the electrical characteristics of NbN/Al-AlNx/NbN tunnel junctions fabricated on sapphire-C or fused-silica substrates. NbN/Al-AlNx/NbN multilayers were prepared by reactive dc-magnetron sputtering deposition of lower and upper NbN layers, and rf-magnetron sputtering deposition of the Al layer, followed by radical-nitridation. The junctions showed low sub-gap leakage currents in the current-voltage characteristics at 4.2 K. The quality parameter, Rsg/Rn, where respective Rsgand Rnare the sub-gap resistance at 2 mV and junction resistance at 5 mV, was above 15. The gap voltage (Vg) was around 3.3 mV, which was reduced in comparison with Vgvalues of ~4.3 mV for junctions on MgO substrates. The reduced Vgwas caused by superconducting properties of lower NbN layers deposited on the sapphire or fused-silica substrates. Uniformity in critical current (Ic) was relatively good, and the maximum-to-minimum spread in Icwas ±1.7% for 200 junctions on a sapphire substrate and ±2.0% for the junctions on a fused-silica substrate.
A novel process suitable for fabrication superconducting circuitry based on the all refactory material NbN is described. In this process, and in-situ trilayer film composed of NbN/MgO/NbN is used to fabricate Josephson tunnel junctions. Reactive ion etching processes are used to delineate devices and pattern insulators and metallizations. Fabricated junctions have yielded good tunneling characteristics with reasonable current density uniformity and reproducibility. Devices with gap voltages close to 3mV have been achieved for high quality junctions (Vm 20 mV). In addition to the trilayer, there are two wiring layers, two resistor depositions, and two insulation layers, constituting a full NbN based fabrication technology. Using this process, we fabricated and successfully tested thin film DC SQUID and time domain reflectometer (TDR) circuits. Preliminary measurements suggest that the critical temperature of these circuits is well within the operating temperature of commercial two-stage closed cycle refrigerators. (Author)
All-NbN Josephson tunnel junctions with sputter-deposited magnesium oxide barriers have been fabricated using single-crystal NbN films for the base electrodes. Fabricated Josephson junctions have shown good tunneling characteristics with large gap voltages (5.6-5.8 mV), narrow gap widths (0.1-0.2 mV, from 30 to 70%), and small subgap leakage currents (V/sub m/=20-30 mV, measured at 3 mV). The results of a measurement of a subgap structure for a fabricated junction suggest that the excess leakage currents of fabricated junctions are due to multiparticle tunneling through locally thin areas in the MgO barriers.
Microwave-Induced Characteristics of (NbN/TiNx)N/NbN{({\rm NbN/TiN}_{\rm x})}_{N}/{\rm NbN}(NbN/TiNx​)N​/NbN StackedJosephson Junction Arrays
Stacked (NbN/TiNx)N/NbN (N = 2, 3, 4, 5 and 10) Josephson junctions and their arrays have been fabricated on a Si chip and their microwave-induced characteristics were measured at a temperature around 10 K. To achieve a vertical and smooth edge of an NbN - TiNxmultilayer, an inductively coupled plasma reactive ion etching (ICP-RIE) technique was used. Current-voltage characteristics measured for stacks of (NbN/TiNx)N/NbN Josephson junctions without microwave indicated that critical currents for those junctions distribute in a certain range. We defined Ic- spread as (IcMAX- IcMIN)/IcAverage to measure the distribution of Ic. The Ic- spread was minimized when the thickness of intermediate electrodes was d = 50 nm. We also evaluated values of Ic- spread for stacks prepared at different fabrication runs as a function of the number of junctions in a stack N. As a result, we found N = 3 is the maximum for practical use. For arrays, we obtained a constant-voltage step height of about 2 m A for 2 048 stacks of double-barrier junctions (N = 2), and about 1 mA for 1 024 stacks of triple-barrier junctions (N = 3).
The authors report on the fabrication and testing of all-refractory NbN/MgO/NbN SIS (superconductor-insulator-superconductor) tunnel junctions for use as high-frequency mixers. Progress in the development of techniques for the fabrication of submicron-area tunnel junctions is described. Junction structures which have been investigated include mesa, crossline, and edge geometries. Using reactive sputtering techniques, NbN tunnel junctions with critical currents in excess of 10/sup 4/ A/cm/sup 2/ have been fabricated with V/sub m/ values as high as 65 mV and areas down to 0.1 mu m/sup 2/. Specific capacitance measurements on NbN/MgO/NbN mesa-type tunnel junctions give values in the range 60-90 fF/ mu m/sup 2/. These SIS tunnel junctions have been integrated with antennas and coupling structures for mixer tests in a waveguide receiver at 207 GHz. Preliminary mixer results are reported.>