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Japanese Journal of Applied Physics

Overdamped NbN Josephson Junctions Based on Nb/AlOx/Nb Trilayer Technology

Takao Iwai, Takashi Aoyama, Ryutaro Oke, Hiroyuki Akaike, Akira Fujimaki, Hisao Hayakawa

Published Aug 1, 1999• 5 citations
Publisher

Abstract

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.

DOI
10.1143/jjap.38.l929

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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.
Self-Shunted NbN Junctions With NbNx/AlN{\rm NbN}_{x}/{\rm AlN}NbNx​/AlN Bilayered Barriers for 4 K Operation
We report the fabrication and electrical characteristics of self-shunted Josephson junctions consisting of NbN electrodes, an AlN barrier, and an NbNx normal layer for an SFQ circuit application. To fabricate such junctions with a high characteristic voltage V c, we have studied their preparation conditions including the surface treatments of MgO substrates. Junction characteristics at 4 K were dependent on the electrical properties and film thickness of the NbNx layer; the underdamped junctions changed into overdamped ones with an increase in the film thickness. The overdamped junctions showed a characteristic voltage Vc of 0.8 mV and a critical current density Jc of 22 A/cm2 at 4.2 K.
We present characteristics of overdamped Josephson junctions consisting of Nb/AlOx/Al/ AlOx/Nb structures. The junctions were fabricated using a well-developed Nb/AlOx/Nb-junction technology and showed well-defined Josephson characteristics at 4.2 K. The characteristic voltage Vc [the product of the critical current Ic and the effective normal resistance Rn(eff)] of junctions, which determines high-frequency performance of the junction, was in the range of 0.1–0.5 mV, and the critical current density Jc in the range of 102–104 A/cm2. Maximum-to-minimum variations in Ic over a wafer were ±4% for junctions with Vc=0.15 mV and ±13% for junctions with Vc=0.5 mV.
We describe the fabrication processes and electrical characteristics of two types of NbN junctions. One is a self-shunted NbN/NbNx/AlN/NbN Josephson junction, which is expected to improve the density of integrated circuits; the other is an underdamped NbN/AlNx/NbN tunnel junction with radical-nitride AlNx barriers, which has highly controllable junction characteristics. In the former, the junction characteristics were changed from underdamped to overdamped by varying the thickness of the NbNx layer. Overdamped junctions with a 6-nm-thick NbNx film exhibited a characteristic voltage of Vc =0.8mV and a critical current density of Jc =22A/cm2 at 4.2K. In the junctions with radical-nitride AlNx barriers, Jc could be controlled in the range 0.01-3kA/cm2 by varying the process conditions, and good uniformity of the junction characteristics was obtained.
Overdamped Nb/Al-AlOx/Nb Josephson junctions are an intermediate state between the SIS and SNS Josephson junctions. With a proper choice of fabrication parameters, stable and reproducible nonhysteretic curent-voltage characteristics have been obtained Featuring critical current densities up to 70 kA/cm2and characteristic voltages up to 450 μV at 4.2 K, overdamped junctions are suitable for many aplications ranging from RSFQ electronic circuits to programmable and ac voltage standards. Tests on single junctions arrays of series-connected junctions have shown a suitable behaviour when driven by a microwave with frequencies around 75 GHz. The quantized steps have proven to be stable and flat at nV level. These junctions also shown an improved temperature dependence, which can be interesting in view of the development instrumentation with an integrated quantum voltage reference operated in a closed cycle refrigerator.
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Abstract In this work, we report the electrical properties of NbN internally shunted Josephson junctions with NbN x barriers. Cross-sectional scanning transmission electron microscopy analysis shows that all layers have the same cubic structure; NbN/NbN x /NbN trilayers were epitaxially grown on MgO substrates. The resistivity of the NbN x films could be varied in the range of 1–104 mΩ cm by controlling both the N2 partial pressure and the deposition time during reactive sputtering. The temperature dependence of the critical current density (J c) and characteristic voltage (I c R n) of the junctions with different barrier resistivities were measured for various barrier thicknesses. For the 10 nm-thick NbN x layer with resistivities of 73.44, 385.72, and 711 mΩ cm, the coherence length of the barrier was determined to be 5.55 ± 0.07, 4.88 ± 0.06, and 1.40 ± 0.13 nm, respectively, corresponding to carrier diffusion rates of 2.741 ± 0.004, 1.211 ± 0.002, and 0.008 ± 0.001 cm2 s−1, respectively. Thus, the reduction in barrier resistivity leads to a larger coherence length and a faster diffusion rate, which will further increase the J c and I c R n of the junction. By adjusting the barrier resistivity and thickness, the J c of the junction can be easily tuned over more than four orders of magnitude, and an I c R n value of 0.97 ± 0.07 mV was obtained at 10 K. The results indicate that the all-NbN self-shunt junction is a promising candidate in high-speed and high-temperature applications.