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Japanese Journal of Applied Physics

Microwave Characteristics of NbN/NbNx/NbN Josephson Junctions for Quantum Voltage Standards

Hirotake Yamamori, Akira Shoji

Published Oct 1, 2005• 3 citations
Publisher

Abstract

All NbN Josephson junctions with polycrystalline NbN films for both electrodes and barriers have been fabricated and their microwave characteristics for a quantum voltage standard were investigated. A superconductor and a normal metal were prepared by changing the partial pressure of N2during the rf sputter deposition of NbN films. For the Josephson junctions normal resistance was controlled by changing the partial pressure of N2during the NbNxbarrier deposition, and critical current was controlled by changing barrier thickness. As a result, it was found that the product of current and normal resistance can be optimized to maximize the current range of the constant-voltage step for quantum voltage standards.

DOI
10.1143/jjap.44.l1326

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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.
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.
Both underdamped junctions and overdamped junctions utilizing semiconductive amorphous a- barriers have been fabricated by controlling the partial pressure of when the a- barrier is sputtered. While a NbN film is generally used as a superconductive electrode, we have used NbN films for both electrodes and barriers to fabricate overdamped junctions. The films for the barrier are an amorphous semiconductor and have high density of localized states. Typical overdamped junctions have of more than and products of 0.5 mV. On the other hand, typical underdamped junctions have critical current density of to and products of about 1.5 mV. The barrier height of the a- barrier is estimated to be about 0.3 eV from the slope of the barrier thickness dependence of critical current . Then, stacked double barrier junctions have been also fabricated.
High quality NbN-NbOX-Pb Josephson devices have been fabricated by r.f. diode sputtering. The critical temperatures of the NbN films were as high as 16.8 K. The junction geometry was defined by photolithography and the oxide barrier was obtained by thermal oxidation or by a plasma oxidation technique. Very low leakage currents have been obtained, corresponding to values of the parameter Vm = 80 mV (Vm = IO× 2 mV/I (2mV)). The junction microwave quality factor Q and other a.c. properties have been measured in the frequency range 20 - 300 GHz by looking at the devices self-resonant modes. Q values up to 120 at 4.2 K have been found in the low frequency range, indicating potential performances of these devices for microwave applications such as Josephson voltage standards. Theoretical analysis of the results shows that the main contribution to the losses is related to the NbN films surface impedance.
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.
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.