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IEEE Transactions on Applied Superconductivity

Low-loss epitaxial NbN/MgO/NbN trilayers for THz applications

Akira Kawakami, Zhen Wang, Shigehito Miki

Published Mar 1, 2001• 30 citations
Publisher

Abstract

To improve the performance of Josephson oscillators and SIS mixers over 700 GHz, we have developed a fabrication process to grow epitaxial NbN/MgO/NbN trilayers by reactive dc-sputtering. Trilayers were fabricated on a single-crystal MgO substrate at ambient temperature. NbN and MgO films were deposited by reactive dc-sputtering with an Nb and Mg target, respectively. The MgO inter-layer thickness was changed up to 480 nm, but, T/sub C/ and 20-K resistivity of the NbN upper-layer showed no remarkable dependency and they were 15.7 K and about 60 /spl mu//spl Omega/cm, respectively. To evaluate the RF performance of the epitaxial NbN films, we fabricated Josephson junctions with a microstrip resonator constructed from an epitaxial NbN/MgO/NbN trilayer. The I-V characteristics of the junction exhibited resonance steps up to 2.5 mV, which suggests that the epitaxial NbN films have low loss up to 1.2 THz.

DOI
10.1109/77.919289

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To improve the performance of Josephson oscillators and SIS mixers over 700 GHz, we have developed a fabrication process to grow epitaxial NbN/MgO/NbN trilayers by reactive dc-sputtering. Trilayers were fabricated on a single-crystal MgO substrate at ambient temperature. NbN and MgO films were deposited by reactive dc-sputtering with an Nb and Mg target, respectively. The MgO inter-layer thickness was changed up to 480 nm, but, T/sub C/ and 20-K resistivity of the NbN upper-layer showed no remarkable dependency and they were 15.7 K and about 60 /spl mu//spl Omega/cm, respectively. To evaluate the RF performance of the epitaxial NbN films, we fabricated Josephson junctions with a microstrip resonator constructed from an epitaxial NbN/MgO/NbN trilayer. The I-V characteristics of the junction exhibited resonance steps up to 2.5 mV, which suggests that the epitaxial NbN films have low loss up to 1.2 THz.
To improve the performance of superconducting analog devices at frequencies exceeding the Nb gap frequency, we developed a fabrication process to grow epitaxial NbN/MgO/NbN trilayers. To evaluate the RF performance of the trilayers, we fabricated Josephson junctions with an epitaxial NbN/MgO/NbN microstrip resonator. A Josephson junction was placed in the center of the resonator and it was used as both an oscillator and a detector. The width and length of the microstrip resonator were 10 and 1000 /spl mu/m, respectively. Current steps of up to 1.9 mV resulting from coupling with the resonator were observed in the I-V characteristics. We found that the height of the current steps depended on the loss of the resonator. By assuming that the sum of the dielectric loss and the radiation loss is much smaller than the conductor loss, we estimated the surface resistance of the epitaxial NbN thin films. The surface resistance of the epitaxial NbN films was estimated to be approximately 2.6-88 m/spl Omega/ at 0.1-1.1 THz.
We have designed, fabricated and tested a quasi-optical superconductor–insulator–superconductor (SIS) mixer employing distributed NbN/MgO/NbN tunnel junctions and NbN/MgO/NbN microstriplines at submillimetre-wave frequencies. These trilayers were fabricated by dc- and rf-magnetron sputtering on an MgO substrate at ambient temperature so that the NbN and MgO films were grown epitaxially. Our SIS mixer consists of an MgO hyperhemispherical lens with an antireflection cap and a self-complementary log-periodic antenna made of a single-crystal NbN film, on which the distributed SIS junctions and the two-section impedance transformers were mirror-symmetrically placed at the feed point of the antenna. As designed, the junctions are 0.6 μm wide and 15.5 μm long, which is sufficient to absorb the incoming signal along this lossy transmission line, assuming a current density of 10 kA cm −2 . The mixer showed good I – V characteristics, with subgap-to-normal resistance ratios of about 13, although weak-link breaks were observed above the gap voltage. The minimum double side band receiver noise temperature was 334 K at 678 GHz, including the input noise of about 200 K as estimated by the standard technique. The noise temperature gradually rose to 672 K at 820 GHz. This behaviour may be caused by RF losses from the weak-link parts, due to the steps between the distributed junctions and the microstriplines being less than 1 μm wide.
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 have developed fabrication processes for epitaxial NbN/MgO/NbN trilayers and epitaxial NbN/MgO/NbN tunnel junctions. We estimated the junction specific capacitances (CS) for designing fully epitaxial NbN–superconductor-insulator-superconductor (SIS) mixers by using Fiske steps. Tunnel junctions measuring 40×3 and 20×3 μm2 were fabricated, and Fiske steps caused by external magnetic fields were clearly observed in the I–V characteristics. The CS was estimated as a function of the product of the normal-state resistance and the junction area (RNA), and they were 73–120 fF/μm2 at 8800–90 Ω μm2. We fabricated and tested a fully epitaxial NbN/MgO/NbN–SIS mixer. The mixer showed low-noise properties over the Nb gap frequency, and the lowest DSB receiver noise was 260 K at 795 GHz.
The most developed integrated receivers for THz radiation nowadays are based on Josephson junction superconductor-insulator-superconductor devices. In this kind of devices, the highest receivable frequency is determined by the energy gap of the superconducting electrodes and limited to approximately 700 GHz in case of Nb/AlOx/Nb multilayers. We have developed a technology for NbN/AlN/NbN Josephson junctions on sapphire substrates which allow operation at frequencies above 1 THz. The trilayers are deposited in situ in a three-chamber dc-magnetron sputtering system at temperatures as high as 775 °C. Each layer is reactively sputtered in an argon and nitrogen atmosphere. By variation of the partial gas pressures and discharge current of the plasma, optimum deposition conditions for the tri-layers were found. So far, gap voltages as high as 5.1 mV at 4.2 K have been achieved which allows the operation frequency of the JJ devices to exceed 1 THz.
We have fabricated both NbN/Ta x N/NbN SN*S-type, NbN/MgO/NbN SIS-type Josephson junctions and microwave band-pass filters on different substrates (Si, Sapphire, MgO). NbN films have been deposited on both sides of (100) oriented, 250 µm thick, MgO substrates with a high crystalline texture quality. The aim was to investigate the performances and the maximum achievable operating frequency in an NbN based RSFQ modulator front-end of an ADC in the 4 K-10 K temperature range. We observed that Ta x N thin films can be tuned from an insulating phase to a superconducting phase (Tc ∼4K) by varying the nitrogen content during sputter deposition while the barrier height of MgO can also be controlled by deposition conditions and by tri-layer postdeposition annealing. Junction properties (Jc∼10-25 kA/cm 2 ), Mac Cumber parameter and RnIc product measured up to 1 mV are shown to be controlled by the reactive sputtering conditions. We have designed three pole band-pass filters and resonators in a micro-strip configuration and studied the junction coupling with the filters. We will show that a sigma-delta NbN technology is a suitable solution for analogue-to-digital conversion in the future generations of telecommunication satellites to achieve high sampling frequency and large bandwidth at high carrier frequency signal.