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Superconductor Science and Technology

The dependence of operating frequency of NbN Toggle Flip-flop on parameters of NbN Josephson junctions

Yingyi Shao, Huiwu Wang, Minghui Niu, Wei Peng, Jie Ren, Zhen Wang

Published Jun 6, 2024• 3 citations
Publisher

Abstract

Abstract We performed a comprehensive investigation of the relationship between the maximum operating frequency of the NbN superconducting frequency divider and the key parameters of the NbN Josephson junction (JJ). We designed a superconducting frequency divider that uses a single NbN JJ as a pulse generator, several NbN Josephson transmission lines, an NbN Toggle Flip-flop, and a load. By comparing the bias voltage V in of the pulse generator with the output voltage V out of the load, we determined if the circuit was working correctly and calculated the maximum operating frequency f max. Additionally, we employed JSICsim software for simulations to analyze the impact of key parameters of the NbN JJ, such as critical current density J c, gap voltage V g, characteristic voltage V c, quality factor Q, and specific capacitance C s, on f max. The simulation results demonstrate that f max increases with increasing J c and Q. Specifically when J c exceeds 300 kA cm−2 and Q surpasses 4, the superconducting frequency divider can achieve a f max of 1 THz. Furthermore, we successfully fabricated a superconducting frequency divider using a 10 kA cm−2 process and tested a f max of 260 GHz, with a deviation of approximately 6% from the simulation results.

DOI
10.1088/1361-6668/ad54f4

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Abstract We performed a comprehensive investigation of the relationship between the maximum operating frequency of the NbN superconducting frequency divider and the key parameters of the NbN Josephson junction (JJ). We designed a superconducting frequency divider that uses a single NbN JJ as a pulse generator, several NbN Josephson transmission lines, an NbN Toggle Flip-flop, and a load. By comparing the bias voltage V in of the pulse generator with the output voltage V out of the load, we determined if the circuit was working correctly and calculated the maximum operating frequency f max. Additionally, we employed JSICsim software for simulations to analyze the impact of key parameters of the NbN JJ, such as critical current density J c, gap voltage V g, characteristic voltage V c, quality factor Q, and specific capacitance C s, on f max. The simulation results demonstrate that f max increases with increasing J c and Q. Specifically when J c exceeds 300 kA cm−2 and Q surpasses 4, the superconducting frequency divider can achieve a f max of 1 THz. Furthermore, we successfully fabricated a superconducting frequency divider using a 10 kA cm−2 process and tested a f max of 260 GHz, with a deviation of approximately 6% from the simulation results.
A rapid-single-flux-quantum toggle-flip-flop logic gate was fabricated using high-temperature superconducting interface-engineered Josephson junctions. It was shown that the gate can operate as a voltage divider up to 155 GHz at 15 K and 19 GHz at 27 K. At the same time, the temperature dependence of the IcRn product and the maximum divided voltage was compared. As a result, it was found that the ratio of these values is 0.4–0.1 for 15 K>T>27 K. Circuit simulation with noise sources reveals this peculiar temperature dependence of the maximum divided voltage.
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We designed and fabricated a rapid-single-flux-quantum T flip-flop (TFF) with high- T c interface-engineered Josephson junctions. Y 1 Ba 2 Cu 3 O 7− d and Sr 2 AlTaO 6 were deposited for the superconducting layer and the insulating layer, respectively. The Josephson junction was formed through an interface treatment process using Ar ion milling and vacuum annealing. We simulated a TFF circuit and designed a physical layout using WRspice and Xic. The fabricated TFF has a minimum junction width of 3 μm. Through the measurement of the voltage divider operation, the maximum operation frequency was estimated to be 53 GHz at 22 K and 106 GHz at 12 K.
The static frequency divider is commonly used as a performance benchmark for both superconductor and semiconductor digital device technologies. We present results of a static divide-by-two circuit, an NbN (1 kA/cm/sup 2/) SFQ T-flip-flop (TFF) operating to 97 GHz. Details of the measurement and operating criterion are discussed. Measurements of junction capacitance, a particularly important factor effecting device performance, are presented for TRW's NbN process. Simulations of expected device performance are shown to explain measured performance reasonably well. NbN results are presented alongside a those of a recent 8kA/cm/sup 2/ Nb divider operating at 300 GHz, as well as published Nb TFF results.
Abstract We report on the electrical properties of NbN/TaN/NbN Josephson junctions grown on thermally oxidized silicon substrates, along with the design and fabrication of superconducting single-flux-quantum (SFQ) circuits based on these NbN superconductor/normal metal/superconductor (SNS) junctions. The critical current density ( J c ) of the junctions was found to be relatively sensitive to the barrier thickness, decreasing from 108.0 ± 8.1 kA cm −2 for a 15 nm barrier to 12.8 ± 1.9 kA cm −2 for a 30 nm barrier. For a J c of approximately 24.5 ± 2.1 kA cm −2 and a barrier thickness of 25 nm, the NbN SNS junctions are self-shunted and exhibit nonhysteretic current–voltage ( I – V ) characteristics. Especially for junctions with diameter ( φ ) ranging from 0.8 to 1.6 μ m, their critical current ( I c ) falls within the range of 110–450 μ A, making them suitable for SFQ circuits. By considering the impact of excess current and incorporating it as an additional term in the conventional resistively and capacitively shunted junction model, the I – V curves of NbN SNS junctions can be precisely described, successfully minimizing the deviation between simulations and test results. The DC-SFQ and SFQ-DC interface circuits can both operate normally, and the bias margins of cell circuits such as Josephson transmission line, confluence buffer, D flip-flop, and splitter are greater than 40%. Compared to Nb superconductor/insulator/superconductor junctions, their self-shunting characteristics and relatively thick 25 nm barriers can also enhance the integration of circuits and increase the yield to complex circuits.
We report a developed single-flux-quantum (SFQ) circuit fabrication process for vertically-stacked interface-treated Josephson junctions by using high-temperature superconductor (HTS) and also report the 500 GHz operation of a toggle flip-flop (TFF). As the first trial of the circuit operation, we fabricated TFF circuits, because TFF is used as a benchmark circuit to indicate potential of the circuit operating speed. To develop the circuit fabrication process, we utilized the counter YBCO layer as a wiring layer. We obtained resistively-shunted-junction behavior of single junction fabricated by new process. In addition, we observed current-voltage characteristics at the input port and the output port of the TFF, and confirmed that the input voltage coincided with twice the output voltage up to 1.1 mV at 4.2 K, which corresponds to the correct operation up to 500 GHz. This frequency is the highest value in HTS circuits ever observed, which suggests the high potential of the vertically-stacked JJs for the circuit operation.