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IEEE Transactions on Magnetics

High frequency properties of all-NbN nanobridges with gap structure in I-V curves

K. Hamasaki, Tsuyoshi Yakihara, Z. Wang, T. Yamashita, Yoji Okabe

Published Mar 1, 1987• 15 citations
Publisher

Abstract

All-NbN nanobridges with gap structure in I-V curves have been reproducibly constructed using RIE and lift-off techniques. The nanobridges had a width of 2 μm, and a thickness ofcRNproducts of ∼3 mV, and low excess current. Small-area dc SQUIDs were made using the nanobridges, and analyses of the response to magnetic flux were performed. The current-phase relationship of the nano-bridges was found to be close to sinusoidal. The maximum LC resonant voltage was about 1.2 mV, corresponding to a frequency of 580 GHz. The IF peak was obtained up to the bias voltage of about 4 mV in 101 GHz Josephson mixing.

DOI
10.1109/tmag.1987.1064833

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All-NbN nanobridges with gap structure in I-V curves have been reproducibly constructed using RIE and lift-off techniques. The nanobridges had a width of 2 μm, and a thickness ofcRNproducts of ∼3 mV, and low excess current. Small-area dc SQUIDs were made using the nanobridges, and analyses of the response to magnetic flux were performed. The current-phase relationship of the nano-bridges was found to be close to sinusoidal. The maximum LC resonant voltage was about 1.2 mV, corresponding to a frequency of 580 GHz. The IF peak was obtained up to the bias voltage of about 4 mV in 101 GHz Josephson mixing.
Using RIE (reactive in etching) and lift-off techniques, all-NbN nanobridges with gap structure in the I-V (current-voltage) curves have been reproducibly constructed for testing as millimeter-wave mixers. The nanobridges were constructed on NbN/MgO/NbN edge junctions. They were characterized by measurements of their I-V curve and dI/dV versus V and by mixing experiments at 105.9 GHz. These devices had high normal-state resistance, and showed almost ideal Josephson response to external magnetic flux and to millimeter-wave irradiation. The I-V curve remained nonhysteretic from T/sub c/ to 4.2 K, and millimeter-wave-induced steps in the I-V curve were observed up to approximately 3 mV. The cooling of the NbN nanobridges was quite good due to their small size and VTB geometry and the high thermal conductivity of the MgO films. Hence the Josephson response of these devices to millimeter waves was comparatively less limited by self-heating.>
The testing of NbN(g) nanobridges with Al direct shunt resistors for use as millimeter-wave Josephson mixers is discussed. The NbN(g)/Al nanobridges have a nearly sinusoidal current-phase relation, high resistance, well-defined gap structure at approximately 4 mV, large I/sub J/ R/sub N/ products of approximately 3 mV, and nonhysteresis I-V curves. By means of the Al direct shunt, the effective noise temperature T/sub N//sup eff/ of the NbN(g) nanobridges, which was calculated by transition-state theory, was reduced to 4.2 K as a result of reducing the self-heating and noise rounding effects. The NbN(g)/Al nanobridges show almost ideal Josephson response to millimeter-wave radiation at 106 GHz. The well-pronounced Josephson steps were observed up to approximately 4 mV. The observed systematic decreasing of the step heights with increasing the RF current can be accounted for by the effect of the thermal noise if an effective noise temperature of T/sub N/=15 K (at i/sub RF/=3) is assumed. The IF peaks in the Josephson mixing were obtained up to the bias voltage of approximately 1.2 mV. The NEP was about 3*10/sup -20/ W/Hz at 4.2 K.
We have fabricated two types of NbN nanobridge dc Superconducting QUantum Interference Devices (SQUIDs). From the analyses of the response to magnetic field, it is found that the current-phase relationship for the planar-type nanobridge and the nanobridge fabricated by field evaporation is close to sinusoidal. For V\gtrsim720 µV, The spectral densities of the voltage noise show linear dependence on the voltage for the SQUIDs with nanobridges whose length is about the superconducting coherence length of NbN. The SQUIDs with nanobridges with lengths of several times the NbN coherence length exhibit parabolic voltage dependence.
In the recent years, nanoscale superconducting quantum interference devices (nano-SQUIDs) played a fundamental role in the study of small spin systems. Nano-SQUIDs typically employ nano-Dayem bridges having dimensions (length LLL and/or width WWW) greater than coherence length $\ensuremath{\xi}$ of the superconducting film. They exhibit characteristics different from those of standard SQUIDs because the current-phase relationship (CPR) is nonsinusoidal, rendering most of the theoretical predictions based on the standard SQUID theory usually unreliable. Here, we present a noise theory of dc nano-SQUIDs based on Dayem nanobridges. We have computed the main characteristics of this quantum device including current-voltage and voltage-magnetic flux characteristics, magnetic flux-to-voltage transfer factor, and spectral densities of voltage and magnetic flux noise for LLL/$\ensuremath{\xi}$ ratios ranging from 1 (sinusoidal limit) to 3.5 (hysteretic limit). The CPRs have been computed by using the theory of Josephson weak links based on the Ginzburg--Landau equation. The results show a dependence of the magnetic flux noise spectral density on (LLL/$\ensuremath{\xi}$)4/3{}^{4/3}4/3 involving a degradation of about a factor of five between the two extreme cases and are consistent with experimentally measured magnetic flux noises reported in the literature. These results provide useful information for both device physics and their applications.
Reduction of the size of the superconducting quantum interference device (SQUID) ring structure improves its sensitivity to magnetic moments. Nanobridge junctions are advantageous for SQUID miniaturization. However, many planar SQUIDs with nanobridge junctions have shallow flux modulation depths because of their nonideal Josephson current-phase relationships. Here, we fabricated a three-dimensional niobium nitride (NbN) miniaturized SQUID in which the nanobridge junction structures are much thinner than the superconducting ring. We grew a thick superconducting NbN film on a silicon wafer and embedded an insulating slit in the middle of the film. By setting two thin nanobridge junctions across this insulating slit, we were able to manufacture NbN miniaturized SQUIDs that, in principle, can become nano-SQUIDs. The flux modulation depths of these devices reached 36% at 4.2 K. In addition, the fabricated devices show reversible current-voltage characteristics in the absence of any shunting resistance.