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Japanese Journal of Applied Physics

Fabrication and Noise Properties of NbN Nanobridge dc Superconducting QUantum Interference Devices (SQUIDs)

Akinobu Irie, Hiroya Abe, M. Hátle, Katsuyoshi Hamasaki

Published Dec 1, 1991• 1 citation
Publisher

Abstract

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.

DOI
10.1143/jjap.30.3925

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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.
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.
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.
Abstract NbN nanobridge dc SQUIDs were fabricated by the field evaporation method and their noise characteristics were evaluated. The fabricated device has a magnetic flux‐voltage conversion efficiency Φ of about 2.5 mV/Φ0 and hence the device has a higher gain than that of the tunneling‐type device with a resistive shunt. The energy resolution is as small as 40 h. When the fabricated SQUID was installed in an FLL circuit and the dynamic range was measured, it is found that an output is proportional to the input for the input magnetic flux of ±32Φ0 without removing the lock. The magnetic flux resolution of the FLL circuit used is 9 × 10−10 Φ02/Hz in the white noise region (at 3 kHz). In the low‐frequency region less than 100 Hz, the 1/f noise described by 5.8 × 10−8 (1/f)Φ02/Hz was observed. This value of the magnetic flux resolution is large by one order of magnitude than that inherent to the device and the noise of the electronics is predominant.
For the fabrication of nanoscale SQUIDs, for example having loop diameters of order 1 µm or smaller, the conventional trilayer window-type Josephson junctions are too large. Instead, we consider the use of nanobridge junctions with dimensions of order 100 nm. The problem of hysteresis in the junctions' current–voltage characteristic (IVC) is avoided by depositing a gold overlayer, which serves as both a thermal and electrical shunt. Noise measurement of dc SQUIDs which incorporate such Nb nanobridges as the Josephson junctions are reported. Devices having different IVCs were fabricated and characterized. The best device has a flux noise of 5 × 10 −6 Φ 0 Hz −1/2 at 4.2 K, and there is no significant increase in the noise level in a static magnetic field up to 2 mT. Techniques for increasing the voltage modulation depth, and hence improving the flux noise performance, are discussed.
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.
Integrated NbN direct current superconducting quantum interference device (DC-SQUID) magnetometers were developed based on high-quality epitaxial NbN/AlN/NbN Josephson junctions for SQUID applications operating at high temperatures. We report the current–voltage and voltage–flux characteristics and the noise performance of the NbN DC-SQUIDs for temperatures ranging from 4.2 to 9 K. The critical current and voltage swing of the DC-SQUIDs decreased by 15% and 25%, respectively, as the temperature was increased from 4.2 to 9 K. The white flux noise of the DC-SQUID magnetometer at 1 kHz increased from 3.9 μΦ0/Hz1/2 at 4.2 K to 4.8 μΦ0/Hz1/2 at 9 K with 23% increase, corresponding to the magnetic field noise of 6.6 and 8.1 fT/Hz1/2, respectively. The results show that NbN DC-SQUIDs improve the tolerance of the operating temperatures and temperature fluctuations in SQUID applications.