Instrumentation & Measurement Magazine 23-2 - 7

c = 11.68 Å. Currently, in addition to various single-crystal substrates used for sputtering epitaxial HTS films [35], bicrystal
substrates made of SrTiO3, MgO, Y-ZrO2 (YSZ), Si, NdGaO3,
LaAlO3 and sapphire (α-Al2O3) are available. For microwave
applications, materials with a dielectric constant ε ≅ 10: MgO,
α-Al2O3 and Si are attractive. Unfortunately, bicrystal junctions made on sapphire and silicon demonstrate the Josephson
effect at temperatures well below the boiling point of liquid
nitrogen, 77.4 K. High-quality bicrystal JJs with high critical
currents and characteristic voltages at nitrogen temperatures
can be produced on magnesium oxide. But arrays of such junctions have been little studied so far. Among the remaining
materials, NdGaO3 and YSZ are most interesting. The advantage of NdGaO3 compared to YSZ is the perfect matching of
the crystal lattice with YBCO, which makes it possible to fabricate HTS films of very high quality, as well as low attenuation
coefficients at high frequencies. On the other hand, the advantage of YSZ substrates is their wide availability and, most
importantly, the possibility of manufacturing substrates with
several bicrystal grain boundaries, which is very important for
fabrication in multi-junction Josephson circuits.
At the same time, additional restrictions should be noted
when bicrystal substrates from YSZ are used. Firstly, this is a
high dielectric constant ε = 26 and there are significant dielectric losses, which are two to three orders of magnitude higher
than the losses in sapphire at frequencies of about 100 GHz.
Secondly, the arrays of series-connected JJs made on a bicrystal substrate have the shape of a meander. The noted features
limit the possibilities in the design of the circuits and prevent
the uniform distribution of the high-frequency bias current
along the array. These reasons make it difficult to use the
known schemes for incorporating bicrystal JJs into the microwave lines.
Despite these limitations, HTS arrays of bicrystal JJ structured on yttria-stabilized-zirconia bicrystal substrates are
used now in dc voltage standards. This is due, firstly, to the
ideal IVCs of single junctions at nitrogen temperatures (Fig. 1),
both with no mm wave power and when being irradiated [36].
In the latter case, the steps are observed with a current range
exceeding Ic in full accordance with the Resistively Shunted
Junction (RSJ) model of the Josephson junction. Secondly, it
is connected with the ability to synchronize arrays containing
hundreds of JJs. For this purpose, a two-layer structure containing 120 nm YBCO epitaxial film covered in situ by 30 nm
Au film was used to fabricate series arrays of shunted HTS
junctions. Junction shunting is necessary to reduce the spread
of Rn, which is one of the conditions for the realization of (2).
The JJs are formed at the intersection of the bicrystal grain
boundary with thin-film bridges of HTSs with a width w=6
μm. The width of the meander, i.e., the size of the meander in
the direction perpendicular to the electric field vector, is chosen to be λeff / 2, where λeff is the effective wavelength in the
YSZ substrate. Thus, each strip of the meander forms an external electromagnetic field with a half-wave resonator at a
frequency of about 75.2 GHz with RF current maximum at its
center in the region of the JJs [9].
April 2020

Fig. 1. IVC of a 5 μm wide junction at 77 K: (a) without external radiofrequency current and (b) with microwave irradiation at 93.37 GHz.

With this optimal design and quasioptical method of irradiation of the arrays [37], a maximum first step at a Josephson
voltage of about U ≅ 0.1 V on the array of 620 bicrystal junctions
at temperature of 79 K and frequency of f= 77.465 GHz was
achieved (Fig. 2) [9]. The resulting characteristic frequency
fc ≈ 60 GHz was smaller as f is optimum for the observation
of the first voltage step under mm-wave irradiation. Enlarged portions of the I-V curves demonstrate the amplitude
and the steepness of the step with nV resolution. Sub-arrays
containing 62 junctions each were also synchronized at the
same frequency and power. Steps from 0.01 V to 0.1 V were
observed. It is important to note that the average critical current is Ic = 0.55 mA with a standard deviation of 19 μA or 3.5%.
The average resistance of one shunted junction is about 0.18 Ω
with a standard deviation of 2.5 mΩ, or less than 1.5%. These
parameters approach the best results typical of the advanced
technology of LTS JJs arrays. Hence, the arrays of bicrystal
junctions are challenging for application in dc and ac programmable voltage standards.
There are two possibilities for enhancing the packing density of the junctions and correspondingly increasing the output
voltage. One is to use substrates with more than one grain
boundary. The grain boundaries are parallel and only spaced
about 10 to 20 μm, so the meander can cross them all, resulting
in a k-times enhanced junction rate per length, if k is the number of grain boundaries. Arrays prepared on such substrates
showed promising results for k=2 and were described in [38].
The other option is to push the meander together. While the
width of the junctions may be determined by optimal junction
size considerations and cannot be reduced, the space between
the junctions could be as small as possible. It seems possible to
increase the output voltage U from HTS circuit up to 1 V by using bicrystal substrates with k = 10 grain boundaries.

Bicrystal Junctions in Metrology
A commercially available table top dc voltage standard N421 based on the successful development of the arrays of

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Instrumentation & Measurement Magazine 23-2

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