Spread of critical currents in thin-film YBa2Cu3O7-x bicrystal junctions and faceting of grain boundary

Abstract

The statistical distributions of critical currents in series arrays of YBa₂Cu₃O_7-x grain-boundary junctions have been studied by low-temperature laser scanning microscopy. A set of arrays has been fabricated on [110] NdGaO₃ bicrystal substrates with misorientation angles from 2 × 10° up to 2 × 26° and patterned to the widths from 1.7 up to 5 micrometers. The critical current values of the individual junctions in the array have been obtained by focusing a laser beam on each junction and measuring the bias current at which the maximum laser-induced voltage response has appeared on the array. The measured critical current distributions have been demonstrated to be close to a log-normal Gauss function. A spread of this distribution has been found to increase with a bicrystal angle. YBa₂Cu₃O_7-x grain-boundary topography has been studied by atomic force microscopy. From results of these measurements we suppose that the maximum values of critical current density might be assigned to the symmetrical facets of grain boundary.

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 13, NO. 2, JUNE 2003 603
Spread of Critical Currents in Thin-Film
YBa?Cu?O? ?Bicrystal Junctions and
Faceting of Grain Boundary
Pavel Shadrin, C. L. Jia, and Yuri Divin
Abstract—The statistical distributions of critical currents in se-
ries arrays of YBa?Cu?O?
grain-boundary junctions have been
studied by Low-Temperature Laser Scanning Microscopy. A set of
arrays has been fabricated on (110) NdGaO?bicrystal substrates
with misorientation angles from 2
terned to the widths from 1.7 up to 5 micrometers. The critical
current values of the individual junctions in the array have been
obtained by focusing a laser beam on each junction and measuring
the bias current at which the maximum laser-induced voltage re-
sponse has appeared on the array. The measured critical current
distributions have been demonstrated to be close to a log-normal
Gauss function. A spread of this distribution has been found to in-
crease with a bicrystal angle. YBa?Cu?O?
pography has been studied by Atomic Force Microscopy. From re-
sults of these measurements we suppose that the maximum values
of critical current density might be assigned to the symmetrical
facets of grain boundary.
10 up to 226 and pat-
grain-boundary to-
Index Terms—Bicrystal boundary, critical current, Josephson
junction, meandering.
I. INTRODUCTION
T
of high-quality Josephson junctions. One of the promising di-
rection in this field is a technology of bicrystal grain-boundary
Josephson junctions (GBJJ). By this technique it is possible
to produce high-performance junctions with characteristics
close to that predicted by the resistively shunted junction (RSJ)
model.
Unfortunately, due to a small coherence length in high-tem-
perature superconductors (HTS), electrical properties of GBJJ
are very sensitive to defects of any kind. At the same time, a
real grain boundary (GB) in HTS film is a complicated 3D ob-
ject. Due to an island-growth mechanism, the GB in the HTS
film is meandering with respect to a bicrystal boundary in the
substrate [1] and, as it was supposed, this meandering might re-
sult in significant local differences in transport properties along
GB. A considerable spread of the critical currents of the 24
YBa Cu OGBJJ has been found [2]. It is an open question
how this spread is related to the meandering of a GB.
HE key problem in the development of high temperature
superconducting electronics is a reproducible fabrication
Manuscript received August 5, 2001. This work was supported in part by the
International Science and Technology Center under Grant 1912.
P.ShadriniswiththeInstituteofRadioengineeringandElectronicsofRussian
Academy of Sciences, Moscow 101999, Russia (e-mail: psh@mail.cplire.ru).
C. L. Jia and Y. Divin are with the Institute of Solid-State Physics, Juelich
Research Center, D-52425 Juelich, Germany (e-mail: C.Jia@fz-juelich.de;
Y.Divin@fz-juelich.de).
Digital Object Identifier 10.1109/TASC.2003.813959
In this report we present the results of our study of the spread
of critical currents in the YBa Cu O
misorientation angles by Laser Scanning Microscopy (LSM)
and the results of our study of the topography of these GBJJ
by Atomic Force Microscopy (AFM).
GBJJ with different
II. EXPERIMENTAL DETAILS
Wehaveusedthesamplelayout,whichissimilartotheprevi-
ously described one [3]. To study a statistical distribution of the
critical current of GBJJ we have prepared a special set of sam-
ples. The thin-film YBa Cu O
is oriented along the grain boundary and crosses a GB many
times, forming a GBJJ at each cross. Thus, we have got an array
of 100 GBJJ of the same width connected in series. Bicrystal
substrates of (110) NdGaO with symmetrical misorientation
angles 210.5 , 212 , 2
[4] were used for preparation of samples.
For all our samples we have used YBa Cu O
depositedonthebicrystalsubstrateofNdGaO bydcsputtering.
Allthewidthvaluesinthisreportaretheaveragewidthvaluesin
thearray. Adispersion of thejunction widthin an arraydoes not
exceed
0.2m. We have measured several arrays consisting
of GBJJ with different width from 2, 5 up to 15 m.
Some GBJJ prepared within the same series as ones used for
statistical measurements were examined with HREM to check
the fine structure of GB. It was observed with atomic resolu-
tion that grain boundary contain no amorphous layer; the me-
andering amplitude is of order of 0.1–0.5
facets of symmetrical and asymmetrical microstructures are of
order 1–10 nm long.
It is difficult to determine a spread of the critical currents
within an array from the dependence of the differential re-
sistance
from the transport current
due to overlappingof the different peaks. The problem becomes
moreserious withincreasingnumber ofGBJJ inthearray. Low-
temperature scanning electron microscopy was used to solve
this problem [2]. In this work we use a laser probing for the
same task. For LSM measurements we have used an experi-
mental set-up similar to the one described previously [3].
The high-
samples were mounted on the table of the
laser-scanning microscope in a special optical cryostat. Radia-
tion from an Ar-ion laser with a wavelength of 488 nm and a
power level of up to 34 mW was focused by a long-distance
objective on the surface of the superconducting sample into
a spot of around 1.2
m diameter. The voltage response
V of the sample was measured by standard lock-in technique
meander-shaped microstrip
14 , 218.4 and 226.6
thin film
m, and elementary
through the array
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604IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 13, NO. 2, JUNE 2003
Fig. 1.
of YBa Cu O
average width of 2.5 ?m. For a linear current scale (a) this distribution looks
nonsymmetrical, but for a logarithmic one (b) it fits the log-normal Gaussian
curve.
The statistical distribution of the critical currents ?
GBJJ with the misorientation angle of 2 ? 14
in the array
and the
and recorded as a function of the beam position (
sample.
Using the LSM electrical imaging technique, it is possible
to measure the
-value of each junction in the array directly.
The focused laser beam induces local heating of the junction
from
toand suppresses the value of critical current from
to(). The voltage response
to the difference of two –
curves—for
stant bias current. If the bias current is lower than
have no response. When bias is equal to
appears and increases with an increase of the bias current. The
maximum response is observed when
increase of biasing, we can observe a slow decrease of the re-
sponse. So, increasing the biasing and observing the amplitude
of the laser-beam-induced voltage response for serial array of
Josephson junctions, we can find the value of critical current
for each of the junctions by recording the bias current that cor-
responds to the maximum response.
) on the
is equal
—at con-and
, we
, the first response
. With further
III. RESULTS AND DISCUSSION
After measuring the sets of LSM voltage images at the var-
ious bias currents we can find a critical current for each of the
junction in the array. Typical resulting statistical data are pre-
sented in Fig. 1. These data have been measured for an array of
2.5 m-wide junctions on 2
14 substrate.
If the critical current distribution is plotted in a linear scale of
thecurrent currents[see Fig.1(a)],some nonsymmetricaldistri-
bution appears. The situation is different if we use a logarithmic
scale for the current axis [Fig. 1(b)]. The resulting distribution
was found to be close to the log-normal Gaussian curve
(1)
where
Gaussian distribution,
The origin of the observed log-normal distribution of the crit-
ical current density seems to come from the tunneling mecha-
nism of the superconducting transport between two misoriented
high-Tc films.Thegrainboundarybetween filmswitha defined
misorientation angle can be considered as a tunnel barrier with
is an average critical current,
is the area under the curve.
is a spread of the
Fig. 2.
GBJJ with different misorientation angles at the temperature of 77 K. The
spreads of the corresponding Gaussian distributions are shown by bars.
The average values of critical current density for arrays of 5 ?m-wide
some barrier thickness . The critical current density is expo-
nentially dependent from the barrier thickness with some char-
acteristic thickness
.
In the real high-
bicrystal junction, due to the faceting of
the grain boundary, the local misorientation angles are spread in
somerange.Thismight resultinthespreading oftherealbarrier
thicknesses
and the characteristic thicknesses
turn will result in the exponential spread of the local current
densities.
The inhomogeneous current distributions with some perco-
lation length
of several micrometers have been observed in
realYBa Cu O
grain-boundary junctions [5]. Choosing the
width of the junctions around the percolation length gives us a
possibility to increase an amount of the junctions with RSJ-like
behavior [6] but leads to the extended spread of the critical cur-
rent densities.
Similar situation was observed for all samples under study
with the bicrystal angles from 2
widths from 2.5 up to 15 m. The parameters of measured sta-
tistical distributions for a set of arrays of 5- m wide GBJJ with
different bicrystal angles are shown in Figs. 2 and 3.
AsonecanseeinFig.2,anaveragecriticalcurrentdensityhas
ageneraltendencytodecreaseexponentiallywithanincreaseof
the bicrystal angle for the angles 2
18.4 .Therearetwoexclusionsfromthisexponentialfall-down,
at the angles of 2
14 and 2
erage critical current density for the angle 2
pared with that of for 210.5 and 2
plained as a result of more well-ordered structure that arises at
the coherent angle and a better quality of bicrystal boundary in
the substrate. Practically the same values of the average current
densities and the spreads for junctions with 2
18.4 might be due to their equal difference from the misorien-
tation angle of 45 , where minimum current density for GBJJ
might be expected, e.g., from the d-wave symmetry of the order
parameter.
ThespreadofGaussiandistribution ,showninFig.3,seems
to be more regular characteristic of GBJJ. For junctions of fixed
width this parameter depend on the bicrystal angle only. Again,
like for the average values of the critical current densities, the
angular dependence of the spread
shape with respect to 45 .
, which in
10.5 to 226.6 and the
10.5 , 212 and 2
26 . A higher value of the av-
14 , when com-
12 data, may be ex-
26.6 and 2
might have a symmetrical
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SHADRIN et al.: SPREAD OF CRITICAL CURRENTS IN THIN-FILM YBa Cu OBICRYSTAL JUNCTIONS605
Fig. 3.
misorientation angle of GBJJ for the same set of samples as in Fig. 2.
The spread of the log-normal Gaussian distribution as a function of the
Fig. 4.
(upper image) and maximal (lower image) values of the critical current in the
array of 100 junctions. The bicrystal angle is 2 ? 12 .
The AFM topography images of 5 ?m-wide GBJJ with the minimal
The topography images of junctions with maximal and min-
imal values of the critical currents in the array of 100 GBJJ with
the bicrystal angle 2
12 have been measured with AFM. In
Fig. 4 one can see an image for junction with minimal current
(above) and maximum current (below). The bicrystal boundary
in the substrate is oriented horizontally for both images along
thearrows.InFig.4(a)thegrainboundaryintheYBa Cu O
filmcanbeseenasawavydarklinenearthecenteroftheimage.
One can see that most of the GB for this sample has deviations
fromtheorientationofbicrystalboundaryinthesubstrateonthe
angles close to
12 . The orientation of such segments is indi-
cated by dotted lines. It means, that most part of microfacets for
this junction has nonsymmetrical orientation: the planes (100)
or (010) for one side and the planes with higher indexes on the
other side. This profile is very unusual for the -axis bicrystal
GBJJ. Typically, GBJJ profile is much more meandered and has
a general orientation along the boundary in substrate. In addi-
tion, a presence of dark line on the upper AFM image means
that there is some groove along the GB in the YBa Cu O
film.
In Fig. 4(b) one can see the junction with maximum critical
current in the same array. It is not so easy to find the position
of GB in this image, due to an absence of significant groove
along the GB. Hardcopy limitation adds additional difficulties
to image analysis. But, at careful consideration, it is possible to
identify the profile of GB. In this case the profile of GB is close
to a straight line and there is practically no angular deviation
from the orientation of bicrystal boundary in the substrate. It
means, that most part of the microfacets for this junction has a
symmetrical orientation. This profile of GB is also very unusual
for the -axis bicrystal GBJJ.
TheobserveddeviationoftheGBprofilefromthetypicalone
forthesetwojunctionsmaybeusedforexplanationoftheirelec-
trical characteristics. One may assume, that the maximum crit-
ical current density corresponds to the symmetrical orientation
of the microfacets. On the other hand, the microfacets termi-
nated by the planes (100) or (010) from one side of the junction
may correspond to the minimal critical current density.
Indeed, from geometrical point of view, the atomic period
of YBa Cu O
for both sides is equal only for symmetrical
orientation of GB. For nonsymmetrical orientation we will get
some irrational ratio of atomic periods at both sides of GB. This
factmeans,thatcrystallographicboundarystructureofsymmet-
rical facets is more well-ordered and a tunnel barrier is this case
might have a smaller thickness with the respect to that of for the
asymmetric ones.
IV. SUMMARY
The statistical distributions of critical current densities for
the arrays of YBa Cu O
grain-boundary Josephson junc-
tions with different misorientation angles and junction widths
were measured witha help oflaser local probing technique. The
experimental data are in good agreement with the log-normal
Gauss distribution.
The profiles of the grain boundaries for the junctions with
maximum and minimum critical currents in the array were ob-
tainedfromAFMtopographyimages.Itmightbeassumedfrom
the comparison of these images, that maximum values of the
critical currentdensity inthe YBa Cu O
correspond to the symmetrical facets of grain boundary.
bicrystal junctions
ACKNOWLEDGMENT
The authors are grateful to Dr. Poppe and Prof. I. M.
Kotelyanskii for helpful discussions.
REFERENCES
[1] J.Alarcoetal.,“Microstructureofanartificialgrainboundaryweaklink
in an YBaCuO thin film,” Ultramicroscopy, vol. 51, pp. 239–246, 1993.
[2] R. Gerdemann et al., “Spatially resolved analysis of high-Tc grain
boundary Josephson junctions and arrays,” J. Appl. Phys., vol. 76, no.
12, pp. 8005–8015, Dec. 1994.
[3] P. M. Shadrin and Y. Y. Divin, “Spread of critical currents in thin-film
YBa Cu O
bicrystal junctions and faceting of grain boundary,”
Physica C, vol. 372, p. 80, 2002.
[4] Cryotec Co., Moscow, Russia.
[5] P. M. Shadrin and Y. Y. Divin, “Submicrometer electrical imaging of
grain boundaries in high-Tc thin-film junctions by laser scanning mi-
croscopy,” Physica C, vol. 297, p. 69, 1998.
[6] Y. Y. Divin et al., Proceedings of EUCAS 97, H. Rogalla and D. H. A.
Blank, Eds.Bristol: IOP Publishing Ltd., 1997, pp. 467–470.
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