Spread of critical currents in thin-film YBa2Cu3 O7-x bicrystal junctions

Abstract

A spread of the critical currents in a series array of up to 100
YBa₂Cu₃O_7-x bicrystal junctions has
been studied by Laser Scanning Microscopy. The values of the critical
current I_c of individual junctions in the array have been
obtained by focusing a laser beam on each junction and measuring the
current at which the maximum laser-induced voltage response ΛV on
the array has appeared. The distribution of critical currents in
logarithmic scale was close to a Gaussian one. The I_c-spread
has been found to increase with the increase of misorientation angle of
bicrystal substrate and the decrease of the width of the junctions in
the array

Full text

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. I I, NO, I, MARCH 2001
Spread of Critical Currents in Thin-Film
YBa2Cu307-x Bicrystal Junctions.
Pavel Shadrin and Yuri Divin
Abstract- A spread of the critical currents in a series array
of up to 100 YBazCu307-, bicrystal junction junctions has been
studied by Laser Scanning Microscopy. The values of the
critical current I, of individual junctions in the array have
been obtained by focusing a laser beam on each junction and
measuring the current at which the maximum laser-induced
voltage response AV on the array has appeared. The
distribution of critical currents in logarithmic scale was close
to a Gaussian one. The I,-spread has been found to increase
with the increase of misorientation angle of bicrystal substrate
and the decrease of the width of the junctions in the array.
Index Terms-Josephson
scanning microscopy, local probing.
junctions, grain boundary, laser-
I. INTRODUCTION
he reproducible fabrication of high-quality Josephson
T junctions based on high temperature superconductors
(HTS) is the key problem in development of superconducting
electronics. One of the promising direction in this field is a
development of the technology of bicrystal grain-boundary
Josephson junctions (GBJJ). By this technique it is possible
to produce high-performance junctions
characteristics close to predicted for the resistively shunted
junction (RSJ) model.
Unfortunately, a real grain boundary (GB) in HTS film is
a complicated 3D object. Due to an island-growth
mechanism, the GB in the HTS film is meandering with
respect to a bicrystal boundary in the substrate an4 as it was
supposed, this meandering might result in significant local
differences in transport properties along GB [l]. A
considerable spread of the critical currents of the 24"
YBa2Cu307.x GBJJ has been found [2]. It is an open question
how this spread is related to the meandering of a GB.
In this report we present the results of our study of the
spread of critical currents in the YBa2Cu307-x GBJJ with
different misorientation angles by
Microscopy (LSM) and the results of our study of the
topography of these GBJJ by Atomic Force Microscopy
WM).
with the
Laser Scanning
Manuscript received September 18,2000. This work was supported in part
by German Ministry of Sciences under Grant No. 13N7335/8.
Pavel Shadrin is with the Institute of Solid-State Physics, Juelich Research
Center, D-52425 Juelich, Germany (telephone: (49) 2461-61-2394), e-mail:
*P.Shadrin@fi-juelich.de (on leave from the Institute of Radioengineering and
Electronics, Russian Academy of Sciences, Moscow, Russia )
Yuri Divin is with the Institute of Solid-State Physics, Juelich Research
Center, D-52425 Juelich, Germany (telephone: (49) 2461-61-2394), smail:
Y.Divin@fz-juelich.de
11. EXPERIMENTAL
DETAILS
A. AFM Study o f Bictystal Substrates and Junctions
We have examined the quality of SrTi03 and NdGa03
bicrystal substrates used for fabrication of HTS GBJJ by
AFM and optical microscopy. It was found, that any of these
substrates saer from the defects of different nature. After
careful selection and comparison of the substrates the group
of the has been chosen with minimum set of defects,
consisting from the groove, that arises along the substrate
GB during chemo-mechanical polishing, and small voids,
due to inclusions of gas or d i r t into the GB. A quality of
these selected bicrystals can be characterized by 3
parameters: a depth of the groove along the GB, an average
size and a linear density of the voids.
Some of the available (1 10) NdGa03 bicrystal substrates
with misorientation angles 2x10°, 2x12", 2x14" and 2x18"
[3] got relatively small density of the voids, and these
substrates have been taken for GBJJ preparation. The AFM
topography image of the one of the best 2x14" (1 10) NdGa03
bicrystal substrates is shown in Fig.1. The total height
modulation in the 2.4x2.4 pm2 area near the GB of this
bicrystal substrate was only 3 nm. The GB in the image is
oriented nearly
Fig 1. AFM image of the high-quality NdGaOs bicrystal substrate with angle
2x14". Topography is shown by a grayscale with the amplitude from black to
white equals to 3 nm. One can see the grain boundary as a dark line - a groove
of 40 nm width and 0.7 nm depth. Black spot near the center is a void of 20Onm
diameter and 1,5 nm depth. The linear density of such voids for grain boundary
on this substrate is less then 1 for 100 pm. On the lek bank ofthe bicrystal one
can see terraces of NdGaOl surface with the height of 0,5 nm and the period of
100 nm.
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Fig. 2. AFM image ofthe grain boundary and adjacent area ofthe YBazCusO,.
film, sputtered on the substrate, shown in Fig. 1. The gain boundary oriented
vdically near the canter of the image. One should pay attention to a step-flow
growth of YBCO film and small amplitude of meandering - of order of 100 nm.
vertically and it is seen in the image due to a shallow grove
along it. A groove of 40 nm width has a depth as small as
0,7 nm. A black spot near the center is the only void
observed on more than 100 pm GB length. It has around 200
nm diameter and 1,5 nm depth. On the left bank of the
bicrystal one can see terraces of NdGaO, surface with height
of 0,5 nm and the period of 100 nm, which are due to a
small miscut (0.3”) of the surface of the substrate. The
presence of these terraces demonstrates high quality of the
surface of the NdGaOs substrates used for this study.
The image in Fig.2 shows an AFh4 signals, proportional
to the x-axis derivative of the topography of YBa2Cu307.,
film, that was dc spattered on this substrate (sample
YD006141). One can see some brick-like precipitates with
dfferent orientation on opposite sides of GB. The GB itself
is oriented nearly vertically and located near the center of
the image. The film growth demonstrates a step-flow
character with clearly observed terraces. The meandering of
grain boundary has amplitude of order of 100 nm.
B. LSMset-up
To study a statistical distribution of the critical current of
GBJJ, we have prepared special samples in the form of serial
array of thin film YBa2Cu307-x GBJJ. Meander-shaped
microstrips along the grain boundary crosses a GB many
times, forming a GBJJ at each cross. This structure
schematically shown in Fig. 3. Thus, we have got samples
consisting from a large number of GBJJ of the same width
connected in series.
The exact spread of the critical current values withm an
array is difficult to determine from the dV/dI versus I curves
due to overlay of the different peaks. Ths problem becomes
Fig. 3. Schematic view of the principle of measurements. The meander shaped
line of width D along the bicrystal grain boundary forms a series array of GBJJ.
Voltage response AV(x,y) induced by focused laser beam irradiation is
measured at constant bias current using standard lock-in technique. Seis of
these “snake”-like struciures with the width 1.7 - 15 pm on bicrystal substrates
with different misorientation angles were prepared.
more serious with increasing number of GBJJ in array. Low-
temperature scanning electron microscopy was used to solve
this problem [2]. In this work we use a laser probing for the
same task.
Schematic view of the principle of measurements is shown
in Fig. 3. Laser beam, focused on the surface of the film into
a spot of submicrometer size, induces its local heating and
changes of the total voltage on the sample. When the
position (x,y) of focused beam is scanned across the sample,
the corresponding two-dimensional distribution AV(x,y) of
voltage response is measured and this distribution gives an
information about local electrical properties of the sample.
We have used experimental set-up similar to the one
described previously [4], but with improved optical and
electronic parts. The high-Tc samples were mounted on the
table of the laser-scanning microscope in the special
continuous-flow optical cryostat. Radiation 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-&stance objective on the
surface of the superconducting sample into a spot of around
1.2 pm diameter. The voltage response AV of the sample
was amplified and recorded as a function of the beam
position (x,y) on the sample. Electrical images AV(x,y)
consisting of 512 x 512 points with 8 bit signal resolution
and spatial resolution of order of 1.5 km were obtained for
all Y1Ba2Cu307-, samples under study. The voltage response
amplitude is digitized into 256 gray colors scale.
For all our samples we have used YBa2Cu307-x thin film
deposited on the bicrystal substrate of NdGa0,. Width value,
pointed everywhere in this report are the average width of
the set of junctions, and a dispersion of width for all samples
in the set does not exceed S . 3 p. We have measured
several “snakes” consisting of GBJJ with different width
from 2,5 up to 15 p. In addition, series of samples on the
substrates with different misorientation angles (from 2x10’
to 2x14’) were prepared using exactly the same dc spattering
deposition technology. Each of them consists of snakes of
different widths - from 1,7 up to 5 p.

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t
I
I
I
1
I
0
50
100
150
260
Distance, pm
Fig. 4. Response of pad of a serial array of 10 pn junctions. T=85 K, I=800
PA, mhomogenuity of the current distribution along the GB is clearly seen for
m o s t ofjundions.
As an example, one of such images is shown in Fig.4.
This is a typical image of low-temperature electrical
response for a part of a snake with junctions width 10 pm.
The temperature is 85 K, bias current is 800 pA. The snake
itself is oriented horizontally; a grain boundary crosses them
near the center of the image. Amplitude of the laser-beam-
induced voltage response is encoded by gray scale. One can
see that the maximum response is located on the grain
boundary. Because we are rather close to Tc, some weak
voltage response from the Y1Ba2Cu307-x film can be
observed. It is easy to see, that for most of GBJJ distribution
of voltage response (that is proportion to the local current
density) along the GB is inhomogeneous. For some junction
it is detected, that response is concentrated to one small spot,
some have 2 spots.
So, one can say, that a current do not flow throw GB
homogeneously, but concentrated at some microshorts with
smallest local resistance. If there are 2 such microshorts on
one junction, it will works as a SQUID. Due to this fact, for
further consideration we have choose arrays of junctions
with smaller width of 5 and 2.5 pm. Because Josephson
penetration depth hJ for such junctions at temperatures 77 K
and above is bigger than the width, we can neglect magnetic
effects.
Using of such LSM electrical images it is possible to
directly measure IC of each junction. It is easy to see that the
value of the voltage response is depended upon the bias
current Ib and the temperature T. Let us consider a small
Josephson junction. Heating by laser beam increases the
temperature from TI to TZ. The correspondmg values of
critical current are Icl and Ic2 (Icl>Ic2). Our voltage response
is equal to the difference at constant current of two I-V
curves- for TI and T2. If the bias current is lower than Ic2,
we have no response. When bias is equal to Ic2, the first
response appears and increases with the increasing bias
current. The maximum response is observed at Ib=Icl. With
further increasing of biasing, we can observe a slow decrease
of the response. So, changmg the biasing and observing the
amplitude of the laser-beam-induced voltage response for
serial array of Josephson junctions, we can for each of them
find the value of critical current, that is the bias current that
corresponds to the maximum response.
111. RESULTS AND DISCUSSION
After measuring the sets of response distribution images
(like in Fig. 4) at different bias current we can combine them
to the total pictures (Fig. 5). Here, an array of 2.5 pm GBJJ
from the sample WO6141 was measured at 77 K with bias
$$
t
a
%
3
100
I
1
0
I
I
I
50
100
150
200
250
300
Distance, pm
Fig. 5. Combined image for series of eledrical images like in Fig. 4 with
different bias current. Array of 2.5 pn GBJJ was measured at 77 K w i t h bias
current increased in 60 times in logarithmic scale.
current increased in 60 times in logarithrmc scale. The array
was fabricated on 2x14' NdGaO3 bicrystal substrate, shown.
in Fig. 1. The scans, recorded at different bias current values
are displaced vertically. More dark regions corresponds to
larger voltage signal For each of 63 junction under study,
looking on the column from down side to upper, one can see
no response at bias current below critical, then fast
increasing, maximum at critical current and slow
decreasing. So, with LSM we were able to find a critical
current for each of junction in the snake.
Resulting statistical data, which were obtained from these
combined pictures, are presented in Fig.6. If the statistical
chart of critical current dstribution is plotted in linear scale
of current (see Fig,6a), some non-symmetrical distribution
appears. The situation is different, if we use a logarithm
scale for the current axis (Fig.6b). The resulting dstribution
is close to the classical Gauss curve. The same situation was
observed for all samples under study - for all angles and
width from 2.5 up to 15 pm We have measured 3 bicrystal
snake samples, prepared with exactly the same technological
process, but on substrates with different misorientation
angles. Resulting parameters of the statistical distribution for
5 pm snakes are shown in the Table I.
a
N
1
b
15
10
5
0
0 5W 1000 1500 2000 2500
IC, arb.un.
100 300 1DM3 3000
IC, arb.un.
Fig. 6. Statistical distribution of critical currents, obtained fromthe Fig.5. For a
linear current scale (a) this distribution looks non-symmetrical, but for a
logarithmic one it fits the Gaussian curve.

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TABLE I
FOR ARRAY OF S W - W I D E GBJJ.
TABLE I1
SPREAD OF THE IC DISTRIBUTION
SPREAD OF THE IC DISTRIB~ON
FOR SAMPLES ON SIMILAR 2x14’
SUBSTRATES.
YD006142 YD006262 YD006141
YD006141 YD002222
bicrvstal anele
TcT, K
Cater ofpeak IC, FA
Spread
2X10.5’
4 s
30
29%
2x12’
5
18
3 7%
2x14’
4
37
44%
Y
2,5 C”
5 pm
55%
42%
71%
44%
An observed results demonstrate that an increasing of the
spread is proportional to the increasing of the misorientation
angle of the substrate, in spite of a fact that 2x14’ sample
has higher quality of bicrystal boundary and lower amplitude
of meandering.
For a comparison, we have used another sample that was
prepared on the substrate from the same series, but with
other film preparation procedure (sample YD002222). It was
founded by AFM examination, that in this case film has
growth islands, and amplitude of meandering is larger, but
of the same order. After measuring statistical distribution of
critical currents, dispersions of statistical distribution are
calculated (see Table 11).
It is obvious, that for junctions of larger width the spread
of critical current should be smaller, due io an averaging.
Not surprising, that for the sample YD006141 with more
homogeneous film structure the spread is essentially lower
for 2.5 and nearly the same for 5 pm junction arrays. For a
sample YD8033 11 on 2x14’ substrate of lower quality for 10
pm junction array dispersion has a value 47%, that close to
the data for high-quality substrate, demonstrated above. One
can make a conclusion, that for a junction wide enough for
averaging of internal junctions inhomogeneities, the spread
of distribution depends mostly on the substrate bicrystal
angle and only a weak dependence on substrate GB
characteristics can be observed.
So, comparing data of Table I and Table 11, one can see,
that homogeneity of the film structure effects the width of
the Gaussian distribution for a small junction width (2.5 pm)
only. For more wide junctions (5 pm and more) width of
obtained Gaussian distribution depends mostly on the
misorientation angle of the substrate.
Iv. SUMMARY
Statistical distribution of critical current for high-quality
GBJJ with Merent misorientation angle and junction width
were measured with laser local probing technique.
It was demonstrated, that these distributions fit to
Gaussian curve with logarithmic scale of bias current axis.
The IC spread was found to be increasing with the
misorientation angle of the substrate from 29 % for 2x10,5”
to 44% for 2x14” for junctions of 5 pn width. and does not
depends on the growth mechanism of the film.
ACKNOWLEDGMENT
We are grateful to M. Lyatti for the help in data
processing, Dr. poppe and prof. I. M. Kotelyanskii for
helpful discussions~
REFERENCES
[l]
[2]
J. Alarco et al. “Microstructure of an artificial grain boundary weak link
in an YBCOthin film.” LllWamicroscopy, 51, pp 239-246, 1993
R. Gerdemann, R. Gross et al. “Spatially resolved analysis of high-Tc
grain boundary Josephson junctions and arrays.” J. Appl. Phys. 76 (12),
pp. 8005-8015, DE 1994
[3] Cryotec Co. Moscow, Russia.
[4] P. M. Shadrin , Y. Y. Divin, “Submicrometer eledriwl imaging of grain
boundaries in high-Tc t h i n - f i l m junctions by laser scanning microscopy.”
Physica C, 297,69 (1998)