IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 9, No. 2, JUNE 1999
2993
Effects of Oxygen Content on YBCO Josephson Junction Structures
Jason P. Sydow, Michael Berninger and Robert A. Buhrman
Cornell University, Ithaca NY
Brian H. Moeckly
Conductus Inc. Sunnyvale CA
Abstract-The high degree of crystal stress and strain
present at, and in the vicinity of, high angle grain
boundary (GBJ), ramp edge COdoped SNS (CO-SNS),or
interface engineered junctions (IEJ) can lead to localized
and highly non-uniform regions of basal plane oxygen
loss in YBa2Cu30,.6 (YBCO). These oxygen inhomogeneities will, to a greater or lesser degree, affect or
cause the superconducting weak link behavior demonstrated by these types of junctions. In order to examine
the impact of the localized oxygen microhanostructure
on the weak link behavior of these three junction technologies, we have utilized ozone anneals to provide a partial pressure of atomic oxygen far in excess of that produced by standard O2 anneals.
I. INTRODUCTION
The enhanced partial pressure of atomic oxygen provided
by ozone is used to achieve a greater level of oxygenation in
stressed YBCO systems, such as doped or crystalographically
strained YBCO, than that which is routinely obtained
through standard -5OOC, 1 atm. 0 2 , post fabrication anneals.
As we shall see below, by performing ozone anneals on various HTSC Josephson junction structures, we are able to observe the more intrinsic nature of the weak link present in a
particular device architecture without the added complication
of the oxygen disorder and loss which is often a secondary
effect of the fabrication procedure.
111. CO-SNSJUNCTIONS
Fig. 1 presents IV characteristics for a CO-SNS junction
with a normal layer comprised of 30% CO doped YBCO.
Previous work with CO-YBCO thin films has demonstrated
that ozone annealing can increase the T, of this material to
75K [4]. Fig. 1 demonstrates that by substantially increasing
the oxygen content and order of the N-layer and subsequently
increasing T,, ozone annealing has transformed the device
f?om a resistively shunted junction (RSJ) with substantial
excess current at T=35K, to a flux flow type characteristic.
After ozone annealing, and at 35K, the device is SS'S, where
S' is the CO-YBCOwith a T, close to 75K.
Fig. 2 presents IV characteristics for one junction at a variety of temperatures, following a series of annealing treatments, each lasting 1.5 Hr. Current and voltage have been
scaled by I, and I,R, respectively. I, was determined by a 1
pV criterion while R, was taken from the asymptotic value at
P31,. Fig. 2 provides further evidence that the primary eEct
of ozone annealing is to increase the oxygen content well
beyond that achieved by standard 0 2 anneals, and
substantially raise the temperature at which the device
functions in the RSJ regime, up to 80K. Furthermore we note
that by scaling the data, and choosing temperatures which
yield a similar I,R, product, we obtain remarkably similar IV
characteristics. This result indicates that the intrinsic junction
structure, i.e. the thickness of the N-layer and the relative
importance and effect of the S-N interface, is not altered by
0.5
11. OZONE ANNEALING
GBJ, CO-SNS, and IEJ devices were fabricated at Conductus Inc. The fabrication procedure for each junction architecture is provided elsewhere [ 11-[3]. Current-voltage (IV) characteristics were collected before and after various annealing
treatments including ozone annealing. During anneals, samples were mounted with silver paste to a Haynes 214 alloy
block which was heated by an enclosed halogen lamp. For O2
and Ar anneals, the chamber was pumped down to -Sx104
Torr, and backfilled to slightly over 1 atm. For 0 2 / 0 3 anneals, O2 was flowed through a commercial ozone generator
creating an 0 2 / 0 3 mix with -2% 0 3 , by weight. The 0dO3
mixture then flowed through the annealing chamber at
slightly greater than 1 atm. Upon cool-down, the flow was
maintained until the sample was near room temperature and
then removed from the chamber.
Manuscript received Sept. 14, 1998. This research was supported in part
by the Office of Naval Research and made use of the facilities of the
Cornell MSC facilities supported by NSF under Award No. DMR-9632275.
1051-8223/99$10,00
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Fig. 1 IV characteristics at 35K for a CO-SNSjunction before and after
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Additional time at 500C in O2 is required to complete the
oxygen loss necessary to regain the initial conditions.
35K, Post 1st 500C O2
80K, Post 500C Ozone
JUNCTIONS
IV. GRAINBOUNDARY
The typical effect of ozone annealing on the IV characteristic for a GBJ at 4.2K (5 pm wide, -200 nm thick, misorientation angle -23") is demonstrated in Fig. 4. I, increased from
0.80 mA to 1.78 mA while R, decreased from 1.63 to
0.57 C2 for a net decrease in I,R, product ftom 1.31 to 1.02
mV. The general similarity of I,R, products in spite of the:
more dramatic change in I, and R, is clarified in Fig. 5 which.
portrays scaled IV characteristics for the same data. Again, E;
in the case for CO-SNS junctions, the scaled data agrees;
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Fig. 4 1V characteristics at 4.2K for a GBJ before and after ozone annealing at 500C for 1.5 Hours.
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Temperature (K)
70
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Fig. 3 I,R, versus Temperature for three CO-SNS junctions following a
series of O2 and ozone anneals.
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Fig. 5
Scaled IV characteristics at 4.2K for the GBJ portrayed in Fig. 4.
2995
before and after ozone annealing. We attribute this to the fhct
that the GBJ consists of at least two components; a relatively
immutable barrier layer closely associated with the physical
grain boundary which does not substantially change with
ozone annealing, and regions of de-oxygenated YBCO on
either side of the grain boundary which impact I, and R,, but
not I,R, and the intrinsic RSJ behavior of the device. An
important difference between the CO-SNSand GBJ devices is
that the operating temperature is not affected by ozone annealing as demonstrated by Fig. 6 which portrays IcRn versus
temperature after a series of anneals. This figure demonstrates
that I,R, is relatively constant across the entire temperature
range before and after ozone annealing. I,R, only begins to
decrease with the substantial amount of oxygen loss induced
by the 250 and 300C Ar anneals. Following these Ar anneals
the 1,'s have substantially decreased to 116 and 33 pA respectively. The consistency of the I,R, product across a broad
range of values for J, is portrayed in Fig. 7. The scaling of
1,R, with J, in the lower J, region has been observed in previous investigations, and has been attributed to a variety af
mechanisms [ 11, [5-71. The newly observed plateau behavior
for I,R, in the case of optimally and nearly optimally oxygenated devices can be explained by the limiting effect of the
intrinsic barrier in the immediate vicinity of the grain boundary, which is not altered by ozone annealing. The eff'ect of
ozone annealing is to increase the amount of completely oxygenated YBCO material adjacent to the grain boundary, and
thus increase the effectivejunction area, which increases I, and
decreases R, while holding I,R, constant. As de-oxygenation
of the grain boundary continues with Ar anneals at higher
temperatures, the volume of oxygen deficient YBCO spreads
outwardly in a non-uniform manner from the grain boundary,
eventually thickening the barrier, and decreasing the 1,R.
product. It is important to note that 1-10 pm wide GBJ's
have shown similar plateau behavior when oxygenated by
ozone annealing and electromigration. The independence of
junction width on the plateau discounts explanations
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Temperature (K)
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Fig. 6 I,R, versus temperature for a single GBJ following a series of
anneals. 02 and 0 2 / 0 3 anneal were conducted for 1.5 Hr. while Argon
anneals were lengthened to 3 Hr.
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Fig. 8 Scaled 4.2K IV characteristics for an IEJ before and after ozone
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2996
fi-om 2.32 to 0.45 Ohms, and I,R, remains relatively constant, increasing from 1.51 to 1.56 mV.
There is an increase in excess current following ozone annealing, but the RSJ behavior is largely unchanged. Fig. 9
portrays the I,R, product versus J, at 4.2K for all five junctions on the test chip which contained the device discussed
above. A plateau behavior, reminiscent of that observed in
GBJ’s is present, but with less data supporting its existence.
The I,R, product is substantially more robust under ozone
anneal than was the case for CO-SNS devices, indicating that
like the GBJ’s, the IEJ’s contain a robust barrier layer which
induces some secondary oxygen loss in the adjacent electrode
that in turn affects the I, and R, of the junction.
Fig. 10 portrays I,R, versus J, data for a separate test chip
with a higher average I,, presumably due to a thinner barrier
layer. In this case a series of 3 hr. Ar anneals were used to
gradually de-oxygenate the IEJ’s following an initial ozone
anneal. It is interesting to note that temperature excursions as
mild as 175C are sufficient to induce oxygen loss in the
highly oxygenated, ozone annealed samples as demonstrated
by the factor of 4 decrease in J, for the devices. A similar susceptibility to oxygen loss at such moderate temperatures following optimal or near optimal oxygenation through ozone
annealing is observed in Fig. 7 for GBJ’s and has been reported for CO-YBCOthin films [4]. This behavior appears to
be common in a variety of stressed YBCO systems. An initial plateau behavior for this set of devices is actually surpassed through the effects of ozone annealing. This indicates
that complete oxygenation of the electrode material may not
be possible even with the elevated partial pressure of atomic
oxygen provided by ozone, e.g. a higher I,R, plateau at
-1.5 mV similar to that observed in Fig. 9 may exist.
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I,R, versus J, for five separate IEJ’s at 4.2K. Chip #L97357
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As Fabricated
As Delivered
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ANDCONCLUSIONS
VI. DISCUSSION
The ability to examine and compare various HTSC
As Fabricated
As Delivered
Post 500C Ozone
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Josephson junction architectures independent of substantial
variations in oxygen content, common in these structures, is
necessary in order to accurately determine the most appropriate technology for a given application. We have shown that
three different types of HTSC junctions behave differently,
especially when comparing GBJ and IEJ to CO-SNS junctions. The operating temperature in which a CO-SNS junction will demonstrate RSJ behavior is highly dependent upon
oxygen content. Once at an appropriate temperature for RSJ
operation, scaled IV characteristics obtained for widely varying levels of oxygenation are similar; remarkably, even the
amount of excess current scales. GBJ’s, on the other hand,
maintain RSJ like behavior over a wide range of temperatures
and oxygen content. After ozone annealing a greater number
of optimally oxygenated regions of the YBCO electrodes
make contact with the intrinsic barrier localized at the grain
boundary. The consistency of the I,Rn product is attributed to
a model in which the effective area of the junction is increased
as the oxygen content is increased. A similar behavior is observed for IEJ’s, but in this case it is not clear that an oxygen
saturation of the YBCO electrodes can be achieved for all
critical current densities and barrier thicknesses.
1000
1
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1o5
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Jc (Amps/crn2)
Fig. 10 I,R. versus J, for five separate IEJ’s at 4.2K Chip #L97359
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