Low-temperature
formation
in reduced O2 pressure
of epitaxial
T12Ca2Ba2Cu301,, thin films
W. Y. Lee, S. M. Garrison,a) M. Kawasaki, b, E . L. Venturini,@ B. T. Ahn, R. Boyers, J.
Salem, R. Savoy, and J. Vazquez
IBM Almaden Research Center, San Jose, California 95120
(Received 2 1 October 199 1; accepted for publication 7 December 199 1)
Epitaxial Tl,Ca,Ba,CusO,, films on (100) LaAlOs are prepared by post-annealing
2Tl:2Ca:2Ba:3Cu precursor films at 830-860 “C in =0.03-O. 15 atm of 0,. These films (0.21.1 pm thickness) are smooth and shiny to the eye, and have a sharp zero resistance
and onset diamagnetic transition temperature of 117-121 K. Transport critical current densities
of 1.6X lo6 A/cm2 at 77 K and N lo5 A/cm2 at 100 K are obtained for a 0.38~pm-thick
film in magnetic fields up to 100 Oe. Strong flux pinning at low temperatures is inferred from
the weak-field dependence of the critical current density calculated from magnetic
hysteresis loops. At 5 K, the best film has a magnetic critical current density of 9x lo6
A/cm* in zero field, decreasing gradually to 1.5 x lo6 A/cm2 in a 5-T field.
Several Tl-based
superconductors
have been
reportedIT since the initial discovery of superconductivity
in the Tl-Ca-Ba-Cu-0 system3 a few years ago. Because of
their structural similarities, extensive intergrowths of these
superconductors were reported.4*5 The presence of intergrowths and the lack of suitable phase diagrams for guiding the fabrication processes have made the preparation of
T12Ca2Ba2Cu30,, (2223) superconductors extremely difficult. In addition, the rapid loss of Tl during high-temperature annealing6 further complicates the preparation of
thin films of this Tl-based superconductor. We reported
previously&’ the preparation on various substrates of 2223
films with a zero-resistance T, of > 120 K by annealing in
sealed quartz tubes to minimize the loss of Tl. It was empirically determined in our work and confirmed by other
groups1°-12 that an annealing temperature of 880-900 “C
was required to prepare 2223 films under z 1 atm 0, pressure. However, the films thus prepared typically are very
rough due to the high-temperature annealing required. The
high-temperature annealing also makes the preparation of
high-quality films with thickness much less than 1 ,um extremely difficult because of the significant film-substrate
interaction that occurs during annealing. It is desirable to
lower the annealing temperature to prepare thinner and
smoother films which may have properties more suitable
for device applications. Recently, we have investigated the
0, pressure-temperature stability regime for 2223 using a
solid-state electrochemical technique.13 For samples with
an overall cation composition of 2T1:2Ca:2Ba:3Cu, our
studies revealed that the maximum 0, pressure at which
2223 is thermodynamically stable decreases with decreasing temperature. This result implies that 2223 thin films
can be prepared at a temperature lower than the 880900 “C used earlier, provided the oxygen pressure is below
1 atm. In this letter, we report the first successful preparation of visibly smooth, high-quality 2223 films as thin as
0.2 pm using low-temperature/low 0, pressure conditions.
The 2223 films thus prepared on LaAlO, are shown to be
“‘Conductus, Inc., Sunnyvale, CA 95086.
“IBM T. J. Watson Research Center, Yorktown Heights, NY 10598.
“Sandia National Laboratories, Albuquerque, NM 87 150.
772
Appl. Phys. Lett. 60 (6), 10 February 1992
epitaxial with a transport critical current density of
1.6~ 106/cm2 at 77 K.
Detailed descriptions of the techniques used for sputter
deposition and post-annealing of 2223 amorphous precursor films were given previously.“’ Briefly, these films were
sputter deposited, in a t-f diode facing target sputtering
system, from sintered targets prepared from oxide powder
mixtures with an initial cation ratio of 2T1:2Ca:2Ba:3Cu.
Previously, the films were wrapped with 2223 pellets in
gold foil and annealed at N 900 “C under N 1 atm O2 in a
sealed quartz tube to minimize the loss of Tl. In the present
work, a much lower O2 pressure of 0.03-O. 15 atm was used
for annealing the 0.2-l-pm-thick
films on 1 X 1 cm2 (100)
LaAlO, at 830-860 “C. The 0, pressure required to form
the 2223 films at the desired annealing temperature was
determined from the O2 pressure versus temperature stability diagram reported recently for the 2223 phase.13
To minimize
the nucleation and growth of the
T12Ca,Ba2Cu20s (2122) phase during annealing, a heating
rate of > 5 “C/min was used in this work.
The superconducting properties of these films were
characterized using a low-frequency ac four-point probe,14
an ac susceptometer* as well as a susceptometer- and a
SQUID-based magnetometer. The film surface was aligned
perpendicular to the applied field to enhance the small
output signals from the sample. A demagnetization factor
(D) of 0.987*0.002, calibrated using 0.2-l-pm-thick
Pb
films,15 was obtained in this sample orientation for the ac
susceptometer. To examine the detailed effects of applied
ac field on ac susceptibility, however, only external susceptibility (D = 0) is shown in this letter. Complete hysteresis
loops were measured up to f 1 and f 5 T dc fields in the
susceptometer magnetometer and the SQUID magnetometer, respectively, at several temperatures, allowing 5 min at
each field for partial relaxation of the trapped flux. The
Bean critical state model16 was used to calculate the critical
current density (Jo) of these films from their hysteresis
loops. Transport critical current density (J,) was measured under O-100 Oe magnetic field, using a standard
four-point-probe technique, from e 15-pm-wide and lOOpm-long microbridges formed by laser ablation. A criteria
of 1 pV/cm was used for the determination of Jc Film
0003-6951/92/060772-03$03.00
@ 1992 American Institute of Physics
772
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structure was investigated using a four-circle x-ray diffractometer with Cu Ka radiation. In-plane orientation was
determined by examining diffraction intensities as the sample was rotated about the substrate normal from planes
that are not parallel to the substrate surface (referred to
here as “4 scans”“). For c-axis-oriented 2223 films, 4
scans were collected for the 2223 (1015) family of peaks.
The corresponding 4 values indicatefihe
2223 [loo] inplane direction. The LaA103 (101) peaks, measured at a
different d spacing than the 2223 (1015) peak, indicated
the LaAIOz [lOO] in-plane direction. Finally, electron microprobe and scanning electron microscopy were used to
characterize the composition and morphology of these
films, respectively.
Results of four-point resistivity measurements show a
zero-resistance T, of 117-121 K for 0.2-1.1~pm-thick films
annealed at 850 “C under -0.1 atm O2 for -9 h. Meissner
data recorded in 5 Oe show an onset of superconductivity
near 120 K, and a transition width (lo%-90%)
of p-5 K
in the E l-pm-thick films. These films have a room-temperature resistivity of 3-9 X 10 - 4 R cm and show excellent
metallic behavior as is evident from their =2.5X drop in
resistivity from room temperature to the onset of transition
and the extrapolation to zero in their normal-state resistivity versus temperature curves. Similar results can also be
achieved for the films annealed at ~830°C under co.06
atm O2 pressure, but a longer annealing duration (e.g., 13
h at 830 “C) is required. The films < 1 pm in thickness are
smooth and shiny to the eye, although shallow cavities
( e 1 pm in width) distributed over a smooth background
can be detected with a scanning electron microscope.
The real (x) and imaginary (x”) parts of the external
susceptibility are given in Fig. 1 as a function of temperature (5-150 K) and ac field (0.01-10 Oe rms) for a Alpm-thick film annealed in 0.1 atm O2 at 850 “C for 9 h. For
ac fields ~0.04 Oe, x’ decreases precipitously while x”
shows a sharp maximum at 121 K, which is close to the
zero-resistance T, obtained with the four-point-probe technique. Both x’ and x” are seen to decrease to a minimum
and thus show a complete diamagnetic shielding at -55117 K, depending on the ac field used. The increase in x’
transition width and decrease in the temperature where the
y” maximum occurs with increasing ac fields are much
more gradual in these films than in the films annealed at
-900 “C in Z 1 atm O2 pressure.’ This weaker field dependence of y’ and x” suggests a stronger grain coupling and
thus a higher critical current density for these films.
The values of zero-field J, measured from the laserablated microbridges are shown in Fig. 2 as a function of
temperature (T) for films with thicknesses of 0.20, 0.38,
and 1.1 pm. The values for a 0.8~pm-thick film were also
measured and are similar to those for the 1.1~pm-thick
film. No significant magnetic field dependence of these
J,‘s is observed under the 0-lOO-Oe fields used. The J,
values of these films are 23~ lo6 A/cm2 at 4.2 K. They
decrease to 0.7-1.6X lo6 A/cm2 at 77 K and remain at
-0.1 X 10h A/cm2 at 100 K. A much more rapid decrease
in J, at temperatures greater than 100 K was observed,
especially for the 0.2~pm-thick film. This can be attributed
773
Appl. Phys. Lett., Vol. 60, No. 6, 10 February 1992
g
2
-500 ‘;
P
5 -lOOOz
j
-1500X’
-2000
0
20
I
40
1
60
I
100
I
80
Temperature
1
120
I
140
IKI
FIG. 1. External susceptibility vs temperature for a t l-pm-thick film
deposited on LaAIOl and annealed at 850°C in =O.l atm O2 for 9 h.
to a greater degree of damage to the thinner films during
patterning. For example, the resistivity transition width of
the 0.2~pm film became broader and its zero-resistance
T, degraded from E 117 to 107 K after patterning. The
damage to the 0.38~ym film was smaller (120-112 K
T,), and no such damage to the 0.8- and 1. l-pm-thick
films was observed. For the films with thickness > 0.2 pm,
logarithmic plots of J, vs 1 - T/T, give two straight lines
with a slope of 1.2-1.5 and 2.1-2.2 for T below and above
110 K, respectively. The near-quadratic temperature dependence of J, for T close to T, suggests that tunneling in
superconductor-normal metal-superconductor or proximity junctions is the primary conduction mechanism for
these films.
The magnetic hysteresis loops obtained from the susceptometer magnetometer for the 0.2- and 0.38~pm-thick
films gave a J,, of -9 X lo6 A/cm2 in zero field and
2X lo6 A/cm2 in a 1-T field at 4.2 K. The values for the
0.8- and 1. l-pm-thick films are -2 x lower. Similar results
were obtained from the SQUID magnetometer for several
N l-pm-thick films. Here, the best film had a J,, of 9 X lo6
A/cm2 in low field, which decreased gradually to 1.5 X lo6
A/cm2 in 5 T at 5 K. The J,, values were found to decrease much faster with increasing magnetic field at 40 and
77 K, although they remained at E lo6 A/cm2 at 77 K in
Temperature (K)
FIG. 2. Transport critical current density vs temperature from 15pmwide microbridges formed by laser ablation for 0.2-, 0.38-, and I.l+mthick films on LaAlO,.
Lee et al.
773
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2
5
F
5
LaA103
(101)
-5
z
f
El
E
-
0
90
180
$ (degrees)
270
360
FIG. 3. X-ray diffraction intensity from the 2223 (1015) and the LaAIO,
(101) family of peaks vs the angle around the norrn;irof a= I-pm-thick
film deposited on LaAlO, and annealed at 850 “C in 20.1 atm O2 for 9 h.
zero field for the best film. The weaker field dependence of
J,, at 5 K is apparently due to the effects of stronger flux
pinning (relative to the thermal energy) at this temperature.
X-ray diffraction patterns obtained width a 8-28 scan
show that these films, like the films annealed at -900 ‘C6,’
are strongly textured with the c axis oriented perpendicular
to the substrate. This result, coupled with the electron microprobe data, indicates that these films are almost solely
the 2223 phase, although weak signatures of the 2122
phase were detected. A typical 4 scan showing fourfold
symmetry is given in Fig. 3 for an l-pm-thick film with a
zero-resistance T, of 120 K. The overlap of the 2223
(1015) and LaAlO, (101) peaks seen in this figure indicates2223 [100]~~LaA10,[100], demonstrating that cubeon-cube epitaxy has occurred. The clean spectrum between
peaks indicates the lack of misoriented grains ( < 0.3%) in
the plane of the film. All films (0.2-l-pm thickness) in this
study exhibited this epitaxial relationship, which is not surprising since the in-plane lattice constants of 2223 and
LaAlO, are similar: 3.82 A (Ref. 18) and 3.79 A,19 respectively. Similar epitaxial growth of the 2122 impurity phase
with 2122 [100]~~LaA10,[100] can also be concluded by
performing a 4 scan for the 2122 (105) peak. The epitaxial
growth of 2 122 and 2223 on LaAlOs has also been reported
for the films prepared by laser ablation*’ and by sputter
deposition. t ‘,*I The amount of 2122 impurity in the 2223
film can be reduced by using the highest heating rate available, but probably cannot be eliminated completely as implied by the 2223 stability diagram.13 The detailed effects of
the presence of this epitaxial 2122 impurity phase on the
J, of the 2223 films are still under investigation.
In summary, the preparation of T12Ca2Ba2Cu3010thin
774
Appl. Phys. Lett., Vol. 60, No. 6, 10 February 1992
films on ( 100) LaAlO, at 800-860 “C in 0.03-0.15 atm O2
pressure is reported. Four-point resistivity and ac susceptibility measurements both indicate a T, of 117-121 K for
the 0.2-l. l-pm-thick films annealed at 850 “C in 0.1 atm
0, for 9 h. X-ray diffraction patterns obtained from both
8-20 and 4 scans show that these films are epitaxially
grown on LaAlO,. The epitaxial growth results in a high
transport J, of 1.6X lo6 A/cm* at 77 K for a 0.38~pm-thick
film. The J, values estimated from magnetic hysteresis
measurements are as high as 9 X lo6 A/cm - * in low fields
and remain at 1.5 X lo6 A/cm - * in 5 T at 5 K, suggesting
strong flux pinning at this low temperature.
Part of this work was conducted under the auspices of
the Consortium for Superconducting Electronics. We are
grateful to J. Lacey of IBM T. J. Watson Research Center
for his skillful assistance in the critical current density
measurements. Work at Sandia National Laboratories was
supported by the U. S. Department of Energy, Office of
Basic Energy Science, under Contract No. DE-ACO476DPOO789.
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