Thin films of YBa2Cu3O(7-beta) have been grown by sequential evaporation of Cu, Y, and BaF2 on SrTiO3 and MgO substrates. The onset temperatures were as high as 93 K while T sub c was 85 K. The Ba/Y ratio was varied from 1.9 to 4.0. The Cu/Y ratio was varied from 2.8 to 3.4. The films were then annealed at various times and temperatures. The times ranged from 15 min to 3 hr, while the annealing temperatures used ranged from 850 C to 900 C. A good correlation was found between transition temperature (T sub c) and the annealing conditions; the films annealed at 900 C on SrTiO3 had the best T sub c's. There was a weaker correlation between composition and T sub c. Barium poor films exhibitied semiconducting normal state resistance behavior while barium rich films were metallic. The films were analyzed by resistance versus temperature measurements and scanning electron microscopy. The analysis of the films and the correlations are reported

Bhasin, Kul B.

Rohrer, Norman J.

Valco, George J.

Warner, Joseph D.

English

NASA Technical Reports Server

e4h,
NASA Technical Memorandum 101388
Sequentially Evaporated Thin Y-Ba-Cu-O
Superconductor Films: Composition
and Processing Effects
George J. Valco and Norman J. Rohrer
Ohio State University
Columbus, Ohio
and
Joseph D. Warner and Kul B. Bhasin
Lewis Research Center
Cleveland, Ohio
Prepared for the
35th National Vacuum Symposium
sponsored by the American Vacuum Society
Atlanta, Georgia, October 3-7, 1988
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SEQUENTIALLY EVAPORATED THIN Y-Ba-Cu-O SUPER_R FILMS:
_SITIC_ AND PROCESSING EFFECTS
George J. Valco and Norman J. Rohrer
Ohio State University, Columbus, Ohio 43210
Joseph D. Warner and Kul B. Bhasin
NASA Lewis Research Center, Cleveland, Ohio 44135
ABSTRACT
Thin films of YBa2Cu307_ 8 have been grown by sequential
evaporation of Cu, Y, and BaF_ on SrTiO 3 and MgO substrates. The
onset temperatures were as hlgh as 93K, while T¢ was 85K. The Ba/Y
ratio was varied from 1.9 to 4.0. The Cu/Y ratio was varied from
2.8 to 3.4. The films were then annealed at various times and
temperatures. The times ranged from 15min to 3 hours, while the
annealing temperatures used ranged from 850°C to 900°C. There was
found a good correlation between transition temperature (T) and the
.... O C
annealing condltlons; the f11ms annealed at 900 C on SrT10_ had the
best Tc'S. There was a weaker correlation between composltion and
T . Barium poor films exhibited semiconducting normal state
c
resistance behavior while barium rich films were metallic. The
films were analyzed by resistance versus temperature measurements
and scanning electron microscopy. The analysis of the films and the
correlations are reported.
INTRODUCTION
With the widespread effort to investigate and develop elec-
tronic applications of the superconducting oxides, a large variety
of techniques have been used to form high temperature superconduct-
ing thin films. One such technique consists of the sequential
evaporation of a multi-layer stack containing the constituents of
the superconductor followed by a_]ea!ing in an oxygen ambient. When
performed by electron beam evaporation from a multi-hearth gun, this
technique allows deposition of films with little spatial variation
of stoichiometry as all components of the film are evaporated from
the same point in space. The stoichiometry of the films is also
easily adjusted by controlling the thickness of the individually
deposited layers. This technique has been employed with a variety
of starting materials; Y, Ba and Cu metals themselves I-3, a
combination of the metals and oxides 2 4 5' ' , a combination of metals
and BaF23 and a combination of oxides and BaF26
In the work reported here, we have performed sequential
evaporation of Cu, Y and BaF 2 to study the formation of supercon-
ducting films on SrTiO 3 and MgO substrates. We have varied the
stoichiometry of the films by adjusting the thicknesses of the
individual layers. Films of fixed composition have also been
differently annealed to assess the influence of annealing condi-
tions. The fabricated films were characterized through measurement
of their resistance as a function of temperature and through
scarming electron microscopy. The results of these characteriza-
tions are presented here.
DEPOSITICkNOFFILMS
Deposition of the films was performed in a CHAIndustries
electron beamevaporator. The system is equipped with a four hearth
gun, allowing deposition of the multi-layer stack without breaking
vacuum. Thickness of the layers was controlled via an Inficon XTC
thickness monitor and rate controller. The depositions were
calibrated by measurementsof step heights using a surface profilo-
meter.
A cross sectional drawing of the structure of a typical as
deposited film is shown in Figure i. First approximately 510Aof
copper was deposited on the substrate. This was followed by an
approximately 480i thick layer of yttrium which was followed by an
approximately 1920Athick layer of barium fluoride. For most of our
depositions, this multi-layered sequencewas repeated four times for
a total of twelve layers. For the thicknesses listed above, the
film is characterized by a barium/yttrium atomic ratio of 2.25 and a
copper/yttrium atomic ratio of 3.01, which has produced our best
results on SrTiO_ substrates. Wehave investigated the properties
of f11mswlth barlum/yttrlum ratlos ranging from 1.9 to 4.0 and
copper/yttrium ratios ranging from 2.8 to 3.5. Wehave used barium
flouride rather than elemental barium since barium flouride is less
reactive.
SUBSTRATE
_BaF 2
_y
ANNEAL IN
OX_'GEN SUBSTPo\TE
Fig. i. On
deposited film.
in oxygen.
th left: Typical twelve layer structure of the as
On the right: YBa2Cu3OT_ _ formed after annealing
ANNEALING
The deposited films were annealed in a hot wall, programable,
quartz tube furnace. The furnace was preheated to the annealing
temperature and purged with oxygen prior to inserting the samples.
Temperatures ranged from 850°C to 900°C. The samples were pushed
into the furnace with either a fast push of approximately 30sec or a
slow 5min push. The duration of the anneals ranged from 15min to
3hr. The temperature was then ramped to 450°C at a rate of
-2°C/min. The sampleswere held at 450°C for 6hr and then the
temperature was rampedto roomtemperature at -l°C/min. During the
high temperature portion of the anneal the ambient consisted of
ultra high purity oxygen bubbled through roomtemperature water to
assist in removal of fluorine from the films. Dry oxygen was used
during all other portions of the annealing process.
CHARACTERIZATION
Ohmic contacts were formed on the films to allow measurementof
their resistance as a function of temperature. Most of the samples
were rectangular in shape with widths of approximately 5mmand
lengths of approximately icm. The contacts for these samples were
deposited by evaporation of I/Jmof silver through shadowmasksto
produce four stripes across the width of the samples. For some
irregularly shaped samples, shadowmaskswhich produced four dots
were used. The contacts were annealed in dry oxygen at 500°C for
lhr. The temperature was rampedto 250°Cat a rate of -2°C/min and
then to room temperature at -l°C/min.
The samples were mounted to a sampleholder and gold ribbon
bonds were madebetween the silver contacts and bonding posts. A
four probe DCmeasurementwas employedto determine the resistance.
The samples were cooled in a closed cycle helium refrigerator.
Measurementswere performed from roomtemperature to well below the
transition temperature for superconducting films or to approximately
10K for non-superconducting films.
Scanning electron microscopy (SEM)was employed to observe the
morphology of the films. In addition, several of the films on
SrTiO3 substrates were analyzed by x-ray diffraction spectroscopy(XDS) to observe orientation and the presence of other phases.
RESULTS
Figure 2 is a plot of the distribution of composition for
several films. Eachpoint on the plot represents the combination of
Cu/Y ratio and Ba/Y ratio used for a deposition. Each of these
films was madewith a 12 layer deposition and was between 1.0 and
1.2;nn thick before annealing. All of the samplesshownin this plot
were annealed with the procedure described above for 45min at 900°C.
A few films with composition well outside the range plotted here
(larger Ba/Y ratios) were also deposited. They are not plotted
since they had a small residual resistance below the "transition"
temperature even though they showed a fairly sharp onset of
superconductivity and transition. Films of manyother compositions
were also deposited but were not annealed at the sameconditions and
are thus not plotted here.
Associated with each point on the plot are two numbersand a
letter. The first number is the onset temperature in Kelvin. The
second number is the temperature below which the resistance of the
sample was zero. The letter "m" refers to a metallic behavior of
the normal state resistance with temperature while the letter "s"
2.4
O
n"
m
2.3
2.2
2.1
2.0
93/81 m
93/85m <
93/85m -_
"l87/31 s •
83/21 s
• 93/83m
• 92/77m
• 83/51 m
89/21 s
89/48s
1.9 I I •
2.9 3.0 3.1 3.2
Cu/Y Ratio
Fig. 2. Composition of several films
annealed at 900°C for 45min. The numbers
give onset/T c . m: metallic normal state,
s: semiconducting normal state.
indicates that the
normal state resistance
of that sample showed a
semiconducting behavior
with temperature.
The data in this
plot show that there is
a minimum Ba/Y ratio
below which the samples
have a semiconducting
normal state resist-
ance-temperature (R-T)
characteristic. Those
films deposited with a
Ba/Y ratio of greater
than approximately 2.2
have a metallic normal
state R-T characteris-
tic. This was also
true of the films,
mentioned above, with
the large Ba/Y ratios
(=3.0 and =4.0).
Over the ranges studied, the Cu/Y ratio does not affect the
qualitative character of the normal state resistance. As would be
expected, adjusting the thicknesses of the individual layers to make
the films copper rich or poor does degrade the onset temperature and
critical temperature of the resulting films.
Our best films on SrTiO 3 are those deposited with a Ba/Y ratio
of approximately 2.25 and a Cu/Y ratio of =3.0. The R-T character-
istic below room temperature for one of these films is plotted in
Figure 3. This film had a room temperature resistivity of approxi-
mately 2mQ-cm. The onset temperature of this film was 93K and the
critical temperature was 85K. The 90% to 10% transition width for
this sample was 3.6K. As can be seen from Figure 2, there is a
range of ratios near these values which produce films with nearly
the same onset and critical temperatures.
Films of several compositions were annealed at different
temperatures or for different durations. Data for one of these
experiments, involving four samples, is listed in Table I. These
samples were annealed for 45min at three temperatures. Two samples
were annealed at 850°C. One was quickly pushed into the furnace
while the other was pushed into the furnace over a period of 5min.
The other two samples were annealed at 875°C and 900°C, both with
slow pushes. The last of these samples is included in Figure 2.
The onset temperature, critical temperature, 90%-10% transition
width and 99%-1% transition width are tabulated. The transition
widths are referenced to the resistance just above the onset.
¢9
E
O
v
3
(a
r_ I
÷
+
++++
#
@
÷
÷
÷
÷
÷
÷
t
I ' i
100 150
Temperature (K)
+
÷
÷
÷
0 I ' I ' I '
0 50 200 250 300
Fig. 3. Resistance as a function of temperature for a Y-Ba-Cu-O
thin film with composition Ba/Y=2.25 and Cu/Y=3.01. Annealed at
900°C for 45min. Onset temperature=93K. Tc=85K.
Table I. Transition widths for three annealing temperatures.
Annealing &T &T
Temperature Onset T 90%-_0% 99%-_%
(°C) (K) (K) (K) (K)
850 fast push 55 21 14.7 25.1
850 slow push 69 41 10.3 18.4
875 slow push 83 49 13.0 21.5
900 slow push 83 51 12.6 20.2
The tabulated data shows that both the onset temperature and
critical temperature are improved with the higher temperature
anneals. In addition, for the same onset temperature, a higher
temperature anneal results in a slightly sharper transition. The
push rate also has a dramatic effect on the R-T characteristic. The
sample which was slowly pushed into the furnace had significantly
higher onset and critical temperatures.
For several compositions of films on SrTiO., two samples were
annealed for 45min or 60min respectively at 85_°C. No significant
or consistent differences in their R-T characteristics were
observed.
One of the films with a critical temperature o_ 85Kwas
remeasured at two later times. The resistance as a function of
temperature from 70K to 120K from these measurements is shownin
Figure 4. The initial measurement is marked by the squares. The
second measurement, plotted with crosses, was made14 days later.
The normal state resistance had increased by approximately .8Q from
the first measurement. The transition had degraded only slightly
over this period. The sample wasmeasured a third time 33 days
after the first measurement,shownby the circles in Figure 4. The
normal state resistance was seen to have increased very slightly
from the secondmeasurement. There wasno additional change in the
superconducting transition temperatures.
5
E 4
c-
o
v 3
e-
2
_C
7o
o_
o
9O
+o
[]
I ! I
80 100 110 120
Tern perature (K)
Fig. 4. Three measurements of the resistance as a function of
temperature for a Y-Ba-Cu-O film. Squares: Initial measurement.
Crosses: Second measurement, 14 days. Circles: Third measurement,
33 days.
Fig. 5. Scanning electron micrograph
of a 1.21nn film deposited in 12 layers.
A scanning electron
micrograph of the film of
Figure 3 is shown in
Figure 5. This film was
deposited in twelve lay-
ers and had a thickness
of approximately 1.21nn
prior to annealing. The
film is polycrystalline
and can be seen to have
both ordered and unor-
dered grains. Observa-
tions of this and other
areas of the film as well
as other samples appear
_ .-__=;,h,.L._.L "',_.,-,¢..
OF POOR qUALITY
q'_ "-I'-_"L , ,-'_.'7. _ 7_
to indicate that the film consists of a layer of ordered grains in
contact with the substrate and an overlayer of unordered grains.
The x-ray diffraction spectrum for this sample also shows incomplete
ordering.
Fig. 6. Scanning electron micrograph
of a 0.gMm film deposited in 18 layers.
In addition, super-
conducting films have
been formed with deposi-
tions consisting of both
more and less layers.
Figure 6 shows a scanning
electron micrograph of a
film deposited in 18
layers which had a thick-
ness of 0.9_m prior to
annealing. This film
appears to be more com-
pletely ordered than the
thicker films formed with
twelve layers.
Superconducting films on MgO substrates are not as readily
formed. When the same annealing conditions used for SrTiO 3 are used
for MgO, films with semiconducting normal state resistance tempera-
ture characteristics result. These films still show sharp onset
temperatures in the vicinity of 90K, but the transitions have long
resistive tails and zero resistance is achieved only below 15K, if
at all. Better results are achieved by annealing at 850°C for times
of two to three hours, yielding metallic normal state resistance
characteristics, an onset of 93K and a critical temperature of 50K.
S_YAND CONCLUSIONS
The technique of multi-layer sequential evaporation of BaF2, Y
and Cu layers has been employed for the formation of high tempera-
ture superconducting films. The effects of varying dne composition
and annealing conditions have been studied. For films on SrTiO 3
annealed at 900°C for 45min, variation of the Ba/Y ratio signifi-
cantly altered the resistance-temperature characteristics. Barium
poor films exhibited semiconducting normal state resistance-
temperature characteristics while barium rich films were metallic.
The transition temperatures varied with both the Ba/Y and Cu/Y
ratios. Films which were superconducting at 85K were formed with
Ba/Y=2.25 and Cu/Y=3.0. For a given composition, films on SrTiO.
had higher transition temperatures when annealed at 900°C than a[
850=C. Conversely, films on MgO were better when annealed at 850°C
for longer times. The normal state resistance of the films
increased over a period of two weeks but the transition temperature
degraded only slightly.
..
3.
.
o
o
REFERENCES
B-Y. Tsaur, M.S. Dilorio and A.J. Strauss, Appl. Phys. Lett.,
51, 858 (1987).
C.X. Qiu and I.Shih, Appl. Phys. Lett., 52, 587 (1988).
A. Mogro-Campero and L.G. Turner, Appl. Phys. Lett., 52, 1185
(1988).
C-A. Chang, C.C. Tsuei, C.C. Chi and T.R. McGuire, Appl. Phys.
Lett., 52, 72 (1988).
Z.L. Bao, F.R. Wang, Q.D. Jiang, S.Z. Wang, Z.Y. Ye, K. Wu,
C.Y. Li and D.L. Yin, Appl. Phys. Lett, 51, 946 (1987).
N. Hess, L.R. Tessler, U. Dai and G. Deutscher, Appl. Phys.
Lett., 53, 698 (1988).
National Aeronautics and
Sipace Administration
1 Report No.
NASA TM-101388
Report Documentation Page
2. Government Accession No.
4 Title and Subtitle
Sequentially Evaporated Thin Y-Ba-Cu-O Superconductor Films:
Composition and Processing Effects
7. Author(s)
George J. Valco, Norman J. Rohrer, Joseph D. Warner,
and Kul B. Bhasin
9, Performing Orgamzatfon Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135-3191
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546-0001
3 Rec_p_ent's Catalog No.
5 Report Date
6 Performing Organization Code
8 Performing Organizahon Report No
E-4448
10. Work Unit No
506-44-21
11. Contract or Grant No.
13 Type of Report and Period Covered
Technical Memorandum
14, Sponsoring Agency Code
15 Supplementary Notes
Prepared for the 35th National Vacuum Symposium sponsored by the American Vacuum Society, Atlanta,
Georgia, October 3-7, 1988. George J. Valco and Norman J. Rohrer, Ohio State University, Columbus,
Ohio 43210; Joseph D. Warner and Kul B. Bhasin, NASA Lewis Research Center.
16. Abstract
Thin films of YBa2Cu307__ have been grown by sequential evaporation of Cu, Y, and BaF2 on SrTiO3 and MgO
substrates. The onset temperatures were as high as 93K, while T c was 85K. The Ba/Y ratio was varied from 1.9
to 4.0. The Cu/Y ratio was varied from 2.8 to 3.4. The films were then annealed at various times and
temperatures. The times ranged from 15 min to 3 hours, while the annealing temperatures used ranged from
850 °C to 900 °C. There was found a good correlation between transition temperature (T c) and the annealing
conditions; the films annealed at 900 °C on SrTiO3 had the best Tc's. There was a weaker correlation between
composition and Tc. Barium poor films exhibited semiconducting normal state resistance behavior while barium
rich films were metallic. The films were analyzed by resistance versus temperature measurements and scanning
electron microscopy. The analysis of the films and the correlations are reported.
18. Distribution Statement
Unclassified - Unlimited
Subject Category 33
17. Key Words (Suggested by Author(s))
Superconductivity; Superconductor; Oxide superconductor
Thin films; High temperature superconductor;
YBa2Cu30×; YBazCu3Oy_x; E-beam; Sequential
evaporation
19 Security Classff. (of this report) 20. Security Classd (of this page) 21. No of pages
Unclassified Unclassified 10
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Sequentially evaporated thin Y-Ba-Cu-O superconductor films: Composition and processing effects

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