Growth of the 110 K superconducting phase and characterization of Pb-doped Bi-Sr-Ca-Cu-O thin films prepared by sputtering

Journal of the Less-Common

Metals, 144 & 165 (1990) 687-694

687

GROWTH OF THE 110 K SUPERCONDUCTING PHASE AND CHARACTERIZATION OF Pb-doped Bi-Sr-Ca-Cu-0 THIN FILMS PREPARED BY SPUTTERING

S.LABDI. H.RAFFY, S.MEGTERT, A.VAURES Laboratoire de Physique Orsay, France

des

Solides,

Bat.510,

Unrversite

Bat.504,

Universite

Paris

XI, 91405

P.TREMBLAY Laboratoire France

de

Geochimie,

Paris

XI,

91405

Orsay,

We report on the preparation of (Bif_xPbx)-Sr-Ca-Cu-0 thin films with Tc up to 108K. Lead doping was used to promote the 1 IO K phase. The highest Tc were obtained for deposits with x-15%-20% after 14h (8000A thick film) or 11 h (3000A) annealing at 855°C860°C. X-ray diffraction studies and SEM examinations show that films of the 2223 phase are highly c-axis oriented like films of the 2212 phase.

1. INTRODUCTION In

a previous

study,

Bi(Pb)-Sr-Ca-CuO works2,3.

we

ceramics

We emphasized

reportedlon

enhanced

the

growth

by lead doping,

their large surface to volume

ratio as compared

Here, we present characterization

our results concerning

of the film composition

phase

exacerbated

in

with other

in the application

to ceramics.

lead doped films which have been annealed

2223

in agreement

that a major problem encountered

method to thin films was the loss of lead during annealing,

prepared

of the

of this

in thin films by

In this study, we have

under a Pb partial pressure.

the superconducting

transitions

by EDX and X-Ray diffraction

and the

analyses

at the

different stages of the study.

2. EXPERIMENTAL 2.1. Sample deposition Bi(Pb)SrCaCuO

deposits

have been obtained

by single target d.c. sputtering

on

(100) MgO substrates. Targets are prepared in a standard way, described in reference 4, starting from appropriate carbonate

amounts of Bi203,

powders. Three different

x = Pb/(Pb+Bi), (1.57,0.53):2:2:2.9

and the following

targets

PbO, CuO oxide and CaC03,

were

nominal

SrCO3

used with different lead content ,

cationic

(Bi,Pb):Sr:Ca:Cu

composition

for target A (x=25%), (2.8,1.2):3:3:4 for target B (x=30%),

OO22-5088/90/$3.50

0 Elsevier Sequoia, Printed in The Netherlands

688

(2,1.3):1.8:2:2.7

for target C (x=40%).

During deposition,

the substrates

films are amorphous. crystallization

are kept at room temperature.

In this study, their thickness

and the superconducting

The as-deposited

ranges from 250019 to 8000~. The

properties

are obtained

by a post-deposition

thermal treatment.

2.2. Annealing During the heat treatment, Bi(Pb)SrCaCuO surface6.

samples are in the immediate

oxide ceramic

neighborhood

of a pellet5 of

in order to get a lead rich atmosphere

near the film

The pellet cationic composition

with 15% to 25% lead substitution

was chosen as close as possible to 2:2:2:3,

for Bi, because of the possible

cation exchanges

between the film and the pellet. In order to get a good quality of the film surface, there is no direct physical contact between the pellet and the film, because of the roughness of the pellet surface. It was noted that after 300 h use at 86O”C, 30% of its lead content. 840-865”C,

Films were annealed

the pellet had lost about

at a temperature

Ta, in the range

for a time ta which was varied from 1 h to over 20 h. After ta, the samples

were cooled at room temperature 2.3. Superconducting D.c. resistance

in a few minutes.

transition measurements

measurements

with four mechanically

out on the samples to determine their superconducting

pressed contacts are carried

transition.

3. RESULTS AND DISCUSSION 3.1. Superconductucting

transitions

We have studied the influence of annealing and of the target lead-doping

Fig. 1 shows the normalized resistance A thick films prepared increasing

conditions,

on the superconducting

from target

temperature

Ta and time ta,

properties of the films.

R(T)/R(l5OK) vs temperature

A and annealed

at the same

of a set of 8000 Ta = 86U”C, fo!

times ta : 1.5 h (Al), 5 h (A2), 10 h (A3), 14 h (A4). After ta = 1.5 h, there is

already a 14% drop of R(T) at 110 K. This percentage

increases with ta. It appears that

the best sample (A4) with the highest critical temperature,

Tc = 108 K, and the highest

dR/dT slope (extrapolation

origin) is obtained after 14 h

annealing.

of the normal state curve to the

The same study has been carried

out on 3000 A thick films where the

optimum time was found to be 11 h (Tc=106 K). Work is in progress for 1000 A thick samples.

689

50

70

90

110

Temperature

130

150

(K)

FIGURE 1 Normatized resistance versus temperature for a set of samples, 8OOOAthick, annealed at Ta=860°C for times ta : 1.5 h (Al), 5 h (A2), IO h (A3). 14 h (A4) respectively. A study of the effect of the annealing Typical

temperature

results are given in Fig. 2. , where

T, has also been performed.

the normalized

resistance is plotted for two

8000 A thick samples (target A), annealed for the same time, at Ta = 840°C for sample A5 and Ta = 860°C for sample A4. From all the experimental

results, it appears that the

formation of the 2223 phase can be obtained in significant quantities temperature

in a rather narrow

window, 855”C-860°C. 1 0.8

I

xA4 +A5

z z 0.6 P go.4 Igo.2 0 50

70

90

110

Temperature

130

150

(K)

FIGURE 2 Normalized resistance versus temperature for sample A4 annealed sample A5 annealed at Ta = 840°C, for 14 h.

at Ta = 860°C and

690

Temperature

(K)

FIGURE 3 Normalized resistance versus temperature for samples A4, Bl, Cl prepared targets A, B, C respectively and annealed at 860°C for 14 h. The influence

of target

lead-doping

and composition

shows R(T) curves for three samples made

is illustrated

from

in Fig. 3 which

with targets A, B, C respectively,

annealed

under identical conditions (ta = 14 h, Ta = 860°C). It can be seen that target A with 25% nominal lead content gave the best results.

3. 2. Lead composition

characterization

In order to establish a eorrelai~~n between R(T) results and annealing composition analyses

of the samples

and their Pb content

has been determined

by EDX

(see table 1)

___________---_-c___--------_----------______---__--__~ Before annealing Ta ta Film ( Bi , Pb ): Sr :Ca : Cu (“C) (hf

Al A2 A3 A4 A5 Bl Cl

conditions, the

(1.75,0.34):1.90:2.0:3.15

”

1,

” fl.70,0.40~:, .85:2.0:3.15 ” (2.00,0.54):1.8;:2.0:2.7 (2.00:0.86):1.60:1.9:2.6

Film composition conditions. Films respectively.

before Al-A5,

860 860 860 860 840 860 860

1.5 5 10 t4 14 14 14

After annealing ( Bi , Pb

(1.80, (1.90, (2.10, (1.75, (1.80, (2.05, (2.10,

): Sr :Ca

:Cu

0.15):1.90:1.90:3.4 0.19):1.90:1.90:3.2 0.28):1.85:1.85:3 0.46):1.80:2.00:3.1 0.19):1.90:2.00:3.25 0.29):1.80:2.15:2.75 0.23):1.85:2.00:2.85

TABLE 1 and after heat treatments Bl, Cl were prepared with

under targets

different A, B, C

691

The evolution of lead content with annealing time can be seen from the results obtamed from samples Al to A4. The as-deposited

films have a composition

and 19% for A4. After 1.5 h annealing

x=17% for Al-A3

close to 2223 with

(sample Al),

7.5% and R(T) shows only a small drop at 110K; x increases

x decreases

again

to

for a longer

annealing:

9% for ta = 5 h (A2), 12% for ta = 10 h (A3) and 20% for A4 after 14 h

annealing.

The latter corresponds to a complete R(T) transition at 108 K. Consequently,

it appears that in our annealing beginning

process, the film looses part of its Pb content at the

of the heat treatment,

loss which is progressively

compensated

by a lead

supply from the pellet. It can be mentioned also that after applying the same treatment to a pure Bi film, about 7% of lead content were found in this film. With x values from EDX analyses, we find (Fig. 4) that critical temperatures 1OOK are obtained 15%22%. samples

for annealed

in agreement A5 (Ta=840°C

deposits

having

with results reported

: too low annealing

a lead content

on ceramics

temperature)

x=30% : too rich lead content in the as-deposited

Tc over

x in the range

1 13. For example,

and sample

Cl

both

(target C,

film), with only x=10% after annealrng,

have a Tc value equal to 70 K. 11

6

Superconducting

FIGURE 4 transition temperature T, (R=O) as a function of Pb% = x.

3.3. X-ray diffraction analysis. X-ray diffraction identification.

experiments

were made for sfructural characterizafion

and phase

Typical X-ray diagrams obtained for samples Al, A3, A4, with increasing

T, from Al to A4, are shown in Fig. 5. a, b, c. The first remarkable thing is that all films

692

h$i& Y

a

20

40

28Cdeg.l

40

0

2-a (deg.1

.

2 8

FIGURE 5 X-ray diffraction patterns where the high Tc phase peaks (0) and the low Tc

b

phase peaks (without dot) have been indexed a- sample Al (ta = 1.5 h, Ta = 860°C) b- sample A3 (ta = 10 h, Ta = 860°C) c- sample A4 (ta = 14 h, Ta = 860°C) @.=I.542 A )

28 (deg.1

are highly oriented as shown by the predominant feature is also revealed magnetoresistance

in the transport

intensity of the

properties

where a large anisotropy

(001) reflections.

(This

under magnetic field, such as the

is observed

as well in 2212 films and in

2223 films3). As shown by an X-ray texture like scan where a large part of the diffraction rings (0012 reflections) was measured, we can conclude that the high and the low Tc

‘I /,I - oH0012

I,,

1.2_‘,

1 ;+L

I,,

, /,

‘I,

,_

0012 @A

0.8 : x ; 0.6 : : - 0.4 :

$

j 0 @ CD

09 80’

Wg% o.2 :$/ 0 -3

, -2

-1 Scan

, 0 Angle

, ,

1

r$, 2

l

,; 3

(deg.)

FIGURE 6 Texture scan pattern made on 0012 peaks ( Fig. 5c ) for sample

A4

693

phase are highly substrate

and equally

well oriented

plane. The desorientation

with the c-axis

which can be estimated

perpendicular

to the

from Fig.7 is about 2

degrees (half maximum width) for both phases. So we can compare the amount of each phases

by comparing

their respective diffraction intensities. We use here the ratio :

r = H(0014)/((H(0014)+L(0012))

to estimate the volume fraction of the high Tc phase,

where H and L refer to the high and low T, phase peak intensity respectively.

This gives

r = 15% for Al with 14% R(T) drop at 110 K, r = 55% for A3 (ta = 10 h, and R(T) drop 85%) and r = 90% for A4 with Tc = 108 K after 14 h annealing

These results indrcate

that more than 55% of 2223 phase are needed to achieve a superconducting above 1OOK, in agreement

transition

with reference 7 on films prepared by laser ablation.

The lattice parameter c deduced from these patterns is (37,15 * 0,02)A for the high Tc phase and (30,75 + 0,02)A for the low T, one. We can also detect in the X-Ray diffraction patterns, some parasite phase peaks: a Ca2PbO4 (110) peak at 28=17.6” and a peak at 28=27.83” mentioned possibly corresponding

to a (Bi, Pb) free oxide (CaSr)3Cu508.

in reference 9 as

This peak is smeared

out with increasing annealing times. 3. 4. SEM observations Fig. 7a and 7b gives SEM micrographs

for samples Al and A4. It can be seen that

after a short annealing time (Al) the system is still at the beginning of the crystallization. There are needles annealed

5 to 10 pm long with random

orientations.

In constrast

the film

for 15 h at 86O”C, with T, =108 K, appears to be very well oriented with large

plate-like crystals parallel to the substrate plane

FIGURE 7.a SEM micrograph for sample Al (Ta=860”C, ta=l.5 h)

FIGURE 7.b SEM micrograph for sample A4 (Ta=860”C, ta=l4 h)

694

4. CONCLUSION Using

single

target

sputtering, 90%

of the 2223

prepared

lead-doping

target. To promote the high Tc phase formation, in the film during the thermal

phase

superconducting

more

the lead content

than

we have

containing

starting

from

with

25%

it is necessary to maintain

treatment of the deposits. We found that

to reach a zero resistance above 100 K the film must contain between lead after annealing.

thin films

a 2223

The optimum annealing temperature

15% and 25%

is in the range 85O”C-86O”C,

with ta =14 h for 8OOOAthick films. Both the high Tc phase and the low Tc phase films are highly and equally

c-axis oriented.

properties from transport

Work is in progress to compare

measurements

the physical

under magnetic fields (J,(T, H, 6) and R(T, H,

8) with 6 = (H, c)) in films of each phase.

ACKNOWLEDGMENTS We would support,

like to thank

J.C.Toledano

M.Helmer

for SEM micrographs,

J.Arabski

for technical

for fruitful discussions.

REFERENCES 1) H. Raffy, J. Arabski, A. Vaures, S. Megtert, R. Reich and P. Monod J. Less Comm Met. 151(1989)385 2) See references 4 and 6-l I in ref.1 3) M. Takano, J. Takada, K. Oda, H. Kitaguchi, Y. Miura, Y. Ikeda, Y. Tomii, H. Mazaki, Jpn. .Appi. Phys. 27 (1988) 1041. 4) H. Raffy, S. Labdi, A. Vaures, J. Arabski, S. Megtert, Physica C 162-164 (1989) 613 ; H. Raffy, J. Arabski, A. Vaures, S. Megten, Solid State Comm. 68 (1988) 235. 5) S.Labdi and H.Raffy to be published 6) M. R. Tseng, J. S. Chu, Y. T. Huang and P. T. Wu, J.Appl.Phys.67 7) J. C Toledano, preprint.

A. Litzler, J. Primot, J. Schneck,

8) To be published, and H.Raffy,S.Labdi,

(1990) 2657

L. Pierre, D. Morin, C. Daguet,

O.Laborde, P.Monceau,

LT19 Proceedings

9) Y.T.Huang, R.G.Liu, S.W.Lu, P.T.Wu, and W.N.Wang, Appl.Phys.Lett.56

(1990) 779