BiSrCaCuO films made by coevaporation: Influence of the initial composition

Journal of the Less-Common Metals, 164 & 165 (1990) 695-702

695

BiSrCaCuO FILMS MADE BY COEVAPORATION: INFLUENCE OF THE INITIAL COMPOSITION

P. Luzeau,

X.Z. Xu and M. Laguts

CNRS UA421, M.Nanot

ESPCI,

10 rue Vauquelin,

ESPCI,

Ceramique

et Materiaux

10 rue Vauquelin, BiSrCaCuO

of the

electron

gun,

and the

In situ

oxidation

plasma

source.

discuss

the

resistive

proportion by Rutherford also prepared

the

ultrahigh

copper

was deposited

were

and

Backscattering,

stoichiometry

of the bismuth.

oxygen

conditions,

and we

on the

related

initial

at the atomic The main

is related

This problem

an

Knudsen

conditions.

layer

difficulty

to the variations

to the

composition

and on the deposition

by flux modulation

by this way are promising.

right

from

obtained

depends

by

from

by an atomic

in different

2223 phases

composition

vacuum

evaporated

was performed

annealed

of 2212

under

The

components

were

This

Films were

coefficient

prepared

of the films

The films

obtained

FRANCE

elements.

other

determined

reach

were

pure

transition.

Mineraux

75231 Paris

films

coevaporation

results

FRANCE

and F.Queyroux

Laboratoire

cells.

7523 1 Paris

scale.

The

in order

of the

to

sticking

will be discussed.

INTRODUCTION The Bi cuprate presents

one

superconductors. superposition

family

of the

most

This

BizSr2Ca,Cu,+t0, lamellar

structure

of the following (BiO),

defects

corresponding

(Sr) (CuO)

difficulty prepare

epitaxial

including (MBE) MBE

to obtain

0022-50881901$3.50

by the

periodic

described

of

to different

techniques

sputtering one

n= 1,2,3 to a Gibbs

values value

BiZSr~CanCu~+rOx

deposition

is presently

cuprate

and

(Sr) . . . . . . . . .

related

with a single

films of a single

[3,4] and magnetron

to other

[ 1). On the other hand, stacking

intergrowth

corresponding pure phases

conventional

technique

(Ca) (CuO) compound

[2]. This is probably

close for the phases

2D behaviour

can be simply

to the

observed

exhibits compared

layers:

in the case of the Bi$r&aCt+O, frequently

structure

free

phase

as molecular

only way supposed

are

energy

of n, leading

of n. Various

[5], as well as laser

of the

compounds

very

to a basic attempts

to

were published, beam ablation

epitaxy [6]. The

to allow

the

0 Elsevier Sequoia, Printed in The Netherlands

696

control of multilayer devices at the atomic layer scale. In this communication, we present results obtained by MBE and flux modulation in order to make Bi2SrzCa,Cu, incorporation

+ I 0 x films. The specific problems related to the bismuth in the films is discussed both in the case of flux modulation

and in the case of permanent EXPERIMENTAL The MBE chamber

flux deposition.

is based on the EVA32 (Riber)

equipment.

bismuth, the strontium and the calcium are evaporated from Knudsen using Joule effect, while the copper is evaporated from an electron

The cells gun.

Deposition rates range typically around 0.4 As-‘. The oxidation of the films is performed using an atomic oxygen plasma source’ (OPS) [7]. This source is based on a DC excited plasma in a U Pyrex tube full with molecular oxygen (figure 1). A small hole allows the effusion of excited species and especially atomic oxygen which is present at a partial pressure of 10% [S]. The typical pressure in the tube is 1.5 Torr, leading to an estimated flux of oxygen atoms on the substrate

of 3 1015 at.cm-2.s-1 .

OXYGEN INLET

Schematic

_

OXYGEN OUTLET

Figure 1 drawing of the oxygen plasma source (OPS)

1. The oxygen plasma source was developped in collaboration company, Thomson-CSF company and the LGLP laboratory University (France).

with Riber of Orsay

691

THIN FILMS A first

WITH POSTANNEALING set of BiSrCaCuO

MBE process, defined

with the simultaneous

rates.

The substrates the sticking different

temperature

used in these

depositions

requested In

large,

deposition

T(Sr)=390’C, contain

value.

effectively

quantity

incorporated

of Sr (figure

and vanishes

(100)

should

MgO single

range

in the

conditions

typically

order

of

used

A.s-l),

of magnitude

of Bi divided

the

below

influence

by the

decreases

above 700°C, while the deposition

If the

The Bi deficiency

shows a direct

2). The Bi concentration

crystals. of unity, were

v(Cu)=O.l

one order

at well to 600°C.

same

which

of bismuth.

temperature quantity

400°C

are all of the order

T(Ca)=400°C,

being

The substrate

of all the constituents

were

a very small amount

the Bi concentration

expected the

rates

typical

using the conventional

was in the range

of the constituents

evaporation the

(T(Bi)=SOO’C, films

deposition

The substrate coefficients

magnitude. deposited

films were prepared

steeply

is the

on [Bi],

incorporated above 500°C

flux is kept constant.

Ts('C) 500

600

700

0

I.0

0

0.8

g 0.6

0 0 0

0.4

0.2

0

1

1.1

1.3

1,2

I.4

IOOO/Ts (K-l) Figure Relative

Bi concentration

versus

2

substrate

temperature

in BiSrCaCuO

films

698

Sr ! 9: ” . I

200

4th

360

I :. Bi i l: :.

5bo

CHANNEL

Figure 3 RBS spectra for two BiSrCaCuO thin films deposited but with (continuous line) and without the atomic

in the same conditions oxygen plasma source

(dotted line) This Bi deficiency bismuth (the temperature

could be explained by the high vapor pressure of of the Bi Knudsen cell itself ranges between 45O’C

and SOO°C, of the same order of magnitude as the substrate is indeed very difficult to deposit pure Bi on a substrate vacuum, molecular

due to its vapor pressure. oxygen (without

On the other hand,

OPS) the sticking coefficient

temperature). It at 500°C under

in the presence

of

of Bi is of the order

of unity owing to the oxidation of the film. The very low sticking coefficient of the bismuth is thus mainly related to the atomic oxygen pressure. Figure 3 presents the comparison of two films prepared exactly in the same conditions except for the presence of atomic oxygen. Rutherford BackScattering (RBS) measurements are presented which show both the concentration and the depth profile of the different elements. The bismuth quantity deposited in the presence of atomic oxygen (OPS working) is around 20 times lower than the bismuth deposited in absence of atomic oxygen. The bismuth deficiency is also correlated to the simultaneous deposition rate of copper. The bismuth concentration in the films decreases when the copper deposition rate increases.

699

The

Bi deficient

films

superconductive

upon

resistance

temperature

under

versus

air

transitions

in the

around

of bulk

network

of each between

25% , or by a 3D network

3% to 8% . The 2D percolation and is strongly

Bi [9]. In the present diffraction

shows

percolation

case,

the presence

hand,

the 2223 regions

crystals

favoured

of a very small sizes

two

are larger

probably

observed

respectively

to the

et al. [9], this ratio either

by a 2D

of each phase initialy

larger

of part

of the resitivity than

than the of the

no Pb and the Xray

2223 concentration. 2 pm (figure

to intergrowth

of

in the range

by the Pb substitution contain

the

at 860°C

and a 2223 concentration

the best description

the grain

correspond

means

become

4 presents

with a 2223 concentration

the films

is thus probably

On the other

The

can be described

the two phases,

can

75mn annealing

by J.C.Toledano

transition

they Figure

BiPbSrCaCuO.

As it was shown

the resistances

film thickness

but

at Tonset = 1lOK and 83K correspond

2223 and 2212 phases. percolation

amorphous,

at high temperature. for such a film, after

presence

observed

between

are

annealing

regions

The 3D of this film. 5) and thus

smaller

than the

grain thickness.

T Figure Temperature after

annealing

dependence at 860 ‘C

(K)

4

of the electrical

resistivity

of a Bi deficient

film

700

SEM micrograph annealing

Figure 5 of a Bi deficient BiSrCaCuO

thin film after

a 860°C

FLUX MODULATION The lamellar structure of the cuprates and especially the BiSrCaCuO family suggests to make them by a sequential deposition of each lamellar unit. Flux modulation is a convenient way to perform this sequential deposition as well as for other cuprates superconductors [lo]. Moreover flux modulation allows to solve the difficulty incorporation. Each source is periodically sequence

x as illustrated

encountered with the bismuth opened and closed following a

by the figure 6. The stoechiometry

of the films may

then be adjusted

simply by varying the duration

of the opening of each source

and for instance films deposited

the Bi concentration obtained are inthe correct range. The in this way exhibit Xray diffraction, showing partial

crystallization as grown. These flux modulated depositions results are recent and further experiments are performed to reach in situ deposition of superconductive

films.

701

Sr

cu

period

Figure Outline

of the flux modulation

cell shutter

S

6

technique.

Each

crenel

corresponds

to the

opening.

CONCLUSION In this communication flux deposition Bi quantity is hardly

Films

in the case

because

of atomic

to use permanent oxygen,

in the film is then very low. The bismuth Bi flux does

the occurence

prepared

the difficulty

in the presence

only by a lowering

the incident

We suggest observed

incorporated explained

increasing

we have shown

of BiSrCaCuO

not always

of a self-blocking

of arsenic

by flux

of its sticking

in the case

modulated

they are not Bi deficient

deposition and because

coefficient

increase

are

deficiency because

film much

similar grown more

they allow a lamellar

to what by MBE. growth

ACKNOWLEDGMENTS We want to acknowledge and J. Perriere

A.Dubon

for SEM and EDX measurements,

for RBS analysis

of the films.

is

promising

the films.

and A. Cheene

the

the Bi incoporation.

mechanism of GaAs

because

of

702

REFERENCES [I]

J.M. Tarascon, Y. Lepage, W.R. McKinnon, E. Tselepis, P. Barboux, B.G. Bagley and W. Ramesh, Phys. Rev. B, No. 39, 4319 (1989)

[Z]

J.L. Tallon, R.G. Buckley, P.W. Gilberd, M.R. Presland, I.W.M. Brown, M.E. Bowden, L.A. Christian and R. Goguel, Nature, 333, 153 (1988)

[3]

Y. Nakayama, I. Tsukuda, A. Maeda and K. Uchinokura, Jpn. J. Appl. Phys., Vol. 28, No. 10, Ll809 (1989)

[4]

J.N. Eckstein, D.G. Schlom, E.S. Hellman, K.E. Von Dessonneck, Z.J. Chen, C. Web, S. Turner, J.S. Harris, M.R. Beasley and T.H. Geballe, J. Vat. Sci. Tech. B7, 319 (1989)

[5]

H. Adachi, S. Kohiki, K. Setsune, T. Mitsuyu and K. Wasa, J. Appl. Phys., 27, 1883 (1988)

[6]

T. Kawai,

[7]

P. Luzeau, X.2. Xu, M. LaguZs, J.-P. Contour, N. Hass, M. Nanot, F. Queyroux, M. Touzeau and D. Pagnon, JVST (1990) (in press)

[8]

M. Touzeau, D. Pagnon, P. Luzeau, A. Schuhl, R. Cabanel, MRS Fall Meeting Boston (Nov/Dec 1989)

[9]

J.C. Toledano, A. Litzler, J. Primot, J. Schneck, D. Morin and C. Daguet, Phys. Rev. B (1990) (in press)

ISTEC Journal, Vol. 2, No. 3 (1989)

[IO] R. Cabanel, A. Schuhl, G. Hyselen and G. Creuzet, ICMC '90 Meeting (Garmisch Partenkirchen, May 1990)