Growth characterization of a-axis oriented “1-2-3” films

PHYSICA

Physica C 180 (1991) 6 5 - 6 8 North-Holland

GROWTH CHARACTERIZATIONOF a-AXIS ORIENTED "I-2-3" FILMS R.L. WANG*, J. REINER+, J. REMMEL, E. BRECHT, J. GEERK, O. MEYER, AND G. LINKER

Kernforschungszentrum Karlsruhe, Institut fur Nukleare Festkorperphysik, P.O.B. 3640, D-7500 Karlsruhe 1, Germany

The growth o r i e n t a t i o n of YBaCuO and EuBaCuO HTSC t h i n films has been studied on d i f f e r e n t (100) oriented substrates l i k e SrTi03, LaAI03, MgO and Zr(Y)02. The main deposition parameter cont r o l l i n g the growth d i r e c t i o n was the substrate temperature, Ts. At Ts values of I00-150°C below those optimized f o r c-axis growth the films grow a-axis oriented. The growth q u a l i t y of the films is characterized by t y p i c a l mosaic spreads of 0.2 ° and Xmin values in channeling experiments around 5%. The superconducting properties of such films are degraded. In a growth process consisting of a t h i n template layer deposition at reduced Ts and continued growth at elevated Ts the growth d i r e c t i o n and q u a l i t y is preserved and the properties are improved (Tc=84 K).

substrate

1. INTRODUCTION The

growth

quality

of

HTSC t h i n

films

like,

may be imposed on the f i l m

e.g.,

on

(110)SrTi03

growth

substrates

where

depends on quite a number of parameters includ-

sputter-deposited YBaCuO f ilms also grow ( i i 0 )

ing those c h a r a c t e r i s t i c of a deposition tech-

oriented ( i ) . HTSC t h i n films of d i f f e r e n t o r i e n t a t i o n are

nique or which must be applied using a special preparation method.

Of

importance to

all

techniques is

its

o r i e n t a t i o n with respect to s u b s t r a t e - f i l m

matching or

the

similar

substrate material

substrate-film

reaction

and

and pro-

of

interest

in basic research and f o r

cations

due to

"1-2-3"

structure

properties.

We

the

appli-

large anisotropy of

reflected have

in

detected

the

the

physical

earlier

that

bably the substrate temperature during deposi-

reductions of Ts change the growth d i r e c t i o n of

tion,

YBaCuO films on (lO0)SrTi03 substrates from c-

Ts.

Gas flow

during d i f f e r e n t rates,

dynamics,

gas

deposition steps,

pressures deposition

or geometrical arrangements of

sources

axis to a-axis o r i e n t a t i o n ( I ) . studied

this

behaviour

in

Now, we have

more detail

for

and substrates may be in more d e t a i l determined

EuBaCuO and YBaCuO on d i f f e r e n t (i00) oriented

by a s p e c i f i c method.

substrates

film s

with

the

regarding the

In most cases t h i n HTSC

"1-2-3"

structure

c-axis textured, i . e . ,

with the c-axis perpen-

d i c u l a r to the substrate surface. is

mainly observed on substrates

orientation crystals.

optimized

superconducting properties grow

but

also

Such growth with

on randomly cut

But also special

(100) single

o r i e n t a t i o n s of

a

(SrTi03,

report results special the

LaAI03, MgO, Zr(Y)02).

from these i n v e s t i g a t i o n s with

emphasis on SrTi03

properties

We

of

the

substrates.

f ilms

Since

deposited

at

reduced Ts are degraded, a special procedure of growth

at

different

temperatures

with

deposition of a t h i n template layer at low Ts has been applied f o r properties improvement.

*On leave from Chinese Academy of Science, B e i j i n g , P.R. China +Kernforschungszentrum Karlsruhe, I n s t i t u t f~r Technische Physik 0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.

R.L. Wang et al. / Growth characterization of a-axis oriented "1-2-3"films

66

2. EXPERIMENTAL PROCEDURE

findings in general refer to both compounds i f

For the film deposition we have employed the

not otherwise specified. We have found that Ts

inverted cylindrical magnetron (ICM) technique

during f i l m deposition is the most important

with a special arrangement of

parameter controlling the growth direction of a

form of

an inner cylinder.

the target

in

This geometrical

film.

In fact,

in our experiments i t

was the

arrangement largely avoids negative

ion bom-

only parameter reversing the growth direction

bardment which is

the

from c-

detrimental

for

film

to

a-axis orientation. This process

growth. The f i l m preparation has been completed

occurs gradually. At Ts around 800°C (we quote

in two steps, i . e .

the heater block temperature measured with a

in situ deposition at ele-

vated Ts and subsequent oxygen take up in the

thermocouple) the films grow c-axis oriented.

cooling process to RT. Details of the ICM-tech-

With decreasing Ts mixtures of c- and a-axis

nique are described elsewhere (2). We have kept

oriented grains are observed, with the concen-

all

tration

parameters constant

in

both preparation

steps, except Ts during deposition, which was

ratio

changing in

almost p u r e a-axis

deposition rate which may influence the growth

observed.

direction was around 0.3 nm/s. h a v e characterized

of

a-axis

orientation with decreasing Ts. At Ts of 700°C

the variable parameter in our experiments. The

We

favour

oriented f i l m

growth is

In Fig. 1 we show X-ray d i f f r a c t i o n diagrams

the

film

growth

direction and growth quality by X-ray d i f f r a c t -

of

films

deposited at

two different Ts.

At

730°C (hO0) lines clearly separated from the

ion measurements and ion beam channeling ex-

substrate

periments. Two parameters serve as measures of

However, a small (005) peak is also observed in

growth quality, namely the FWHMof X-ray rock-

the diagram s t i l l

ing curves, A (mosaic spread), and the r a t i o of

tration of c-axis oriented grains of about 4%.

random and

Xmin, f r o m ion

At Ts of 650°C there is no indication for the

backscattering spectra. The optimization of a-

presence of (00~) lines, but the (hO0) lines of

aligned yields,

yielding a volume concen-

the i.e.,

and of the content of c-axis oriented grains.

We have observed that down to Ts of 700°C the

sition at lower Ts and temperature increase for

a-axis

to

growth.

axis oriented f i l m growth was performed with

a template layer depo-

shift

demonstrate a-axis

respect to a minimization of A and Xmin values In the process of

film

lines

lower d i f f r a c t i o n angles,

the a-axis l a t t i c e parameter increases. lattice

parameter also

in

the

mixed

films was constant and close to bulk value, but

the f i l m growth we have varied the times of

increased rapidly for

template layer growth and temperature increase

quality of the a-axis Films in comparison to c-

in the range of 30 s to 5 min. The total depo-

axis films

s i t i o n time was around 30 min corresponding to

values of the mosaic spread ~ are below 0.4 °

is

not

lower Ts.

largely degraded. Typical

and best films have values below 0.2 ° . An even

f i l m thicknesses of 500 to 600 nm.

more sensitive check of

growth quality

channeling measurements. In

3. RESULTS In this

contribution we report results of

YBaCuO and EuBaCuO thin f i l m growth on (100) SrTi03 substrates.

No significant differences

have been observed, therefore the

The growth

following

Fig.

random and aligned backscattering

are

2 we show spectra of

2 MeV He particles of a EuBaCuO film deposited at 700°C. Unfortunately, the signals from Eu and Ba atoms cannot be well separated. Never-

R.L. Wang et aL / Growth characterization of a-axis oriented "l-2-3" films

67

18 T z = 730

°C

t4 lO

EuBaCuO/SrTiO3 Xm~= 54 °/o

~. 5

o

~v~i"

o

o

6

-J

RANOOM

/

7 cu

/

L_

2 0

/Ba

b 20

T s = 650 %

¢-

[I00] ALIGNED

15

,/Eu

i

.,4

io~

c

5 200

c H

300 Channel

400

FIGURE 2 Random and aligned backscattering spectra of a EuBaCuO film on SrTi03 demonstrating the growth quality of a-axis oriented films deposited at low Ts.

o.15 0.05 4.

15

3'5

25

applied by Inam et al.

Theta ((:leg)

(3) of a thin template

layer deposition at reduced Ts and continued FIGURE 1 X-ray d i f f r a c t i o n diagrams of a-axis films deposited at different substrate temperatures, Ts, of 730°C (a) and 650°C (b); (c) is the enlarged diagram of (b) to demonstrate the low concentration of c-axis oriented grains.

growth. In this process the a-axis orientation

theless, the Xmin value in the Eu/Ba sublattice

s l i g h t l y improved. Best Xmin values are 4% and

behind the surface peak is 5.4%. In films on

4.5% for EuBaCuO and YBaCuO films, respective-

growth at higher Ts for the bulk of the film. In the second step of this procedure we used Ts values close to those optimized for c-axis Film is

preserved and the growth quality is

even

LaAI03 substrates the values were even better

ly. Best A values are close to 0.1°; i t should

appraoching 4%.

be mentioned, however, that the whole shape of

This

is

close

to

what is

observed in c-axis oriented films, though our

particle

best values here were in the range of 3%.

characterization of the growth of a film than

Despite the good crystallographic growth in

distribution

is

a

more complete

merely the FWHM of the rocking curve. In this

terms of A and Xmin values the properties of

respect the distribution of the a-axis films

the a-axis films, especially of those deposited

deposited in one step at reduced Ts appeared

at lowest Ts, where the increase of the l a t t i c e

steeper. A typical rocking curve of a f i l m pre-

parameter was observed, are degraded, i . e . , the

pared in the "template" procedure is displayed

transitions to superconductivity are broadened

in the upper part of Fig. 3. In the lower part

with depressed zero resistance values. This may

we show a rocking curve with the detector set

be

due

to

the

incorporation

of

a

defect

structure probably on the oxygen sublattice. Heating in oxygen atmosphere at

for

the

detection of

the

(005) line

which

appeared as a minute peak in the ®-2® diagram.

temperatures

Due to the broader distribution of the c-axis

above Ts did not show any distinct annealing

grains the volume percentage of this misorien-

effects. We therefore adopted a procedure f i r s t

tations s t i l l

amounts to 0.4%. The difference

/

R.L. Wang et al.

68

Growth characterization of a-axis oriented "l-2-3" films conductivity are sharp with Tco values of 84 K.

180F

EuBaCuO/SrTi03

(200)~

We show a sample R vs. T measurement in Fig. 4.

--

The films are metallic with a rather linear R 140r

vs T relationship, which does not extrapolate FWHM- 022°

to zero. The high r e s i s t i v i t i e s at 100 K in the

iO0 I

T 0 -~

range of 800-1000 p~cm mainly are due to two

60 I

d i f f e r e n t in-plane orientations of the c-axis

2oi

J L

as has been demonstrated in TEM measurements

(4).

°'a~ i/

(o0s)

6. CONCLUSIONS

=~.I°

The substrate temperature during deposition, Ts, is one of the most important parameters in the

control

of

HTSC thin

film

growth.

On

d i f f e r e n t (100) oriented substrates the reduc_

20 2P Omega (deg)

IB

a4

tion of Ts is s u f f i c i e n t to convert c-axis to a-axis oriented growth. The growth quality of

FIGURE 3 Rocking curves of a EuBaCuO f i l m prepared in the "template" procedure with the detector set for the detection of the (200) and (005) l i n e s .

the a-axis films in terms of Xmin and A with values of 4 to 5% and 0.2 ° , respectively, is similar to c-axis films.

The surfaces of the

a-axis films are extremely shiny and smooth, in

the d i s t r i b u t i o n widths shows that for

correct

estimation

grains

with

of

the

different

concentration

orientations

a of

integral

i . e . , superior to c-axis films. The "template" procedure allows the preparation of

metallic

a-axis films with reasonable Tc values of 84 K.

i n t e n s i t i e s should be used from rocking curves rather than from ®-2® scans. The

properties

of

the

"template"

a-axis

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