Atomic layer by atomic layer growth of BiSrCaCu oxide superconducting thin films by molecular beam epitaxy

II

Physica C 185-189 (1991) 2013-2014 North-Holland

ATOMIC LAYER BY ATOMIC LAYER GROWTH OF Bi-Sr-Ca-Cu OXIDE SUPERCONDUCTING THIN FII.HS BY ~OI.ECULAR BEAM EPITAXY

S. SAKAI, Y. KASAI, H.TANOUE, H. NATSUHATA, P. BODIN and T. OOHIRA Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba-shi, Ibarakl 305 Japan

We have prepared BI-Sr-Ca-Cu oxide superconducting thin films by molecular beam epltaxy. All metal fluxes were sequentially turned on and off by the shuttering technique. When the fil=¢ of the 2212 phase were grown on NgO(lO0) substrates, observation of RHEED patterns showed that, depending on the substrate temperature, there were two kinds of epitaxy. 1. INTRODUCTION Growing

oxide superconducting

films by

an

atomically layer-by-layer way is important for fundamental

studies

applications.

We

superconducting

well

as

have thin

prepared films

as

for

Bi-Sr-Ca-Cu-O

by

the

molecular

Streaks

were observed

Energy

Electron

Diffraction

patterns

from

an

to

last

stage.

the

early stage

The f i l m s

w i t h t h e 2212 phase were a l s o grown

summarized i n Table I .

powerful

particularly

NgO(100)

Figure

for making films of the single phase No.

of Bi2SrmCanCun+1Oo+mn. 1-m

2

substrates.

Table

I.

along

the

pattern

during

It can

azimuth

as

supply an ozone flux of 2xlO~cm-ms -~

to the

pattern

observed

substrate, while the NBE chamber is maintained

after

at a low pressure of about lxlO-%bar.

in

and

Bi

were

evaporated

from

Figure

lb

and

epitaxially

Ic

[010]

from

to t h e <100> d i r e c t i o n

crucible. turned closing

shuttered

electron-beam-heated

The beams o f

on and o f f

sequentially

the shutters,

irradiated

the metal sources

while

t h e ozone beam was

No.

4

in

patterns

observed

diffraction c-axis 70K

data

oriented. for

substrate,

a

showed The

film whose

that

highest grown

the

films

were

Tc obtained

on

temperature

a

was

different spacings

I.

along

that

the

the

film

[100]

is

or

parallel

grown from

2212 phase

but

that

show t h a t

show

azimuth

the

the

~ilms

orientation

!1101

I

~ is

" The s t r e a ~

direction

of the

is parallel to the d~rection

of the ~g~) substrate.

was

ep~taxy

ha~

0921-4534D1/$03.50 O 1991 - Elsevier Science Publishers B.V. All fights rcse~:ed.

and 2b

and during growth. that

in F i g u r e the

2a

the ~g~

show

LaAlOa(lO0) 750"C.

a

of t h e ~g~ s u b s t r a t e .

Figures

These

epitaxially

The 2212 phase films were made, and X-ray

shows

and s p a c i n g s

that

before growth

respectively.

3. RESULTS AND DISCUSSION

the

~Ig-O a z i m u t h

of t h e 2212 phase

Table

on ~gO(lO0}

c o n t i n u o u s l y d u r i n g growth.

and

on

Figure 2 shows the RHEED patterns of sample

were

by o p e n i n g and

the

lc

indicate

grown

direction

of sample

a l o n g the same

Figure

alon~

are

F i g u r e lb shows a

g r o w t h . The s t r e a k p a t t e r n s

Figures

is

results

shows a p a t t e r n

observed la.

shuttered effusion cells and Cu was evaporated a

la

grown.

MgO<100> azimuth

growth

ozone source of the cryocooler type. 4

Ca

Figure

b a r e ~gO(lO0) b e f o r e growth.

As an active oxygen source, we used a pure

Sr,

The

1 shows t h e RItEED p a t t e r n

in

observed

2. EXPERIMENTAL PROCEDURE

of the growth

RHEED p a t t e r n s

The

t h e f i l m was e p i t a x i a l l y

The

is

(RttEED)

that

on

shuttering

High

indicated

beam epitaxy (NBE) with shuttered techonolgy. ~ sequential

in the Reflective

two

As shown ~n Table ~ the

modes,

dependin~

on

~be

$. Sakai et at / Atomic layer by atomic layergrowth of Bi-Sr.Ca.Cu oxide superconducting thin films

2014

Table I

Growth c o n d i t i o n s and p r o p e r t i e s of 2212 f i l m s grown

Sample Ts a) (°C) P~> (mbar) t °) (nm) No.1 No.2 No.3 No.4

610 670 670 750

7x10 -s lx10 -~ lxlO -~ 2xlO -~

on ~gO(lO0) s u b s t r a t e s .

azimuth parallel to rqgo<100>

90 85 90 70

[I00] or [010] [100] or [010] [100] or [010] [110]

phase

{2212) (2212) (2212) (2212)÷ minor CuO

~ ) s u b s t r a t e temperature,~>background p r e s s u r e , ° > t h i c k n e s s , d ) t r a n s i t l o n

!i!i!i}!i!!i!i!iii!/i! ii!ill!

i!ilil i

Tc a) {K) 51 40 37 47

temperature.

iiiiiiiiiii!iii!/i/////i ? : / ? :?si

i~ iI~ iI/'!~~ II!i!ii/ii~ iii/i l i?/:i//i i!ii i ilaii i¸).....(/{/il//}i/i!:il!i(i{

b

....

i

. . . . . . ~~

Figure 1 RHEED patterns of sample No.2. Patterns observed along the MgO azimuth (a) on the ~gO(lO0) before growth, and (b) during growth. {c)Pattern observed along the ~gO azimuth after the growth.

substrate temperatures. At the lower temperatures, 610°C

and 670°C,

the [100] or [010] direction of the 2212 phase

was parallel to the ~gO direction. At the higher

temperature

direction

of

of

750 °C

the

[110]

the 2212 phase was parallel

to

the ~gO direction.

ilii!ii !ili!i!!:!iif!ii!!i!i/iii!i!ii¸iiii!i!iliii!iiiiil!iiiii~i/ii !i!¸~ii!ii¸!i¸!i!i!ili!iiiil!ii!if!ili !i!iii iIIiiiilii:iiiI !!!ijii! l ili/i/!!ii!iiiiili iiiii~iI~iili!i~i!IiIi!!i!i !ii!!i! ii~~ ii/i i iii!!i!ii!il!il!il!i!iii/i!iii!iilii/iii/!!/ii/iii!!ii!ii!i~il!i!!i~ ii!i!i!iilli!ii!il!ii!i!i!i !ii!~ii!~ !il~i!li!ii!i!~ili!i!i!i!!i!il!!ii!i!i!i!i!i!i!i!i!i~i!~i!li!i!li!!~i!i!i!i!i!i!i!i!i!il!iil!i!ii!i!!il/i! llllll

II

4. SUmmARY Bi-Sr-Ca-Cu oxide superconductor films have

been prepared by ~BE with a pure ozone source. The films have been grown in a layer-by-layer way by employing the From

RHEED

patterns,

shuttering technique.

we

found

two

different

kinds of epitaxy for the 2212 films grown on ~gO(lO0)

substrates.

The epitaxy depended on

Figure 2 RIIEED patterns of sample No.4, obsurv~d along the MgO azimuth (a) on the MgO(lO0) and (b) during growth.

the substrate temperature. REFERENCES 1. S . S a k a i , Y.ffasai et a l . , a c c e p t e d for p u b l i c a t i o n in Proceedings of 3rd I n t e r n a t i o n a l Symposium on S u p e r c o n d u c t i v i t y (Nov., 1990, Sendai).

2. K.Uchinokura. Y.Nakayama et a l . , to be published in Proceedings of Conference on Science and Technology of T h i n - f i l m S u p e r c o n d u c t o r s , (April, 1990, Denver). 3. D.G.Schlom, A.F.Marshall et a l . , J . C r y s t a l Growth 102 (1990) 361. 4. S.11osokawa and S . I c h i m u r a , a c c e p t e d for p u b l i c a t i o n in Rev. Sci. I n s t r u m . .