Formation of CuxMo6S8−y thin film on an MoS2 substrate by a chemical transport technique

Mat. R e s . B u l l . , Vol. 20, p p . 611-617, 1985. P r i n t e d i n t h e USA. 0025-5408/85 $3.00 + .00 C o p y r i g h t (c) 1985 P e r g a m o n P r e s s Ltd.

FORMATION

Hirofumi

OF C U x M O 6 S 8 _ y

THIN

BY A C H E M I C A L

TRANSPORT

Hinode,

Department

TECHNIQUE

Satoshi

Yamamoto,

and M a s a o

Taniguehi

of C h e m i c a l

of Technology,

F I L M ON AN MoS 2 S U B S T R A T E

Engineering,

Ookayama,

Meguro-ku,

Masataka

Wakihara

Tokyo

Institute

Tokyo

152,

Japan

( R e c e i v e d O c t o b e r 26, 1984; R e f e r e e d )

ABSTRACT Thin films of C u x M o 6 S s - y were p r o d u c e d by t r a n s p o r t of a g a s e o u s c o p p e r c o m p o u n d onto an MoS 2 s u b s t r a t e by using a c h e m i c a l t r a n s p o r t technique. The films w i t h t h i c h n e s s of about i0 to 30 ~m were always g r o w n on the substrate, and it was found that the t h i c k n e s s of the film may be e s t i m a t e d by that of the initial MoS 2 substrate. The m i c r o - s t r u c t u r e of the films was investig a t e d by using both the x-ray d i f f r a c t i o n and the s c a n n i n g e l e c t r o n m i c r o s c o p e techniques. It was found that the CuxMo6Ss_y lattice g r o w t h p r e f e r r e d the hexagonal c-axis d i r e c t i o n on the substrate. The copper c o n t e n t of the bulk s p e c i m e n was d e t e r m i n e d by the atomic a b s o r p t i o n m e t h o d and the g r a d i e n t of cepper c o m p o s i t i o n in the film was i n v e s t i g a t e d by the e l e c t r o n probe m i c r o analysis. No c o n c e t r a t i o n g r a d i e n t of c o p p e r was o b s e r v e d w i t h i n the p r o d u c t film.

Introduction A n u m b e r of ternary m o l y b d e n u m sulfides known as Chevrel phase c o m p o u n d s (i) of f o r m u l a MxMo6S8 ( M = m e t a l ) w h i c h contain b u i l d i n g b l o c k s of Mo6S8 c l u s t e r have b e e n r e c e n t l y i n v e s t i g a t e d from the p h y s i c a l and crystal s t r u c t u r a l v i e w points (2,3,4,5). Many of these c o m p o u n d s have s u p p e r c o n d u c t i n g p r o p e r t i e s showing a h i g h u p p e r c r i t i c a l m a g n e t i c field. In c o p p e r m o l y b d e n u m sulfide, the copper ions have the c h a r a c t e r i s t i c high m o b i l i t y among the Mo6S 8 clusters. Also, the sulfides, i.e. the C u - c l u s t e r c o m p o u n d s 611

612

H. HINODE, et al.

Vol. 20, No. 6

C U x M O 6 S 8 .have a w i d e range of h o m o g e n e i t y w i t h r e s p e c t to the copper c o ~ p o s i t o n ( 2 < x < 4, at 1000°C (6)) . Therefore, the C u - c l u s t e r c o m p o u n d s have b e e n e x p e c t e d to w o r k as an e l e c t r o d e m a t e r i a l for s e c o n d a r y b a t t e r i e s b e s i d e s the a p p l i c a t i o n for a new s u p p e r c o n d u c t i n g m a t e r i a l . The s y n t h e t i c techniques for p r o d u c i n g Chevrel phase thin film are u s u a l l y m u c h m o r e c o m p l i c a t e d than those for bulk s p e c i m e n p r e p a r a t i o n . Until now, the Chevrel phase films ( c a 1-3 ~m t h i c k ) have b e e n p r e p a r e d by using the t e c h n i q u e of RF s p u t t e r i n g (7,8, 9,10). W h e n s u b s t r a t e t e m p e r a t u r e was lower than 300°C, however, these films could not be i d e n t i f i e d by x-ray d i f f r a c t i o n analysis due to their a m o r p h o u s nature. C o n s e q u e n t l y , in order to c r y s t a l lize the p r e p a r e d films, they were u s u a l l y a n n e a l e d at 900 ° or 1000°C in s e a l e d q u a r t z tube under h e l i u m a t m o s p h e r e or in vaccum. For a s u b s t r a t e t e m p e r a t u r e was h i g h e r than about 700°C, there exists an i d e n t i f i e d s e c o n d phase. In the p r e s e n t work, using a c h e m i c a l t r a n s p o r t t e c h n i q u e the C u - c l u s t e r c o m p o u n d thin film was p r e p a r e d by the t r a n s p o r t of g a s e o u s copper iodide from a lower t e m p e r a t u r e zone to an MoS 2 s u b s t r a t e in a h i g h e r t e m p e r a t u r e zone f o l l o w e d by the r e a c t i o n of the t r a n s p o r t e d c o p p e r iodide w i t h the MoS 2. This m o t h o d s i m p l i f i e s the p r e p a r a t i o n of C u - c l u s t e r c o m p o u n d thin films. Its m i c r o - s t r u c t u r e was i n v e s t i g a t e d by u s i n g x-ray d i f f r a c t i o n and s c a n n i n g e l e c t r i o n m i c r o s c o p e ( S E M ) techniques. The copper content of the p r e p a r e d film was d e t e r m i n e d by both the atomic a b s o r p t i o n and the e l e c t r o n probe m i c r o - a n a l y s i s ( E P M A ) methods.

Experimental Preparation

of the MoS2

substrate

M o l y b d e n u m sheet w i t h a t h i c k n e s s of 0.02 cm was cut into c o u p o n s 0.5 cm x 1.5 cm. A small hole was d r i l l e d at the top of each coupon for the i n s e r t i o n of a s i l i c a s u s p e n s i o n hook. The 12 : 2 0 m g l c m 3 800 coupon was p o l i s h e d w i t h emery p a p e r down to 4/0 grade, and subsequently washed with distilled r w a t e r and acetone. The s t o i c h i o m e e~ tric MoS2 was p r o d u c e d on the Mo coupon by using the H2S / H 2 gas high t e m p e r a t u r e s u l f i d a t i o n ~ method. The s u l f i d a t i o n c o n d i t i o n was c o n f i n e d to the stable range of the MoS 2 phase, d e t e r m i n e d by the e q u i l i b r i u m sulfur p r e s s u r e c o m p o s i t i o n i s o t h e r m (ii) ( l o g P s 2 FIG. 1 = - 2.70, at 950°C, 24-48 h ). The S c h e m a t i c figure for the t h i c k n e s s of the MoS 2 film was t e m p e r a t u r e g r a d i e n t and the c a l c u l a t e d from the w e i g h t change p o s i t i o n of the silica tube. as 10 to 25 ~m a p p r o x i m a t e l y , and

Vol.

the

20, No.

thickness

Synthesis

613

CUxMO6S8_y THIN FILMS

6

varied

of the

with

the

CuxMo6Ss-y

reaction

thin

time.

film

T h e M o S 2 s u b s t r a t e a n d c o p p e r p o w d e r ( ca. 1.0 g ) w i t h i o d i n e ( 20m~/cm3 ) as the t r a n s p o r t i n g a g e n t w e r e s e a l e d in an e v a c u a t e d ( 10-JTorr ) s i l i c a t u b e ( ID 1 2 m m, OD 1 6 m m, l e n g t h 2 0 0 m m ) . The M o S 2 s u b s t r a t e c o u p o n w a s s u s p e n d e d on a s i l i c a h o o k w i t h o u t t o u c h i n g the s i l i c a w a l l , a n d w a s p o s i t i o n e d at the h i g h t e m p e r a t u r e zone ( ca. 3 cm ) s i n c e the t r a n s p o r t of g a s e o u s c o p p e r i o d i d e t a k e s p l a c e f r o m the c o l d zone to the h o t zone. The t e m p e r a t u r e g r a d i e n t a n d the r e a c t i o n t u b e in the p r e s e n t e x p e r i m e n t s are l i s t e d in T a b l e I, w h e r e a s the t y p i c a l t e m p e r a t u r e g r a d i e n t of the f u r n a c e a n d the p o s i t i o n of the s i l i c a r e a c t i o n t u b e is s h o w n in Fig. I. Structural

investigation

and Analysis

of the p r e p a r e d

thin

film

The p r o d u c e d f i l m s w e r e i d e n t i f i e d by x - r a y d i f f r a c t i o n m e t h o d , a n d its m i c r o - s t r u c t u r e w a s i n v e s t i g a t e d by u s i n g s c a n n i n g e l e c t r o n microscope ( S E M ) . It has b e e n k n o w n t h a t the c o p p e r in C U x M ° 6 S S - y c a n be e x t r a c t e d w i t h h y d r o c h l o r i c a c i d (12). So the prepared film was crushed and ground into powder, and a definite a m o u n t of the s a m p l e w a s i m m e r s e d in 6M H C I s o l u t i o n for i0 days. A n d the e x t r a c t e d s o l u t i o n w a s a n a l y z e d by the a t o m i c a b s o r p t i o n m e t h o d . The g r a d i e n t of c o p p e r in the p r o d u c t f i l m w a s i n v e s t i g a t e d by the e l e c t r o n p r o b e m i c r o - a n a l y s i s (EPMA) .

Results

and Discussion

As l i s t e d in T a b l e i, in all the p r e s e n t e x p e r i m e n t a l c o n d i t i o n s , the C U x M O 6 S S _ y c o m p o u n d w a s f o r m e d , a n d t h i s w a s i d e n t i f i e d by x - r a y d i f f r a c t i o n a n a l y s i s d e s c r i b e d h e r e i n a f t e r . H o w e v e r , w h e n the h o t z o n e t e m p e r a t u r e w a s h i g h e r t h a n 9 0 0 ° C , the p r e p a r e d C U x M O 6 S 8 y f i l m p e e l e d o f f the s u b s t r a t e , r e p r e s e n t e d by the s y m b o l s Z in T a b l e i. The p r o d u c e d f i l m p e e l e d o f f m o r e v i g o r o u s l y ,

TABLE Related values thin films temperature g r a d i e n t ( °C )

* * * *

600 600 600 700 700 800 800

-

700 800 900 800 900 900 i000

1

for the p r e p a r a t i o n

reaction

time ( hr )

72, 162 60, 72 24, 77 48, 60, 115 24, 48, 60 24, 5! 72

of C u - c l u s t e r

compound

x in C U x M O 6 S S _ y atomic absorption EPMA 3.2+0.3 3.1±0.3 3.1±0.3 2.5±0.3

2.5 2.5

H. H I N O D E , et a l .

614

w h e n the MoS 2 s u b s t r a t e was thinner. This w a s t h o u g h t to be due to the d i f f e r e n c e b e t w e e n the c o e f f i c i e n t of t h e r m a l e x p a n s i o n of C U x M O 6 S S _ y a n d the s u b s t r a t e MoS2. The fine C U x M O 6 S S - y thin f i l m s w e r e f o r m e d on the s u b s t r a t e b e l o w 900°C. W h e n the t e m p e r a t u r e g r a d i e n t was 600 - 700°C, the f o r m a t i o n of C U x M O 6 S 8 - y was i n c o m p l e t e and a p o r t i o n of the MoS 2 remained unreacted even when it was h e a t e d for 72 hours, and in this case, m o r e than 3 days w e r e n e e d e d to comp l e t e the reaction. H o w e v e r , the s u r f a c e of the p r o d u c t film was v e r y smooth, and the film a d h e r e d s t r o n g l y to the s u b s t r a t e . C o n s i d e r i n g the r e a c t i o n time a n d the h a r d n e s s of the p r o d u c t film, we found that a t e m p e r a t u r e g r a d i e n t of 600 - 800°C was m o s t s u i t a b l e for our e x p e r i mental condition.

20, No.

!014)

I

(226

:| (3[I)

I

I

2 (12,,,

I

I

I

I

I

(bl

Cu4 Mo6S8

10

L ~i

n

I

I

I

20

30

40

510

610

70

28 (deg) FIG.

2

X-ray diffraction a) p r o d u c t film b) C u 4 M o 6 S 8

patterns

of

! i0

~m

6

(a)

A typical x-ray diffraction p a t t e r n of the p r o d u c e d f i l m is s h o w n in Fig. 2a; as this figure shows, the n u m b e r

i

Vol.

m

FIG.

3

Scanning electron microscope photographs a) the c r o s s - s e c t i o n of the film b) the top s u r f a c e

of the p r o d u c t

film,

Vo].

20, No.

6

615

CUxMO6Ss_y THIN FILMS

of p e a k s is less than that of the p o l y c r y s t a l l i n e C u 4 M o 6 S 8 c o m p o u n d (Fig. 2b ). The x - r a y p o w d e r d i f f r a c t i o n (a) i= p a t t e r n of the s c r a p e d and g r o u n d p o w d e r from the (I00) surface of film was i d e n t i fied w i t h that of the p o l y crystalline CuxMo6S8_y. This p h e n o m e n o n r e v e a l e d that: 1008/ 200 the C u - C h e v r e l p h a s e film was f o r m e d w i t h o u t any byI I I I ! p r o d u c t s . Since (003) peak of the thin film was v e r y (002) (b) strong, the s p e c i f i c lattice g r o w t h of the CUxMO6S8_. in the thin film p r e f e r r e d Y t h e h e x a g o n a l c-axis direction. (~05) The surface m i c r o - s t r u c t u r e of the film was i n v e s t i g a t e d (I00) by u s i n g SEM. Fig. 3a shows (00q)~ ~102)( Ll wO l 1)07 (1~ 06) 0 (2O0) the c r o s s - s e c t i o n of the film, r e v e a l i n g that the CuI0 2~3 3'o Lo 5'0 6'0 70 c l u s t e r c o m p o u n d has g r o w n 2e (deg) in the form of c o l u m n a r crystals. The suface SEM p h o t o g r a p h ( Fig. 3b ) shows FIG. 4 that the c o l u m n surface is h e x a g o n a l in shape in some X - r a y d i f f r a c t i o n p a t t e r n s of places, w h i c h is c o n s i s t e n t a) s u b s t r a t e MoS 2 film w i t h the x - r a y d i f f r a c t i o n b) p o l y c r y s t a l l i n e p o w d e r MoS 2 results. For RF s p u t t e r i n g , there r e p o r t s (7,10) that the samples, w h e n p r e p a r e d by d e p o s i t i o n on s u b s t r a t e at lower t e m p e r a t u r e ( 200 - 400°C ) and a n n e a l e d in v a c c u m at 900°C for 2 hours, p r e f e r r e d the (i00) r h o m b o h e d r a l ( i.e. (i01) h e x a g o n a l ) d i r e c t i o n . The o r i e n t a t i o n of the our film s p e c i m e n to the h e x a g o n a l c-axis d i r e c t i o n can be a t t r i b u t e d to the c r y s t a l l o g r a p h i c a l o r i e n t a t i o n of the MoS 2 s u b s t r a t e . The x - r a y d i f f r a c t i o n p a t t e r n of the s u b s t r a t e MoS 2 film and the p o l y c r y s t a l l i n e p o w d e r sample are shown in Fig. 4 a and b, r e s p e c t i v e l y . The a p p e a r a n c e of s t r o n g (i00) p e a k in Fig. 4a reveals that the o r i e n t a t i o n of MoS 2 film is r e m a r k a b l y p a r a l l e l to the c-axis d i r e c t i o n , and the m e c h a n i s m of g r o w t h of this type of s u l f i d a t i o n is s i m i l a r to that of the s u l f i d a t i o n of v a n a d i u m foils (13) . MoS 2 has a l a y e r e d s t r u c t u r e , and the s u l f u r and m o l y b d e n u m layers are e s s e n t i a l l y l o c a t e d in a d i r e c t i o n p a r a l l e l to the c-axis. So in the MoS 2 s u b s t r a t e film, all layers are p e r p e n d i c u l a r to the surface. The t r a n s p o r t e d c o p p e r i n t e r c a l a t e s into the van der Waals gap of the MoS 2 s a n d w i c h e s , and s u b s e q u e n t l y reacts w i t h the o r i e n t e d MoS 2 to form C u - c l u s t e r compound. For the f o r m a t i o n of Mo6S 8 cluster, it is a s s u m e d that the s u l f u r and m o l y b d e n u m layers in the o r i g i n a l s t r u c t u r e are r e a r r a n g e d , and these layers in the c l u s t e r of the p r o d u c t C u - C h e v r e l p h a s e s p e c i m e n stack

Z

jL_j

616

H. HINODE, et al.

a l o n g the h e x a g o n a l surface. The f o r m a t i o n as f o l l o w s ; Cu + 1 / 2 1 2 xCuI

(g)

c-axis,

i.e.

each

layer

is p a r a l l e l

to the

film

is p r o p o s e d

to be

reaction

of the

(g)

~

CuI

>

CUxMO6S 8 + x/212

+ 6 MOS 2

As s h o w n in Fig. 5, the E P M A s u g g e s t s t h a t the c o p p e r c o n c e n t r a t i o n is c o n s t a n t in the d i r e c t i o n of t h i c k n e s s , a n d the m o l y b d e n u m c o n t e n t is" also constant. Also, from the EPMA, the c o p p e r cont e n t x in C u x M o 6 S 8 - y w a s c a l c u l a t e d as 2.5, a n d this v a l u e a g r e e s w e l l w i t h t h a t of the a t o m i c absorption method ( Table 1 ). F r o m the r e s u l t s of the a t o m i c a b s o r p t i o n m e t h o d a n d the E P M A l i s t e d in T a b l e i, it s e e m s t h a t the c o p p e r c o n t e n t of the p r o d u c t f i l m is d e p e n d e n t on the t e m p e r a t u r e of the c o l d zone, i.e. the c o m p o s i t i o n of c o p p e r i n c r e a s e s w i t h d e c r e a s e of t e m p e r ature.

thin

Vol. 20, No. 6

(g)

( cold

zone

+ 2 S2

) ( hot

zone

)

5;m J

i

100% 25 % Mo

Cu

U

i

i

0

4

'i

FIG.

5

The g r a d i e n t s of Cu a n d Mo c o n t e n t in the C u - c l u s t e r t h i n f i l m by the EPMA.

The t h i c k n e s s of the f i l m w a s d e t e r m i n e d to be 17 ~m f r o m the p h o t o g r a p h s h o w n in Fig. 3a, and this v a l u e a g r e e s w e l l w i t h t h a t of the E P M A ( Fig. 5 ). In the c a s e of this C u - c l u s t e r film, the t h i c k n e s s of the s u b s t r a t e MoS 2 was a b o u t 13 ~m. The t h i c k n e s s of the p r o d u c t f i l m was a b o u t 30 ~m w h e n 21 ~m t h i c k M o S 2 w a s used. T h u s it s e e m s t h a t the t h i c k n e s s of the C u - c l u s t e r c o m p o u n d t h i n f i l m i n c r e a s e s w i t h i n c r e a s e w i t h the i n c r e a s e of t h a t of the initial MoS 2 substrate.

Re fe r e n c e s i.

R. C h e v r e l , M. S e r g e n t ~, 515 ( 1971 ) .

2.

K. Y v o n , A. P a o l i , R. F r d k i g e r B33, 3066 ( 1 9 7 7 ) .

3.

~.

Fischer,

Appl.

Phys.

a n d J. P r i g e n t ,

i_66, 1

J. S o l i d

and R. C h e v r e l ,

(1978)

.

State

Acta

Chem.

Cryst.

CUxMO6S8_y THIN FILMS

Vol. 20, No. 6

4.

G. J. D u d l e y , K. Y. C h e u n g Chem. 32, 259 ( 1 9 8 0 ) .

5.

K. Y v o n

6.

S. Y a m a m o t o , K. M a t s u i , M. W a k i h a r a Res. B u l l . i_88, 1 3 1 1 ( 1 9 8 3 ) .

7.

C. K. B a n k s , L. K a m m e r d i n e r C h e m . 15, 271 ( 1 9 7 5 ) .

8.

P. P r z y s l u p s k i , R. H o v y n , J. S z y m a s z e k S t a t e c o m m u n . 28, 869 ( 1 9 7 8 ) .

9.

R. O h t a n i , B. 575 ( 1982 ) .

a n d A.

Paoli,

R.

a n d B. C.

Solid

State

a n d H.

617

Steel,

Commun.

24,

a n d M.

L.

Luo,

41

State

( 1967 ) . Mater.

J.

Solid

State

a n d B.

Gren,

Solid

a n d H.

L.

Luo,

Mater.

i0.

B. R. Zhao, R. O h t a n i 185 ( 1 9 8 3 ) .

a n d H.

L.

Luo,

Thin

ll.

Y. S u z u k i , Res. B u l l .

12.

M. T a v o s , L. E. D e l o n g , S o l i d S t a t e C o m m u n . 30,

13.

M. T a n i g u c h i , ( 1983 ) ; Jpn.

a n d M.

D. C. J o h n s t o n 551 ( 1979 ) .

Solid

Taniguchi,

Zhao

T. U c h i d a , M. W a k i h a r a 16, 1085 ( 1 9 8 1 ) .

J.

Res.

Solid

Bull.

Films

Taniguchi,

a n d M.

17,

ii0,

Mater.

B. M a p l e ,

M. W a k i h a r a a n d Y. K o j i m a , P r o c e e d i n g Inst. M e t a l s , S u p p l . , 441 ( 1 9 8 3 ) .

JIMIS-3