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Characterization of Y-Ba-Cu-Oxide superconductor thin films prepared by excimer laser evaporation
PhysicaC 153-155 (1988) 1429-1430 North-Holland, Amsterdam
C H A R A C T E R I Z A T I O N OF Y - B a - C u - O X I D E LASER EVAPORATION
Hanns-Ulrich
HABERMEIER
and G ~ n t e r
SUPERCONDUCTOR
THIN
FILMS
PREPARED
BY E X C I M E R
MERTENS
M a x - P l a n c k - I n s t i t u t fHr F e s t k ~ r p e r f o r s c h u n g , H e i s e n b e r g s t r . Federal R e p u b l i c of G e r m a n y
1D
70oo
Stuttgart-8o,
We p r e p a r e d s u p e r c o n d u c t i n g Y - B a - C u - O x i d e films u s i n g the t e c h n i q u e of p u l s e d laser e v a p o r a t i o n of the c o n s t i t u e n t s from a s i n t e r e d s u p e r c o n d u c t i n g t a r g e t onto h e a t e d SrTiO 3 single crystal s u b s t r a t e s in a h i g h v a c u u m c h a m b e r . D e p e n d i n g on the s u b s t r a t e t e m p e r a t u r e d u r i n g e v a p o r a t i o n and the t e m p e r a t u r e time p r o f i l e in a s u b s e q u e n t a n n e a l i n g p r o c e d u r e we o b t a i n e d s p e c i m e n s w i t h c r i t i c a l t e m p e r a t u r e s e x c e e d i n g 80 K and a t r a n s i t i o n w i d t h less than 5 K. X- ray d i f f r a c t o m e t r y shows that our s p e c i m e n s c o n s i s t of single phase o r t h o r h o m b i c m a t e r i a l w i t h a strong t e x t u r e ( c- axis of the o r t h o r h o m b i c crystal is p e r p e n d i c u l a r to the film plane ).
I. I N T R O D U C T I O N The d i s c o v e r y of high T - s u p e r c o n d u c tivity in the m e t a l l i c oxide s y s t e m L a - B a - C u - O by Bednorz and M U l l e r ( I ) and the s u b s e q u e n t d i s c o v e r y of s u p e r c o n d u c t i v i t y in the 9o K range by Chu et al. has i n i t i a t e d an e n o r m o u s g r o w t h of r e s e a r c h a c t i v i t y in this field by m a n y groups to e x p l o r e both the p h y s i c a l m e c h a n i s m r e s p o n s i b l e for this p h e n o m e n o n and its obvious t e c h n o l o g i c a l p o t e n t i a l . H o w e v e r , d u e to the b r i t t l e n e s s and the low c r i t i c a l c u r r e n t s bulk c e r a m i c m a t e rials are not yet suitable for m a n y trad i t i o n a l large scale a p p l i c a t i o n s such as m a g n e t s or power t r a n s m i s s i o n lines. C o n s e q u e n t l y , the p r e p a r a t i o n of these m a t e r i a l s as thin films was one of the m a j o r c h a l l e n g e s in m a t e r i a l s s c i e n c e in this f i e l d . P a r t i c u l a r l y , f o r a p p l i c a tions in an all s u p e r c o n d u c t i n g e l e c t r o n ics t e c h n o l o g y or any h y b r i d s u p e r c o n ducting/semiconducting c i r c u i t s it is m a n d a t o r y to d e v e l o p a t e c h n o l o g y w h i c h y i e l d s films w i t h a T c o m p a r a b l e to C. that of the bulk m a t e r l a l and no d e g r a dation during lithographic processes like e x p o s u r e to p h o t o r e s i s t s , a q u e o u s s o l u t i o n s of r e s i s t d e v e l o p e r s and e t c h ing solutions. So far, all c l a s s i c a l t e c h n i q u e s to form m u l t i c o m p o n e n t thin films have b e e n used to prepare h i g h T c c e r a m i c oxides like e l e c t r o n b e a m coe v a p o r a t i o n , s p u t t e r i n g from a m u l t i c o m p o n e n t t a r g e t , c o s p u t t e r i n g from 3 d i f f e r ent t a r g e t s , m o l e c u l a r b e a m epitaxy, laser e v a p o r a t i o n ( 2 ) and p l a s m a spraying. In this p a p e r we report on the succ e s s f u l d e p o s i t i o n of s u p e r c o n d u c t i n g Y - B a - C u - O x i d e thin films using the techn i q u e of p u l s e d laser e v a p o r a t i o n as i n t r o d u c e d by D i j k a m p et al. ( 2 ) , d e t e r m i n e some c r i t i c a l p r e p a r a t i o n p a r a m e ters to o b t a i n high q u a l i t y thin films
0921-4534/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
and c o r r e l a t e p r e p a r a t i o n p a r a m e t e r s w i t h m e t a l l u r g i c a l and e l e c t r o n i c p r o p e r t i e s of the films. 2. S P E C I M E N P R E P A R A T I O N As t a r g e t s for the laser e v a p o r a t i o n we used p e l l e t s ( 2 cm d i a m e t e r , 2 - 4 m m thick ) of the n o m i n a l c o m p o s i t i o n Y B a 2 C u 3 0 7 _ x w h i c h w e r e p r a p a r e d in the u s u a l w a y by g r i n d i n g and sintering. The targets c o n s i s t of single p h a s e orthorhombic material with a critical t e m p e r a t u r e of about 90 K and a t r a n s i tion w i d t h less than 1 K. The t a r g e t was g l u e d onto a r o t a t a b l e c o p p e r s t a g e m o u n t e d in a h i g h v a c u u m c h a m b e r and i r r a d i a t e d d u r i n g the d e p o s i t i o n e x p e r i ment through a quartz window with a f o c u s s e d b e a m of a KrF e x c i m e r laser. The laser light b e a m had an angle of o a p p r o x i m a t e l y 30 w i t h r e s p e c t to the s u r f a c e p l a n e of the target. The laser was used at a r e p e t i t i o n rate of 5 Hz to a total of s e v e r a l t h e o u s a n d shots. The v a c u u m s y s t e m is p u m p e d d o w n to a base p r e s s u r e of a p p r o x i m a t e l y Io -4 Pa by a c o n v e n t i o n a l t u r b o m o l e c u l a r pump and was e q u i p p e d w i t h a m a s s s p e c t r o m e t e r and a n e e d l e v a l v e to b l e e d rea c t i o n gases like o x y g e n into the chamber d u r i n g a n d / o r a f t e r the d e p o s i t i o n of the films. The s u b s t r a t e s - u s u a l l y single c r y s t a l SrTiO. slices of 1oxlox I m m size and ( 1oo ~ o r i e n t a t i o n are used - w e r e m o u n t e d a p p r o x i m a t e l y 3 cm a p a r t from the t a r g e t onto a h e a t i n g stage w h o s e t e m p e r a t u r e c o u l d be a d j u s t ed to any v a l u e b e t w e e n r o o m t e m p e r a t u r e and 55o°C. I n d e p e n d e n t of the s u b s t r a t e t e m p e r a t u r e d u r i n g film g r o w t h a b l a c k g r a y i s h d e p o s i t c o u l d be o b s e r v e d on
H.-U. Habermeier and G. Mertens / Thin films prepared by excimer laser evaporation
1430
the s u b s t r a t e s w h i c h was i n s u l a t i n g at room t e m p e r a t u r e . X-ray a n a l y s i s showed that the films w e r e a m o r p h o u s w i t h no traces of c r y s t a l l i n e m a t e r i a l ; b y chemical a n a l y s i s it c o u l d be c o n f i r m e d that the c o m p o s i t i o n of the m e t a l l i c c o n s t i t u e n t s of film and t a r g e t d e v i a ted less than 5%.The films were e x p o s e d to an a n n e a l i n g p r o c e d u r e in flowing o x y g e n a t m o s p h e r e in a c o n v e n t i o n a l furnace d e s i g n e d for thermal o x i d a t i o n of s i l i c o n wafers. The a n n e a l i n g c o n d i t i o n s of the films c o n s i s t in a 2 hour anneal at 9oo°C and a slow c o o l i n g to r o o m t e m p e r a t u r e . I n our e x p e r i m e n t s the cooling to r o o m t e m p e r a t u r e was simply done by s w i t c h i n g off the p o w e r of the furnace and u s i n g the heat c a p a c i t y of the total s y s t e m to o b t a i n a slow cooling. In Fig.1 the t e m p e r a t u r e time p r o f i l e of our furnace is r e D r @ s e n t e d . 9w
ence of the s u b s t r a t e t e m p e r a t u r e on the p r o p e r t i e s of the films s y s t e m a t i c a l l y . J.0gO 7~
~ ~
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2.~e ,.mm .~
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m.'
T£HPF..~qTtJR~ [K3
FIGURE 2 T r a n s i t i o n to s u p e r c o n d u c t i v i t y for a 5oo nm thick Y - B a - C o - ~ film m e a s u r e d r e s i s t i v e l y ( TD= 465 C ).
....
..~J
2
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a
5
10
n 15
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n 2o
2S
FIGURE I T e m p e r a t u r e - time p r o f i l e of the furnace used in our e x p e r i m e n t s . In some cases r a p i d t h e r m a l a n n e a l i n g was used to form c r y s t a l l i n e s u p e r c o n d u c t i n g films from the a m o r p h o u s deposits.
3. E X P E R I M E N T A L
RESULTS
AND DISCUSSION
S p e c i m e n s w i t h the c o r r e c t s t o i c h i o m etry of the m e t a l l i c c o n s t i t u e n t s s h o w e d all a room t e m p e r a t u r e r e s i s t i v i t y in the range of I- lo m ~ m and a t r a n s i t i o n to s u p e r c o n d u c t i v i t y b e t w e e n 7oK and 9oK. In Fig. 2 and Fig.3 two typical e x a m p l e s are given for films w i t h a t h i c k n e s s of 5oonm ( Fig.2 ) and 400 nm (Fig.3) g r o w n at s l i g h t l y d i f f e r e n t s u b s t r a t e ~ e m p e r a tures ( 465°C for Fig. 2 and 45o~C for Fig. 3 ) but e x p o s e d to the i d e n t i c a l post d e p o s i t i o n treatment. X- ray diff r a c t i o n on b o t h films showed single phase o r t h o r h o m b i c m a t e r i a l w i t h a text u r e ; t h e c-axis of the lattice is strongly o r i e n t e d p e r p e n d i c u l a r to the film plane. So far we i n v e s t i g a t e d the influ-
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FIGURE 3 R e s i s t i v e l y m e a s u r e d t r a n s i t i o n to s u p e r c o n d u c t i v i t y for a Y b - B a - C u ~ O film ( t h i c k n e s s 4o0 nm,T D = 450 C). Our r e s u l t s can be s u m m a r i z e d as follows: - r o o m t e m p e r a t u r e d e p o s i t i o n of thin films and s t a n d a r d a n n e a l i n g does not r e s u l t in s u p e r c o n d u c t i m g films, o - s u b s t r a t e t e m p e r a t u r e s a r o u n d 33o C r e s u l t in s u p e r c o n d u c t i n g films w i t h low T ( 7oK ) and a b r o a d t r a n s i t i o n widthC( 3o K ). - s u b ~ t r a t e t e m p e r a t u r e s b e t w e e n 4oo°C and 480 C and s t a n d a r d a n n e a l i n g r e s u l t in h i g h q u a l i t y films w i t h T c 85 K , t h e size of the g r a i n s i n c r e a s e from 7onm ( TD= 4oo°C ) to 9onm (TD= 48o°C ). REFERENCES ( I ) J.G.Bednorz B 64, 189 ( ( 2 ) D.-~ijkamp, Appl. Phys.
and K . A . M H l l e r , Z . P h y s . 1986 ) T. V e n k a t e s a n , X . D . W u Lett. 5_!I, 619 ( 1 9 8 7 ) .
C H A R A C T E R I Z A T I O N OF Y - B a - C u - O X I D E LASER EVAPORATION
Hanns-Ulrich
HABERMEIER
and G ~ n t e r
SUPERCONDUCTOR
THIN
FILMS
PREPARED
BY E X C I M E R
MERTENS
M a x - P l a n c k - I n s t i t u t fHr F e s t k ~ r p e r f o r s c h u n g , H e i s e n b e r g s t r . Federal R e p u b l i c of G e r m a n y
1D
70oo
Stuttgart-8o,
We p r e p a r e d s u p e r c o n d u c t i n g Y - B a - C u - O x i d e films u s i n g the t e c h n i q u e of p u l s e d laser e v a p o r a t i o n of the c o n s t i t u e n t s from a s i n t e r e d s u p e r c o n d u c t i n g t a r g e t onto h e a t e d SrTiO 3 single crystal s u b s t r a t e s in a h i g h v a c u u m c h a m b e r . D e p e n d i n g on the s u b s t r a t e t e m p e r a t u r e d u r i n g e v a p o r a t i o n and the t e m p e r a t u r e time p r o f i l e in a s u b s e q u e n t a n n e a l i n g p r o c e d u r e we o b t a i n e d s p e c i m e n s w i t h c r i t i c a l t e m p e r a t u r e s e x c e e d i n g 80 K and a t r a n s i t i o n w i d t h less than 5 K. X- ray d i f f r a c t o m e t r y shows that our s p e c i m e n s c o n s i s t of single phase o r t h o r h o m b i c m a t e r i a l w i t h a strong t e x t u r e ( c- axis of the o r t h o r h o m b i c crystal is p e r p e n d i c u l a r to the film plane ).
I. I N T R O D U C T I O N The d i s c o v e r y of high T - s u p e r c o n d u c tivity in the m e t a l l i c oxide s y s t e m L a - B a - C u - O by Bednorz and M U l l e r ( I ) and the s u b s e q u e n t d i s c o v e r y of s u p e r c o n d u c t i v i t y in the 9o K range by Chu et al. has i n i t i a t e d an e n o r m o u s g r o w t h of r e s e a r c h a c t i v i t y in this field by m a n y groups to e x p l o r e both the p h y s i c a l m e c h a n i s m r e s p o n s i b l e for this p h e n o m e n o n and its obvious t e c h n o l o g i c a l p o t e n t i a l . H o w e v e r , d u e to the b r i t t l e n e s s and the low c r i t i c a l c u r r e n t s bulk c e r a m i c m a t e rials are not yet suitable for m a n y trad i t i o n a l large scale a p p l i c a t i o n s such as m a g n e t s or power t r a n s m i s s i o n lines. C o n s e q u e n t l y , the p r e p a r a t i o n of these m a t e r i a l s as thin films was one of the m a j o r c h a l l e n g e s in m a t e r i a l s s c i e n c e in this f i e l d . P a r t i c u l a r l y , f o r a p p l i c a tions in an all s u p e r c o n d u c t i n g e l e c t r o n ics t e c h n o l o g y or any h y b r i d s u p e r c o n ducting/semiconducting c i r c u i t s it is m a n d a t o r y to d e v e l o p a t e c h n o l o g y w h i c h y i e l d s films w i t h a T c o m p a r a b l e to C. that of the bulk m a t e r l a l and no d e g r a dation during lithographic processes like e x p o s u r e to p h o t o r e s i s t s , a q u e o u s s o l u t i o n s of r e s i s t d e v e l o p e r s and e t c h ing solutions. So far, all c l a s s i c a l t e c h n i q u e s to form m u l t i c o m p o n e n t thin films have b e e n used to prepare h i g h T c c e r a m i c oxides like e l e c t r o n b e a m coe v a p o r a t i o n , s p u t t e r i n g from a m u l t i c o m p o n e n t t a r g e t , c o s p u t t e r i n g from 3 d i f f e r ent t a r g e t s , m o l e c u l a r b e a m epitaxy, laser e v a p o r a t i o n ( 2 ) and p l a s m a spraying. In this p a p e r we report on the succ e s s f u l d e p o s i t i o n of s u p e r c o n d u c t i n g Y - B a - C u - O x i d e thin films using the techn i q u e of p u l s e d laser e v a p o r a t i o n as i n t r o d u c e d by D i j k a m p et al. ( 2 ) , d e t e r m i n e some c r i t i c a l p r e p a r a t i o n p a r a m e ters to o b t a i n high q u a l i t y thin films
0921-4534/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
and c o r r e l a t e p r e p a r a t i o n p a r a m e t e r s w i t h m e t a l l u r g i c a l and e l e c t r o n i c p r o p e r t i e s of the films. 2. S P E C I M E N P R E P A R A T I O N As t a r g e t s for the laser e v a p o r a t i o n we used p e l l e t s ( 2 cm d i a m e t e r , 2 - 4 m m thick ) of the n o m i n a l c o m p o s i t i o n Y B a 2 C u 3 0 7 _ x w h i c h w e r e p r a p a r e d in the u s u a l w a y by g r i n d i n g and sintering. The targets c o n s i s t of single p h a s e orthorhombic material with a critical t e m p e r a t u r e of about 90 K and a t r a n s i tion w i d t h less than 1 K. The t a r g e t was g l u e d onto a r o t a t a b l e c o p p e r s t a g e m o u n t e d in a h i g h v a c u u m c h a m b e r and i r r a d i a t e d d u r i n g the d e p o s i t i o n e x p e r i ment through a quartz window with a f o c u s s e d b e a m of a KrF e x c i m e r laser. The laser light b e a m had an angle of o a p p r o x i m a t e l y 30 w i t h r e s p e c t to the s u r f a c e p l a n e of the target. The laser was used at a r e p e t i t i o n rate of 5 Hz to a total of s e v e r a l t h e o u s a n d shots. The v a c u u m s y s t e m is p u m p e d d o w n to a base p r e s s u r e of a p p r o x i m a t e l y Io -4 Pa by a c o n v e n t i o n a l t u r b o m o l e c u l a r pump and was e q u i p p e d w i t h a m a s s s p e c t r o m e t e r and a n e e d l e v a l v e to b l e e d rea c t i o n gases like o x y g e n into the chamber d u r i n g a n d / o r a f t e r the d e p o s i t i o n of the films. The s u b s t r a t e s - u s u a l l y single c r y s t a l SrTiO. slices of 1oxlox I m m size and ( 1oo ~ o r i e n t a t i o n are used - w e r e m o u n t e d a p p r o x i m a t e l y 3 cm a p a r t from the t a r g e t onto a h e a t i n g stage w h o s e t e m p e r a t u r e c o u l d be a d j u s t ed to any v a l u e b e t w e e n r o o m t e m p e r a t u r e and 55o°C. I n d e p e n d e n t of the s u b s t r a t e t e m p e r a t u r e d u r i n g film g r o w t h a b l a c k g r a y i s h d e p o s i t c o u l d be o b s e r v e d on
H.-U. Habermeier and G. Mertens / Thin films prepared by excimer laser evaporation
1430
the s u b s t r a t e s w h i c h was i n s u l a t i n g at room t e m p e r a t u r e . X-ray a n a l y s i s showed that the films w e r e a m o r p h o u s w i t h no traces of c r y s t a l l i n e m a t e r i a l ; b y chemical a n a l y s i s it c o u l d be c o n f i r m e d that the c o m p o s i t i o n of the m e t a l l i c c o n s t i t u e n t s of film and t a r g e t d e v i a ted less than 5%.The films were e x p o s e d to an a n n e a l i n g p r o c e d u r e in flowing o x y g e n a t m o s p h e r e in a c o n v e n t i o n a l furnace d e s i g n e d for thermal o x i d a t i o n of s i l i c o n wafers. The a n n e a l i n g c o n d i t i o n s of the films c o n s i s t in a 2 hour anneal at 9oo°C and a slow c o o l i n g to r o o m t e m p e r a t u r e . I n our e x p e r i m e n t s the cooling to r o o m t e m p e r a t u r e was simply done by s w i t c h i n g off the p o w e r of the furnace and u s i n g the heat c a p a c i t y of the total s y s t e m to o b t a i n a slow cooling. In Fig.1 the t e m p e r a t u r e time p r o f i l e of our furnace is r e D r @ s e n t e d . 9w
ence of the s u b s t r a t e t e m p e r a t u r e on the p r o p e r t i e s of the films s y s t e m a t i c a l l y . J.0gO 7~
~ ~
,~0
~ w
" " 4.~0_ 3~
~ ~
2.~e ,.mm .~
/ Q'
~,,TK Z6 ~ v 1907
m.'
T£HPF..~qTtJR~ [K3
FIGURE 2 T r a n s i t i o n to s u p e r c o n d u c t i v i t y for a 5oo nm thick Y - B a - C o - ~ film m e a s u r e d r e s i s t i v e l y ( TD= 465 C ).
....
..~J
2
[ ,
a
5
10
n 15
.hE ( . M ~
~
n 2o
2S
FIGURE I T e m p e r a t u r e - time p r o f i l e of the furnace used in our e x p e r i m e n t s . In some cases r a p i d t h e r m a l a n n e a l i n g was used to form c r y s t a l l i n e s u p e r c o n d u c t i n g films from the a m o r p h o u s deposits.
3. E X P E R I M E N T A L
RESULTS
AND DISCUSSION
S p e c i m e n s w i t h the c o r r e c t s t o i c h i o m etry of the m e t a l l i c c o n s t i t u e n t s s h o w e d all a room t e m p e r a t u r e r e s i s t i v i t y in the range of I- lo m ~ m and a t r a n s i t i o n to s u p e r c o n d u c t i v i t y b e t w e e n 7oK and 9oK. In Fig. 2 and Fig.3 two typical e x a m p l e s are given for films w i t h a t h i c k n e s s of 5oonm ( Fig.2 ) and 400 nm (Fig.3) g r o w n at s l i g h t l y d i f f e r e n t s u b s t r a t e ~ e m p e r a tures ( 465°C for Fig. 2 and 45o~C for Fig. 3 ) but e x p o s e d to the i d e n t i c a l post d e p o s i t i o n treatment. X- ray diff r a c t i o n on b o t h films showed single phase o r t h o r h o m b i c m a t e r i a l w i t h a text u r e ; t h e c-axis of the lattice is strongly o r i e n t e d p e r p e n d i c u l a r to the film plane. So far we i n v e s t i g a t e d the influ-
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FIGURE 3 R e s i s t i v e l y m e a s u r e d t r a n s i t i o n to s u p e r c o n d u c t i v i t y for a Y b - B a - C u ~ O film ( t h i c k n e s s 4o0 nm,T D = 450 C). Our r e s u l t s can be s u m m a r i z e d as follows: - r o o m t e m p e r a t u r e d e p o s i t i o n of thin films and s t a n d a r d a n n e a l i n g does not r e s u l t in s u p e r c o n d u c t i m g films, o - s u b s t r a t e t e m p e r a t u r e s a r o u n d 33o C r e s u l t in s u p e r c o n d u c t i n g films w i t h low T ( 7oK ) and a b r o a d t r a n s i t i o n widthC( 3o K ). - s u b ~ t r a t e t e m p e r a t u r e s b e t w e e n 4oo°C and 480 C and s t a n d a r d a n n e a l i n g r e s u l t in h i g h q u a l i t y films w i t h T c 85 K , t h e size of the g r a i n s i n c r e a s e from 7onm ( TD= 4oo°C ) to 9onm (TD= 48o°C ). REFERENCES ( I ) J.G.Bednorz B 64, 189 ( ( 2 ) D.-~ijkamp, Appl. Phys.
and K . A . M H l l e r , Z . P h y s . 1986 ) T. V e n k a t e s a n , X . D . W u Lett. 5_!I, 619 ( 1 9 8 7 ) .