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ATOM - PROBE ANALYSIS OF YBa2 Cu3 O7-x - SINGLE CRYSTALS
M. Leisch, M. Eisl, E. Schweiger, G. Leising
To cite this version:
M. Leisch, M. Eisl, E. Schweiger, G. Leising. ATOM - PROBE ANALYSIS OF YBa2 Cu3 O7-x - SINGLE CRYSTALS. Journal de Physique Colloques, 1989, 50 (C8), pp.C8-481-C8-486.
�10.1051/jphyscol:1989882�. �jpa-00229980�
COLLOQUE DE PHYSIQUE
Colloque C8, Supplément au noll, Tome 50, novembre 1989
ATOM
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PROBE ANALYSIS OF YBa,Cu,O,-,-
SINGLE CRYSTALSM. LEXSCH, M. EXSL, E. SCHWEIGER and G . LEISING
Institut fiir Festkorperphysik, University of Technology Graz Petersgasse 16, A-8010 Graz, Austria
Résumé
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Des e x p é r i m e n t a t i o n s s u p p l é m e n t a i r e s à l ' a i d e d e s p e c t r o m è t r e s d ' i o n d e champ o n t été e f f e c t u é e s p o u r c a r a c t é r i s e r e t o p t i m i s e r l e s c o n d i t i o n s d e p r é p a r a t i o n d e s m o n o c r i s t a u x YBa2Cu307-x. L ' a n a l y s e d e p r o f i l e n p r o f o n d e u r d e p e t i t e s a i g u i l l e s d é t a c h é e s d é m o n t r e d e s f l u c t u a t i o n s d i s t i n c t e s d e s c o n c e n t r a t i o n s d e s é l é m e n t s p r i n c i p a u x l e s q u e l l e s s o n t en r a p p o r t a v e c l a s t r u c t u r e s t r a t i f i é e d e l a m a t i è r e . La c o m p o s i t i o n d é t e r m i n é e p a r d e e x p é r i e n c e s e s t e n c o n c o r d a n c e a v e c l a p h y s e 1 : 2 : 3 q u i é t a i te s c o m p t é e . Des t r a c e s d e f o n d a n t s u t i l i s é s d e p l u s d a n s l a c u l t u r e du c r i s t a l , t e l q u e PbC12, p e u v e n t ê t r e d é t e c t é e s p r è s d e l a s u r f a c e .
A b s t r a c t - Complementary atom-probe measurements h a v e been c a r r i e d o.ut t o c h a r a c t e r i z e and t o o p t i m i z e t h e p r e p a r a t i o n c o n d i t i o n s o f YBa2Cu307-x
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s i n g l e c r y s t a l s . The d e p t h p r o f i l i n g a n a l y s i s o f c l e a v e d o f f s m a l l n e e d l e s shows d i s t i n c t f l u c t u a t i o n s o f t h e main components c o n n e c t e d t o t h e l a y e r s t r u c t u r e o f t h e m a t e r i a l . The b u l k c o m p o s i t i o n a s d e t e r m i n e d f r o m atom-probe s p e c t r a i s g e n e r a l l y i n a g r e e m e n t w i t h t h e e x p e c t e d 1 : 2 : 3 p h a s e . A d d i t i o n a l l y u s e d f l u x media i n t h e growth p r o c e d u r e l i k e PbCl c a n b e f o u n d 2 embedded i n n e a r s u r f a c e l a y e r s .1
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INTRODUCTIONS i n c e t h e d i s c o v e r y o f one o f t h e most t h o r o u g h l y i n v e s t i g a t e d h i g h t e m p e r a t u r e s u p e r c o n d u c - t o r m a t e r i a l YBa2Cu307-x (O<x<l) /1/ it was r e a l i z e d t h a t it is r e m a r k a b l y e a s y t o p r e p a r e c e r a m i c s a m p l e s w i t h t r a n s i t i o n t e m p e r a t u r e s above 9 0 K. I n c o n t r a s t , it i s much more d i f f i - c u l t t o p r o d u c e s i n g l e c r y s t a l s 1 2 1 . N e v e r t h e l e s s , s i n g l e c r y s t a l s w i t h s u i t a b l e d i m e n s i o n s f o r t r a n s p o r t , m a g n e t i c and o p t i c a l measurements a r e o f b a s i c i n t e r e s t . The p r o b l e m s i n c r y s t a l growth o f YBa2Cu307-x a r e b a s e d on i t s l i m i t e d t h e r m a l a n d c h e m i c a l s t a b i l i t y and on its low s o l u b i l i t y i n a p p r o p r i a t e f l u x media. I n i t i a l growth e x p e r i m e n t s were performed from a melt o f t h e p s e u d o t e r n a r y s y s t e m CuO-BaCu02-YBa2Cu307
.
A p p r o p r i a t e amounts o f t h e com- p o n e n t s were g r o u n d e d , p e l l e t i z e d and m e l t e d i n a l u m i n a c r u c i b l e s , l i n e d by g o l d s h e e t s . U s i n g t h e w e t t i n g and c r e e p i n g o f t h e m e l t on t h e g o l d s u r f a c e , p l a t e l i k e c r y s t a l s up t o a d i m e n s i o n o f 1 x l x O . l mm c a n b e f o u n d i n t h e g a p s between t h e m e t a l s h e e t s 131. The p r e v i o u s c h a r a c t e r i z a t i o n o f t h e o b t a i n e d c r y s t a l s h a s been done by e l e c t r o n m i c r o p r o b e a n a l y s i s , X-ray d i f f r a c t i o n and by t r a n s p o r t measurements. P r e l i m i n a r y atom-probe f i e l d - i o n m i c r o s - copie i n v e s t i g a t i o n s p e r f o r m e d on h i g h t e m p e r a t u r e s u p e r c o n d u c t o r s were s u c c e s s f u l i n a p p l y i n g t h e t e c h n i q u e t o t h e s e new m a t e r i a l 14-71. To o b t a i n a d d i t i o n a l i n f o r m a t i o n a b o u t m i c r o c o r n p o s i t i o n and t o h e l p t o o p t i m i z e t h e p r e p a r a t i o n c o n d i t i o n s complementary atom- p r o b e s t u d i e s h a v e been c a r r i e d o u t .Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989882
2
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EXPERIMENTALThe s t a r t i n g p o i n t i n t h e s a m p l e p r e p a r a t i o n h a s been a p r e s e l e c t i o n o f s u i t a b l e c r y s t a l s f r o m p a r t s o f t h e c h a r g e a s removed f r o m t h e c r u c i b l e . A t y p i c a l a r r a n g e m e n t o f t h e p l a t e l i k e c r y s t a l s a s grown i n t h e c a v i t i e s c a n b e s e e n i n t h e e l e c t r o n m i c r o g r a p h i n f i g . 1 . The com- p o s i t i o n o f t h e c r y s t a l s h a s been c h e c k e d by e l e c t r o n m i c r o p r o b e a n a l y s i s . The Y:Ba:Cu r a t i o s w e r e 1 : 2 : 3 i n e a c h c r y s t a l w i t h i n t h e a c c u r a c y o f t h e e l e c t r o n m l c r o p r o b e measurement. The rest o f t h e melt e . g . i n t h e c o r n e r s o f t h e m i c r o g r a p h c o n t a i n s m a i n l y CuO.
F i g . 1
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S c a n n i n g e l e c t r o n m i c r o g r a p h o f YBa2Cu307-x s i n g l e c r y s t a l s a s grown from t h e meltS e l e c t e d c r y s t a l s h a v e been removed c a r e f u l l y . I n o r d e r t o p r o v e t h e q u a l i t y o f t h e s i n g l e c r y s t a l s t h e y a r e i n v e s t i g a t e d by X-ray d i f f r a c t i o n i n t h e Laue geometry. F i g . 2 shows t h e Laue d i f f r a c t i o n p a t t e r n o f a s a m p l e i n t r a n s m i s s i o n mode w i t h t h e beam p a r a l l e l t o t h e
F i g . 2
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Laue d i f f r a c t i o n p a t t e r n o f a YBa Cu 0 s i n g l e c r y s t a l , beam p a r a l l e l 0 0 1 - d i r e c t i o n . 2 3 7-xThe s h a r p n e s s o f t h e s p o t s i s a measure f o r f l u c t u a t i o n s o f t h e o r i e n t a t i o n o f subdomains.
T h e s e a r e o b v i o u s l y s m a l l a s c a n b e s e e n i n F i g . 2 and it c o n f i r m s t h e good q u a l i t y o f t h e s i n g l e c r y s t a l .
The s p e c i m e n s f o r t h e f i e l d i o n mass s p e c t r o m e t r i c a n a l y s i s were p r e p a r e d by m e c h a n i c a l means by s e l e c t i n g s u i t a b l e f r a g m e n t s which were c l e a v e d o f f f r o m t h e d i f f e r e n t s i n g l e c r y s t a l s . N e e d l e l i k e f r a g m e n t s were mounted on b l u n t n i c k e l t i p s w i t h a c o n d u c t i n g , s i l v e r c o n t a i n i n g epoxy r e s i n /8/. T h i s was done u n d e r a s t e r e o l i g h t m i c r o s c o p e u s i n g a m i c r o m a n i p u l a t o r . The
f i n a l a l i g n m e n t o f t h e mounted t i p s was a c h i e v e d by b e n d i n g t h e n i c k e l s u p p o r t n e e d l e i n a s u i t a b l e way.
F o r t u n a t e l y most o f t h e n e e d l e l i k e f r a g m e n t s from t h e YBa2Cu307-x s i n g l e c r y s t a l s c a n b e k s e d d i r e c t l y f o r t h e atom-probe a n a l y s i s w i t h o u t f u r t h e r e l e c t r o p o l i s h i n g . F o r comparison a few t i p s were a d d i t i o n a l l y e t c h e d i n a commonly u s e d e l e c t r o l y t e c o n t a i n i n g p e r c h l o r i c a c i d d i - l u t e d i n e t h y l e n e g l y c o l - m o n o b u t y l e t h e r . The a n a l y s e s were c a r r i e d o u t i n a p u l s e d v o l t a g e TOF atom-probe w i t h a n e l e c t r o s t a t i c s e c t o r (163') and e q u i p p e d w i t h a vacuum l o c k f o r f a s t c h a n g e o f t h e s p e c i m e n s . F o r f i e l d i o n i m a g i n g a c h a n n e l p l a t e s c r e e n a s s e m b l y c a n b e f l i p p e d i n f r o n t o f t h e t i p . Hereby o r i e n t a t i o n and q u a l i t y o f t h e t i p s c a n b e checked.
3 - RESULTS AND DISCUSSION
A u n i f o r m f i e l d e v a p o r a t i o n b e h a v i o u r c a n b e o b s e r v e d which makes it p o s s i b l e t o - a n a l y z e t h e m a t e r i a l w i t h a p u l s e d v o l t a g e atom-probe s p e c t r o m e t e r . The s p e c i m e n s have been k e p t a t l i q u i d n i t r o g e n c a m p e r a t u r e and p u l s e - d c r a t i o s above 0 . 1 5 h a v e been u s e d . Only a few c l u s t e r i o n s h a v e been o b s e r v e d . An atom-probe mass s p e c t r u m from one o f t h e s i n g l e c r y s t a l f r a g m e n t s i s shown i n F i g . 3 . I n t h e s p e c t r u m s i g n a l s c o r r e s p o n d i n g t o a t o m i c and m o l e c u l a r oxygen, c o p p e r , b a r i u m , y t t r i u m and o x i d e s o f c o p p e r and y t t r i u m c a n b e o b s e r v e d . T h i s s p e c t r u m h a s been ob- t a i n e d a f t e r r e m o v a l o f more t h a n 2 0 a t o m i c l a y e r s and s h o u l d b e r e p r e s e n t a t i v e f o r t h e b u l k . The e l e m e n t a l c o n c e n t r a t i o n s c a l c u l a t e d f r o m a t l e a s t 2000 i o n s a r e i n good a g r e e m e n t w i t h t h e c o r r e c t e d r e s u l t s from e l e c t r o n m i c r o p r o b e measurements a s w e l l w i t h t h e known s t o i c h i o - m e t r v
.
160
Ions
120
Mass
Camul
f i g . 3 - Atom-probe mass s p e c t r u m from a YBa2Cu307-x - s i n g l e c r y s t a l s a m p l e a f t e r r e m o v a l o f more t h a n 20 s u r f a c e l a y e r s , o r i e n t a t i o n o f t h e specimen p a r a l l e l t o c - a x i s .
The s p e c t r u m a l s o c o n t a i n s a r a t h e r l a r g e peak a t M/n-28. We assume t h a t t h i s peak c o r r e s p o n d s t o CO. B e s i d e s h y d r o g e n , CO i s a main component o f t h e r e s i d u a l g a s i n t h e baked vacuum v e s s e l . From ramped f i e l d d e s o r p t i o n s t u d i e s /9/ we know, t h a t a t a f i e l d o f a b o u t 2 0 V/nm t h e g a s s u p p l y o f t h e t i p w i t h CO i s enhanced a b o u t a f a c t o r o f 2 0 . The c u t - o f f f i e l d (enhancement=l) h a s been o b s e r v e d a t a n e l e c t r i c f i e l d o f a b o u t 3 0 V/nm. The d c - f i e l d d u r i n g o u r a n a l y s i s was t y p i c a l l y k e p t a t 20 V/nm, t h a t means s t r o n g l y enhanced CO-supply o c c u r s . The c u t - o f f f i e l d f o r CO ( 3 0 V/nm) l i e s n o t i c e a b l y above t h e e v a p o r a t i o n f i e l d o f t h e s u p e r c o n d u c t i n g m a t e r i a l which h a s been o b s e r v e d a t 23-24 V/nm. A s i g n i f i c a n t r e d u c t i o n o f t h e CO c o n c e n t r a t i o n . i n t h e s p e c t r a can be o b t a i n e d when t h e a n a l y s i s i s done above t h e b o i l i n g p o i n t o f CO a c c o r d i n g t o t h e s t u d i e s of G. Zaharchuk e t . a l .
/ l o / .
I n n e a r s u r f a c e l a y e r s a l s o OH and H 2 0 c a n b e de- t e c t e d , p r o b a b l y b e c a u s e o f t h e h y g r o s c o p i c b e h a v i o u r o f t h i s m a t e r i a l . On e t c h e d s p e c i m e n s t h e s e s i g n a l s h a v e n o t been o b s e r v e d . A d d i t i o n a l l y , it 1s n o t e a s y t o g e t a c l e a r s e p a r a t i o n between m o l e c u l a r oxygen a n d d o u b l y c h a r g e d c o p p e r i s o t o p e i o n s , e n v e l o p i n g t h e oxygen s i g n a l . I n f u r t h e r d e p t h p r o f i l i n g a n a l y s i s we f o c u s s e d on t h e s t u d y o f t h e l o c a l c o n c e n t r a t i o n s o f y t t r i u m and barium. The c o n c e n t r a t i o n p r o f i l e s o f t h e m e t a l i o n s show d i s t i n c t f l u c t u a t i o n s which a r e o b v i o u s l y c o n n e c t e d t o t h e l a y e r s t r u c t u r e o f t h e m a t e r i a l . A good example o f t h i s i s t h e a u t o c o r r e l a t i o n - p l o t o f t h e r a t i o Ba:(Y+
B a ) , which i s d e p i c t e d i n F i g . 4 .Correlation Length
F i g . 4
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A u t o c o r r e l a t i o n o f t h e r a t i o Ba:(Y + B a ) ; c a l c u l a t e d f r o m t h e d e p t h p r o f i l e o b t a i n e d f r o m t h e same specimen from which t h e mass s p e c t r u m is p l o t t e d i n F i g . 3 . ( c - o r i e n t a t i o n )The a c t u a l number o f l a y e r s e v a p o r a t e d i s o b t a i n e d from t h e c e r t a i n s t e p s which o c c u r i n t h e o n l i n e
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p l o t o f d e t e c t e d i o n s v e r s u s a c t u a t e d d e s o r p t i o n p u l s e s d u e t o t h e p i l e up e f f e c t . The s e p a r a t i o n o f t h e maxima i n t h e c o n c e n t r a t i o n o f y t t r i u m i s found between 5 t o 7 l a y e r s which i s on t h e a v e r a g e i n n i c e a g r e e m e n t w i t h t h e a c t u a l 6 l a y e r s i n t h e l a t t i c e .I n a c r y s t a l l o g r a p h i c d e s c r i p t i o n o f p h a s e s i n Y-Ba-Cu-0 s u p e r c o n d u c t o r s , t h e e x i s t e n c e o f an o r t h o r h o m b i c p h a s e o f Y BaCu05 h a s been p r o v e d by Hazen e t . a l . /11/. T h i s s o c a l l e d g r e e n ~ h a s e 2 i s d i s p e r s e d between t h e g r a i n s o f t h e 1 : 2 : 3 p h a s e i n a r a t i o o f a b o u t 1 : 3 i n t h e i r s a m p l e . S t r i k i n g i s t h e v e r y l i t t l e d i f f e r e n c e i n t h e l a t t i c e c o n s t a n t s . F o r t h e 2 : l : l p h a s e ( g r e e n p h a s e ) a = 3 . 8 5 9
8 ,
c = 3 . 9 0 41,
(3xc=11.7121)
/11/. F o r t h e 1 : 2 : 3 p h a s e a = 3 . 8 81,
b=3.821
andc = 1 1 . 6 6
1
1 1 2 1 . A s a l r e a d y n o t i c e d a b o v e , a 1 : 2 : 3 c o m p o s i t i o n o f t h e s i n g l e c r y s t a l s h a s been o b s e r v e d by e l e c t r o n m i c r o p r o b e measurements. I n t h e t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e some- t i m e s v e r y s m a l l l a m i n a - l i k e subdomains w i t h d i f f e r e n t c o n t r a s t c a n be o b s e r v e d . The t h i c k n e s s h a s been e s t i m a t e d t o b e below 1 0 nm, a s c a l e which s t i m u l a t e s t o p e r f o r m atom-probe measure- m e n t s . I n F i g . 5 a t y p i c a l d e p t h p r o f i l e of a c l e a v e d o f f f r a g m e n t from a YBa 2 Cu 3 0 7-x c r y s t a l i s shown. F o r s i m p l i c i t y o n l y t h e r a t i o o f Ba:(Y+Ba) i s p l o t t e d . The two d o t t e d l i n e s c o r r e s p o n d t o t h e 2 : 1 : 1 - and t h e 1 : 2 : 3 - p h a s e . Most o f t h e a n a l y z e d f r a g m e n t s a m p l e s show t h e same be- h a v i o u r.
T o t a l
number o f i o n sF i g . 5 - R a t i o o f Ba:(Y+Ba) v e r s u s d e p t h , a p p r o x i m a t e l y 1 0 0 i o n s c o r r e s p o n d t o a d e p t h o f o n e a t o m i c l a y e r . D o t t e d l i n e s i n d i c a t e t h e 1 : 2 : 3 and 2 : l : l p h a s e s .
I n a s u r f a c e r e g i o n o f a p p r o x i m a t e l y 1 0 t o 20 a t o m i c l a y e r s i n d e p t h , t h e measured c o n c e n t r a - t i o n s a r e c l o s e r t o t h e c o m p o s i t i o n o f t h e Y2BaCu05 - p h a s e . A f t e r r e m o v a l o f a b o u t 20 a t o m i c l a y e r s g e n e r a l l y t h e c o m p o s i t i o n c o r r e s p o n d i n g t o t h e 1 : 2 : 3 - p h a s e a s d e t e r m i n e d by t h e p r e - v i o u s EDAX measurements i s o b t a i n e d . E x c e p t i n g one s a m p l e , on t h i s specimen b o t h p h a s e s c a n be f o u n d i n c o e x i s t e n c e o v e r t h e whole d e p t h . The p h a s e boundary was s i t u a t e d i n a x i s i n t h e cen- t r e o f t h e t i p . By t u r n i n g t h e t i p b o t h p h a s e s can be p r o b e d one a f t e r a n o t h e r . We assume t h a t o u r way o f specimen p r e p a r a t i o n i s s e l e c t i v e i n a s p e c i a l manner. If a s m a l l subdomain i s embedded i n t h e c r y s t a l , d u e t o t h e l i t t l e d i f f e r e n c e Sn t h e l a t t i c e p a r a m e t e r s an i n t e r n a l s t r e s s w i l l r e s u l t . By t r y i n g t o c l e a v e t h e c r y s t a l , a f r a c t u r e w i l l o b v i o u s l y happen w i t h a h l g h p r o b a b i l i t y a t t h e p h a s e b o u n d a r i e s . T h i s would e x p l a i n t h e atom-probe r e s u l t s o b t a i n e d .
I n o r d e r t o improve t h e c o n d i t i o n s f o r c r y s t a l growth PbCl h a s been added t o t h e m e l t t o a c t 2
a s f l u x medium. Hereby t h e q u e s t i o n a r i s e s t o what e x t e n d p a r t s from t h e f l u x medium w i l l b e p r e s e n t i n t h e c r y s t a l .
Ions
T o t a l
number o f i o n sF i g . 6 - C o n c e n t r a t i o n p r o f i l e o f C1 and Ba f o r a s a m p l e o f a YBa2Cu307-x c r y s t a l , grown from a m e l t c o n t a i n i n g PbCl a s f l u x medium (200 i o n s p e r d a t a p o i n t ) .
2
On crystal samples grown with PbC12 added, chlorine can be found in the atom probe spectra. The concentration profile for C1 and Ba is plotted in Fig.6. The concentrations of copper and yttrium show no significant variations. It should be noticed that in contrast to the profile in Fig.5 the
Y -
concentration does not vary versus depth. After removal of about 20 atomic layers the chlorine signals disappear completely in the spectra. The overall composition of this particular specimen as measured agrees more with the 2:l:l - phase. Unsolved is the question on which lattice sites the C1-ions are sitting. The sites of Ba seem to be very un- likely because of the different electronic structure of these two elements. Other samples of the same charge show strong clustering during field evaporation. From the few samples in- vestigated an improvement in crystal quality due to the added flux medium cannot be found.4
-
CONCLUSIONIn the atom probe depth profiling analysis a second phase in the form of small laminas with a thickness of about 5nm and with a composition very similar to Y2BaCu05 has been found in YBa2Cu307-x crystals, grown from a pseudoternary melt. After addition of PbC12 as flux medium to the melt, C1 has been observed in near surface layers of cleaved off fragments, accompanied by a depletion of barium.
ACKNOWLEDGEMENTS
We would like to thank Dr.P.Golob and Dr.P.Polt (Forschungsinstitut fur Elektronenmikroskopie) for help with the EDAX-measurements and for TEM - imaging and Kristine Roth for providing the X-ray studies. This work was supported by the "Fonds zur Forderung der wissenschaftlichen Forschung" project number P7065.
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