INTERFACE EXAFS USING GLANCING ANGLES

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INTERFACE EXAFS USING GLANCING ANGLES

S. Heald, J. Tranquada, H. Chen

To cite this version:

S. Heald, J. Tranquada, H. Chen. INTERFACE EXAFS USING GLANCING ANGLES. Journal de

Physique Colloques, 1986, 47 (C8), pp.C8-825-C8-830. �10.1051/jphyscol:19868158�. �jpa-00226061�

INTERFACE EXAFS USING GLANCING ANGLES

S.M. HEALD, J.M. TRANQUADA and H. CHEN

Brookhaven National Laboratory, Upton, NY 11973, U.S.A.

ABSTRACT

The use of g l a n c i n g a n g l e s t o o b t a i n EXAFS s i g n a l s from t h i n i n t e r f a c i a l r e g i o n s is d e s c r i b e d . The technique i s a p p l i c a b l e t o the case of a l i g h t o v e r l a y e r on a heavy s u b s t r a t e f o r which t o t a l e x t e r n a l r e f l e c t i o n can be caused t o occur a t the i n t e r - f a c e . I n t h i s c a s e t h e p e n e t r a t i o n i n t o the s u b s t r a t e i s very small (520-30 A i n many c a s e s ) . Data have been o b t a i n e d on two systems: A 1 on Cu and Ag on Au. The A 1 on Cu samples had 1000 A of A 1 on Cu and measurements were made on t h e i n t e r f a c e s t r u c t u r e a s a f u n c t i o n of a n n e a l i n g temperature. For a n n e a l s above 140°C c l e a r i n d i c a t i o n of t h e growth of CuAIZ a t t h e i n t e r f a c e i s observed. The i n t e r f a c e s e n s i t i v i t y was then v e r i f i e d by v a r y i n g the g l a n c i n g a n g l e t o determine the CuA12 l a y e r t h i c k n e s s . Even f o r CuA12 l a y e r s a s t h i n a s 100 A, t h e EXAFS s i g n a l i s es- s e n t i a l l y pure CuAl2 w i t h l i t t l e contamination from t h e underlying Cu. For Ag on Au t h e r e i s no compound formation and t h e s e techniques were used t o look a t i n t e r - d i f f u s i o n a s a f u n c t i o n of annealing. I n p a r t i c u l a r t h e Au environment i n Ag g r a i n boundaries could be d e t e c t e d .

Understanding t h e s t r u c t u r e of s o l i d s t a t e i n t e r f a c e s i s important f o r s t u d i e s of s o l i d s t a t e r e a c t i o n s , t h i n f i l m adhesion and s t a b i l i t y . T r a d i t i o n a l probes such a s Auger s p u t t e r p r o f i l i n g and Rutherford b a c k s c a t t e r i n g can provide compositional i n f o r m a t i o n b u t no d e t a i l e d s t r u c t u r a l information, and f o r buried i n t e r f a c e s o f t e n have only l i m i t e d depth r e s o l u t i o n . For s i n g l e c r y s t a l i n t e r f a c e s e l e c t r o n micros- copy can provide b e a u t i f u l images, b u t sample p r e p a r a t i o n can be t e d i o u s and many systems i n v o l v e p o l y c r y s t a l l i n e o r amorphous i n t e r f a c e s . I n t h i s paper the a p p l i - c a t i o n s of g l a n c i n g a n g l e x-rays t o t h e study of b u r i e d i n t e r f a c e s a r e discussed w i t h p a r t i c u l a r emphasis on i n t e r f a c e extended x-ray a b s o r p t i o n f i n e s t r u c t u r e (ExAFS) measurements.

Previous g l a n c i n g a n g l e EXAFS s t u d i e s have c o n c e n t r a t e d on s u r f a c e s t u d i e s [ I - 5 1 u s i n g d e t e c t i o n of e i t h e r t h e r e f l e c t e d s i g n a l 11-41 o r the f l u o r e s c e n c e s i g n a l e m i t t e d by t h e s u r f a c e atoms 151. The same techniques can be a p p l i e d t o s o l i d s t a t e i n t e r f a c e systems [ 6 , 7 ] and t h i s paper w i l l summarize our r e c e n t r e s u l t s . S o l i d s t a t e i n t e r f a c e s can a l s o be s t u d i e d i n a b s o r p t i o n i f m u l t i l a y e r s t r u c t u r e s a r e used t o enhance t h e i n t e r f a c e s i g n a l [ 8 , 9 ] b u t t h i s paper w i l l c o n c e n t r a t e on s t u d i e s of s i n g l e b i l a y e r systems. I n p a r t i c u l a r , r e s u l t s on Cu-A1 and Au-Ag b i - l a y e r s w i l l be r e p o r t e d . The Cu-A1 r e s u l t s demonstrate t h e i n t e r f a c e s e n s i t i v i t y of t h e g l a n c i n g angle techniques while t h e Au-Ag system demonstrates the a b i l i t y of t h e s e techniques t o monitor g r a i n boundary i n t e r d i f f u s i o n i n b i l a y e r systems.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868158

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X-RAYS AT GLANCING ANGLES

The primary d i f f e r e n c e between x-ray o p t i c s and normal o p t i c s i s t h a t f o r x-rays t h e i n d e x of r e f r a c t i o n i s s l i g h t l y l e s s than one [ l o ] . It i s u s u a l l y w r i t t e n a s n=l-6-i8 where 6 and 8 a r e both p o s i t i v e . Away from a b s o r p t i o n edges, 8 is r e l a t e d

to p by t h e e x p r e s s i o n 8 = pX/4n. Since t h e index i s l e s s than one, a t g l a n c i n g a n g l e s t o t a l e x t e r n a l r e f l e c t i o n can occur. The c r i t i c a l a n g l e can be determined from S n e l l ' s law t o be 0, = 426 where the a n g l e i s measured w i t h r e s p e c t t o t h e s u r f a c e . For the c a s e s to be d i s c u s s e d Oc

-

-

The key p r o p e r t y i n o b t a i n i n g i n t e r f a c e s e n s i t i v i t y i s the p e n e t r a t i o n of the x- r a y s i n t o t h e s u b s t r a t e . Figure 2 shows t h e p e n e t r a t i o n a s a f u n c t i o n of a n g l e a g a i n f o r t h e A 1 on Cu system. I n t h i s c a s e a 50 A l a y e r of CuA12 was p u t a t t h e

0.004 0.006 0.008 0.010 ANGLE (rod~ons)

J ~ : : a : a : l

Figure 1. C a l c u l a t e d x-ray r e f l e c t i v i - Figure 2. S o l i d l i n e : C a l c u l a t i o n s of t i e s f o r pure A 1 (dashed l i n e ) , pure Cu t h e p e n e t r a t i o n of the i n c i d e n t x-rays (dot-dashed l i n e ) and 1000 A of A 1 on Cu below t h e Al-CuA12 i n t e r f a c e f o r a model ( s o l i d l i n e ) a t 8.6 keV. system of 1000 A A l , 50 A CuA12 and

1000 A Cu a t an x-ray energy of 9.3 keV.

Dashed l i n e : C a l c u l a t i o n of t h e r e l - a t i v e c o n t r i b u t i o n t o t h e Cu f luores- cence s i g n a l of t h e Cu and CuAL2 l a y e r s . i n t e r f a c e i n o r d e r t o look a t the s e n s i t i v i t y t o i n t e r f a c i a l r e a c t i o n s , and the r e l a t i v e c o n t r i b u t i o n t o the f l u o r e s c e n c e s i g n a l f o r t h e CuA12 and Cu l a y e r s i s a l s o c a l c u l a t e d . For a n g l e s j u s t above t h e A 1 c r i t i c a l a n g l e i t i s seen t h a t most of t h e Cu f l u o r e s c e n c e s i g n a l i s due t o t h e CuA12 l a y e r . Since minor components of

the EXAFS s i g n a l can be d e t e c t e d to the 20% l e v e l o r l e s s , i t should be p o s s i b l e t o d e t e c t t h e EXAFS from CuA12 l a y e r s a s t h i n a s 10

-

FLUORESCENCE DETECTION

I n a l l of t h e EXAFS measurements both the r e f l e c t e d and f l u o r e s c e n c e s i g n a l s were c o l l e c t e d . I n almost every c a s e , however, t h e f l u o r e s c e n c e s i g n a l had supe- r i o r s i g n a l / n o i s e . A t f i r s t t h i s may seem s u r p r i s i n g s i n c e i n many c a s e s the r e - f l e c t e d beam has

-

f r a c t i o n of the t o t a l s i g n a l . This minimizes t h e problem s i n c e t h e f l u c t u a t i o n s occur i n t h e e n t i r e s i g n a l , and, thus, a r e a s m a l l e r f r a c t i o n of t h e EXAFS f o r f l u - orescence. T y p i c a l f l u o r e s c e n c e s i g n a l l e v e l s a r e -1-5 x

lo6

A s mentioned, t h e s h a r p i n t e r f e r e n c e s t r u c t u r e found i n the r e f l e c t i v i t y can be a problem when measuring EXAFS. As the energy i s changed the phase of the i n t e r f e r - ence s t r u c t u r e changes r e s u l t i n g i n l a r g e modulations of t h e background. An exam- p l e i s shown i n Fig. 3 which a l s o shows t h e s o l u t i o n . By v a r y i n g t h e a n g l e of i n c i d e n c e i n c o n c e r t w i t h the energy t h e sample can be maintained a t a f i x e d posi- t i o n on t h e r e f l e c t i v i t y curve, and t h e background modulations removed. I n the p r e s e n t work a p i e z o e l e c t r i c element was used f o r f i n e tuning t h e angle. I t had some n o n l i n e a r i t i e s which gave r i s e t o t h e r e s i d u a l c u r v a t u r e seen i n Fig. 3.

Another d i f f i c u l t y i n a n a l y z i n g g l a n c i n g a n g l e measurements was pointed o u t by p a r t e n s and Rabe 121. The r e f l e c t i v i t y and f l u o r e s c e n c e depend on both 6 and 8 while the a b s o r p t i o n c o e f f i c i e n t i s contained only i n 5. They t r e a t e d t h e r e f l e c - t i v i t y c a s e w h i l e i n t h i s paper we w i l l be concerned w i t h t h e f l u o r e s c e n c e s i g n a l . Analogous t o s t a n d a r d f l u o r e s c e n c e t h e s i g n a l i n t e n s i t y can be expressed a s

where P i s t h e a b s o r p t i o n c o e f f i c i e n t to be measured ( h e r e assumed t o be depth in- dependent) and pf i s t h e a b s o r p t i o n c o e f f i c i e n t of t h e f l u o r e s c e n t x-rays. Is i s

t h e s u r f a c e i n t e n s i t y which depends on both the incoming and r e f l e c t e d beams, and P* is t h e e f f e c t i v e a b s o r p t i o n of t h e sample. Below t h e c r i t i c a l a n g l e kt* i s j u s t t h e i n v e r s e of t h e p e n e t r a t i o n depth of t h e s u b s u r f a c e evanescent wave and i s much l a r g e r than p. Thus, t h e measured s i g n a l must be c o r r e c t e d f o r Is/y* t o recover P.

I n g e n e r a l t h i s c o r r e c t i o n is a f u n c t i o n of both 5 and 6 a l t h o u g h w e l l below the c r i t i c a l a n g l e t h e 8 c o n t r i b u t i o n is small and i t can be shown t h a t :

S i n c e

o c

Also because 6 and 8 a r e r e l a t e d by a Kramers-Kronig r e l a t i o n , EXAFS o s c i l l a t i o n s i n 8 a l s o show up i n 6. T h i s complicates applying c a l - c u l a t e d c o r r e c t i o n s a s i n eq. ( 2 ) s i n c e 8 must be known i n o r d e r t o determine 6 .

I I I I I I

0 200 4 0 0 600 800 1 0 0 0

ENERGY FROM EDGE (eV) The advantage of f l u o r e s c e n c e de- t e c t i o n i s t h a t t h e c o r r e c t i o n s a r e F i g u r e 3. The f l u o r e s c e n c e s i g n a l a t t h e l e s s important and thus do n o t need Cu-K-edge (8.980 keV) f o r t h e 180°C an- to be a c c u r a t e l y made. I f a change nealed sample a t 4.0 mrad i n c i d e n t angle. i n 6 c a u s e s a 20% change i n R the For t h e u n c o r r e c t e d c a s e t h e a n g l e i s held f l u o r e s c e n t s i g n a l w i l l a l s o change f i x e d w h i l e f o r t h e c o r r e c t e d c a s e by about 20%. However, i n c r o s s i n g a n g l e and energy a r e scanned t o g e t h e r ( s e e t h e a b s o r p t i o n edge R may o n l y

t e x t ) . change by 10-20% f o r a n g l e s below

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the c r i t i c a l angle. Thus, a c o r r e c t i o n may be equal t o t h e edge s t e p i n R and be only a f r a c t i o n of t h e f l u o r e s c e n c e edge s t e p . A comparison of techniques f o r gen- e r a t i n g t h e a p p r o p r i a t e c o r r e c t i o n s w i l l be t h e s u b j e c t of a l a t e r p u b l i c a t i o n . For t h e d a t a i n t h i s paper t h e c o r r e c t i o n s were n o t a p p l i e d s i n c e they a r e gen- e r a l l y small and a f f e c t p r i m a r i l y t h e o v e r a l l normalization.

FLUORESCENCE DATA

The A 1 on Cu and Ag on Au samples were made by e v a p o r a t i o n onto f l o a t g l a s s sub- s t r a t e s . The chamber p r e s s u r e was 2x10-6 T o r r , and s i n c e only a few seconds e l a p s e d between e v a p o r a t i o n of t h e s u b s t r a t e and s u r f a c e m e t a l s , no d e t e c t a b l e oxide l a y e r was o b s e r v a b l e a t t h e i n t e r f a c e u s i n g Auger s p u t t e r p r o f i l i n g . The samples were annealed i n a i r f o r f i v e minutes a t each temperature. To c a l i b r a t e t h e a n g l e s and check t h e sample alignment x-ray r e f l e c t i v i t y measurements were r u n f i r s t . For t h e A1-Cu system r e f l e c t i v i t y d a t a were i n good agreement with c a l c u l a - t i o n s a s shown i n Fig. 1. These i n d i c a t e d some roughening of the i n t e r f a c e over t h a t of t h e f l o a t g l a s s f o r the unannealed sample and c l e a r i n d i c a t i o n s of i n t e r - f a c i a l r e a c t i o n s could be observed 171. From p r e v i o u s work [11,12] i t i s known t h a t CuAL2 should begin t o form a t t h e i n t e r f a c e a t a n n e a l i n g temperatures around 150°C. EXAFS measurements were made a t v a r i e t y of a n n e a l i n g temperatures i n t h e range 100-220°c.

Figure 4 shows t h e e v o l u t i o n of t h e EXAFS s i g n a l a s a f u n c t i o n of a n n e a l i n g tem- p e r a t u r e f o r an a n g l e of incidence j u s t above t h e A 1 c r i t i c a l angle. P l o t t e d a r e

the X(k) f u n c t i o n s e x t r a c t e d a f t e r removing t h e slowly v a r y i n g background seen i n Fig. 3. I n comparing t h e s p e c t r a w i t h t h e Cu and CuAIZ s t a n d a r d s , t h e c l e a r growth of a CuA12 i n t e r f a c e phase can be seen. I n s p i t e of t h e d i s t o r t i o n s mentioned i n the p r e v i o u s s e c t i o n i t was found t h a t good f i t s t o t h e s p e c t r a could be obtained with a l i n e a r combination of the two s t a n d a r d s . The reason is t h a t w e l l below the c r i t i c a l a n g l e t h e c o r r e c t i o n s f o r t h e f l u o r e s c e n c e s i g n a l a r e small and confined mainly t o t h e edge region. They t h u s a f f e c t t h e o v e r a l l n o r m a l i z a t i o n , b u t n o t the b a s i c s t r u c t u r e of t h e o s c i l l a t i o n s .

To determine the amount of CuA12, measurements a t each a n n e a l i n g temperature were made a s a f u n c t i o n of angle. An example f o r t h e 180°c a n n e a l i s shown i n Fig. 5.

A s t h e a n g l e s become l a r g e t h e p e n e t r a t i o n i n c r e a s e s and t h e s i g n a l r e v e r t s t o t h a t of pure Cu. T h i s v e r i f i e s t h a t t h e i n t e r f a c e CuA12 i s indeed t h i n , and from t h e s e and x-ray r e f l e c t i v i t y measurements i t i s found t o be

-

.

The measurements on t h e Ag-Au b i l a y e r s i n d i c a t e a n o t h e r s t r e n g t h of t h e EXAFS tech- nique, t h e a b i l i t y t o measure r e l a t i v e l y d i l u t e systems. I n t h i s system the con- s t i t u e n t s a r e completely m i s c i b l e w i t h a n n e a l i n g r e s u l t i n g i n simple i n t e r d i f f u s i o n of t h e elements. The i n t e r d i f f u s i o n takes p l a c e by two p r o c e s s e s , bulk d i f f u s i o n and g r a i n boundary d i f f u s i o n w i t h t h e g r a i n boundary mechanism dominant a t low tem- p e r a t u r e s . I n f a c t a t room temperature t h e bulk d i f f u s i o n i s e s s e n t i a l l y z e r o and only t h e g r a i n boundaries c o n t a i n t h e i n t e r d i f f u s i n g s p e c i e s [13]. Therefore, f o r a n Ag on Au sample aged a t room temperature, t h e only Au atoms l o c a t e d n e a r the Ag s u r f a c e should be i n t h e g r a i n boundaries. Glancing a n g l e EXAFS measurements of t h e Ag s u r f a c e r e g i o n can thus be used t o s t u d y t h e Au environment i n Ag g r a i n boundaries. This i s a d i l u t e system s i n c e f o r t y p i c a l g r a i n s i z e s the Au concen- t r a t i o n i s only a b o u t 1%.

Figure 6 shows some examples of d a t a o b t a i n e d a t t h e Au L3 edge f o r t h i s system.

I n t h i s c a s e the F o u r i e r transforms of X(k) a r e p l o t t e d s i n c e they show t h e r e s u l t s most c l e a r l y . Both Au and Ag have t h e same s t r u c t u r e s and l a t t i c e parameters and

i t might be expected t h a t Au w i t h Au neighbors would have t h e same EXAFS a s Au with Ag neighbors. However, t h i s i s n o t t h e c a s e a s t h e r e s u l t s f o r the two s t a n d a r d s i n d i c a t e . Because of t h e i r complica t e d phase s h i f t s and b a c k s c a t t e r i n g amplitudes, t h e l a r g e peak i n t h e transform due t o t h e n e a r e s t neighbors i s s p l i t i n d i s t i n c t l y d i f f e r e n t ways f o r the two environments. I n t h e unannealed sample f o r which Au i s only i n t h e g r a i n boundaries i t i s seen t h a t transform most resembles t h a t of pure

f o r t h e l8O0C annealed sample.

Au while a n n e a l i n g i n c r e a s e s i n t e r d i f - f u s i o n i n t o the b;lk and i n c r e a s e s t h e F i g u r e 4. V a r i a t i o n of t h e i n t e r - number of Ag neighbors. This r e f l e c t s f a c e EXAFS s i g n a l a s a f u n c t i o n of the f a c t t h a t t h e Au i n the Ag g r a i n a n n e a l i n g temperature. The i n c i - boundaries i s c o n c e n t r a t e d and has more dence a n g l e i s 4.4 mrad. For Au neighbors than when i t begins t o be comparison the EXAFS f o r bulk Cu d i s p e r s e d i n t o t h e bulk. For t h e pres-

and CuA12 a r e included. e n t measurements the e f f e c t is somewhat

exaggerated by the s c a t t e r i n g of p a r t of t h e i n c i d e n t beam i n t o t h e underlying Au l a y e r where the pure Au s i g n a l would be e x c i t e d . I t i s e s t i m a t e d t h a t t h i s c o n t r i b u t e s a t most 1 / 3 of t h e f l u o r e s c e n c e s i g n a l , and new samples w i t h t h i n n e r Au l a y e r s a r e being prepared which should e l i m i n a t e t h e problem. I n any case the r e s u l t s demonstrate t h e s e n s i t i v i t y of t h e technique i n being a b l e t o d e t e c t a n -1% c o n c e n t r a t i o n of Au i n t h e f i r s t 30-40 A of t h e Ag l a y e r . I t should be noted t h a t i n t h e Au-Ag system Ag tends to s u r f a c e s e g r e g a t e 1141 and t h e r e f o r e , s u r f a c e s e g r e g a t i o n does n o t appear t o be enhancing t h e s i g n a l . T h i s i s v e r i f i e d by Rutherford b a c k s c a t t e r i n g measurements on s i m - i l a r l y prepared samples.

ACKNOWLEDGMENTS

The a u t h o r s wish t o thank R. Dinardo f o r help i n p r e p a r i n g t h e samples and J. T a f t o , R. Corderman, and A. Hanson f o r t h e i r h e l p i n c h a r a c t e r i z i n g them.

T h i s work was performed i n p a r t a t beam l i n e X - 1 1 a t t h e N a t i o n a l Synchrotron L i g h t Source which is supported by t h e U.S. Department of Energy under C o n t r a c t Nos. DE-AS05-80-3410742 and DE-AC02-76CHOOOl6.

C8-830 JOURNAL DE PHYSIQUE

F i g u r e 6. F o u r i e r t r a n s f o r m s of t h e s u r - f a c e EXAFS s i g n a l s o b t a i n e d on Ag-Au b i - l a y e r s a s a f u n c t i o n of a n n e a l i n g . For comparison s i m i l a r t r a n s f o r m s f o r p u r e Au and a d i l u t e Au i n Ag s t a n d a r d a r e i n c l u d e d . The k range f o r t h e t r a n s f o r m s i s 3-11 A-l.

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