ELECTRONIC SPECTRA OF THE AMORPHOUS AuSi INTERFACE

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ELECTRONIC SPECTRA OF THE AMORPHOUS AuSi INTERFACE

A. Cros, J. Derrien, C. Mouttet, J. Gaspard, P. Lambin, F. Salvan

To cite this version:

A. Cros, J. Derrien, C. Mouttet, J. Gaspard, P. Lambin, et al.. ELECTRONIC SPECTRA OF THE AMORPHOUS AuSi INTERFACE. Journal de Physique Colloques, 1980, 41 (C8), pp.C8-795-C8-798.

�10.1051/jphyscol:19808197�. �jpa-00220301�

JOURNAL DE PHYSIQUE Colloque C8, suppllment au n08, Tome 41, ao2t 1980, page Cfj-795

ELECTRONIC SPECTRA OF THE AMORPHOUS A u S i INTERFACE

A. ~ r o s + , J. Derrien ++ , C. ~outtet', J.P. as pard*, P.

am bin*

*

Abstract.- Auger and Electron Loss Spectra (ELS) have been measured on Au deposited on the ( I l l ) Si surface at room temperature. The absence of LEED pattern and a structural analysis by Auger technique clearly show an amorphous surface composition close to the eutectic composition A U - ~ ~ S ~ l g irrespec- tive of the amount of Au deposited. This is also confirmed by the evolution of ELS spectra during the growth process and during a subsequent sp~ttering.~Evidence is found for the formation on top of the Si substrate of 1. a diffuse interface of % 20 A thickness, 2. a pure Au fflm and 3. an eutectic amorphous surface. The stability of the surface composition is discussed using molecular dynamics calculations. The parameters of the model have been deduced from themdynamical data on the Au-Si system. They reflect a strong attractive hetero-atomic interaction. The calculations show that the surface with eutectic concentration is in equilibrium with a pure gold substrate.

I. Introduction : Due to its importance, both fun- damental and technological, the gold silicon (Au- Si) interface has initiated numerous structural investigations I ^I I. The aim of this paper is to report some E.L.S., Auger and depth profiling data on this interface at the early stages of its forma- tion. Results obtained experimentally reveal the diffusion of Si through the deposited Au film.

Molecular dynamics calculatiors hare been undertaken in order to explain the observed surface composi- tion.

11. Experiments : All experiments were performed in a ultra high vacuum (U.H.V.) vessel equipped with various surface techniques as Auger electron spec- troscopy (AES), low energy electron diffraction (LEED), electron loss spectroscopy (ELS), depth profiling, mass spectroscopy (other experimental details can be found in ref. 1 2 1 ) . Au atoms were deposited onto a clean 7x7 silicon ( 1 1 1 ) surface by means of a Knudsen cell. AES and ELS spectra were recorded at each stage of the metal deposition

(every 0 = 0.1 up to 0 = 100). They were also recorded at each stage of the ion sputtering of the interface, once formed, and show therefore a pro- file of the investigated surface. The Auger tran- sitions characteristic of Ab (N VV % 69 eV)

67

Si(LVV % 92 eV) were used to identify the compo- nents of the surface film.

At room temperature, the evolution of the Auger spectra with increasing gold coverage (0 = 1 cor- responds 7.9. l0I4 at/cm 2 ) gives the following re-

sults : the intensity of the elemental Si peak at 9 2 eV decreases in the range 0 < 0 < 4. Near 9 = 4,

the peak splits into two components located at 90 eV and 94 eV. This reflects a change in the chemical environment of the Si atoms. The lineshape of this peak then remains unchanged but its overall intensity decreases while the intensity of the AU peak at 69 eV increases at higher coverages. Even at 0 % 100, AES indicates the presence of Si atoms

E L E C T R O N E N E R G Y ( e V )

Fig. 1 : AES spectra from the 60-100 eV region.

a) obtained after deposition of gold (8 = 80) b) after 15 mn of ~ r + ion sputtering.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19808197

C8-796 ^{JOURNAL DE PHYSIQUE}

on t o p o f t h e g o l d f i l m ( f i g . 1 ) . Once t h e f i l m i s formed a d e p t h p r o f i l i n g i s p r o c e e d e d ( s p u t t e r i n g r a t e % 2 m o n o l a y e r / w ) . The p r o f i l e o b t a i n e d i s c o n s i s t e n t w i t h AES r e s u l t s d u r i n g t h e i n t e r f a c e f o r m a t i o n and c o n f i r m s t h e p r e s e n c e o f I ) a S i r i c h s u r f a c e monolayer, 2) a p u r e g o l d f i l m , 3) a d i f f u - s e i n t e r f a c e of a few m o n o l a y e r s o n t o p of t h e S i s u b s t r a t e ( f i g . 2 ) .

The change i n t h e Auger S i l i n e r e v e a l s t h e m o d i f i - c a t i o n o f t h e v a l e n c e e l e c t r o n i c s t a t e s o f S i s u r - f a c e atoms s i m i l a r t o many S i - m e t a l a l l o y s 131 and shows a s u r f a c e c o m p o s i t i o n of che topmost l a y e r

t e r b a n d t r a n s i t i o n s r a p i d l y d i s a p p e a r when 8 i n - c r e a s e s . Two Au i n d u c e d p e a k s A ( 7 . 5 eV) and B (?. I I eV) a r e s e e n i n t h e f i r s t monolayer r a n g e . W i t h A u o v e r d o s e t h e Apeak d i s p l a y s a s h i f t t o l o w e r e n e r g y . T h i s p r o g r e s s i v e e n e r g y s h i f t ( f r o m 7 . 5 eV t o 6 eV) v e r s u s c o v e r a g e ( f r o m 1 t o ?. 12 l a y e r s ) r e f l e c t s t h e v a r i a t i o n of t h e i n t e r f a c e c o m p o s i t i o n and a l l o w s i t s e s t i m a t i o n 131. We found i n p a r t i c u - l a r t h a t t h e f i r s t l a y e r c o m p o s i t i o n f a l l s n e a r t h e e u t e c t i c Au 0.8lSi0. 19' The b r o a d and r e s o l v e d peak B which a p p e a r s i n t h e monolayer r a n g e and r e m a i n s unchanged w i t h i n c r e a s i n g c o v e r a g e i s a s s i g n e d t o a s u r f a c e a l l o y e x c i t a t i o n a s s u g g e s t e d by e x p e r i m e n t a l r e s u l t s 131 .The f i n a l p o s i t i o n o f t h e A peak i n a t h i c k f i l m c o r r e s p o n d s t o a c h a r a c t e r i e

t i c l o s s o f p u r e g o l d .

F i g . 2 : Peak t o peak a m p l i t u d e o f Auger t r a n s i t i o n s

-

d u r i n g ~ r + s p u t t e r i n g .

c l o s e t o t h e Au-Si e u t e c t i c c o m p o s i t i o n .

The p r o g r e s s i v e d i s a p - p e a r a n c e o f t h e 7 x 7 LEED s u p e r s t r u c t u r e when Au i s d e p o s i t e d s u g g e s t s t h a t t h e s u r f a c e a l l o y i s amorphous, a c o n c l u s i o n s u p p o r t e d b y t h e t e n d a n c y o f S i and Au t o form amorphous a l l o y s .

The ELS r e s u l t s a r e summarized i n f i g . 3. At v e r y low c o v e r a g e s , one s e e s immediate c h a n g e s i n t h e S i s p e c t r u m . The well-known b u l k and s u r f a c e s t r u c - t u r e s c o r r e s p o n d i n g t o plasmon e x c i t a t i o n s a n d i n -

F i g . 3 : ELS s p e c t r a o f room t e m p e r a t u r e d e p o s i t s ( a ) 7 x 7 c l e a n s u r f a c e - MU ⁼ b u l k s i l i c o n plasmon

MU = s u r f a c e plasmon - S2 and S3 P = p e a k s due t o g

e l e c t r o n i c t r a n s i t i o n s from s u r f a c e s t a t e s . E2= b u l k s i l i c o n i n t e r b a n d t r a n s i t i o n .

(b) 8 7 1 ; ( c ) 8 = I . Note t h e i n i t i a l p o s i t i o n o f t h e A peak f o r t h i s g o l d - s i l i c o n mixed l a y e r i s a t 7.5 eV.

(d) 8 4 ; ( e ) 8 = 12. The A peak h a s s h i f t e d now t o i t s f i n a l p o s i t i o n c h a r a c t e r i s t i c o f p u r e g o l d . The s p e c t r u m r e m a i n s unchanged a t h i g h e r c o v e r a g e s .

( f ) s p e c t r u m o f a p u r e g o l d s a m p l e .

10 20 3 0

S P U T T E R I N G T I M E (rncrr)

111. Theory : The s u r f a c e c o m p o s i t i o n o f t h e Au-Si s y s t e m h a s been i n v e s t i g a t e d u s i n g m o l e c u l a r dyna- m i c s c a l c u l a t i o n s which h a v e been made i n two s t e p s . F i r s t , a s y s t e m of 500 atoms i n a c u b i c c e l l w i t h boundary c o n d i t i o n s i n t h r e e d i r e c t i o n s h a s b e e n g e n e r a t e d on t h e computer i n t h e l i q u i d phase a s s u - ming a Lennard-Jones 12-6 i n t e r a c t i o n p o t e n t i a l . A f t e r t h e thermodynamical e q u i l i b r i u m h a s b e e n r e a c h e d , t h e l i q u i d was quenched a t a t e m p e r a t u r e j u s t below t h e m e l t i n g p o i n t . I n t h e second s t e p , t h e p e r i o d i c b o u n d a r y c o n d i t i o n s were s u p p r e s s e d i n one d i r e c t i o n i n o r d e r t o o b t a i n two f r e e s u r - f a c e s . A randomly g e n e r a t e d b i n a r y m i x t u r e h a s b e e n s i m u l a t e d a t t h e s u r f a c e and t h e s y s t e m was r e l a x e d assuming a g a i n t h e Lennard-Jones p o t e n t i a l s

I n o r d e r t o a c c o u n t f o r t h e Au-Si s y s t e m , t h e U i j p a r a m e t e r s were t a k e n e q u a l t o t h e v a l u e s f o r Au-Au and S i - S i p a i r s , which a r e

' A ~ - A ~ ⁼ 1.44 A

0 S i - S i ⁼ 1.17 1

and f o r Au-Si p a i r s we have used t h e B e r t h e l o t ' s r u l e

u _{Au-Si = ?('Au-Au} ^I

+ O s i - s i )

j u s t f o r t h e s a k e o f s i m p l i c i t y and b e c a u s e o f t h e l a c k o f d e t a i l e d s t r u c t u r a l d a t a . I n a n o t h e r s y s t e m Ag-Ge f o r which t h e p a r t i a l p a i r c o r r e l a t i o n func- t i o n s h a v e b e e n measured 14 ( , a l a r g e d e v i a t i o n from B e r t h e l o t ' s r u l e was found.

The Au-Si s y s t e m shows s t r o n g l y a t t r a c t i v e h e t e r o - a t o m i c i n t e r a c t i o n s . From thermodynamical d a t a on A u ~ - ~ S i x a l l o y s , one can e v a l u a t e t h e r e g u l a r s o l u t i o n p a r a m e t e r R from which t h e c r o s s i n t e r - a c t i o n p o t e n t i a l i s c a l c u l a t e d :

- - 1 + E

' ~ u - s i - 2 ("u-AU Si-Si - R )

The thermodynamical d a t a ( 5 1 show t h a t R depends s t r o n g l y on t h e a l l o y c o m p o s i t i o n . W e have c h o s e n a v a l u e o f Q = - 0 . 5 eV/atom which c o r r e s p o n d s t o a S i c o n c e n t r a t i o n e q u a l t o 20 %. The Au-Au i n t e r - a c t i o n p o t e n t i a l was c a l c u l a t e d from t h e h e a t o f s u b l i m a t i o n o f Au 161 g i v i n g

= 0 . 6 eV/atom

'AU-AU

F o r S i - S i b o n d s , t h e f o l l o w i n g v a l u e s can b e o b t a i - ned u s i n g r e c e n t thermodynamical d a t a 17i

E = 2.0 eV/atom f o r c r y s t a l l i n e S i S i - S i

"i-Si = 1.1 eV/atom f o r l i q u i d S i a t t h e m e l t i n g p o i n t

It seems t h a t t h e S i - S i i n t e r a c t i o n p o t e n t i a l i s s t i l l s m a l l e r i n Metal-Non M e t a l (M-NM) a l l o y s .

F i r s t , t h e number o f n e a r e s t n e i g h b o u r s i s d i f f e - r e n t i n t h e M-NM a l l o y t h a n i t i s i n c r y s t a l l i n e and l i q u i d S i . From t h e e x t r a p o l a t i o n of t h e i n t e r - a c t i o n p o t e n t i a l s i n c r y s t a l l i n e S i ( 4 n e i g h b o u r s ) and i n l i q u i d S i (7-8 n e i g h b o u r s ) , one o b t a i n : a v a l u e o f E Si-Si e q u a l t o 0 . 7 eV/atom i n M-Si s y s - tems. S e c o n d l y , some M-NM s y s t e m s , which have a phas,: d i a g r a m s i m i l a r t o t h a t o f t h e A U ~ - ~ S ~ a l l o y ( i . e . a deep e u t e c t i c n e a r x = 0 . 2 ) , d o n o t

show f i r s t n e i g h b o u r NM-NM bonds i n t h e amorphous s t a t e 181. T h i s i n d i c a t e s t h a t t h e i n t e r a c t i o n e n e r g y between NM-NM atoms i s s m a l l e r t h a n t h e h e t e r o - a t o m i c i n t e r a c t i o n . On t h e o t h e r h a n d ,

thermodynamic c a l c u l a t i o n s f o r l i q u i d Fe-Si and N i - S i s y s t e m s h a v e emphasized a n i m p o r t a n t dependence o f t h e i n t e r a c t i o n p o t e n t i a l between Si-Si atoms on t h e c o m p o s i t i o n o f t h e c o o r d i n a t i o n s p h e r e 191. I n o u r c a l c u l a t i o n , t h e i n t e r a c t i o n p o t e n t i a l E

S i - S i was c o n s i d e r e d a s a f r e e p a r a m e t e r .

The above e x p e r i m e n t a l r e s u l t s can be i n t e r p r e t e d a s f o l l o w s : t h e r e i s a p a r t i a l s e g r e g a t i o n o f S i a t t h e s u r f a c e o f t h e Au d e p o s i t e d l a y e r . Because o f t h e s t r o n g a t t r a c t i v e h e t e r o - a t o m i c p o t e n t i a l , t h e c h e m i c a l i n t e r a c t i o n s a r e opposed t o t h e s e g r e - g a t i o n o f t h e S i atoms i n such a way t h a t a n e q u i - l i b r i u m c o m p o s i t i o n i s r e a c h e d a t t h e s u r f a c e . T h i s c o m p o s i t i o n was found e x p e r i m e n t a l l y c l o s e t o t h e e u t e c t i c c o m p o s i t i o n , i . e . around 20 % o f S i . We h a v e s i m u l a t e d t h i s c o m p o s i t i o n on t h e computer u s i n g a random p r o c e s s and t h e s y s t e m was r e l a x e d u s i n g m o l e c u l a r dynamics c a l c u l a t i o n s . I n t h e s e c a l c u l a t i o n s , two v a l u e s o f t h e Si-Si i n t e r a c t i o n p o t e n t i a l have b e e n c o n s i d e r e d : ESi-Si= 0.006 and E ~ ~ 0.54 eV/atom. I n b o t h c a s e s , t h e s u r f a c e - ~ ~ = c o m p o s i t i o n was found s t a b l e , i . e . i t d o e s n o t change d u r i n g t h e t i m e e v o l u t i o n o f t h e s y s t e m . The a v e r a g e v a l u e s o f t h e S i c o n c e n t r a t i o n a t t h e s u r f a c e c a l c u l a t e d from t h e 200 l a s t i t e r a t i o n s were r e s p e c t i v e l y 19 % a n d 20 % f o r t h e two c a l c u - l a t i o n s . The F i g . 4 shows t h e s u r f a c e c o m p o s i t i o n o b t a i n e d a f t e r t h e computer r u n i n t h e f i r s t c a l c u - l a t i o n ( E ~ ~ - ~ ~ = 0 . 0 0 6 eV/atom). The t e m p e r a t u r e was e q u a l t o 600 K.

The m o l e c u l a r dynamics c a l c u l a t i o n s a l l o w t o ex- p l a i n t h e , s t a b i l i t y o f a s u r f a c e a l l o y w i t h a com- p o s i t i o n c l o s e t o t h e Au-Si e u t e c t i c . The s i m u l a - t i o n o f t h e amorphous monolayer was p r i m a r i l y done i n o r d e r t o c a l c u l a t e t h e e l e c t r o n i c d e n s i t y o f s t a t e s a t t h e s u r f a c e of t h e Au-Si s y s t e m . T h i s i s c u r r e n t l y worked o u t and w i l l b e p u b l i s h e d s e p a r a - t e l y .

c8-798 JOURNAL DE PHYSIQUE

F i g . 4 : S u r f a c e c o n f i g u r a t i o n o f t h e Au-Si s y s t e m o b t a i n e d b y m o l e c u l a r dynamics. The s u r f a c e compo- s i t i o n i s 20 % o f S i . .The atoms a r e r e p r e s e n t e d by t h e i r i n t e r s e c t i o n w i t h a p l a n e l o c a t e d a t a d i s - t a n c e e q u a l t o a g o l d atom r a d i u s ( 0 A ~ - A ~ f ) t h e s u r f a c e . The S i atoms have b e e n shaded.

R e f e r e n c e s

1

1

C r y s t a l Growth 5 (1978) 49 and r e f e r e n c e s c i t e d t h e r e i n .

12 1 S a l v a n F . , Cros A. and D e r r i e n J . , J . de P h y s i q u e L e t t r e s ( t o b e p u b l i s h e d ) .

13) H i r a k i A , , Iwami M . , S h i n u z u A. and S h u t o K . , M a t e r i a l s S c i e n c e and E n g i n e e r i n g 2, 289 ( 1 9 7 6 ) . 141 B e l l i s s e n t - F u n e l M.C., T h e s i s , U n i v e r s i t Q de

Grenoble ( 1 9 7 7 ) .

15) C a s t a n e t R . , C h a s t e 1 R. e t Bergman C., Mate- r i a l s S c i e n c e and E n g i n e e r i n g %, 93 ( 1 9 7 8 ) . 16 1 Honig R. E . , RCA Review 567 (Dec. 1962).

171 Mathieu J . C . ( p r i v a t e communication).

181 Sadoc J . F a n d Dixmier J., M a t e r i a l s S c i e n c e and E n g i n e e r i n g 3, 187 ( 1 9 7 6 ) ; and G a s k e l l P.H., J . Phys. C g, 4337 ( 1 9 7 9 ) .

191 L a t y P . , Joud J . C . and Desr6 P., S u r f . S c i . 60,

109 ( 1 9 7 6 ) .