ELECTRON SPECTROSCOPY STUDY OF Mo/Si(111) INTERFACES

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ELECTRON SPECTROSCOPY STUDY OF Mo/Si(111) INTERFACES

T. Nguyen, R. Cinti

To cite this version:

T. Nguyen, R. Cinti. ELECTRON SPECTROSCOPY STUDY OF Mo/Si(111) INTERFACES.

Journal de Physique Colloques, 1984, 45 (C5), pp.C5-435-C5-439. �10.1051/jphyscol:1984565�. �jpa-

00224184�

ELECTRON SPECTROSCOPY STUDY OF M O / S ~ ( 1 1 1 ) I N T E R F A C E S

T.T.A. Nguyen and R.C. C i n t i

Laboratoire d ' ~ t u d e s d e s Propridtds Electroniques d e s S o l i d e s , C.N.R.S., B.P. 166, 38042 GrenobZe Cedex, France

Rdsumd - L ' Q v o l u t i o n d e s c a r a c t d r i s t i q u e s Q l e c t r o n i q u e s d e l ' i n t e r f a c e formQe pendant l ' b v a p o r a t i o n s e q u e n t i e l l e d e f r a c t i o n s d e monocouches d e Mo s u r une s u r f a c e de S i ( l l 1 ) p r o p r e a Q t Q s u i v i e p a r s p e c t r o s c o p i e d e p h o t o Q l e c t r o n s X (XPS) e t d f Q l e c t r o n s Auger sous rayons

(XAES). L 1 Q n e r g i e d e l i a i s o n du Mo 3d5/2 s u b i t un ddplacement n d g a t i f de 0,4 eV dans l a r Q g i o n de l a sous- monocouche e t augmente e n s u i t e rapidement pour a t t e i n d r e l a v a l e u r du massif v e r s l e s 6 monocouches, t a n d i s que l e S i 2p demeure B l a mEme p o s i t i o n e t que l ' d n e r g i e c i n Q t i q u e d e l a r a i e S i KLL n'accuse qu'une l Q g S r e augmentation.

Les t r a n s i t i o n s Auger S i LVV i n d i q u e n t une p e r t u r b a t i o n d e s Q t a t s 3p en prQ- sence du m Q t a l . Des couches s t a b l e s e t homogsnes d e MoSi2 s e forment quand des ddp6ts d e

15 monocouches s o n t r e c u i t s aux tempdratures T > 500°C. Par rap- p o r t au c a s mieux d t u d i g du N i / S i , l e s t r a n s f e r t s d e charge dans l e s atomes m g t a l l i q u e s s o n t p l u s f a i b l e s e t d e sens opposd, e t l ' i n t e r f a c e s e r a i t beau- coup p l u s a b r u p t e .

A b s t r a c t - XPS and X-ray e x c i t e d AES (XAES) have been used t o f o l l o w t h e evo- l u t i o n of t h e e l e c t r o n i c s t r u c t u r e of t h e i n t e r f a c e formed when submonolayer amounts of Mo atoms a r e evaporated onto c l e a n S i ( l l 1 ) s u r f a c e . The Mo 3dg/2 l e v e l undergoes a chemical s h i f t of - 0.4 eV, towards lower binding energy, f o r s u r f a c e coverage 8 6 1 monolayer (ML) and resumes i t s bulk v a l u e f o r 8 2 6 ML, while t h e S i 2p peak remains a t t h e same p o s i t i o n and t h e S i KLL f e a t u r e i n c r e a s e s weakly i n k i n e t i c energy. S i LVV Auger t r a n s i t i o n s i n d i c a t e p e r t u r b a t i o n s of t h e S i 3p s t a t e s by t h e presence of Mo atoms. Annealings of t h i n d e p o s i t s

15 ML) a t T > 500°C lead t o t h e formation of s t a b l e and homo- geneous MoSi2 l a y e r s . I n comparison with t h e more i n v e s t i g a t e d c a s e of Ni/Si i n t e r f a c e s , t h e main d i f f e r e n c e s a r e a weaker chemical s h i f t of t h e Mo core- l e v e l s , a n o p p o s i t e charge t r a n s f e r i n t h e metal atoms and i n d i c a t i o n of a much more a b r u p t i n t e r f a c e .

INTRODUCTION

Surface techniques have been i n c r e a s i n g l y used f o r probing t h e atomic and e l e c t r o n i c p r o p e r t i e s a t t h e i n t i m a t e r e g i o n of metal-semiconductor c o n t a c t s . These s t u d i e s h a v e brought important p r o g r e s s t o t h e b a s i c understanding of Schottky b a r r i e r s and r e - newed t h e i r concept / I / . I n c l a s s i c a l manner, t h e y begin with t h e c r e a t i o n of a c l e a n semiconductor s u r f a c e , a f t e r t h a t monolayer f r a c t i o n s of metal a r e d e p o s i t e d i n se- quence and t h e composite s u r f a c e analysed. Annealings of t h e d e p o s i t favour t h e ma- t e r i a l i n t e r m i x i n g o r t h e formation of s i l i c i d e s / 2 / . Near noble t r a n s i t i o n m e t a l s have been w e l l explored. Their i n t e r f a c e s a r e g e n e r a l l y r e a c t i v e but t h e chemical na- t u r e and composition depend g r e a t l y on t h e d e p o s i t e d metal 131. R e f r a c t o r y m e t a l s a r e a s y e t l e s s s t u d i e d . W e g i v e i n t h e following t h e XPS and XAES measurements of t h e bonding between Mo and S i 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 i n t h e MoSi2 phase. The r e s u l t s show n o t a b l e d i f f e r e n c e s with t h e N i c a s e f o r which s i m i l a r s t u d i e s have been undertaken / 4 , 5 / .

EXPERIMENTS

Experiments were performed i n a n ultrahigh-vacuum VG Escalab equipment w i t h a base p r e s s u r e of 1 x 10-10 mB. Surface p r e p a r a t i o n and e v a p o r a t i o n were made i n t h e prepa-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984565

C5-436 JOURNAL DE PHYSIQUE

r a t i o n chamber. The p o l i s h e d S i ( l l 1 ) d i s c s ( N t y p e ,

10 ~2.cm-I) were clamped on mo- d i f i e d VG l a r g e s i z e h o l d e r s by means of W c l i p s . Sample h e a t i n g was done by e l e c - t r o n bombardment of t h e sample holder and t h e temperature was checked with a n I R r a - d i a t i o n thermometer (IRCON). C l a s s i c a l A r ion-bombardment a n n e a l i n g technique was used f o r sample c l e a n i n g 151. Small t r a c e s of Cu appeared a f t e r t h e f i r s t annealings and p r o g r e s s i v e l y disappeared i n t h e f o l l o w i n g c y c l e s . The e v a p o r a t i o n source was a h a i r p i n Mo f i l a m e n t (MRC 99.95 % p u r i t y ) d i r e c t l y heated by c o n s t a n t c u r r e n t s . The s u r f a c e coverage 0 was c a l i b r a t e d u s i n g t h e a r e a p l o t of s u b s t r a t e s i g n a l ( S i 2p) v e r s u s d e p o s i t s i g n a l (Mo 3d d o u b l e t ) / 6 / , t h e f i r s t two breaking p o i n t s of t h i s p l o t were taken a s 0

1 and 2 r e s p e c t i v e l y . During e v a p o r a t i o n , t h e p r e s s u r e r a i - sed t o

3 x loe9 mB i n t h e p r e p a r a t i o n chamber, and c o n t r o l scans showed s l i g h t con- t a m i n a t i o n by CO. For

I 5 M L d e p o s i t , t h i s contamination was estimated t o be equiva- l e n t t o

0 . 3 ML. I t completely disappeared upon h e a t i n g a t & 500°C. The e l e c t r o n spectrometer was c a l i b r a t e d w i t h r e f e r e n c e t o t h e Au 4 f 7 / 2 l i n e (Eb

84 eV), and a r e s o l u t i o n of 1.3 eV (FWHM of Ag 3d5/2) was used f o r r e c o r d i n g c o r e - l e v e l and valen- ce-band s p e c t r a . The p r e c i s i o n of t h e measurements was found t o be w i t h i n t 0.05 eV.

K i n e t i c e n e r g i e s were r e f e r e n c e d t o t h e Fermi l e v e l EF.

RESULTS

The f o l l o w i n g f i g u r e s show s i g n i f i c a n t s p e c t r a obtained a t d i f f e r e n t s u r f a c e covera- ges 0 and from a

15

d e p o s i t submitted t o a

mn a n n e a l i n g a t 600°C. For t h i s l a t t e r s u r f a c e , two curves a r e g i v e n , one ( a ) d e t e c t e d a t normal emission, t h e o t h e r (b) a t a n a n g l e of

20" from t h e s u r f a c e p l a n e . As we w i l l s e e l a t e r , they c o r r e s - pond t o a MoSi2 l a y e r .

A s e t of t h e Mo 3d5/2 c o r e - l e v e l s i s shown i n F i g . 1 . The dashed curve i s r e l a t i n g t o a cleaned Mo p o l y c r y s t a l l i n e sample and t h e two upper ones (a and b) t o an annea- led d e p o s i t (MoSi2 l a y e r ) .

1 I

2 2 8 226

BINDING ENERGY ( e V )

Fig. 1 - Mo 3d5/2 l i n e s a t d i f - f e r e n t coverages 8 (ML) and f o r annealed l a y e r ( s e e t e x t ) .

F i g . 2 - Mo 3d5/2 c o r e - l e v e l binding energy Eb a s a f u n c t i o n of coverage 8 . H o r i z o n t a l b a r s a r e Eb i n bulk Mo and MoSi2 l a y e r .

The v a r i a t i o n of t h e p o s i t i o n of t h i s peak mea- sured on 3 d i f f e r e n t S i samples a r e grouped i n F j g . 2. T h i s v a r i a t i o n p r e s e n t s 3 d i s t i n c t r e - glons : a c o n s t a n t energy s h i f t AEb

- 0 . 4 eV a t 8 2 1 ML, a r a p i d change of Eb and the higher 8 p a r t (0 2 ⁶ ML) where t h e Mo 3dg/2 resumes i t s bulk energy. I n t h e MoSi2 phase, t h i s f e a t u r e i s s h i f t e d a g a i n of about - ^{0.2 eV.}

The S i 2p s t r u c t u r e (Fig. 3 ) , on t h e o t h e r hand, remains n e a r l y c o n s t a n t when 8 i n -

c r e a s e s , and even a t 8=15 ML, t h e v a r i a t i o n i s w i t h i n t h e experimental e r r o r . I n t h e

MoSi2 l a y e r (curves a and b ) , t h i s l e v e l t a k e s a chemical s h i f t of AEb

+ 0.4 eV.

171,

equal t o 1616.5 eV f o r c l e a n S i , i n c r e a s e s w i t h 8 and a c q u i r e s

0.4 eV more a t 0

15 M L and 0.5 eV w i t h t h e s i l i c i d e l a y e r .

I n t h e valence-band s p e c t r a ( F i g . 4 ) , t h e Mo 4d s t r u c t u r e i n c r e a s e s r a p i d l y w i t h 8 and becomes pro- g r e s s i v e l y t h e dominant f e a t u r e t h a t moves slowly

>

towards EF. A t 6 & 6 ML t h e energy d i s t r i b u t i o n cur- c- v e s (EDC) t a k e t h e same shape a s t h e one given by a

cleaned p o l y c r y s t a l l i n e Mo. Upon 30 mn a n n e a l i n g a t 600°C, t h e EDC of a 15 ML d e p o s i t completely changes and p r e s e n t s c h a r a c t e r i s t i c f e a t u r e s of t h e b u l k MoSi2 spectrum measured by Weaver e t a l . 181 u s i n g

s y n c h r o t r o n r a d i a t i o n photoemission.

The XAES v a l e n c e band s p e c t r a taken with t h e cons- t a n t - r e t a r d - r a t i o mode (CRR

10, A 1 r a d i a t i o n ) a r e

102 100 98

g i v e n i n t h e u n c o r r e c t e d N(E) form. The S i LW li-

BINDING ENERGY ( e V )

nes ( F i g . 5) show a p r e f e r e n t i a l d e c r e a s e of t h e s t r u c t u r e near 93 eV upon metal a d s o r p t i o n . The Mo MNV and MVV t r a n s i t i o n s a r e atomic l i k e and a r e not F i g . 3 - S i 2p binding energy given in this paper'

f o r c l e a n S i , 15 ML d e p o s i t and MoSi2 l a y e r .

L , . . * , * <

12 10 8 6 4 1 0-E,

8 0 100

BINDING

eV

F i g . 4 - Valence band s p e c t r a .

Fig. 5 - S i LVV Auger l i n e s a t i n c r e a s i n g Mo coverages and a f t e r a n n e a l i n g . DISCUSSION

Let u s f i r s t examine t h e chemical bonding between Mo atoms and t h e S i s u r f a c e d u r i n g

t h e i n t e r f a c e formation and next t h e s i l i c i d e l a y e r produced upon a n n e a l i n g .

C5-438 JOURNAL DE PHYSIQUE

The v a r i a t i o n of t h e Mo 3d5/2energy p o s i t i o n i n f u n c t i o n of s u r f a c e coverage 8 ( F i g . 2) r e f l e c t s t h e chemical environment change of Mo atoms. T h i s curve b e a r s two important i n d i c a t i o n s : a n e g a t i v e chemical s h i f t AEb of about - 0.4 eV f o r 9 < ^{1 ML}

and a f a i r l y s h o r t t r a n s i t i o n r e g i o n , t h e Mo 3dg/2 l e v e l a l r e a d y resumes i t s bulk v a l u e a t 9 2 6 ML. The f i r s t o b s e r v a t i o n i n d i c a t e s t h a t charge i s gained by Eo atoms i n t h e r e a c t i o n w i t h S i s u r f a c e ; i t s amount i s small, e s p e c i a l l y a s i n t h i s c a s e t h e c o o r d i n a t i o n e f f e c t of t h e Mo adatoms has t o be t a k e n i n t o account and t h i s e f - f e c t would c o n t r i b u t e i t s e l f f o r about - 0.2 eV / 9 / t o t h e measured s h i f t . A few Mo o v e r l a y e r s needed f o r a t t a i n i n g t h e c h a r a c t e r i s t i c Eb of t h e bulk would be t h e conse- quence of a n a b r u p t j u n c t i o n . With a n electron-mean-free-path of

20 A, t h e 6 t h o v e r l a y e r c o n t r i b u t e s t o about 25 % of t h e t o t a l emission and a z e r o s h i f t d e t e c t e d a t t h i s coverage i m p l i e s n e g l i g i b l e s h i f t i n s e v e r a l l a y e r s underneath. This ques- t i o n however remains t o be c l a r i f i e d , because Rossi e t a l . 1101 i n a synchrotron r a - d i a t i o n photoemission study have assumed a n i n t e r m i x i n g of t h e two s o l i d s i n o r d e r t o e x p l a i n t h e l a c k i n d e t e c t i n g t h e Mo 4d band a t i n c r e a s i n g coverage 8.

The Mo r e a c t i o n with S i i s q u i t e d i f f e r e n t from t h a t of t h e o t h e r systems i n v e s t i g a - t e d b e f o r e by XPS where t h e chemical s h i f t s a t low 8 observed f o r N i / 4 , 5 / , Pd / 4 / and Cu 1111 d e p o s i t e d on S i a r e a l l i n t h e o p p o s i t e d i r e c t i o n and g e n e r a l l y much mo- r e important. A p o s s i b l e cause i s t h e f i l l i n g degree of t h e e x t e r n a l d s h e l l , Mo i s h a l f f i l l e d and t h e o t h e r c i t e d m e t a l s , on t h e c o n t r a r y , have d e l e c t r o n f i l l i n g much more important.

As f o r t h e S i c o r e l e v e l s , t h e i r energy s h i f t i s v e r y weak. The S i 2p remains n e a r l y c o n s t a n t , w i t h i n t h e experimental e r r o r (f 0.05 eV). The unobserved XPS s h i f t of t h e S i 2p has a l s o happened i n t h e c a s e s c i t e d above 15,111. It can be due t o t h e addi- t i o n of s e v e r a l e f f e c t s : important c o n t r i b u t i o n of t h e s i g n a l from t h e bulk, a li- mited energy r e s o l u t i o n and a l s o t h e weakness of t h e s h i f t . The chemical s t a t e of S i i s b e t t e r i n d i c a t e d by t h e S i KLL s p e c t r a because of a g r e a t e r r e l a x a t i o n ener- gy of Auger t r a n s i t i o n s 171 but l i t t l e i n f o r m a t i o n i s obtained a t t h i s s t a g e s . The S i 2p and KLL c u r v e s a r e much more h e l p f u l f o r t h e s i l i c i d e c h a r a c t e r i z a t i o n . The v a l e n c e band ECD's show a r a p i d predominance of t h e Mo d band near t h e Fermi l e - v e l EF and a slow displacement of t h i s s t r u c t u r e towards EF. The m o d i f i c a t i o n of t h e S i v a l e n c e band i s more p e r c e p t i b l e on t h e S i L W Auger s p e c t r a which mark a s e n s i - b l e d e c r e a s e of t h e s t r u c t u r e a t

93 eV when Mo atoms a r e d e p o s i t e d . The shape of t h i s CVV Auger l i n e has been well s t u d i e d and i t i s demonstrated 1121 t h a t t h e s t r u c - t u r e near 93 eV i s due t o t h e pp component, while t h e o t h e r one a t lower energy i s of s p t y p e . The p r e f e r e n t i a l d e c r e a s e of t h e former peak i n d i c a t e s a r e d i s t r i b u t i o n of S i 3p e l e c t r o n s , i n t h e i n t e r a c t i o n with Mo.

Consider now t h e e f f e c t s of h e a t t r e a t m e n t s on d e p o s i t s of

15 ML. t h i c k . Annealing between Ta

200 and 400°C l e a d s t o inhomogeneous compositions i n t h e analysed l a y e r a s p o i n t e d out by t h e d i f f e r e n c e s i n energy p o s i t i o n s of t h e Mo and S i c o r e - l e v e l s f o r r e s p e c t i v e l y normal and g l a n c i n g d e t e c t i o n s . For

2 500°C, t h e analysed l a y e r becomes homogeneous, and t h e e l e c t r o n i c s p e c t r a remain unchanged upon i n c r e a s i n g temperature u n t i l

850°C, showing t h e completion of a s t a b l e phase a l r e a d y a t t h e lower p a r t of t h i s temperature range. This l a y e r i s MoSi2. Two arguments support t h i s a f f i r m a t i o n : a ) MoSi2 i s t h e f i n a l phase obtained when a Mo d e p o s i t on S i sam- p l e i s annealed /2,13/ ; b) t h e corresponding v a l e n c e band EDC ( F i g . 4 , upper curve) p r e s e n t s t h e c h a r a c t e r i s t i c f e a t u r e s of t h e bulk Most2 spectrum o b t a i n e d by Weaver e t a l . 181. I n t h i s s i l i c j . d e , Mo 3dg/2 undergoes a s h i f t of - 0.2 eV, S i 2p g a i n s

+ 0.4 eV and t h e ~i

k i n e t i c energy i n c r e a s e s of 0.5 eV. The S i Auger parameter s h i f t , which i s twice t h e r e l a x a t i o n energy / 7 / , i s t h e n equal t o 0.9 eV. As t h e S i and Mo c o r e - l e v e l s h i f t s show a n unbalanced charge t r a n s f e r , p a r t of t h a t charge i s r e d i s t r i b u t e d i n t h e m e t a l l i c bond formation.

F i n a l l y , i n comparison with a p r e v i o u s s t u d y performed i n t h e same c o n d i t i o n s with

N i / 5 / , t h e d i f f e r e n c e i s r e v e a l e d by t h e f o l l o w i n g o b s e r v a t i o n s . I n t h e a d s o r p t i o n

phase, t h e chemical s h i f t s a r e - 0.4 eV f o r Mo 3d5/2 and + 0.9 eV f o r N i 2 ~ 3 1 2 . The

d e p a r t u r e i s s t i l l more important w i t h d i s i l i c i d e s , - 0.2 eV f o r MoSi2 and + 1.3 eV

f o r Nisi2. Moreover, with N i t h e i n t e r f a c e i n t e r m i x i n g i s c e r t a i n l y much more impor-

ACKNOWLEDGEMENTS

We wish t o thank t h e L a b o r a t o i r e d l I n t e r a c t i o n s Hyperfines/D.R.F., C.E.A. Grenoble, f o r t h e e x p e r i m e n t a l f a c i l i t i e s and Drs. G . CHAUVET and R. BAPTIST f o r h e l p f u l d i s - c u s s i o n s .

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