BONDS IN AMORPHOUS Nb1-xSix STUDIED BY X-RAY EMISSION AND X-RAY PHOTOELECTRON SPECTROSCOPY

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BONDS IN AMORPHOUS Nb1-xSix STUDIED BY X-RAY EMISSION AND X-RAY PHOTOELECTRON

SPECTROSCOPY

W. Zahorowski, A. Simunek, G. Wiech, K. Söldner, R . Knauf, Saemann Ischenko

To cite this version:

W. Zahorowski, A. Simunek, G. Wiech, K. Söldner, R . Knauf, et al.. BONDS IN AMORPHOUS Nb1-xSix STUDIED BY X-RAY EMISSION AND X-RAY PHOTOELECTRON SPECTROSCOPY.

Journal de Physique Colloques, 1987, 48 (C9), pp.C9-1025-C9-1028. �10.1051/jphyscol:19879184�.

�jpa-00227300�

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JOURNAL DE PHYSIQUE

Colloque C9, suppl6ment au n012, Tome 48, d6cembre 1987

BONDS I N AMORPHOUS Nbl-,Six STUDIED BY X-RAY EMISSION AND X-RAY PHOTOELECTRON SPECTROSCOPY

W. Z A H O R O W S K I ( ~ ) , A. S I & N E K ( ~ ) , G. WIECH, K. SOLDNER*,

R. KNAUF* and G. SAEMANN-ISCHENKO*

Sektion Physik, Ludwig-Maximilians Universitdt Munchen.

0-8000 Munchen 2 2 , F.R.G.

"~hysikalisches Institut, Universitdt ~rlangen-Niirnberg, 0-8520 Erlangen, F.R.G.

A b s t r a c t

X-ray e m i s s i o n bands ( S i K , S i L , N b LS2) and XPS s p e c t r a o f amorphous a-Nbl-x S i x ( x = 0 . 2 0 , 0 . 8 0 ) , c r y s t . s i l i c o n , a - S i and NbSi2 were combined t o i d e n t i f y t h e o r i g i n o f a l l t h e i r s p e c t r a l f e a t u r e s . The r e s u l t s show t h a t t h e s t r e n g t h o f Si3p-Nb4d m i x i n g i s s t a b l e and t h e p-d s t a t e s a c c o u n t f o r t h e bond i n t h e Nb-Si a l l o y s . The Si-Nb bonds seem t o be s i m i l a r i n Nbl-% S i x a n d NbSiz and i n v a r i a b l e w i t h i n a wide r a n g e o f x .

The bonding and e l e c t r o n i c p r o p e r t i e s o f t r a n s i t i o n - m e t a l s i l i c i d e s h a v e e x t e n s i v e l y b e e n s t u d i e d ( 1 , ~ ) . I n t h e p r e s e n t communication we r e p o r t on e l e c t r o n i c s t a t e s o f b o n d i n g e l e c t r o n s i n t h e amorphous s y s t e m

a-Nbl-, S i x , 0 ~ x 4 1

.

To o u r knowledge t h i s i s t h e f i r s t s t u d y o f t h e bonds i n t h e amorphous Nb-Si a l l o y s y s t e m . We a p p l i e d X-ray e m i s s i o n s p e c t r o s - copy (XES) and p h o t o e m i s s i o n s p e c t r o s c o p y (XPS, UPS) t o d e l i n e a t e t h e r o l e o f t h e s i l i c o n s and p and t h e niobium d e l e c t r o n s i n bonding and band s t r u c t u r e p r o p e r t i e s .

XES i s g o v e r n e d by s e l e c t i o n r u l e s and t h e r e f o r e we o b s e r v e p r e d o m i n a n t - l y S i S p , S i 3 s , d and Nb4d e l e c t r o n s i n t h e S i K - , S i L- and Nb L - e m i s s i o n b a n d s , r e s p e c t i v e l y . But e a c h o f t h e s e s p e c t r a may a l s o e x h i b i t s p e c t r a l f e a t u r e s d e r i v e d from o t h e r e l e c t r o n i c s t a t e s , t h u s i n d i c a t i n g t h e e n e r - gy p o s i t i o n s o f t h e s e s t a t e s on t h e XES e n e r g y s c a l e . I n o u r XPS e x - p e r i m e n t s , however, t h e S i 3 p e l e c t r o n s a r e v i r t u a l l y u n d e t e c t a b l e s i n c e i n t h e c a s e o f AlKa e x c i t a t i o n t h e c o r r e s p o n d i n g p h o t o i o n i z a t i o n c r o s s s e c t i o n o ( S i 3 p ) i s v e r y s m a l l when compared t o t h a t o f o t h e r e l e c t r o n s i n t h e bond. For t h e m a g n i t u d e o f t h e c r o s s s e c t i o n s a we h a v e t h e re- l a t i o n o(Nb4d)-o ( S i 3 s ) > > o ( S i 3 p ) and owing t o t h e d i f f e r e n t b i n d i n g e n e r g i e s o f Nb d- and S i s - e l e c t r o n s i n Nbl,,Six t h e c o r r e s p o n d i n g

("on leave from Institute of Physics. Polish Academy of Sciences, Al. Lotnikow 32/46. PL-02 668 Warsaw.

Poland

(2'~ermanent address : Institute of Physics, Czechoslovak Academy of Sciences. 18040 Prague.

Czechoslovakia

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879184

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JOURNAL DE PHYSIQUE

f e a t u r e s i n t h e XPS s p e c t r a a r e n a t u r a l l y s e p a r a t e d .

D e t a i l s c o n c e r n i n g t h e s a m p l e p r e p a r a t i o n and a n a l y s i s a s w e l l a s t h e e x p e r i m e n t a l e q u i p m e n t s a r e d e s c r i b e d e l s e w h e r e ( 3 ) . The XPS s p e c t r a were measured w i t h a Leybold EA 10-100 s p e c t r o m e t e r u s i n g unmonochro- m a t i z e d A 1 Ka r a d i a t i o n . For m e a s u r i n g t h e XES s p e c t r a a lm Johann- t y p e s p e c t r o m e t e r ( f l u o r e s c e n c e e x c i t a t i o n ) and a 2m concave g r a t i n g g r a z i n g i n c i d e n c e s p e c t r o m e t e r ( e l e c t r o n e x c i t a t i o n ) were u s e d . Our XES and p h o t o e m i s s i o n measurements o f N ~ I - ~ S ~ ~ a r e p r e s e n t e d i n F i g . 1 f o r x = 0 . 2 , and i n F i g . 2 f o r x = 0 . 8 . I n a d d i t i o n i n t h e u p p e r p a r t o f F i g . 1 t h e UPS, XPS a n d XES r e s u l t s f o r m e t a l l i c niobium a r e d i s p l a y e d , and i n t h e lower p a r t o f F i g . 2 t h e XES ( S i K- a n d S i L- e m i s s i o n b a n d s ) and XPS r e s u l t s f o r amorphous s i l i c o n ( a - S i ) a r e shown I n b o t h f i g u r e s t h e s p e c t r a a r e a l i g n e d t o a common b i n d i n g - e n e r g y s c a l e .

Comparing t h e s p e c t r a i n F i g s . 1 and 2 we o b s e r v e t h a t t h e i n c r e a s e o f t h e s i l i c o n c o n c e n t r a t i o n x ( i ) d o e s n o t i n f l u e n c e t h e p o s i t i o n s o f t h e maximum a t - 4 . 1 eV (= 1835.1 eV p h o t o n e n e r g y ) and t h e s h o u l d e r a t -7.8 eV (1831.4 eV) i n t h e S i K-emission band and o f t h e peak a t -8.5 eV i n t h e XPS s p e c t r a , ( i i ) b r o a d e n s t h e main p e a k s o f t h e S i K-emis- s i o n band from 2.9 eV t o 4.9 eV (FWHM) and o f t h e Nb LB2-emission band from 3.7 eV t o 4.2 eV (FWHM)

,

and ( i i i ) s h i f t s t h e p o s i t i o n s o f t h e maxima o f t h e Nb L - e m i s s i o n bands t o lower b i n d i n g e n e r g i e s (from - 1 . 3 eV t o - 1 . 0 eV) and t h o s e o f t h e XPS s p e c t w t o h i g h e r b i n d i n g e n e r g i e s

(from - 1 . 3 eV t o - 2 . 0 eV).

I t i s known t h a t b o n d i n g between a t r a n s i t i o n e l e m e n t a n d a n e l e m e n t w i t h o n l y s and p e l e c t r o n s c o u p l e s t h e t r a n s i t i o n e l e m e n t d s t a t e s w i t h t h e o t h e r e l e m e n t ' s p s t a t e s a n d r e d u c e s t h e s p - h y b r i d i z a t i o n o f t h e l a t t e r ( 2 , 4 ) . I n o u r c a s e t h e Nb4d s t a t e s h y b r i d i z e w i t h t h e S i 3 p s t a t e s t o form a b o n d i n g h y b r i d which i s more t i g h t l y bound t h a n e a c h o f t h e s t a t e s from which i t i s formed. T h i s e f f e c t m a n i f e s t s i t s e l f i n t h e s t a b l e p o s i t i o n o f t h e maximum of t h e S i K e m i s s i o n which r e f l e c t s p-d b o n d i n g and l i e s a t h i g h e r b i n d i n g e n e r g i e s t h a n t h e main p e a k s o f t h e Nb L e m i s s i o n o f m e t a l l i c niobium ( F i g . 1 , t o p ) and t h e S i K emis- s i o n o f amorphous ( o r c r y s t a l l i n e ) s i l i c o n ( F i g . 2 , b o t t o m and F i g . 3 ) . On t h e o t h e r hand t h e s i l i c o n s and p o r b i t a l s d e h y b r i d i z e and t h e r e - f o r e t h e f e a t u r e a t -7.2 eV ( = 1 8 3 2 . 0 eV) i n t h e S i K-emission band o f c r y s t a l l i n e s i l i c o n ( c - S i ) ( F i g . 3) which i s c h a r a c t e r i s t i c o f s p 3 hy- b r i d i z a t i o n is not pronounced i n t h e amorphous Nb-Si a l l o y s . F o r b o t h v a l u e s x=O.Z and x = 0 . 8 we o b s e r v e f i x e d p o s i t i o n s of t h e s i l i c o n s band i n t h e XPS s p e c t r a a t - 8 . 5 eV and a s m a l l c o n t r i b u t i o n o f t h e s - p h y b r i d a t - 7 . 8 eV i n t h e XES S i K-emission b a n d s .

For h i g h Nb c o n c e n t r a t i o n s t h e Nb4d s t a t e s do n o t form o n l y Nb-Si bonds b u t a l s o Nb-Nb b o n d s . These d s t a t e s show up a t - 1 . 3 eV i n t h e XPS s p e c t r u m and . t h e XES Nb L-emission band ( F i g . 1 , lower p a r t ) .

For h i g h s i l i c o n c o n c e n t r a t i o n s t h e Nb-Nb bonds a r e b r o k e n . The Nb-Si bonds a t h i g h e r b i n d i n g e n e r g i e s b r o a d e n and s h i f t t h e main maximum of t h e XPS s p e c t r a from - 1 . 3 eV t o - 2 . 0 eV. The b r o a d e n i n g i s a l s o o b s e r - ved i n t h e Nb L - e m i s s i o n band o f NbO. 2 S i 0 . 8 ( i n c r e a s e o f FWHM from 3.7 eV t o 4 . 2 eV)

.

F o r NbO. 2 S i 0 . 8 , however, ( i ) t h e Nb-Si bonds d e c r e a s e i n number and c o n s e q u e n t l y t h e i n t e n s i t y o f t h e main maximum o f t h e s p e c t r a ( a t a b o u t - 1 . 3 eV) i s r e d u c e d and ( i i ) new bonds i n t h e

Nbl-xSir s y s t e m , S i - S i b o n d s , a r i s e . We suppose t h a t t h e s e S i - S i bonds s i g n i f i c a n t l y i n c r e a s e t h e i n t e n s i t y o f t h e l o w - b i n d i n g e n e r g y p o r t i o n ( a t a b o u t -2.5 eV) o f t h e S i K-emission bands and c o n s e q u e n t l y b r o a d e n t h e s e s p e c t r a (from 2.9 eV t o 4.8 eV).

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F i g . 1 . u p p e r p a r t : UPS and XPS v a l e n c e F i g . 2 . band s p e c t r a and Nb L- e m i s s i o n band o f u p p e r p a r t :

bcc-Nb. l o w e r p a r t : XPS v a l e n c e band XPS v a l e n c e band s p e c t r u m s p e c t r u m and Nb L- and S i K- e m i s s i o n s and Nb L- and S i K- e m i s s i o n bands o f a-Nbo. 8 S i o . 2 . bands o f a-Nbo. 2 S i o , 8.

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l o w e r p a r t :

I815 1830

ws

w S i K- e m i s s i o n b a n d o f c r y s t a l l i n e

Ftmton h m g y (eV1 s i l i c o n .

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C9-1028 JOURNAL DE PHYSIQUE

To g e t f u r t h e r i n s i d e i n t o t h e b o n d i n g p r o p e r t i e s o f a-Nbl-xSix we p r e s e n t i n F i g . 3 t h e S i K-emission bands o f c - S i and o f NbSi2 ( 5 ) . I n c - S i e a c h atom i s t e t r a h e d r a l l y c o o r d i n a t e d . I n t h e s i l i c i d e NbS12

( C r S i 2 s t r u c t u r e ) e a c h atom i s c o o r d i n a t e d by t e n atoms a t d i f f e r e n t d i s t a n c e s : Nb atom i s s u r r o u n d e d by t e n S i atoms a t d i s t a n c e s 2.59

( f o u r atoms) and 2.76 8 ( s i x a t o m s ) ; S i atom i s s u r r o u n d e d by f i v e Nb atoms and f i v e S i atoms (two Nb atoms and two S i atoms a t 2.59

a,

t h r e e Nb atoms and t h r e e S i atoms a t 2.76

2)

( 6 ) . The atoms i n t h e f i r s t co- o r d i n a t i o n s p h e r e a r o u n d S i form a d i s t o r t e d t e t r a h e d r o n .

The S i K-emission band o f NbSi2 ( F i g . 3 ) e x h i b i t s a f e a t u r e a t 1835 e~

d e r i v e d from t h e n i o b i u m - s i l i c o n d-p bond and a f e a t u r e a t 1831.8 eV which i s t y p i c a l f o r s p 3 h y b r i d i z a t i o n . I n c - S i and a - S i t h e f e a t u r e s c o r r e s p o n d i n g t o t h e s p 3 h y b r i d a r e o b s e r v e d a t 1832.0 eV and 1831.8 eV, r e s p e c t i v e l y . The two f e a t u r e s o b s e r v e d f o r NbSi2 have e q u i v a l e n t s i n t h e S i K-emission bands o f a-Nbo. 2 S i o , 8 and a-Nbo. 8 S i o . 2 . The e n e r g y p o s i t i o n s a t 1835.1 eV and 1831.4 eV a r e c l o s e t o t h e l o c a t i o n s o f t h e

c o u n t e r p a r t s i n t h e S i K-emission band o f NbSi2 (1835.0 eV a n d 1831.8 eV, r e s p e c t i v e l y ) . I t a p p e a r s t h e r e f o r e t h a t t h e Si-Nb bond i s s i m i l a r i n c r y s t a l l i n e NbSi2 and amorphous Nbl+ S i x .

Moreover, t h e two f e a t u r e s i n t h e S i K-emission bonds o f a-Nbo,2Sio,8 and a - N b o . 8 S i o . a r e f i r m l y f i x e d i n t h e i r e n e r g y p o s i t i o n s . The s t a b i - l i t y o f t h e l o c a t i o n s o f b o t h t h e s e f e a t u r e s s u p p o r t s t h e c o n j e c t u r e t h a t t h e Si-Nb bonds i n t h e amorphous s y s t e m Nbl-x S i x do n o t v a r y w i t h i n t h e wide r a n g e o f x .

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(1984)

-

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0 . B i s i and L.W. C h i a o , P f i s . Rev. B25,

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4943 (1982)

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S t r u c t u r e R c p o r t s

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