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OXYGEN AUGER EMISSION AND FINAL STATE SCREENING IN OXIDES
P. Humbert, J. Deville
To cite this version:
P. Humbert, J. Deville. OXYGEN AUGER EMISSION AND FINAL STATE SCREENING IN OX- IDES. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-927-C9-930. �10.1051/jphyscol:19879165�.
�jpa-00227279�
OXYGEN AUGER EMISSION AND FINAL STATE SCREENING IN OXIDES
P. HUMBERT and J.P. DEVILLE
Equipe d'Etude des Surfaces, CNRS UA-795, Universit6 Louis Pasteur, 4, Rue Blaise Pascal, F-67000 Strasbourg, France
R b w n s ' - L ' i n t e r a c t i o n e f f e c t i v e U e n t r e l e s deux Zacunes de l'e'tat f i n a l d e s t r a n s i t i o n s OKLL peut
e^P&f
calcule'e d p a r t i r des e'nergies de l i a i s o n e t des e'nergies cinktiques Auger obtenues par XPS/XAES. Des re'sultats cohe'rents ne sont obtenus qu'en tenant compte duns ce mode'le du caractDre Ziant ou non-liant des e'tats 02p. Nous en dgduisons que l e s t r a n s i t i o n s Auger OKLL quasi-atomiques faisant i n t e r v e n i r l a bande de valence sont de'terminkes par l a d e n s i t k d ' k t a t s locale.Abstract - Using a three-step model t o describe t h e Auger process, t h e e f f e c t i v e i n t e r a c t i o n U between t h e two f i n a l - s t a t e holes of t h e 0 KLL t r a n s i t i o n s can be deri@ from binding and Auger k i n e t i c energies measured by XPS/XAES. Consistent r e s u l t s were only found i f t h e bonding character of t h e O2p-derived o r b i t a l s was included i n t h e model. We thus concZude t h a t t h e quasi-atomic 0 KLL Auger t r a n s i t i o n s involving t h e valence band are determined by t h e local d e n s i t y of s t a t e s .
1 . I n t r o d u c t i o n
I t i s g e n e r a l l y n o t e d 'chat t h e o v e r a l l shape o f t h e KLL Auger s p e c t r a o f oxygen, e i t h e r r e c o r d e d on b u l k o x i d e s o r on oxygen chemisorbed on metal s u r f a c e s , i s q u i t e s i m i l a r . I n p a r t i c u l a r , Auger t r a n s i t i o n s i n v o l v i n g t h e v a l e n c e band ( i . e . 0 KVV o r 0 K L I V ) r e v e a l no b a n d - s t r u c t u r e induced f e a t u r e s i n t e r m s o f c o n v o l u t i o n o f t h e d e n s i t y o f o c c u p i e d v a l e n c e s t a t e s . On a n o t h e r hand t h e r e i s a s t r o n g s i m i l a r i t y between t h e KLL s p e c t r a o f oxygen and a t o m i c neon whose 2p s h e l l i s c o m p l e t e l y f i l l e d . The KLL s p e c t r u m o f oxygen i s t h e r e f o r e termed q u a s i - a t o m i c i n c o n t r a s t t o b a n d - l i k e Auger s p e c t r a . The q u a s i - a t o m i c n a t u r e o f t h e s e 0 KLL s p e c t r a can be e x p l a i n e d by e x t r a - a t o m i c s c r e e n i n g , due t o t h e p o l a r i z a t i o n o f t h e s u r r o u n d i n g e l e c t r o n g a s , r a t h e r t h a n by i n i t i a l - s t a t e i o n i c i t y .
The e f f e c t i v e i n t e r a c t i o n between t h e t w o f i n a l s t a t e h o l e s , c a l l e d U ( k l ) , has an i m p o r t a n t r o l e i n t h e q u a s i - a t o m i c b e h a v i o u r o f t h e ( j k l ) Auger t f d f i s i t i o n s and i s e x p e c t e d t o v a r y w i t h t h e chemical e n v i r o n m e n t o f t h e oxygen atoms as a r e s u l t o f s c r e e n i n g c o n t r i b u t i o n d i f f e r e n c e s . T h u s , i n a p r e l i m i n a r y s t u d y / I / , we a t t e m p t e d t o c h a r a c t e r i z e v a r i o u s o x i d e s by t h e i r U v a l u e s t h a t we d e r i v e d e x p e r i m e n t a l l y f r o m ESCA measurements u s i n g a three%$ep model t o d e s c r i b e t h e Auger p r o c e s s . The r e s u l t s suggested t h a t e x t r a - a t o m i c r e 1 a x a t i o n i n c r e a s e s w i t h i n c r e a s i n g i o n i c i t y f o r n o n - t r a n s i t i o n - m e t a l o x i d e s . M o r e o v e r , v e r y l a r g e s c r e e n i n g e f f e c t s w e r e observed f o r t r a n s i t i o n metal o x i d e s (Tl~lOs) and w e r e e x p l a i n e d by an e f f i c i e n t s c r e e n i n g c o n t r i b u t i o n o f t h e metal d-band. We r e c e n t l y c o n f i r m e d t h i s g r e a t e r e f f i c i e n c y o f d-band s c r e e n i n g o v e r s- o r p-band s c r e e n i n g by a f u r t h e r i n v e s t i g a t i o n o f a number o f TMOs /2/. We showed f u r t h e r m o r e t h a t d-band s c r e e n i n g b r e a k s o f f as t h e metal d-band i s f i l l e d and a c t u a l l y s e p a r a t e d f r o m t h e v a l e n c e band. I n t h i s p a p e r , we c a r r y on o u r work on non-TMOs t o c l a r i f y t h e d i f f e r e n c e s i n s c r e e n i n g b e h a v i o u r observed on t h e s e o x i d e s .
2 . E x p e r i m e n t a l
A p a r t f r o m MgO ( s i n g l e c r y s t a l ) , a l l samples (A1 0
,
BeO, GeO$, SiOa, T i O a ) w e r e powders. S t r u c t u r e and q u a l i t y o f t h e s e h i g h ? w i t $ i a t e r i a l s w r e c h cked yArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879165
C9-928 JOURNAL DE PHYSIQUE
X-ray d i f f r a c t i o n . The s i n g l e c r y s t a l was cleaned i n s i t u by low energy i o n bombardment. Powders were n o t s u b j e c t e d t o any c l e a n i n g t r e a t m e n t .
B i n d i n g and Auger k i n e t i c energy d a t a were o b t a i n e d w i t h a VG ESCA M k I I device, equipped w i t h d a t a a c q u i s i t i o n f a c i l i t i e s u s i n g A1 o r Mg
$
r a d i a t i o n s . Voltage c a i i b r a t i u n was achieved a c c o r d i n g t o t h eNPL
s t a n d a r d procedure /3/. The Ar2p o r C l s peaks were used t o c o r r e c t f o r c h a r g i n g e f f e c t s /4/ so t h a t a l l e n e r g i e s c o u l d be r e f e r e n c e d t o t h e Fermi l e v e l o f t h e sample.S i n c e non-monochromatized r a d i a t i o n s were used, we p a i d a t t e n t i o n t o t h e p o s s i b l e o v e r l a p o f X-ray s a t e l l i t e s w i t h t h e 02s peak and t h e valence band d i s t r i b u t i o n . I n such cases, s u b t r a c t i o n o f t h e o v e r l a p p i n g s a t e l l i t e was achieved u s i n g t o t h e d a t a g i v e n i n r e f e r e n c e 5. I n a d d i t i o n , v a l e n c e band s p e c t r a underwent i n e l a s t i c background s u b t r a c t i o n . T h i s s h o r t paper does n o t a l l o w us t o p r e s e n t e i t h e r t h e c o r e l e v e l b i n d i n g energy values, o r t h e valence band s p e c t r a we o b t a i n e d on t h e s e oxides. Our photoemission r e s u l t s were i n g e n e r a l agreement w i t h t h o s e found i n t h e l i t e r a t u r e . F o r t h e same reason, we s h a l l p u b l i s h elsewhere o u r r e s u l t s on t h e Auger 0 KLL f i n e s t r u c t u r e o f t h e s e o x i d e s . The r e a d e r i s i n v i t e d t o r e f e r t o o u r p r e v i o u s papers f o r l i n e i d e n t i f i c a t i o n , o v e r a l l shape and u t i l i z a t i o n i n t h e p r e s e n t c o n t e x t o f t h e oxygen Auger s p e c t r a .
3. The t h r e e - s t e p model
A c c o r d i n g t o t h e t h r e e - s t e p model d e s c r i b i n g t h e Auger process /6,7/, t h e k i n e t i c energy o f an Auger e l e c t r o n e m i t t e d i n a (jk1;X) t r a n s i t i o n can be w r i t t e n as :
E A (jkl;X)=EB(j)-EB(k)-EB(l)-F(kl;X)+RC(kl) ( 1 ) where E B ( i ) denotes t h e b i n d i n g energy o f l e v e l i and F(k1;X) t h e unscreened two-hole i n t e r a c t i o n term. R C ( k l ) accounts f o r t h e changes b o t h i n b i n d i n g and r e l a x a t i o n energy o f l e v e l 1 due t o t h e presence o f t h e s p e c t a t o r k - h o l e . B o t h terms i n t r o d u c e d t o c o r r e c t f o r t h e t w o - h o l e f i n a l s t a t e a r e o f t e n grouped t o g e t h e r t o f o r m t h e r e l a x e d t w o - h o l e i n t e r a c t i o n energy UeGff(kl) :
U P f f ( k l ) = F(k1;X)
-
R c ( k l ) ( 2 )- . .
U s i n g U f f ( k l ) i n e q u a t i o n ( 1 ) saves us f r o m making any assumption c o n c e r n i n g F ( k 1 ; ~ ) e a t u e s .
The d i f f e r e n t U ( k l ) v a l u e s o b t a i n e d f o r each o x i d e and f o r each o f t h e t h r e e main Auger l i n e s e f f u ~ (ID), KL L ( I P ) and ULIL1(ls) - can be r e a d i l y d e r i v e d by r e p 1 a c i ng t h e m 2 8 2 e d b i n d i ngl aZd3luger k i n e t i c e n e r g i e s i n e q u a t i o n ( 1
1.
The L l e v e l belongs however t o t h e v a l e n c e band...
~ e t " d s c o n s i d e r U ( L L ) f i r s t . T h i s q u a n t i t y i s independent f r o m t h e c h o i c e made f o r t h e 02p b%&in& h e r g y and g i v e s i n f o r m a t i o n about t h e importance o f e x t r a - a t o m i c s c r e e n i n g e f f e c t s s i n c e t h e f i n a l s t a t e h o l e s o f t h e 0 KL L1 t r a n s i t i o n s a r e c e r t a i n l y l o c a l i z e d . Table 1 l i s t s t h e U ( L i ) v a l u e s obta\ned i n t h e p r e s e n t work on non-TMOs and p r e v i o u s l y on HgO aniffLnO! I t i s n o t e d on one hand t h a t t h e v a l u e s f o r non-TMOs a r e about 3.5 eV h i g h e r t h a n f o r TMOs and, on t h e o t h e r hand, t h a t t h e y a r e q u i t e c l o s e t o t h o s e o b t a i n e d on b o t h f i l l e d d - s h e l l oxides. T h i s r e s u l t i s i n agreement w i t h o u r p r e v i o u s c o n c l u s i o n s /2/ : valence d - e l e c t r o n s i n TMOs a l l o w f o r an e f f i c i e n t s c r e e n i n g o f t h e f i n a l s t a t e h o l e s whereas l e s s r e l a x e d f i n a l s t a t e s
-
and t h u s h i g h e r Ueff v a l u e s - a r e reached i f t h e v a l e n c e band c o n t a i n s no d - e l e c t r o n s .-
F i r s t approachL e t us t u r n now t o t h e t r a n s i t i o n s i n v o l v i n g t h e valence band. F o r most TMOs, t h e 02p p a r t i a l d e n s i t y o f s t a t e s l e a d s t o a s i n g l e , n e a r l y symmetric sharp band and t h e 02p b i n d i n g energy r e q u i r e d t o c a l c u l a t e Ueff can be s a f e l y measured a t t h e band maximum /2/. On t h e o t h e r hand, t h e 02p s t a t e s i n non-TMOs a r e s t r o n g l y c o u p l e d t o t h e c a t i o n s o r p s t a t e s t o f o r m h y b r i d i z e d s t a t e s d i s t r i b u t e d a l l a l o n g t h e v a l e n c e band. I n t h i s case how t o choose t h e 02p b i n d i n g energy v a l u e s t o d e t e r m i n e U ( k l ) f o r 0 KLL t r a n s i t i o n s i n v o l v i n g v a l e n c e e l e c t r o n s i s a m a j o r problem. I n p r e l i m i n a r y s t u d y /I/, we used a s i m p l e approach : t h e r e q u i r e d v a l u e s were measured a t t h e c e n t e r o f t h e v a l e n c e band d i s t r i b u t i o n .
It i s t h e n s u r p r i s i n g t o n o t e t h a t t h e U f(VV) and U ( L V) v a l u e s f o r non-TMOs a r e r e s p e c t i v e l y about 4.0 eV and 1.5
8f
l o w e r thane% L l o s e d d - s h e l l oxides, i n s p i t e o f t h e absence o f valence d - e l e c t r o n s . The p a r t i c u l a r cases o fs t r u c t u r e . I n consequence s i m i 1 a r s c r e e n i n g b e h a v i o u r and U v a l u e s a r e e x p e c t e d . N e v e r t h e l e s s t h e U ( V V ) v a l u e s w e r e f o u n d t o be 9.1 eve% Be0 and 7 . 9 eV f o r IiigG, t o be compared $if'& t h e c o r r e s p o n d i n g v a l u e s o f Hg0 and ZnO g i v e n i n T a b l e 1 . These c o n f l i c t i n g r e s u l t s l e a d t o c a s t i n t o d o u b t t h e way we c h o s e t h e 0 2 p b i n d i n g e n e r g y .
HgO 11.7 13.5 11 .O
ZnO 1 1 . 6 14.5 1 1 . 0
T a b l e 1 .
-
E f f e c t i v e i n t e r a c t i o n U ( k l ) o f Auger 0 XLL t r a n s i - t i o n s i n d i f f e r e n t n o n - t r a n s i t i o n - 6 z f a1 o x i d e s . The U e f f ( V V ) and U f f ( L I V ) v a l u e s a r e c a l c u l a t e d u s i n g t h e b i n d i n g e n e r g y o f t h e l g a s t bound O 2 p - s t a t e s . A1 1 v a l u e s a r e g i v e n i n eV.-
Second approachThe 0 2 p v a l e n c e e l e c t r o n s have been s o f a r c o n s i d e r e d as a w h o l e , w i t h o u t any d i s t i n c t i o n between b o n d i n g o r non-bonding c h a r a c t e r o f t h e v a l e n c e band. On a n o t h e r hand t h e U f f v a l u e s c a l c u l a t e d i n t h e f i r s t approach a r e o b v i o u s l y t o o low t o be i n a g r e e g e n t w i t h o u r p r e v i o u s r e s u l t s on TklOs. T h i s means t h a t t h e 02p b i n d i n g e n e r g i e s have been o v e r e s t i m a t e d . T a k i n g i n t o a c c o u n t t h e s e t w o r e m a r k s , we r e c a l c u l a t e d t h e U f f v a l u e s u s i n g t h e b i n d i n g e n e r g i e s o f t h e l e a s t bound 02p s t a t e s ( n o n - b o n d i n g o? 11-bonding c h a r a c t e r ) . F o r t h i s p u r p o s e , t h e v a l e n c e band d i s t r i b u t i o n s o f t h e d i f f e r e n t non-TMOs have been decomposed i n c o n t r i b u t i o n s o f d i f f e r e n t b o n d i n g c h a r a c t e r a c c o r d i n g t o t h e c o r r e s p o n d i n g l i t e r a t u r e r e f e r e n c e s / 8 - l l / .
The r e s u l t s worked o u t f r o m t h i s second approach a r e l i s t e d i n T a b l e 1 . A much b e t t e r agreement t h a n i n t h e f i r s t approach c a n be n o t e d . The U v a l u e s a r e i n d e e d q u i t e c l o s e t o t h o s e d e t e r m i n e d on HgO and ZnO. They a r e onefthe c o n t r a r y v e r y d i f f e r e n t f r o m t h o s e o b t a i n e d f o r TMOs on a c c o u n t o f t h e l a c k o f v a l e n c e d - e l e c t r o n s . We c o n c l u d e t h a t c o n s i s t e n t r e s u l t s can o n l y be o b t a i n e d i f we t a k e i n t o a c c o u n t t h e b o n d i n g c h a r a c t e r o f t h e h y b r i d i z e d 02p s t a t e s i n t h e a p p l i e d t h r e e - s t e p model.
4. Conclusion
I n t h e c o n t e x t o f t h e t h r e e - s t e p m o d e l , t h e consequences o f t h i s s a t i s f a c t o r y second approach a r e o f g r e a t i m p o r t a n c e . C o n s i d e r i n g o n l y t h e l e a s t bound s t a t e s t o r e p r e s e n t t h e 02p l e v e l i n t h e OKLL t r a n s i t i o n s i n v o l v i n g v a l e n c e e l e c t r o n s means t h a t t h e Auger t r a n s i t i o n p r o b a b i l i t i e s a r e n o t u n i f o r m o v e r t h e w h o l e v a l e n c e band. The f a v o u r e d Auger t r a n s i t i o n s a r e t h e ones t h a t l i i a i n l y i n v o l v e t h e non-bonding 0 2 p s t a t e s , i n o t h e r w o r d s t h e l o c a l d e n s i t y o f s t a t e s . T h i s has a l r e a d y been e x p e r i m e n t a l l y observed f o r b a n d - l i k e s p e c t r a b u t , t o o u r knowledge, not y e t f o r q u a s f - a t a m f c s p e c t r a .
C9-930 JOURNAL DE PHYSIQUE
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