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Chemical shifts in Auger electron spectroscopy from the initial oxidation of Ta(110)
Volume 30A. number 5
PHYSICS LETTERS
3 November 1969
CHEMICAL SHIFTS IN A U G E R E L E C T R O N SPECTROSCOPY FROM THE INITIAL OXIDATION OF Ta(ll0) T. W. HAAS and J. T. GRANT
Aerospace Research Laboratories, Wright-Patterson AFB, Ohio 45433, USA
Received 18 September 1969
Shifts in energies of Auger electrons from a Ta{ll0} surface have been measured during the early stages (less than one monolayer) of oxidation and are correlated with changes in the low energy electron diffraction patterns.
Shifts in e n e r g i e s of p h o t o e m i t t e d e l e c t r o n s f r o m X - r a y i r r a d i a t e d a t o m s have b e e n c o r r e l a te d with changes in v a l e n c e in the e m i t t i n g a t o m s (ESCA) [1]. T h e s e shifts have a l s o been o b s e r v e d in A u g e r t r a n s i t i o n s f r o m h e a v il y o x i d i z e d m e t a l s u r f a c e s e x c i t e d by X - r a y s [2]. In t h e s e c a s e s the e l e c t r o n s o r i g i n a t e f r o m many l a y e r s n e a r the s u r f a c e of the m a t e r i a l [1]. A uger e l e c t r o n s p e c t r o s c o p y [3] e x a m i n e s the e n e r g y l e v e l s in a t o m s in a s i m i l a r way but the i nit i al ionization t a k e s p l a c e with e l e c t r o n b o m b a r d m e n t r a t h e r than with X - r a y s and a c c o r d ingly is m o r e r e s t r i c t e d to s u r f a c e l a y e r s . We have i n v e s t i g a t e d the A u g e r e l e c t r o n s p e c t r u m f r o m a clean T a ( l l 0 ) s u r f a c e and followed changes in the A u g e r s p e c t r u m as oxidation p r o ceeds. The s y s t e m was a l s o used as a low e n e r gy e l e c t r o n d i f f r a c t i o n (LEED) i n s t r u m e n t f o r m o n i t o r i n g the s t r u c t u r e of the oxide f o r m e d [4]. The s t r u c t u r e s of one m o n o l a y e r or l e s s in c o v e r a g e a r e labeled h e r e as cv, 8, 7 c o r r e s p o n d i n g to figs. 7(b), (c), and (e) r e s p e c t i v e l y of ref. 4. T a bl e 1 g i v es the shifts in e n e r g y of the 164 eV N V NVI NVI A u g e r ( C o s t e r - K r o n i g ) t r a n sition f r o m the Ta as e a c h of t h e s e s t r u c t u r e s is i o r m e d . A final shift of 6 eV is found when Table 1 Shift in energy of NV NVI NV:I Auger transition in Ta due to oxidation Oxide structure (LEED)
Chemical shift (eV}
ot
-o .2
T
-i .5
-o.5
272
the s a m p l e is heavily oxidized by heating in oxygen and white oxide l a y e r s a r e f o r m e d . It is e a s y to see f r o m c o n s e r v a t i o n of e n e r g y c o n s i d e r a t i o n s that w o r k function changes in the t a r g e t will not c a u s e shifts in the m e a s u r e d A u g e r e n e r g i e s as the r e t a r d i n g g r i d in the ene r g y a n a l y z e r and the t a r g e t a r e connected e l e c t r i c a l l y . This point has often b e e n o v e r looked in A u g er i n v e s t i g a t i o n s . The d i r e c t i o n of t h ese o b s e r v e d shifts c o r r e spond to an i n c r e a s e in binding e n e r g y of the e l e c t r o n s involved in the A u g er t r a n s i t i o n . A c c o r d i n g to the work of Siegbahn et al.[1], on ESCA this should be the c a s e when the v a l e n c e state of an atom i n c r e a s e s and is e x p e c t e d for the oxidation of a m e t a l . The i m p o r t a n t point to note is that the shift i n c r e a s e s as the oxygen c o v e r a g e i n c r e a s e s and that m e a s u r a b l e shifts a r e o b s e r v e d f o r l e s s than one m o n o l a y e r c o v e r ag es. T h e s e sh i f t s, when c o r r e l a t e d w i t h the L E E D r e s u l t s , should p r o v i d e a valuable additional tool f o r c h a r a c t e r i z i n g the state of s u r f a c e atoms.
References 1. K.Siegbahn, C.Nordling, A.Fahlman et al. "ESCAatomic, molecular and solid state structure studied by means of electron spectroscopy (Almqvist and Wiksells Boktrycheri AB. Uppsala, 1967) Ser. IV. Vol. 20. 2. C. Nordling, E. Sokolowski and K. Siegbahn, Arkiv Fysik 13 {1958} 483. 3. R.E. Weber and W. T. Peria. J. Appl. Phys. 38 (1967) 4355; P. W. Palmberg and T.N. Rhodin. J. Appl. Phys. 39 {1968) 2425. 4. T.W. Haas, A.G.Jackson and M.P.Hooker, J. Chem. Phys. 46 (1967} 3025.
PHYSICS LETTERS
3 November 1969
CHEMICAL SHIFTS IN A U G E R E L E C T R O N SPECTROSCOPY FROM THE INITIAL OXIDATION OF Ta(ll0) T. W. HAAS and J. T. GRANT
Aerospace Research Laboratories, Wright-Patterson AFB, Ohio 45433, USA
Received 18 September 1969
Shifts in energies of Auger electrons from a Ta{ll0} surface have been measured during the early stages (less than one monolayer) of oxidation and are correlated with changes in the low energy electron diffraction patterns.
Shifts in e n e r g i e s of p h o t o e m i t t e d e l e c t r o n s f r o m X - r a y i r r a d i a t e d a t o m s have b e e n c o r r e l a te d with changes in v a l e n c e in the e m i t t i n g a t o m s (ESCA) [1]. T h e s e shifts have a l s o been o b s e r v e d in A u g e r t r a n s i t i o n s f r o m h e a v il y o x i d i z e d m e t a l s u r f a c e s e x c i t e d by X - r a y s [2]. In t h e s e c a s e s the e l e c t r o n s o r i g i n a t e f r o m many l a y e r s n e a r the s u r f a c e of the m a t e r i a l [1]. A uger e l e c t r o n s p e c t r o s c o p y [3] e x a m i n e s the e n e r g y l e v e l s in a t o m s in a s i m i l a r way but the i nit i al ionization t a k e s p l a c e with e l e c t r o n b o m b a r d m e n t r a t h e r than with X - r a y s and a c c o r d ingly is m o r e r e s t r i c t e d to s u r f a c e l a y e r s . We have i n v e s t i g a t e d the A u g e r e l e c t r o n s p e c t r u m f r o m a clean T a ( l l 0 ) s u r f a c e and followed changes in the A u g e r s p e c t r u m as oxidation p r o ceeds. The s y s t e m was a l s o used as a low e n e r gy e l e c t r o n d i f f r a c t i o n (LEED) i n s t r u m e n t f o r m o n i t o r i n g the s t r u c t u r e of the oxide f o r m e d [4]. The s t r u c t u r e s of one m o n o l a y e r or l e s s in c o v e r a g e a r e labeled h e r e as cv, 8, 7 c o r r e s p o n d i n g to figs. 7(b), (c), and (e) r e s p e c t i v e l y of ref. 4. T a bl e 1 g i v es the shifts in e n e r g y of the 164 eV N V NVI NVI A u g e r ( C o s t e r - K r o n i g ) t r a n sition f r o m the Ta as e a c h of t h e s e s t r u c t u r e s is i o r m e d . A final shift of 6 eV is found when Table 1 Shift in energy of NV NVI NV:I Auger transition in Ta due to oxidation Oxide structure (LEED)
Chemical shift (eV}
ot
-o .2
T
-i .5
-o.5
272
the s a m p l e is heavily oxidized by heating in oxygen and white oxide l a y e r s a r e f o r m e d . It is e a s y to see f r o m c o n s e r v a t i o n of e n e r g y c o n s i d e r a t i o n s that w o r k function changes in the t a r g e t will not c a u s e shifts in the m e a s u r e d A u g e r e n e r g i e s as the r e t a r d i n g g r i d in the ene r g y a n a l y z e r and the t a r g e t a r e connected e l e c t r i c a l l y . This point has often b e e n o v e r looked in A u g er i n v e s t i g a t i o n s . The d i r e c t i o n of t h ese o b s e r v e d shifts c o r r e spond to an i n c r e a s e in binding e n e r g y of the e l e c t r o n s involved in the A u g er t r a n s i t i o n . A c c o r d i n g to the work of Siegbahn et al.[1], on ESCA this should be the c a s e when the v a l e n c e state of an atom i n c r e a s e s and is e x p e c t e d for the oxidation of a m e t a l . The i m p o r t a n t point to note is that the shift i n c r e a s e s as the oxygen c o v e r a g e i n c r e a s e s and that m e a s u r a b l e shifts a r e o b s e r v e d f o r l e s s than one m o n o l a y e r c o v e r ag es. T h e s e sh i f t s, when c o r r e l a t e d w i t h the L E E D r e s u l t s , should p r o v i d e a valuable additional tool f o r c h a r a c t e r i z i n g the state of s u r f a c e atoms.
References 1. K.Siegbahn, C.Nordling, A.Fahlman et al. "ESCAatomic, molecular and solid state structure studied by means of electron spectroscopy (Almqvist and Wiksells Boktrycheri AB. Uppsala, 1967) Ser. IV. Vol. 20. 2. C. Nordling, E. Sokolowski and K. Siegbahn, Arkiv Fysik 13 {1958} 483. 3. R.E. Weber and W. T. Peria. J. Appl. Phys. 38 (1967) 4355; P. W. Palmberg and T.N. Rhodin. J. Appl. Phys. 39 {1968) 2425. 4. T.W. Haas, A.G.Jackson and M.P.Hooker, J. Chem. Phys. 46 (1967} 3025.