FINE STRUCTURE IN EELS FROM RARE EARTH SESQUIOXIDE THIN FILMS

HAL Id: jpa-00224013

https://hal.archives-ouvertes.fr/jpa-00224013

Submitted on 1 Jan 1984

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

FINE STRUCTURE IN EELS FROM RARE EARTH SESQUIOXIDE THIN FILMS

L. Brown, C. Colliex, M. Gasgnier

To cite this version:

L. Brown, C. Colliex, M. Gasgnier. FINE STRUCTURE IN EELS FROM RARE EARTH SESQUIOXIDE THIN FILMS. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-433-C2-436.

�10.1051/jphyscol:1984298�. �jpa-00224013�

JOURNAL DE PHYSIQUE

Colloque C2, suppl6ment a u n02, Tome 45, fgvrier 1984 page C2-433

F I N E STRUCTURE I N EELS FROM RARE EARTH SESQUIOXIDE T H I N F I L M S

L.M. ~rown', C , ~olliex* and M. ~as~nier*'

'~aboratoire de Physique des SoZides, Uniuersitl de Paris-Sud, 91405 Orsay, France

** _{ER 210, C.N.R.S.} BeZZeuue, 92195 Meudon Cedex PrincipaZ, France

RESUME: Les s t r u c t u r e s f i n e s des s e u i l s d'absorption e n t r e 0 e t 2 k e V des sesquioxydes d e terres r a r e s 2 lsL?tat d e couches minces o n t 6t6 6 t u d i L s par l a technique d e la spectroscopie des p e r t e s d'6nergie glectronique en u t i l i s a n t un STEM.

ABSTRACT: Pine s t r u c t u r e o f absorption edges between 0 and 2 kev from rare-earth sesquioxide t h i n films w e r e studied by e l e c t r o n energy-loss spectroscopy i n STEM.

Rare-earth t h i n f i l m s (approximately 40 nm t h i c k ) are deposited by Joule evaporation onto NaCQ s u b s t r a t e s , under a vacuum o f ^10-4 Pa. After f l o a t i n g o f f the f i l m s i n water, they are heated i n s i d e an e l e c t r o n microscope by means o f the focused e l e c t r o n beam. I n this way it is easy t o obtain

-

'fundamental' o r band gap, plasmon peaks, N,, and M,, f o r the rare-earth, and K f o r the oxygen. Previous s t u d i e s by Krivanek ( 2 ) show q u i t e d e f i n i t e v a r i a b l e f i n e s t r u c t u r e b u t it is not clear from which s t r u c t u r e the s p e c t r a come.

Summary o f Preliminarv Results

1 ) Band gap: i n mast cases t h e low-loss s p e c t r a f r o m l a r g e c r y s t a l s show a well-defined band-gap. This measure o f t h e gap should correspond t o the value obtained from U-V absorption s t u d i e s . For a l l t h e rare-earth oxides o u r values are i n t h e range 4.5 e V t o 6 eV. They agree w i t h those estimated by Cukier ( 3 ) but not with t h e majority of values obtained by o t h e r methods ( 4 ) which a r e much t o o small and correspond t o w h a t might he expected from impurities o r t o an i n d i r e c t gap.

Sometimes one s e e s energy-loss f i n e s t r u c t u r e within t h e gap, corresponding t o loosely bound e l e c t r o n s associated perhaps with impurities o r impurity r a d i c a l s (H, OH, CO,) b u t without very c a r e f u l f u r t h e r c h a r a c t e r i s a t i o n such s t r u c t u r e s are d i f f i c u l t t o i n t e r p r e t .

2) Plasmon peak: These show t h e same configuration (shape and p o s i t i o n ) as those already reported ( 5 ) . However, one can see very c l e a r l y a point of i n f l e x i o n which separates t h e parabolic rise a f t e r t h e band gap from t h e c o l l e c t i v e exciton o r plasmon.

+on leave from Cavendish Laboratory, Cambridge CB3 OHE, U.K.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984298

C2-434 JOURNAL DE PHYSIQUE

3 ) Rare-earth N,, edger It is d i f f i c u l t t o s t a t e any d e f i n i t e

behaviour o f the N,, edge, which appears as a very broad rounded edge with a v a r i a b l e f i n e s t r u c t u r e depending on c r y s t a l s t r u c t u r e . The i n t e r p r e t a t i o n o f t h i s edge is complicated by t h e c e n t r i f u g a l p o t e n t i a l and by t h e strong coupling between t h e 4f e l e c t r o n s and the hole created by t h e fast electron.

4 ) Rare-earth M,, edge: These edges are characterised by well-defined

'white l i n e s ' as shown by Ahn and Krivanek ( 2 ) . Examples a r e shown i n Fig. 1 f o r La, Tb and Lu at t h e beginning, middle and end o f t h e lanthanide series. Several e f f e c t s can be seen: ( i ) The width of t h e white l i n e s a t about 5 eV is not controlled by t h e r e s o l u t i o n o f the spectmmeter which is better than 1.5 e V under these conditions. ( i i ) The separation between the M, and M, peaks increases systematically across the series; this i s shown graphically i n Fig. 2. ( i i i ) The ratio o f a r e a i n t h e M, peak t o that i n t h e M, peak v a r i e s s y s t e a a t i c a l l y from near u n i t y at the beginning of the series t o near i n f i n i t y at t h e end (Lu), where t h e M, peak h a s p r a c t i c a l l y disappeared. This is shown graphically in Fig. 2. I n c a l c u l a t i n g t h e r a t i o , one has a background s u b t r a c t i o n problem, p a r t i c u l a r l y near t h e end o f t h e series where t h e satellite plasmon r e s u l t i n g from t h e M, white l i n e nearly coincides with t h e M, peak. This r e s u l t s i n r a t h e r l a r g e errors o f t h e M,/M, r a t i o as ahown i n Fig. 2.

From a t h e o r e t i c a l point of view, t h e r e are s e v e r a l f a c t o r s i n understanding t h e s e r e s u l t s :

( i ) Across the lanthanide series, the contraction of the shells, due t o increasing atomic number, w i l l increase t h e s p i n - o r b i t s p l i t t i n g of t h e h o l e i n the 3d s h e l l , i.e. t h e M,-M* separation.

( i i ) I f t h e d e n s i t y of i n i t i a l s t a t e s ( i.e. the spin-orbit s p l i t hole i n t h e 36 s h e l l ) c o n t r o l s the M&, r a t i o , t h a t r a t i o should be 3/2, independent o f atomic number. This is c l e a r l y not the case.

( i i i ) It must be expected that t h e magnetic moment o f t h e hole is coupled by exchange t o the e l e c t r o n s i n t h e 4f s h e l l , and that as t h e 4f shell f i l l s , t h e magnitude o f the ' e f f e c t i v e s p i n ' contributed by the 4f e l e c t r o n s increases.

This w i l l s t r o n g l y a f f e c t the d e n s i t y o f s t a t e s a v a i l a b l e f o r t h e t r a n s i t i o n s . 5 ) Omgen K-edge Throughout t h e s e r i e s , it was found that the near-edge s t r u c t u r e of this edge depends primarily on crystal s t r u c t u r e . It is c h a r a c t e r i s t i c a l l y doubled i n the cubic s t r u c t u r e , t h e magnitude o f t h e doubling being about 5 eV. This appearance is commDn f o r a l l t h e rare earth sesquioxides including Si,O, and Y,O,, Figure 3 shows t h e e f f e c t f o r C-Lu,O,. I n t h e monoclinic ( B ) s t r u c t u r e , the edge is rounded, without peaks, as shown f o r B-Gd20, (Fig.3). I n the hexagonal ( A ) s t r u c t u r e , t h e edge presents f i r s t l y a plateau and f i n a l l y a s m a l l peak, as shown f o r A-La,O, (Fig. 3 ) . Enough s p e c t r a have been studied t o a s s e r t that t h i s s t r u c t u r e does not depend on g r a i n s i z e , nor does it depend markedly on crystal o r i e n t a t i o n , although no c a r e f u l o r i e n t a t i o n s t u d i e s with well-defined angular s e t t i n g s have as y e t been carried out. 'Phe observed e f f e c t s t h u s show a n instance where near-edge s t r u c t u r e ( t h e e l e c t r o n energy-loss equivalent of XANES, sometimes c a l l e d =S) can be c o r r e l a t e d i n a s u r p r i s i n g l y d i r e c t and simple way with c r y s t a l s t r u c t u r e .

The i n t e r p r e t a t i o n of these r e s u l t s seems d i f f i c u l t . The e f f e c t is not due t o c r y s t a l f i e l d s p l i t t i n g , f o r the oxygen atoms i n these s t r u c t u r e s do not sit at p o i n t s with well-defined symrmetry. Furthermore, the magnitude of t h e s p l i t t i n g is large, as l a r g e as the band-gap. The oxygen ions i n t h e s e sesquioxides are not considered t o have mixed valency. However t h e oxygen ions do sit i n d i f f e r i n g rare-earth environments which w i l l lower ^by d i f f e r i n g amounts t h e Coulomb energy o f t h e h o l e created by the fast e l e c t r o n . Because o f t h e l a r g e p o l a r i s a b i l i t y of the rare-earth ion, l a r g e changes i n t h e Coulomb energy can r e s u l t . Detailed c a l c u l a t i o n s are required.

Fig. 1 M,, peaks for A -La,O,, B - ^Tb,O, and C - ^Lu,O,

structures respectively.

0 1 3 4 6 7 91011121314

La Gd Tb Lu

no. of 4f e l e c t r o n s

+

Pig. 2 Variation in separation and area ratio of the M, and M, white lines across the series of lanthanide oxides.

JOURNAL DE PHYSIQUE

CONCLUSIONS

With t h e Gatan spectrometer on a VG H8501 STEM, one can begin t o & e m energy-loss n e a r edge s t r u c t u r e (EWES) which i n t h e case o f t h e oxygen K-edge correlates i n a s u r p r i s i n g l y simple way w i t h t h e crystal s t r u c t u r e .

L. M. Brown acknowledges w i t h p l e a s u r e t h e splendid h o s p i t a l i t y o f t h e ST&M group a t Orsay.

REFEPEUCES

1. M. Gasgnier,Phys. S t a t u s S o l . ( a ) 57 (1980) 11.

2. 0. L. Krivanek, unpublished; see a l s o C. C. Ahn and 0. L. Krivanek, Catalogue o f EELS, o b t a i n a b l e from Arizona S t a t e University, Center f o r S o l i d S t a t e Science.1982

3. M. Cukier, B. Gauthe, V. Wehenkel, J. Physique 41 (1980) 603 4. W. H. Strenlow and E. L. Cook, J. Phys. Chem. R e f . Data, Vo1.2

( 1973) 397

5. C. C o l l i e x , M. Gasgnier, P. Trebbia, 1976, J. d e Physique 37

(1976) 397.

Fig. 3 0 - K peaks f o r A - Laz03, B - GdzO, and C - Lu,O, s t r u c t u r e s r e s p e c t i v e l y .