STRUCTURAL PROPERTIES OF a-Si AND a-Si : H BY EXAFS

(1)

HAL Id: jpa-00226195

https://hal.archives-ouvertes.fr/jpa-00226195

Submitted on 1 Jan 1986

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are published 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.

STRUCTURAL PROPERTIES OF a-Si AND a-Si : H BY EXAFS

A. Filipponi, D. Della Sala, F. Evangelisti, A. Balerna, S. Mobilio

To cite this version:

A. Filipponi, D. Della Sala, F. Evangelisti, A. Balerna, S. Mobilio. STRUCTURAL PROPERTIES OF a-Si AND a-Si : H BY EXAFS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-375-C8-377.

�10.1051/jphyscol:1986873�. �jpa-00226195�

(2)

JOURNAL DE PHYSIQUE

C o l l o q u e C 8 , s u p p l h e n t au n o 1 2 , T o m e 47, d h c e m b r e 1986

STRUCTURAL PROPERTIES OF a-Si AND a-Si:H BY EXAFS(')

A. FILIPPONI, D. DELLA SALA, F. EVANGELISTI, A. BALERNA* and S. MOBILIO*

Dipartimento d i Fisica, ~ n i v e r s i t s degli studi d i Roma "La Sapienza", Piazzale Aldo Moro 2 , I-00185 Roma, Italy

* I N F N - ~ a b o r a t o r i Nazionali di Frascati, C.P. 13, 1-00044 Frascati, Italy

A b s t r a c t . - ^Acomparative EXAFS s t u d y of t h e f i r s t - s h e l l s i g n a l s of c-Si, a - S i and a-Si:H i s p r e s e n t e d . F i r s t - n e i g h b o u r c o o r d i n a t i o n , d i s t a n c e and Debye-Waller f a c t o r a r e r e p o r t e d and d i s c u s s e d .

I n t r o d u c t i o n . - D e s p i t e t h e widespread i n t e r e s t i n t h e c h a r a c t e r i z a t i o n of amorphous S i , both hydrogenated and n o t , t h e u s e of EXAFS t e c h n i q u e f o r s t u d y i n g i t s s t r u c t u r e h a s been s c a r c e [ I ] . This f a c t c o n t r a s t s with t h e c o n s i d e r a b l e amount of work a l r e a d y done on a-Ge [ 2 ] . P o s s i b l e e x p l a n a t i o n s r e s i d e on t h e more advantageous energy p o s i t i o n of t h e Ge K-edge from t h e e x p e r i m e n t a l p o i n t of view and on t h e g r e a t e r d i f f i c u l t y i n a n a l y z i n g s i g n a l s due t o l i g h t b a c k s c a t t e r e r s .

Quite t o t h e c o n t r a r y , s e v e r a l d i f f r a c t i o n works on a-Si and a-Si:H have a l r e a d y been performed [ 3 ] . It i s of g r e a t i n t e r e s t , t h e r e f o r e , t o compare t h i s i n f o r m a t i o n w i t h t h e one t h a t can be o b t a i n e d by EXAFS.

Experimental. - The S i K-edge a b s o r p t i o n s p e c t r a were taken a t LURE ( O r s a y ) , u s i n g t h e beam l i n e equipped w i t h a InSb d o u b l e - c r y s t a l monochromator a t ACO s t o r a g e r i n g . The f o l l o w i n g samples were i n v e s t i g a t e d : two a-Si:H f i l m s w i t h d i f f e r e n t hydrogen c o n t e n t (14% and 21% r e s p e c t i v e l y , a s determined by t h e i n t e n s i t y of t h e Si-Hn wagging band a t 630 p r e p a r e d by glow-discharge; one a-Si f i l m grown by s p u t t e r i n g ; one c-Si sample p r e p a r e d by e v a p o r a t i o n and a n n e a l i n g , t h a t was used a s a model compound.

Data A n a l y s i s - ^The ^EXAFS s i g n a l was e x t r a c t e d by s u b t r a c t i n g a n a t o m i c - l i k e polynomial background from t h e s p e c t r a and t h e n normalized u s i n g t h e f a c t o r

[J-(1-8(E-Eo)/3Eo) 1 - 1 [ 4 ] . The o r i g i n Eo of k s c a l e was p l a c e d i n t h e middle of t h e s t e e p r i s e of t h e a b s o r p t i o n c o e f f i c i e n t . The jump J was e s t i m a t e d a s t h e d i f f e r e n c e of t h e mean v a l u e po of t h e a b s o r p t i o n c o e f f i c i e n t above and below t h e w h i t e l i n e , b o t h e x t r a p o l a t e d a t Eo. J / d i s p r o p o r t i o n a l t o t h e d e n s i t y of t h e sample. A h i g h r e p r o d u c i b i l i t y among d i f f e r e n t measurements on t h e same sample was o b t a i n e d .

EXAFS s p e c t r a and t h e i r F o u r i e r t r a n s f o r m s (FT) a r e r e p o r t e d i n Fig. 1 and 2.

The d i s a p p e a r a n c e of long-range o r d e r i n t h e amorphous samples i s q u i t e e v i d e n t . The s i g n a l of t h e f i r s t c o o r d i n a t i o n s h e l l was back transformed i n t o k-space i n o r d e r t o perform t h e s t a n d a r d EXAFS a n a l y s i s . The minimum and maximum of t h e (''work partially supported by PFEII-ENEA

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986873

(3)

C8-376 JOURNAL DE PHYSIQUE

K

(A-'1

F i g . 1 R

F i g . 2

f i l t e r e d r e g i o n i n R-space were p l a c e d a t t h e v a l u e s where t h e FT reached 30% and 10% of i t s maximum v a l u e , r e s p e c t i v e l y . T h i s p r o c e d u r e was judged t h e b e s t compromise between t h e need of e l i m i n a t i n g t h e background o r t h e c o n t r i b u t i o n from h i g h e r s h e l l s and of r e t a i n i n g maximum i n f o r m a t i o n i n t h e f i l t e r i n g procedure.

The f i l t e r e d s i g n a l s have been compared by u s i n g Ln(A2/A1) t e c h n i q u e and by performing a b e s t f i t w i t h t h e c-Si e x p e r i m e n t a l model. Coincident r e s u l t s were o b t a i n e d which a r e r e p o r t e d i n Table I. The v a l u e s r e p o r t e d a r e t h e a v e r a g e among d i f f e r e n t independent measurements on t h e same sample. The e r r o r s t a k e i n t o a c c o u n t t h e u n c e r t a i n t y of t h e e x p e r i m e n t a l v a l u e s of t h e c-Si used a s a model.

TABLE I

Sample P I PC CN IR C N ~ ~ ( i ) ~~2 ( x 1 0 - 3 i 2 )

R e s u l t s and D i s c u s s i o n . - I n t h e f i r s t column of t h e Table we r e p o r t t h e d e n s i t y of

-- S i atoms a s o b t a i n e d from t h e magnitude of t h e jump a t t h e S i K-edge r e l a t i v e t o t h e d e n s i t y of c-Si. The a-Si:H samples e x h i b i t a r e d u c t i o n of p/pc p r o p o r t i o n a l t o t h e hydrogen c o n t e n t . D e s p i t e t h e absence of H t h e s p u t t e r e d sample t o o has a low d e n s i t y which is probably due t o t h e p r e s e n c e of voids.

The c o o r d i n a t i o n number CNE a s deduced from t h e EXAFS a n a l y s i s is r e p o r t e d i n column t h r e e , r e l a t i v e t o t h a t of c-Si. A r e l a t i v e l y h i g h p r e c i s i o n ( - 5%) i n t h e d e t e r m i n a t i o n of CNE h a s been o b t a i n e d t h a n k s t o t h e a c c u r a c y of t h e a n a l y s i s . Notice t h a t t h e s p u t t e r e d sample e x h i b i t s a l a r g e c o o r d i n a t i o n almost i d e n t i c a l t o

(4)

that of c-Si, despite its low density. This fact points to the presence of a relativity small number of large voids. Quite to the contrary, the reduction in coordination of the a-Si:H samples closely reflects the presence of hydrogen, which does not contribute to the EXAFS signal. This circumstance can be verified by comparing column three with column two, where theapparent coordination number CNIR, as deduced by the IR hydrogen content, is reported.

The average Si-Si bond distance is reported in column four. In close agreement with previous findings on tetrahedrally bonded amorphous semiconductors, it remains constant and equal to that of c-Si.

Finally, in column five we report the "disorder" factor AO' i.e. the difference between amorphous and crystalline samples of the variance o of the bond distance. o is the result of the static contribution us (i.e. variance of the equilibrium distance), the thermal contribution oT and the correlation between the two. The correlation term is generally assumed negligible (it would be exactly zero in the harmonic approrimation). For crystalline Si, os=O by definition, while oT2 should be -3.2.10~~11~ at room temperature, as estimated by using an Einstein model with oscillator frequency equal to 465 c m - 1

.

Assuming that o~ of the amorphous samples is not far from that of c-Si at room temperature, A U ~ then coincides with o$. From Table I we see that the sputtered a-$i has a strained network with a standard deviation of bond distances of 0.06 f 0.01 A (i.e. - ^2.5%).

The presence of hydrogen in the glow-discharge samples relieves part of the strain and reduces as to -0.04 A.

Acknowledgment. - We are grateful to P.Lagarde and A.M.Flank for useful discussions and for providing us with the c-Si sample. The hospitality of the LURE laboratory

is sincerely acknowledged.

References. -

[I] R.Bellissent, A.Chenevas-Paule, P.Lagarde, P.Bazin, and D-Raoux, J. Non-Cryst.

Solids 59-60, 237 (1983)

[2] D.E.Sayers, E.A.Stern, and F.W.Lytle, Phys. Rev. Lett

z,

1204 (1971);

F-Evangelisti, M.G.Proietti, A.Balzarotti, F-Cmin, L-Incoccia and S.Mobilio, Solid State Cmmun. 37, 413 (1981); M.A.Paesler, D.E.Sayers, R.Tsu, and J.Gonzales-Hernandez, Phys. Rev. B 28, 4550 (1983); C.E.Bouldin, E.A.Stern, B.Von Roedern, and J.Azoulay, J. on-cry=. ^Soldds66, 105 (1984)

[3] J.F.Graczyk, Phys. Status Solidi (a) 2, 231 (1979); R.Mosseri, C.Sella and J.Dixmier, Phys. Status Solidi (a) 52, 475 (1979); W.Schulke, Phil. Mag. B S , 451 (1981)

[4] B.Lengeler and P.Eisenberger, Phys. Rev. B 1, 4507 (1980)