AMORPHOUS HYDROGENATED ALLOYS : A COMPARATIVE EXAFS STUDY OF a-Si1-xCx : H, a-Si1- xGex : H, a-SiNx : H AT THE SILICON K-EDGE

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AMORPHOUS HYDROGENATED ALLOYS : A COMPARATIVE EXAFS STUDY OF a-Si1-xCx : H, a-Si1- xGex : H, a-SiNx : H AT THE SILICON K-EDGE

A. Filipponi, P. Fiorini, F. Evangelisti, A. Balerna, S. Mobilio

To cite this version:

A. Filipponi, P. Fiorini, F. Evangelisti, A. Balerna, S. Mobilio. AMORPHOUS HYDROGENATED ALLOYS : A COMPARATIVE EXAFS STUDY OF a-Si1-xCx : H, a-Si1- xGex : H, a-SiNx : H AT THE SILICON K-EDGE. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-357-C8-361.

�10.1051/jphyscol:1986870�. �jpa-00226192�

Colloque C8, supplement au n o 12, Tome 47, decembre 1986

AMORPHOUS HYDROGENATED ALLOYS : A COMPARATIVE EXAFS S'ruuf OF

.- - ^.-

azSil - Cx :H, a-Si, - Gex :H, a-SiNx :H AT THE SILICON K-EDGE'

A. FILIPPONI , P . ^{FIORINI , F} . EVANGELISTI, A. BALERNA' and S. MOBILIO"

Dipartimento di Fisica, ~niversit; "La ~ a p i e n z a " , I-00185 Roma, Italy

" I N F N Laboratori Nazionali d i Frascati, C.P. 1 3 , I-00044 Frascati, Italy

ABSTRACT

a-Sil-,C,

H, a-Si,..,Ge,

R , a-SiNx

R alloys have been studied at the Si K-edge. Si-SI and Si-A (A being C, N, Ge) bond lengths have been determined as a function of composition. The Si-C distance was found to vary significantly, while the Si-Ge and Si-N distances remain almost constant. A high frequency signal has been detected in the a-SiI-,C, :H and a-SiN, :H films at large x due to a second shell contribution. The compositions of the first coordination shell are also reportedand discussed in terms of different local coordination models.

The study of semiconducting binary alloys is of great interest from the fundamental as well as applied point of-view. This is even more true for the hydrogenated amorphous alloys based on a-Si due to the flexibility of the deposition processes, 8.g. glow discharge, and an apparent continuous miscibility of the elements. This seems to be the case also for silicon-carbon and silicon-nitrogen that in the crystalline phase form a definite compound and are not miscible. Up to now no direct structural measurements have been reported on a-Sil-,Cx:II and a-SiNx:H while a-Si,xGex:H bas been already studied at the Ge K-edge [1,21.

In thispaper we report the first EXAFS study performed at the Si K-edge on three silicon based alloys : a-Si,-,C ,:H, a-Sit-,Ge, :H, a-SiN, :H.

EXPERIMENTAL AND DATA ANALYSIS

Three series of samples of a-Sil-,C, :H . a-Sil-,Ge, :X , a-SIN, :H were deposited by glow-discharge from binary gas mixtures of SiB4, GeH,. NH3 -- , CH, on berillium. quartz and silicon substrates.

(')work partially supported by PFE 11-ENEA

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986870

C8-358 JOURNAL DE PHYSIQUE

T h i c k n e s s , o p t i c a l gap and r e f r a c t i v e i n d e x were d e t e r m i n e d by o p t i c a l s p e c t r o s c o p y . The hydrogen c o n t e n t was determined by I R s p e c t r o s c o p y i n t e g r a t i n g t h e i n t e n s i t y of SI-H and Ge-H wagging mode and C-H and N-H s t r e t c h i n g mode. The atomic c o n c e n t r a t i o n o f a-SiN,:H and a-Sil-,Ge,:H was determined from c a l i b r a t i o n c u r v e s o b t a i n e d from ESCA and Auger measurements on p r e v i o u s s e r i e s of samples d e p o s i t e d i n s i m i l a r c o n d i t i o n s . The a t o m i c c o n c e n t r a t i o n of a-Stl-,C,:R was i n s t e a d estimated from t h e o p t i c a l gap and t h e r e f o r e i s less r e l i a b l e .

The S i K-edge a b s o r p t i o n s p e c t r a were measured a t LURE (Orsay) u s i n g t h e ACO X-ray beam, monocromatized by a I n S b ( l l 1 ) d o u b l e c r y s t a l a t s t e p of l . e V .

The EXAFS s i g n a l was e x t r a c t e d

s u b t r a c t i n g a n a t o m i c - l i k e p o l i n o m i a l a-SiNx:H

background from t h e s p e c t r a and

normalized by 2 1 ^{Si-Si Bond}

[ J * ( ~ - ~ * ( E - E O ) / ~ E O ) C ~ [31. The o r i g i n Bo of t h e k s c a l e was p l a c e d a t t h e 3 i n f l e c t i o n p o i n t 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 . he jump J was f

d e t e r m i n e d a s t h e mean v a l u 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 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 E

Bo. The EXAFS m u l t i p l i e d by k2 wer ^L F o u r i e r transformed i n t h e r a n g e ^2-9 fl ²

w i t h a 0.5 8;' wide Hanning window. z

The r e s u l t s a r e shown i n F i g . 1. w

2 a n d ^3. We have a t t r i b u t e d t h e first two p e a k s i n t h e F o u r i e r t r a n s f o r m s of .z

t h e a-SiN,:H and a-Si,-,C,:H a l l o y s t o t h e Si-A and S i - S i d i s t a n c e s i n t h e 3

f i r s t c o o r d i n a t i o n s h e l l . The 2

r e l a t i v e weight of t h e two peaks c h a n g e s g r a d u a l l y w i t h t h e %

c o m p o s i t i o n . A f u r t h e r peak a t h i g h e r

R i s a l s o v i s i b l e a t h i g h A

I 1

c o n c e n t r a t i o n which h a s b e e n I,,,, I , , , , 1 , , , , 1 , , , , 1 , , , , 1 , , , , 1

a t t r i b u t e d t o a second s h e l l

c o n t r i b u t i o n . I n a-SiLnGe,:H t h e two 0 1 2 3 4 5 6

p e a k s due t o t h e f i r s t scell are n o t

s e p a r a t e d because of t h e small Fig. 1 R

d i f f e r e n c e i n t h e bond d i s t a n c e .

The two p e a k s of t h e f i r s t s h e l l were back-transformed i n t o k-space. Each spectrum h a s been f i t t e d w i t h a model EXAFS as f o l l o w s . Experimental a m p l i t u d e s and p h a s e s f o r t h e S i - S i and Si-A p a i r s were o b t a i n e d from t h e EXAFS of p u r e a-Si:H and of t h e sample w i t h t h e h i g h e s t A c o n t e n t . r e s p e c t i v e l y . The SI-Si and Si-C models were a l s o compared w i t h c r y s t a l l i n e S i and SIC i n o r d e r t o have a n a b s o l u t e r e f e r e n c e f o r d i s t a n c e , c o o r d i n a t i o n number and mean s q u a r e r e l a t i v e displacement a2 . The Si-N d i s t a n c e i n t h e model was assumed t o be

1 . 3 , I . . e q u a l t o t h a t i n ,!I-Si3N,. With t h e s e f u n c t i o n s t h e

model s p e c t r a where b u i l t , by assuming a c o n s t a n t c o o r d i n a t i o n around

t h e s i l i c o n atom a t any composition. F i t t i n g p a r a m e t e r s were t h e two

d i s t a n c e s , t h e two do2 v a l u e s and t h e r e l a t i v e c o o r d i n a t i o n

Cr-NA/(NA+Nsi ) o f atom A around t h e S i atom. The r e s u l t s are r e p o r t e d

i n T a b . 1 , 2 , 3 , where x i s t h e a t o m i c c o n c e n t r a t i o n and R are t h e

average bond l e n g t h s . Only t h e do* whioh c o u l d be d e t e r m i n e d w i t h a

s i g n i f i c a n t a c c u r a c y are r e p o r t e d .

Fig. Fig. 3 R (9

TABLE 1 a - S I N x

H

TABLE 2 a - S i , - x G e x : H

TABLE 3 a - S i , - x C x

H

C8-360 JOURNAL DE PHYSIQUE

RESULTS and DISCUSSION

No s i g n i f i c a n t v a r i a t i o n of t h e bond l e n g t h s was d e t e c t e d i n a-SiI-,Ge, :H, i n agreement w i t h p r e v i o u s r e s u l t s o b t a i n e d a t t h e Ge K-edge [11. I n a-SiNx:H t h e SI-N bond d i s t a n c e remains c o n s t a n t . I n a-Sil-,C, :H a g r a d u a l change of 0 . 1 I j i n t h e Si-C d i s t a n c e o c c u r s a t h i g h C c o n c e n t r a t i o n s . F i n a l l y a s l i g h t i n c r e a s e of 0 . 0 2 of t h e S i - S i d i s t a n c e i s observed b o t h i n a-Sil-xC,:H and i n a-SiNx:H.

By comparing t h e a v e r a g e composition of t h e f i r s t - n e i g h b o u r s h e l l a s s e e n by EXAFS w i t h t h e atomic c o n c e n t r a t i o n i n t h e sample it i s p o s s i b l e t o d e r i v e i n f o r m a t i o n on t h e c o m p o s i t i o n a l d i s o r d e r of t h e material 111. Two extreme cases are c o n s i d e r e d and r e p o r t e d i n F i g . ⁴ and 5 f o r t h e a-SiN,:H and a-Sil,Gex:H f i l m s whose a t o m i c c o n c e n t r a t i o n was known.

I n one case t h e f i l m s a r e c o n s i d e r e d c h e m i c a l l y o r d e r e d , i . e . t h e t e n d e n c y t o form a compound i s supposed dominant o v e r d i s o r d e r i n g e f f e c t s . For x v a r y i n g from z e r o t o t h e s t o i c h - i o m e t r i c v a l u e (X=0.5 f o r a-Sit-#eX:H and X = 4 / 3 f o r a-SiNx:H), t h e atom A would be surrounded by S i atoms o n l y , w h i l e t h e -environment of S i i s n o t c o n s t r a i n e d . I n o r d e r t o have l a r g e r v a l u e s of x the r o l e of t h e two atoms must be r e v e r s e d . The e s t i m a t e d c o o r d i n a t i o n o f atom A around t h e S i atom as a f u n c t i o n of t h e a t o m i c c o n c e n t r a t i o n i s r e p o r t e d i n F i g . 4 and 5 f o r t h e two m a t e r i a l s ( d o t t e d l i n e ) . The d a s h e d l i n e i n

0.6 0.6

C r C r

0.4 0.4

0.2 0.2

0.0 0.0

0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1

x/( 1 +XI X

Fig. 4 Fig. 5 Diamonds a r e experimental points from ref. 1 measured at t h e Ge K-edge.

F i g . 4 shows t h e change brought about by t a k i n g a l s o account of t h e p r e s e n c e of hydrogen.

While s t i l l Beeping c o n s t a n t t h e c o o r d i n a t i o n of t h e d i f f e r e n t

atoms ^{( 4} f o r S i and G e and 3 f o r ^N) t h e o p p o s i t e h y p o t h e s i s c o n s i d e r s

n e g l i g i b l e any o t h e r c h e m i c a l c o n s t r a i n t , s o t h a t t h e d i s t r i b u t i o n i s

completely random. A s a consequence t h e p r o b a b i l i t y f o r any atom t o

form a bond w i t h a g i v e n s p e c i e s i s p r o p o r t i o n a l t o t h e atomic

c o n c e n t r a t i o n of t h a t s p e c i e s t i m e s i t s v a l e n c e . The r e l a t i v e

c o o r d i n a t i o n of S i f o r t h i s random bonding model i s r e p o r t e d a s a f u l l

l i n e i n F i g . 4 and 5.

hypothesis closely reproduce the situation of a-SiN,:H and a-Sil-,Ge, :H, respectively. The random composition of the silicon-germanium films agrees with the complete miscibility of the two elements already established in the crystalline alloys. On the other hand the tendency to local chemical order that the silicon-nitride films seems to display is not surprising keeping in mind that in the crystalline phase they are hardly miscible but form stoichiometric S13N4compounds. Finally it is worth mentioning that this tendency toward the chemical order is also found by analysing the IR absorption spectra of a-Six, and a-SiNx:H 141.

ACKNOWLEDGEMENT

We are grateful to P. Lagarde and A . M. Flank for useful discussions and for providing us with the c-SIC data. We thank the Lure laboratories for giving us the opportunity to perform the measurements.

REFERENCES

1) L. Incoccia, S. Mobilio, M. G. Proietti, P. Fiorini, C. Giovannella, and F. Evangelisti. Phys. Rev. B 31, 1028 (1985).

2) S. Minomura, K. Tsuji, M. Wakagi, T. Ishidate, K. Inoue, and M. Shibuya, 3 . Non-Cryst. Solids 59-60, 541-544 (1983).

3) B. Lengeler and P. Eisenberger, Phys. Rev. B 21, 4507 (1980).

4) D. della Sala, C. Coluzza, G. Fortunato and F. Evangelisti,

J. Non-Cryst. Solids 77-78. 933 (1985).