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DIFFRACTION STUDY OF THE a-Si:H STRUCTURE EVOLUTION DURING ANNEALING TREATMENTS
PERFORMED DIRECTLY ON THE X RAY GONIOMETER
J. Dixmier, R. Mosseri, J. Sadoc
To cite this version:
J. Dixmier, R. Mosseri, J. Sadoc. DIFFRACTION STUDY OF THE a-Si:H STRUCTURE EVOLUTION DURING ANNEALING TREATMENTS PERFORMED DIRECTLY ON THE X RAY GONIOMETER. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-237-C4-240.
�10.1051/jphyscol:1981450�. �jpa-00220907�
JOURNAL DE PHYSIQUE
CoZZoque C4, suppldment au n030, Tome 42, octobre 1981 page C4-237
O I F F R A C T I O N STUDY OF THE a - S i : H STRUCTURE E V O L U T I O N DURING ANNEALING TREATMENTS PERFORMED D I R E C T L Y ON THE X RAY GONIOMETER
J . Dixmier, R. Mosseri and J . F . Sadoc*
k b o r a t o i r e de Physique des SoZides, CNRS, 92190 BeZZevue, France
* ~ a b o r a t o i r e de Physique des SoZides, CNRS, Orsay, France
A b k t r a c t . - Energy d i s p e r s i v e X r a y d i f f r a c t i o n p a t t e r n s have been recorded each 15 m i n u t e s d u r i n g t h e a n n e a l i n g t r e a t m e n t of a g.d a.Sig7 HQ powder. Two s e r i e s o f h e a t i n g were performed up t o 350°C and 650°C s e p a r a t e d by a r e t u r n t o room temperature. I. R. a b s o r p t i o n s p e c t r o s c o p y r e v e a l e d b e f o r e a n n e a l i n g a t l e a s t 15 a t % of oxygen i n t h e powder. The atomic d i s t r i b u t i o n f u n c t i o n s (A.D.F.) were c a l c u l a t e d and compared t o t h e p u r e a.Si ADF and a l s o t o t h a t of a.Si50050. I t i s shown t h a t chemisorbed 0 atoms do n o t a f f e c t t h e ADF of a SF-H b e f o r e a n n e a l i n g b u t t h a t S i - 0 c o v a l e n t bond d i s t a n c e s a p p e a r c l e a r l y a t 650°C f o l l o w i n g t h e Hydrogen e f f u s i o n ADF changes.
I n t r o d u c t i o n . - T h e atomic s t r u c t u r e o f a S i H f i l m s a c t u a l y d e s i g n e d f o r p h o t o v o l t a i c d e v i c e s cannot b e e a s i l y determined. T h e i r t h i c k n e s s , from 1000 t o 10.000
8
i s t o o l a r g e f o r e l e c t r o n d i f f r a c t i o n experiments (100 t o 500 8 ) r e q u i r e d and t o o s m a l l f o r X r a y s ( 1 0 microns a t l e a s t ) .Moreover t h e d e s c r i p t i o n of t h e l o c a l o r d e r o f S i atoms and H atoms i n r e l a t i o n s h i p w i t h t h e p r e p a r a t i o n p a r a m e t e r s y i e l d i n g m i x t u r e s of v a r i o u s S i H bonds i s n o t s i m p l i f i e d by d i f f r a c t i o n t e c h n i q u e g i v i n g an a v e r a g e of a l l configurations i n n macroscopic volume. I t i s t h e r e a s o n why we p e r s i s t t o Operate a s f o l l o w s : S t a r t i n g
from m a t e r i a l s of h i g h H c o n c e n t r a t i o n ( d e p o s i t s on non h e a t e d s u b s t r a t e ) we anneal t h e samples i n o r d e r t o g e t s u c c e s s i v e homogeneous composition and, we b e l i e v e , homo- geneous l o c a l s t r u c t u r e .
X r a y a n a l y s i s performed on l a r g e amount of m a t e r i a l s under powder form i n o r d e r t o i n c r e a s e t h e r e s o l u t i o n power.
We p r e s e n t h e r e a t e n t a t i v e s t u d y of t h e e v o l u t i o n of s t r u c t u r e d u r i n g t h e a n n e a l i n g of a glow d i s c h a r g e a-Si-H powder p r e p a r e d by Y. CATHERINE
and G. TURBAN ( L a b o r a t o i r e d e Physique c o r p u s c u l a i r e , U n i v e r s i t 6 d e Nantes, France).
Experiments.- We have used a f u r n a c e e s p e c i a l l y d e s i g n e d f o r t h a t p u r p o s e ( 1 ) . The powder h a s been e n c l o s e d between two f o i l s of mica h o l d by c l i p s i n a s t a i n l e s s s t e e l tube. This t u b e was i n c o n t a c t w i t h a W.wire h e a t e d m e t a l l i c block. A thermocouple measured t h e t e m p e r a t u r e of t h e block. T h i s f u r n a c e was s e t on t h e a x i s o f t h e gonio- m e t e r .
A vacuum chamber with k a p t o n windows i s o l a t e d t h e sample from t h e ambiant a i r . A f t e r a primary pumping was done, a p a r t i a l p r e s s u r e of He was i n t r o d u - ced t o p r e v e n t o x i d a t i o n d u r i n g t h e a n n e a l i n g experiment.
Our h e a t i n g system allowed t h e t e m p e r a t u r e of t h e sample h o l d e r t o r i s e up t o 650°C. I t i s l i k e l y t h a t t h e sample t e m p e r a t u r e was a l i t t l e b i t lower
.
Thed i f f r a c t i o n p a t t e r n o b t a i n e d by t r a n s m i s s i o n h a s been recorded u s i n g t h e energy d i s - p e r s i v e t e c h n i q u e a l r e a d y d e s c r i b e d e l s e w h e r e ( 2 ) .
I t i s i n f a c t n e c e s s a r y i n a dynamic p r o c e s s such a s a n n e a l i n g t o r e c o r d t h e whole diagram a t each moment, l i k e i n a p h o t o g r a p h i c r e c o r d i n g . Otherwise t h e end and t h e b e g i n i n g of t h e spectrum should correspond t o two d i f f e r e n t s t a g e s o f a n n e a l i n g . We took f o r each t e m p e r a t u r e two p a t t e r n s a t 2 0 = 20° and 2 8 = 400 w i t h a c o u n t i n g t i m e of 900 seconds f o r each one.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981450
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Fig. 1 F i r s t s t a g e of a.Si57H43 annea- l i n g . Temperature s t e p s a r e i n d i c a t e d i n d e g r e e C s t a r t i n g from t h e bottom.
The top c u r v e c o r r e s p o n d s t o a b r e a k of t h e h e a t i n g p r o c e d u r e
Fig. 2 Second s t a g e of a.Si57H43 a n n e a l i n g
.
Same c o n d i t i o n s a s i n f i g . 1.
A f i r s t diagram of t h e sample was t a k e n a t 20°C w i t h t h e u s u a l 0 d i s p e r s i v e t e c h n i q u e i n o r d e r t o g e t a r e f e r e n c e f o r t h e n o r m a l i z a t i o n of energy d i s p e r s i v e p a t t e r n s .
T h e s e p a t t e r n s a r e t h e r e s u l t of t h e m u l t i p l i c a t i o n of t h e a c t u a l i n - t e r f e r e n c e c u r v e by t h e p r o f i l e of t h e c o n t i n u o u s w h i t e beam of t h e X r a y tube.
The c o n v e r s i o n f a c t o r i s o b t a i n e d by d i v i d i n g t h e 8 d i s p e r s i v e c u r v e a t 20°C by t h e energy d i s p e r s i v e one.
I n o r d e r t o o b t a i n i n f o r m a t i o n s on t h e hydrogen c o n t e n t and bonding t o S i atoms we have done d e n s i t i e s and i n f r a - r e d t r a n s m i s s i o n measurements on t h e sam- p l e b e f o r e and a f t e r a n n e a l i n g .
The d e n s i t y h a s been measured by f l o a t a t i o n t e c h n i q u e i n a m i x t u r e of bromomethane and e t h y l i c a l c o h o l . The accuracy h a s been e s t i m a t e d t o 1% on a t e s t experiment on c r y s t a l l i n e s i l i c o n .
Fig. 3 Atomic d i s t r i b u t i o n f u n c t i o n (A.D.F.) f o r :
A : p u r e a . S i
B : a.Si57H43 a t 20°C
C : Same sample annealed a t 350°C D : Same sample annealed a t 650°C E : A.D.F. of Si.0 a f t e r ( 6 )
R e s u l t s .
-
Two a n n e a l i n g p r o c e d u r e s have been a p p l i e d t o t h e same sample w i t h o u t removing i t from t h e f u r n a c e .A f i r s t s e r i e of p a t t e r n s were t a k e n a t 20, 200, 250, 350°C and back t o 50°C.
A second s e r i e was performed a day a f t e r a t 20, 410, 496, 560, 600, 650 and back t o 50°C.
The c o r r e s p o n d i n g reduced i n t e r f e r e n c e f u n c t i o n s k ( I ( k ) - l ) a r e g a t h e r e d on f i g . 1 and 2.
The main f e a t u r e s of t h e a n n e a l i n g e f f e c t s a r e a s f o l l o w s . For t h e f i r s t s e r i e t h e t y p i c a l s h o u l d e r on t h e 3 r d h a l o of t h e w e l l known c u r v e f o r a.Si d i s a p e a r s a t 250°C. This i s an unexpected phenomenum s i n c e t h e e f f u s i o n of hydro- gen i s supposed t o l e a d t o t h e p u r e a.Si c u r v e . One should have a t t r i b u t e d i t t o a thermal e f f e c t i f t h e s h o u l d e r would have reappeared a f t e r c o o l i n g down t h e sample t o room temperature.
But a t 50°C t h e 3 r d h a l o i s s t i l l u n s p l i t t e d , l a r g e and d i f f u s e . The f i r s t and second h a l o do n o t show s i g n i f i c a n t m o d i f i c a t i o n s .
During t h e Second s t a g e of a n n e a l i n g an i m p o r t a n t s h i f t of t h e t h i r d h a l o i s observed towards t h e low k v a l u e s .
The maximum goes from 5.7
I-'
t o 5,Ox-'
a t 650°C.A s l i g h t r e v e r s e e f f e c t o c c u r s when c o o l i n g back t o 500C b u t t h e maximum i s s t i l l a t 5 , 3
A-1.
Meanwhile t h e f i r s t a.nd second h a l o e s a r e broadened and t h e maxima and minima a r e l e s s and l e s s marked.The i n f r a r e d s p e c t r o s c o p y r e v e a l s a h i g h c o n c e n t r a t i o n of hydrogen i n t h e a s p r e p a r e d sample b u t a l s o , r e f e r r i n g t o KNIGHTS s t u d y t h e p r e s e n c e of oxygen which amounts a t l e a s t t o 15 % ( 3 ) .
A f t e r a n n e a l i n g a t 650°C hydrogen i s t o t a l y e f f u s e d i . e . t h e r e i s no t r a c e of i t on t h e I . R . a b s o r p t i o n curve.
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The d e n s i t y of t h e s t a r t i n g sample i s 1.79 and a f t e r a n n e a l i n g a t 6500C i t was found t o b e 2.0. We have c a l c u l a t e d t h e F o u r r i e r t r a n s f o r m s of t h e i n t e r f e r e n c e f u n c t i o n s r e l a t i v e t o t h e non annealed sample, a f t e r t h e f i r s t s t a g e a t 350°C and a f t e r t h e second s t a g e a t 650°C.
These t h r e e atomic d i s t r i b u t i o n f u n c t i o n s a r e compared t o t h a t of p u r e a-Si.
The ADF o f a-Si-H i s c o n s i s t 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 ( 4 ) i . e . a well marked peak a t 4.8
fi
which does n o t e x i s t i n p u r e a-Si. This peak h a s been a t t r i -b u t e d t o a d i h e d r a l a n g l e d i s t r i b u t i o n e x h i b i t i n g a maximum i n t h e r a n g e 35-45O.
This i n t e r m e d i a t e v a l u e betJreen 0 and 60°C of t h e c r y s t a l l i n e c a s e c a n be due t o e i t h e r l a r g e r i n g s o r t o d i s t o r t e d s i x b o a t shaped r i n g s which a r e e a s i e r t o d i s t o r d than s i x c h a i r shaped r i n g s .
A f t e r t h e t r e a t m e n t a t 350°C t h e peak a t 4.8
1
d i s a p p e a r s . A f t e r t h e 650°C s t a g e a new f e a t u r e t a k e s p l a c e : one o b s e r v e s a l a r g e peak w i t h a maximum a t 1.68
and a s h o u l d e r on t h e l e f t s i d e o f t h e second peak. The maximum p o s i t i o n i s a t 3.21.
Tnese two d i s t a n c e s correspond t o t h e SiO and S i - S i f i r s t d i s t a n c e i n S i 0 2 ( 5 ) .D i s c u s s i o n . - Using t h e measured d e n s i t i e s one c a n c a l c u l a t e t h e c o o r d i n a t i o n number of t h e f i r s t S i - S i n e i g h b o r s - f o r t h e sample b e f o r e and a f t e r h e a t i n g t r e a t m e n t . One founds 3.25 and 3.32. These numbers have n o t t h e same p h y s i c a l s i g n i f i c a t i o n . We know from I.R. s p e c t r o s c o p y t h a t i n t h i f i r s t sample t h e r e i s a h i g h c o n c e n t r a t i o n o f hydrogen. From t h e formula C = 4
-
where C i s t h e c o o r d i n a t i o n number we can d e r i v e t h e H c o n c e n t r a t i o n . For C = 3 . z , x # 4 3 % i f one suppose t h a t S i atoms a r e o n l y bonded t o H atoms.I n t h e second c a s e , hydrogen h a s been t o t a l y e f f u s e d
.
Moreover,Si-0 d i s t a n c e s a p p e a r on t h e ADF of t h e sample. So, t h e c o o r d i n a t i o n number 3-32 i s t h e weighted a v e r a g e of S i - S i and 0. 0 f i r s t n e i g h b o r s . (2.35 and 2.65 A).
From t h e works of YASAITIS e t a1 ( 6 ) i t i s known t h a t t h e s t r u c t u r e of S i 0 ) can b e regarded a s t h e m i x t u r e of S i and S i 0 4 t e t r a h e d r a a t a small s c a l e but-got of S i and S i 0 a t a l a r g e s c a l e . Taking i n t o account t h i s knowledge we c a n e s t i m a t e a oxygen c o n c e n t r a t i o n from t h e d i f f e r e n c e between 4 and 3.32 f o r 2 t h e m i x t u r e S i - S i and 0-0 p a i r d i s t a n c e . We f i n d 20% of oxygen.
Remembering t h a t I . R . s p e c t r o s c o p y gave a t l e a s t 15 % of oxygen f o r t h e non annealed sample one c a n o b s e r v e t h a t almost t h e same c o n c e n t r a t i o n i s found a f t e r a n n e a l i n g .
Conclusion.-Annealing of a p a r t i a l y o x i d i z e d aSi.H powder undergoes two s t a g e s of s t r u c t u r a l e v o l u t i o n . The Hydrogen d e s o r b t i o n , d u r i n g t h e f i r s t s t a g e up too3500C i s accompanied by t h e d i s a p e a r a n c e of t h e c h a r a c t e r i s t i c p a i r d i s t a n c e a t 4.8 A on t h e H r i c h ADF.
Up t o 650°C oxygen d i f f u s e s from i n t e r s t i t i a l s i t e s t o e f f u s e d H s i t e s and form c o v a l e n t bonds between S i atoms. This i s demonstrated by S i - 0 p a i r d i s - t a n c e s a p p e a r i n g on t h e ADF a f t e r a n n e a l i n g whereas t h e y were n o t v i s i b l e b e f o r e , d e s p i t e t h e p r e s e n c e of t h e same amount of chemisorbed oxygen atoms.
References
(1.) LARIDJANI M., SADOC J.F., Colloque S.F.P. 1981.
( 2 ) MALAUREW J.C., DIXMIER J., Thin S o l i d Films 36, (1976) LlL4.
(3) KNIGHTS J.C., STREET R.A., LUCOVSKY G., 3. of non C r y s t . Sol. 35-36(1980) 279-284.
( 4 ) MOSSERI R. SELLA C., DIXMIER J., Phys. S t a t Sol. ( a ) 52, (1979) 475.
( 5 ) MOZZI R.L., WARREN B.E., 3. Appl. Cryst. 2 (1969) 164.
(6) YASAITIS J.A., KAPLOW R., 3. Appl. Phys., Vol. 43 n o 3 (1972) 995.
