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NUCLEAR SCATTERING MEASUREMENTS OF COMPOSITION PROFILES IN a-Si : H MULTILAYER
STRUCTURES
C. Brassard, R. Groleau, J. l’Ecuyer, J. Martin, J. Currie, P. Depelsenaire, M.
Wertheimer, A. Yelon
To cite this version:
C. Brassard, R. Groleau, J. l’Ecuyer, J. Martin, J. Currie, et al.. NUCLEAR SCATTERING MEA-
SUREMENTS OF COMPOSITION PROFILES IN a-Si : H MULTILAYER STRUCTURES. Journal
de Physique Colloques, 1981, 42 (C4), pp.C4-795-C4-798. �10.1051/jphyscol:19814174�. �jpa-00220799�
JOURNAL D E PHYSIQUE
CoZZoque C4, suppliment au nO1O, Tome 42, octobre 1981 page C4-795
NUCLEAR S C A T T E R I N G MEASUREFlENTS O F COFIPOSITION P R O F I L E S I N a - S i : H M U L T I L A Y E R STRUCTURES
C . Brassard, R . Groleau, J . L'Ecuyer, J . P . Martin. J . F . c u r r i e s , P .
~ e p e l s e n a i r e * , M . ~ e r t h e i m e r * a n d A. Yelon*
Laboratoire de Physique Nucle'aire, Universite' de Montre'al, C.P. 6128, Succ.
"A", MontrdaZ H3C 3J7, Canada
" ~ g ~ a r t e m e n t de GBnie Physique, Ecole Polytechnique, C.P. 6079, Succ "A", iYontre'al H3C 347, Canada
Abstract.- We r e p o r t on some r e c e n t r e s u l t s o b t a i n e d on t h e a n a l y s i s o f com- p o s i t i o n a l p r o f i le s i n amorphous hydrogenated s i l i c o n s t r u c t u r e s using a non- d e s t r u c t i v e n u c l e a r e l a s t i c r e c o i 1 technique (ERD) t o p r o f i l e l i g h t elements t o a depth o f about 1 urn and Rutherford backscattering. We have a p p l i e d these techniques t o t h e study o f t h e parameters which i n f l u e n c e f i l m composi-.
t i o n . These i n c l u d e l i o h t element contamination, an e f f e c t o f t o t a l t h i c k - ness on t h e composition p r o f i l e s and posthydrogenation. Secondly we have observed t h e phenomena o f i n t e r p e n e t r a t i o n o f elements i n m u l t i l a y e r s t r u c - tures. These i n c l u d e dopants i n p - i -n j u n c t i o n s and e l e c t r o d e metals. F i n a l - l y we r e p o r t m o d i f i c a t i o n o f c o n c e n t r a t i o n p r o f i l e s by l a s e r annealing.
I n t r o d u c t i o n .
-
Successful study and a p p l i c a t i o n o f amorphous t h i n f i l m s r e q u i r e a p r e c i s e knowledge o f t h e chemical composition and o f i t s v a r i a t i o n throughout t h e f i l m thickness. We have r e c e n t l y developed a n o n - d e s t r u c t i v e n u c l e a r e l a s t i c r e c o i l technique (ERD) (1,2,3) t o p r o f i l e l i g h t elements i n s o l i d s t o a depth o f about 1 m For h e a v i e r elements we use standard R u t h e r f o r d b a c k s c a t t e r i n g spectroscopy ( 4 ) RBS). We a p p l i e d these techniques t o the study o f t h e chemical composition o f t h e study o f t h e chemical composition o f a-Si:H prepared by four o f t h e most w i d e l y used methods: RF glow discharge, r e a c t i v e s p u t t e r i n g , chemical vapor d e p o s i t i o n , and microwave plasma. ltte present here a s e l e c t i o n o f some o f t h e most s t r i k i n g r e s u l t s encountered.I n c o l l a b o r a t i o n w i t h t h e Groupe de T r a n s i t i o n s de Phases du CNRS de Grenoble we prepared a s e r i e s o f co-deposited s p u t t e r e d f i l m s on c r y s t a l l i n e s u b s t r a t e s which d i f f e r e d one from another o n l y by t h e depostion time and hence by t h i c k n e s s ( 5 ) . The hydrogen composition p r o f i l e s i n Fig. 1 show t h a t the concentrations are n o t
SUBSTRATE DEPTH [gg c f i 2 ]
Fig. 1 Hydrogen percentage p r o f i l e s o f Fig. 2 T y p i c a l hydrogen percentage s p u t t e r e d a-Si:H f i l m s o f d i f f e r i n g t h i c k - p r o f i l e obtained i n a s i n g l e antenna
ness. microwave p l asma.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814174
JOURNAL DE PHYSIQUE
uniform. We note f i r s t t h a t f o r a l l b u t t h e t h i n n e s t f i l m , ( i ) , t h e hydrogen con- c e n t r a t i o n a t the f r e e surface has the same p r o f i l e , r i s i n g g r a d u a l l y t o 9%.However t h e c o n c e n t r a t i o n increases l i n e a r l y w i t h depth towards t h e c o n t a c t w i t h t h e subs- t r a t e . For the t h i n n e s t f i l m s ( i ) and ( i i ) t h e r e i s i n a d d i t i o n a peak 0.1 vm from the c r y s t a l l i n e i n t e r f a c e . Fig. 1 a l s o i n d i c a t e s t h a t hydrogen penetrates t h e c r y s - t a l l i n e Si substrate. We have v e r i f i e d t h i s r e s u l t by exposing a c r y s t a l l i n e subs- t r a t e t o a pure hydrogen p l a s m and found t h a t t h e hydrogen content increases from 0.2% t o 0.5% as a r e s u l t .
I n s t u d i e s o f microwave plasma d e p o s i t i o n ( 3 ) . we have employed two d i f f e r e n t antenna c o n f i g u r a t i o n s which y i e l d e d t h e r e s u l t s shown i n Figs. 2 and 3. The f o r - nier shows a t y p i c a l hydrogen p r o f i l e f o r t h e o r i g i n a l antenna c o n f i g u r a t i o n w h i l e the l a t t e r shows a p r o f i l e obtained w i t h an improved antenna c o n f i g u r a t i o n . Chan- ging d e p o s i t i o n geometry leads t o a remarkable d i f f e r e n c e i n t h e shape o f t h e pro- f i l e s . For i n s t a n c e i n Fig. 2 t h e r e i s a u n i f o r m c o n c e n t r a t i o n near t h e s u b s t r a t e w h i l e t h e r e i s a peak w i t h i n 0.1 vm o f the f r e e surface. On t h e c o n t r a r y t h e p r o - f i l e o f Fig. 3 i s reminiscent o f those we measured i n Fig. 1, on s p u t t e r e d f i l m s .
I n second c o l l a b o r a t i o n w i t h Frenoble we c h a r a c t e r i s e d low pressure chemical vapor deposited a-Si:H f i l m s . P a r t o f t h e i r f a b r i c a t i o n procedure i s t o posthydro- genate t h e f i l m s w i t h a s o f t hydrogen plasma ( 6 ) . We have observed what e f f e c t t h i s l a t t e r plasma has on t h e f i l m as shown i n Fig. 4. The f i r s t sample ( i ) d i d n o t un- dergo t h e treatment, w h i l e t h e second ( i i ) was grown under t h e same c o n d i t i o n s as t h e former (except t h a t i t i s s l i g h t l y t h i c k e r ) and then exposed t o t h e hydrogen plasma. We note t h a t as a r e s u l t , t h e b u l k hydrogen c o n c e n t r a t i o n slopes upwards toward t h e f r e e surface, i n c r e a s i n g on t h e average by 50%, w h i l e t h e peak a t the s u b s t r a t e diminishes and t h e surface peak quadruples.
Fig. 5 shows the r e s u l t o f d e p o s i t i o n i n a microwave discharge i n t h e presen- ce o f vacuum leaks. The e f f e c t s o f a i r and hydrocarbons i n t h e plasma are shown i n t h e f i g u r e f o r a very t h i n f i l m ( z 0.1 ~ m ) deposited on a glass s u b s t r a t e . We n o t e t h a t t h e compositional percentages o f hydrogen, carbon, oxygen and s i l i c o n are 16%, 20%, 11% and 53% r e s p e c t i v e l y . The e l i m i n a t i o n o f leaks and t h e improvement o f t h e base vacuum g r e a t l y reduces t h e carbon and t h e oxygen contamination. The minimum l e v e l o f C o r 0 contamination detectable by
ERD
i s 0.5%.W C )
4
L- W 10-
z t3
;
20.W I-
n
E
lo- 00
0
I r
0 W
*
I100 200 Z
SUBSTRATE DEPTH [go c n i 2 ] w
(3
F i g.3 A t y p i c a l hydrogen percentage p r o f i l e obtained i n a u n i f o r m double antenna ~clicro- wave plasma. Data p o i n t s a r e shown. The e r r o r bars a r e o f t h e s i z e o f t h e p o i n t s . Data p o i n t s and e r r o r bars a r e t y p i c a l and are n o t shown on other graphs f o r c l a r i t y o f t h e f i g u r e s .
(i)
F i g.4 Hydrogen percentage p r o f i l e s o f two Sam- I
-
p l e s o f n e a r l y the same thickness: ( i ) as pre-
0 100 200
pared by CVD technique ( i i ) as the previous
DEPTH [pg
~ m - ~ ]sample then posthydrogenated.
I
I
nI I
SUBSTRATEI I
G L A S S I a-Si : H
DEPTH Cpgcni21
Fig.5 Compositional p r o f i l e s o f chemical F i g . 6 Compositional p r o f i l e s o f chemi- c o n s t i t u e n t s o f impure microwave a-Si :H c a l c o n s t i t u e n t s o f a p - i - n j u n c t i o n
t h i n f i l m s . d e v i c e .
F i g . 7 Gold percentage p r o f i l e o f a 150
8
Fig.8 Hydrogen p e r c e n t a g e p r o f i l e o f l a y e r o f g o l d on two microwave glow a - s i : H glow d i s c h a r g e a-Si:H f i l m f o r f o u r l a s e r f i l m s d e p o s i t e d a t s u b s t r a t e t e m p e r a t u r e s a n n e a l i n g powers: ( - ) 0.4W, (- -) 0.5W, o f ( i ) 5000C, and ( i i ) 4000C r e s p e c t i v e l y . ( - - ) 0.6W, (- . ) 0.7W.C4-798 JOURNAL DE PHYSIQUE
Amorphous p - i - n s t r u c t u r e s a r e c o n s i d e r e d t o be i n t e r e s t i n g candidates f o r l a r g e area s o l a r c e l l s . I n f o r m a t i o n about t h e depth d i s t r i b u t i o n o f t h e v a r i o u s components o f t h e j u n c t i o n i s i m p o r t a n t f o r u n d e r s t a n d i n g o f t h e device. We used ERD i n c o n j u n c t i o n w i t h RBS t o o b t a i n p r o f i l e s o f a wide range o f elements.
S p u t t e r e d amorphous s i l i c o n f i l m s were d e p o s i t e d on c r y s t a l l i n e s i l i c o n subs- t r a t e s a t Grenoble. The dopants were i n c o r p o r a t e d i n t h e plasma i t s e l f d u r i n g de- p o s i t i o n . RBS w i t h 4 MeV 4He and ERD w i t h 30 MeV ?"I were used t o p r o f i l e t h e j u n c t i o n s . The r e s u l t s a r e shown i n f i g u r e 6.
The hydrogen p r o f i l e i s c o r r e l a t e d t o t h e dopant d i s t r i b u t i o n . The boron r e - g i o n n e a r t h e s u r f a c e c o n t a i n s l e s s hydrogen w h i l e t h e a r s e n i c r e g i o n c o n t a i n s more hydrogen, as expected f r o m p r e v i o u s r e s u l t s . (Bruyere e t a l . t o be p u b l i s h e d ) . The Si c o n c e n t r a t i o n , i s f l a t i n t h e i n t r i n s i c r e g i o n and drops o f f i n t h e a r s e n i c r e - gion, s u g g e s t i n g t h a t t h e a r s e n i c r e p l a c e s s i l i c o n atoms.
Argon, p r e s e n t i s t h e plasma gas m i x t u r e , has a b u l k c o n c e n t r a t i o n o f about 4%, which decreases i n t h e A r s e n i c r e g i o n . We n o t i c e i n a d d i t i o n t h a t t h e p r o f i l e s o f B and o f As a r e n o t sharp s t e p s b u t v e r y broad, i n s p i t e o f t h e f a c t t h a t t h e dopants were i n t r o d u c e d suddenly i n t o t h e plasma.
We have observed t h e p e n e t r a t i o n o f e l e c t r o d e m e t a l s i n t o microwave-produced a-Si:H f i l m s , u s i n g t h e RBS o f 14N as shown i n F i g . 7. N o m i n a l l y 150
1
of g o l d was d e p o s i t e d on smooth f i l m s o f a-Si :H d e p o s i t e d a t ( i ) 500% and ( i i ) 400°c r e s p e c t i - v e l y . We see a v e r y s i g n i f i c a n t c o n c e n t r a t i o n o f g o l d a t depths exceeding 3008
and n o t e t h a t t h e p r o f i l e seems s t e e p e r f o r t h e sample produced a t t h e l o w e r t e m p r - a t u r e . N u c l e a r s c a t t e r i n g techniques a t h i g h e n e r g i e s do n o t g i v e any d i r e c t i n - f o r m a t i o n c o n c e r n i n g t h e chemical bonding o f Au and o f S i and hence on t h e forma- t i o n o f g o l d s i l i c i d e s .
L a s e r a n n e a l i n g o f amorphous Si i s c u r r e n t l y t h e s u b j e c t o f i n t e n s e e x p e r i
-
mental s t u d y . I n Fig. 8 we show a s e r i e s o f samples w h i c h were p r e p a r e d and l a s e r annealed by a team f r o m Be1 1 N o r t h e r n Research L a b o r a t o r i e s ( 7 ) . These samples were p r e p a r e d by r.f. plasma d e p o s i t i o n , and annealed and c r y s t a l l i s e d w i t h a scan- ned C.W. argon i o n l a s e r . They were a l l annealed w i t h 40 pm beam s p o t scanning a t a r a t e o f 20 cm s e c - I , u s i n g a d i f f e r e n t l a s e r power l e v e l s . I t i s c l e a r from t h e f i g u r e t h a t i n c r e a s i n g t h e power d e n s i t i e s tends t o e l e i m i n a t e hydrogen f r o m n e a r t h e s u r f a c e b u t n o t a t t h e s u r f a c e . The narrow hydrogen peak a t t h e s u r f a c e i s p r o b a b l y due t o c o n t a m i n a t i o n a f t e r t r e a t m e n t . The h i g h e s t power d e n s i t y seems t o l e a v e a somewhat h i g h e r c o n c e n t r a t i o n t h a n t h e n e x t l o w e r d e n s i t y , b u t t o a f f e c t a s i g n i f i c a n t l y g r e a t e r depth. I n a d d i t i o n , t h e r e a r e suggestions o f some m o d i f i c a - t i o n o f hydrogen c o n c e n t r a t i o n i n t h e b u l k o f t h e sample. The q u a l i t a t i v e c o n c l u - s i o n s have been c o n f i r m e d i n a second s e r i e s o f experiments.
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