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ELECTROREFLECTANCE STUDY OF ELECTRONIC STRUCTURE IN a-Si : H
S. Nonomura, H. Okamoto, T. Nishino, Y. Hamakawa
To cite this version:
S. Nonomura, H. Okamoto, T. Nishino, Y. Hamakawa. ELECTROREFLECTANCE STUDY OF
ELECTRONIC STRUCTURE IN a-Si : H. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-761-
C4-764. �10.1051/jphyscol:19814166�. �jpa-00220790�
CoZloque
C4,suppldment au nO1O, Tome octobre 1981 page
ELECTROREFLECTANCE STUDY OF ELECTRONIC STRUCTURE I N a - S i : H
S . Nonomura, H . Okamoto, T . Nishino and Y . Hamakawa
Faculty o f Engineering Science, Osaka University, Toyonaka, Osaka, Japan
ABSTRACT.- A s e r i e s of e l e c t r o r e f l e c t a n c e ( E R 1 s t u d y has been made on v a r i o u s p a r a m e t r i c system o f a-Si:H. A d r a s t i c a l change i n t h e ER s i g n a l w i t h t h e f i l m p r e p a r a t i o n c o n d i t i o n s h a s been found. High power d e p o s i t i o n changes t h e ER s p e c t r a from a s i n g l e broad peak t o a c r y s t a l l i n e - l i k e ER f e a t u r e . While, t h e ER spectrum sharpened g r a d u a l y w i t h i n c r e a s i n g s u b s t r a t e temperature. These t r e n d can be i n t e r p r e t e d by assuming an e x i s t e n c e of very s m a l l p u r e s i l i c o n c r y s t a l l i t e s surrounded by hydrogen r i c h r e g i o n i n a-Si:H. Based upon t h e experimental d a t a , we d i s c u s s t h e o p t i c a l t r a n s i t i o n p r o c e s s and r e l a t e d e l e c t r o n i c s t a t e s n e a r t h e fundamental edge i n conjunction w i t h t h e c o r r e s p c d i n g change of o p t i c a l a b s c r p t i o n s p e c t r a .
INTRODUCTION.- I t h a s been w e l l experimented t h a t modulation spectroscopy is a u s e f u l l t o o l f o r t h e c h a r a c t e r i z a t i o n o f t h e band s t r u c t u r e o f s o l i d s [ l l . The f i r s t a t t e m p t o f t h e s e measurements on modulated e l e c t r o - o p t i c a l s i g n a l s of plasma d e p o s i t e d a-Si:H has been made by Okamoto e t a l . [ 2 1 . F i g u r e 1 shows an example of e l e c t r o - r e f l e c t a n c e s p e c t r a (ER) of a-Si:H n e a r t h e fundamental o p t i c a l edge (?.1.8eV). The s i g n a l e x h i b i t s a s t r u c t u r e which is c h a r a c t e r i s t i c o f t h e Mo c r i t i c a l p o i n t i n a c r y s t a l l i n e semiconductor. Freeman e t a l . have a l s o r e p o r t e d ER s p e c t r a p o s s e s s i n g o n l y one d i s t i n c t peak i n s u p u t t e r e d a-Si:H[31. ER s p e c t r a o v e r a wide energy region i n plasma d e p o s i t e d a-Si:F:H[41 have been r e c e n t l y demonstrated by Tsu e t a l . .
T h e i r ER s p e c t r a r e v e a l s t r u c t u r e s around 2eV which f a i r l y correspond t o t h o s e r e p o r t e d by O k m t o e t a l . and o t h e r s t r u c t u r e s due t o h i g h e r i n t e r b a n d t r a n s i t i o n i n t h e h i g h energy r e g i o n .
S i n c e o u r f i r s t t r i a l of ER s t u d y on a-Si:H, we have conducted a s e r i e s of s y s t e m a t i c measurements f o r t h e e l e c t r o r e f l e c t a n c e s i g n a l on a-Si:H a s a f u n c t i o n o f v a r i o u s d e p o s i t i o n parameters. I n t h i s p a p e r , among t h e s e d a t a we r e p o r t t h e change i n t h e e l e c t r o r e f l e c t a n c e s p e c t r a w i t h r f power f o r decomposition of monosilane and s u b s t r a t e temperature, and d i s c u s s t h e e l e c t r o n i c s t r u c t u r e n e a r t h e fundamental edge of a-Si:H.
EXPERIMENT.- Hydrogenated amorphous s i l i c o n f i l m s were d e p o s i t e d by RF plasma d e p o s i t i o n from monosilane ( s i H d i l u t e d w i t h hydrogen t o 10%. 4The
WOTON ENERGY h v tev) c a p a c i t i v e l y coupled c l o s s - f i e l d system
2.0 1.8 1.6 1 . 4 [5,61 w i t h a 13.56MHz o s c i l l a t o r was
3 employed f o r t h e decomposition of SiH4.
The d i s t a n c e betweeen t h e p o s i t i v e
-
2 column and t h e s u b s t r a t e was c o n t r o l l e du
-
C from l O c m t o 18cm and t h e r f power was
I v a r i e d from 35W t o 140W. The t o t a l
- e gas flow r a t e was 50sccm w i t h a g a s
E p r e s s u r e o f about 21.3Torr.
9 0
E l e c t r o r e f l e c t a n c e measurements have been c a r r i e d o u t by u s i n g f i e l d
- I modulation b o t h i n t h e Schottky b a r r i e r
6000 7000 MOO 9000 AU (?.50~l/a-Si : ~ / n - t y p e c - ~ i (0. lRan)
MAM LENGTH A
(i)
and SnO /p-i-n c o n f i g u r a t i o n . ~t hasFig. 1 FieLd modulntzd ~ e f l e e t m e IARI o f been p o l n t e d o u t by Brodsky e t a l . t h a t 2 a-Si:H obtained by Okamoto e t aZ. Film ER s i g n a l o f t e n i n v o l v e s a c o n s i d e r a b l e
t h i c k n e s s i s about 4 P and applied modulation t h e n n o r e f l e c t a n c e components[7]. To voltage is 30V.
remove such t h e r m o r e f l e c t a n c e c o n t r i - b u t i o n coming from thermal h e a t i n g due
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814166
C4-762 JOURNAL DE PHYSIQUE
t o t h e c u r r e n t p a s s i n g through t h e f i l m , we have c a r e f u l l y formed t h e j u n c t i o n t o e l t m i n a t e t h e dark thermal c u r r e n t component and a p p l i e d t h e e l e c t r i c f i e l d t o t h e sample i n such a way t h a t c u r r e n t flow is n e g l i g i b l y s m a l l . Furthermore, t h e ER s i g n a l of a-Si f i l m is u s u a l l y accompanied by many i n t e r f e r e n c e f r i n g e s a s shown i n Fig. 1. To minimize such i n t e r f e r e n c e f r i n g e s we have used very t h i n a-Si:H f i l m s with t h e t h i c k n e s s of about 1500A. By checking t h e i n v a r i a n c e o f t h e ER l i n e s h a p e w i t h t h e a n g l e o f l i g h t i n c i d e n c e a t t h e f i l m s u r f a c e , we have confirmed t h a t t h e ER s i g n a l a r e almost f r e e from t h e i n f l u e n c e o f i n t e r f e r e n c e f r i n g e s .
EXPERIMENTAL RESULTS & DISCUSSION.- F i r s t l y , we demonstrate t h e v a r i a t i o n o f ER s p e c t r a n e a r t h e o p t i c a l gap with r e s p e c t t o r f power d u r i n g t h e plasma d e p o s i t i o n of a - S ~ : H f i l m s . F i g u r e 2 shows a s e t of experimental d a t a f o r r f powers of P =35,
r f . 100 and 140W, r e s p e c t i v e l y . ER measurments were c a r r i e d o u t a t room temperature with a c o n s t a n t modulation v o l t a g e of 3V i n t h e S c h o t t k y - b a r r i e r geometry. A s s e e n i n t h e f i g u r e , t h e ER spectrum of Prf=35W has a broad s i n g l e peak and resembles ER s p e c t r a o b t a i n e d with s p u t t e r e d a-Si:H f i l m s . I n c r e a s i n g r f power, t h e spectrum comes t o e x h i b i t s i m i l a r f e a t u r e s t o t h o s e seen n e a r an Mo c r i t i c a l p o i n t i n c r y s - t a l l i n e semiconductors. These experimental b e h a v i o r s a r e i n t e r p r e t e d a s f o l l o w s . O p t i c a l t r a n s i t i o n s between extended s t a t e s i n t h e completely d i s o r d e r e d system would h a r d l y be a l t e r e d by t h e presence of t h e e x t e r n a l modulation e l e c t r i c f i e l d because t h e l i f e t i m e o f an e l e c t r o n i n t h e h y p o t h e t i c a l Bloch s t a t e s i s s o s h o r t . That i s , t h e l i f e t i m e broadening p r e v a i l s t h e i n f l u e n c e of e x t e r n a l e l e c t r i c f i e l d t o diminish t h e modulated r e f l e c t a n c e s i g n a l a s s o c i a t e d w i t h t r a n s i t i o n s between t h e extended s t a t e s . This i s t h e c a s e seen i n a-Si:H prepared w i t h low power plasma d e p o s i t i o n o r u s u a l s p u t t e r i n g , where t h e f i e l d induced change i n t h e d i e l e c t r i c c o n s t a n t might be dominated by t h a t o f t r a n s i t i o n s between d i f f u s e d t a i l s t a t e s and extended s t a t e s a s suggested by J a l a l i and Weiser[91. On t h e o t h e r hand, ER s p e c t r a i n plasma d e p o s i t e d a-Si:H with h i g h e r r f power e x h i b i t broad b u t d i s t i n c t s t r u c t u r e o r i g i n a t e d from t h e f i e l d induced change i n o p t i c a l t r a n s i t i o n s between p a r a b o l i c extended bands. This i m p l i e s t h a t t h e h i g h e r power d e p o s i t i o n promotes s t r u c t u r a l o r d e r n e s s and t h u s weakens t h e l i f e t i m e broadenig.
F i g u r e 3 i n d i c a t e s t h e modulation v o l t a g e V dependence o f AR/R peak h i g h t s . The s l o p e s i n l o g AR/R vs. l o g V p l o t s a r e v e p c l o s e t o 2 i n a l l t h e peaks i n ER s p e c t r a . The e l e c t r i c f i e l d a 8 8 u l d be a p p l i e d almost uniformly over t h e a-Si:H l a y e r when t h e f i l m t h i c k n e s s is very t h i n of =2000A[81 i n c o n t r a s t t o t h e c a s e of c r y s t a l l i n e b u l k semiconductors i n which t h e a p p l i e d e l e c t r i c f i e l d extends o n l y i n t h e s u r f a c e b a r r i e r r e g i o n . This r e s u l t s i n d i c a t e s t h e s q u a r e power dependence o f AR/R or. t h e modulation e l e c t r i c f i e l d . Moreover we cinfirmed t h e l i n e s h a p e i n v a r i a n c e
? I
APPLIED YOCILU. 1 Y4
F@. 2 CPange o f e l e c t r o r e f l e c t m c e s i p a l hR/R with v a r i n g the r f power dur?~ng the plasma deposi- s i a . Arrows i n d i c a t e t h e p o s i t i o n o f t h e energy gap d e t e n i n e d by three point a d j u s t i n g method.
I
I
1 .o 10 .o
APPLIED VOLTAGE I V )
F i g . 3 Modulation voltage Vo deuendeme o f
hR/R peak h i g h t s for various e o s ' i t i o n r f pomp.
The slopes of Log AR/R us. Log V are about 2.
apP
h o l d s good t o a c i r t a i n e x t e n t i n t h e c a s e o f o u r h i g h e r power d e p o s i t e d a-Si:H.
Then we c o u l d d e t e r m i n e t h e p o s i t i o n o f t h e e n e r q y g a p , t h a t i s t h e fundamental e d g e , by t h e t h r c e p o i n t a d j u s t i n g method, which is i n d i c a t e d by arrows i n F i g . 2 . While, f o r t h e l o w e r power d e p o s i t e d a-Si:H, t h e e l e c t r i c f i e l d dependence o f ER s p e c t r a i s i n c o i n c i d e n c e w i t h t h a t t h e o r e t i c a l l y v e r i f i e d by E s s e r i n t h e e l e c t r o - o p t i c a l s i g n a l a s s o c i a t e d w i t h l o c a l i z e d s t a t e s [ l O ]
.
F i g u r e 4 shows t h e s u b s t r a t e s tempera- t u r e T dependence o f ER s p e c t r a i n a-Si:H f i l m s E r e p a r e d w i t h u s u a l r f power f o r photo- RF POUER 35hl
1 5 v o l t a i c d e v i c e f a b l i c a t i o n [ 6 1 . The a r r o w s i n t h e f i g u r e i n d i c a t e t h e p o s i t i o n o f t h e o p t i c a l gap. I t is n o t i c e a b l e t h a t t h e i n c r e a s e o f s u b s t r a t e t e m p e r a t u r e s h i f t s t h e ER peak toward lower e n e r g y s i d e a c c o r d i n g t o t h e change i n t h e o p t i c a l g a p (Eo t ) , and a l s o d e c r e a s e s t h e h a l f w i d t h o f ER g p e c t r a .
T s = 3 O O 0 C
;
The narrowing o f t h e h a l f w i d t h e s mighti n d i c a t e a s h r i n k i n g o f d i f f u s e d t a i l s t a t e s
,
, , '\, w i t h i n c r e a s i n g t h e s u b s t r a t e t e m p e r a t u r e . ,; .,' \ T h i s f a c t i m p l i e s t h a t l o c a l d i s o r d e r i n t h e
1 . 5 1 . 6 1 . 7 1 . 8 1 . 9 2 . 0 amorphous network might b e smeared o u t w i t h
i n c r e a s i n g t h e s u b s t r a t e t e m p e r a t u r e . PHOTO) ENERGY 1:" [ e V ) I n F i g . 5 , we show t h e a b s o r p t i o n co-
e f f i c i e n t s p e c t r a n e a r t h e fundamental edge F'i.7. 4 C h r t g e n f elzctrorefLeczance s i p o l of t h e f i l m s used f o r ER measurements. A s w i t h oariv.2 <ti. e G s t r a : e t.mpcrqture. be seen from t h e f i g u r e , change i n t h e Arrows :niica:e the positior. 9' the o p t i c a l
gap ce,%nsd no a ctra'ght Line intercept s u b s t r a t e t e m p e r a t u r e makes a p a r a l e l s h i f t energy o f &v u s . h v y l o t s . of t h e a b s o r p t i o n s p e c t r a and a l s o a change o f t h e s l o p e i n l o g
a
v s . h~ p l o t s i n lower- . -
- photon e n e r g y r e g i o n . The f o r m e r c o r r e s -
T5* 250'C ponds t o t h e change i n t h e o p t i c a l e n e r g y
P,:. j5% gap w i t h hydrogen c o n t e n t i n t h e a-Si n e t w o r k ,
and t h e l a t t e r f a c t t e l l s t h e s p r e a d i n g s o f t h e l o c a l i z e d d i f f u s e d t a i l s t a t e s w i t h d e c r e a s i n g s u b s t r a t e t e m p e r a t u r e . While, t h e h i g h e r power d e p o s i t i o n g i v e s rise t o a d r a s t i c a l change i n a b s o r p t i o n s p e c t r a a s .
.
found i n ER s p e c t r a shown i n f i g . 2 . P r o b a b l ydue t o t h e s t r u c t u r a l o r d e r n e s s b r o u g h t a b o u t i n t h e a-Si:H n e t w o r k , t h e a b s o r p t i o n co- e f f i c i e n t i n h i g h e r power d c p o s i t c d a-Si:H can b e no l o n g e r f i t t e d t h e dahv v s . hv p l o t s which i s c h a r a c t e r i s t i c i n t h e n o n - d i r e c t o p t i c a l t r a n s i t i o n u n i q u e t o d i s o r d e r e d s y s -
A----- tcm. These o b s e r v a t i o n s a l s o s u p p o r t t h e
1.5 2.0 2 . 5 3 . 0 i n t e r p r e t a t i o n o f t h e r f power o r s u b s t r a t e
temperature dependence o f ER s p e c t r a .
FUOTOh i t i L R G Y o r ( e v l
Amorphous h y d r o g e n a t e d S i i s c o n s i d e r e d t o c o n s i s t o f v e r y s m a l l p u r e s i l i c o n i s l a n d s
~ i 5 ~ AbsoTticn c o e f f i c i e n t s?ectra o f . s u r r o u n d e d by t h e Si-H r i c h r e g i o n [ 7 ] . The a-si r r i prepared )with varing substrate p u r e s i l i c o n i s l a n d c o u l d b e r e g a r d e d a s temperature, 200°C, ZSC°C, 300°C and r f s i l i c o n c r y s t a l l i t e s p u t under a s t r o n g s t r a i n p w e r s , 35V, IOOY, 140K.
f i e l d c a u s e d by t h e s u r r o u n d i n g hydrogen r i c h r e g i o n . A c c o r d i n g l y , t h e o v e r a l l e l e c t r o n i c p r o p e r t i e s c l o s e l y c o r r e l a t e w i t h t h e c r y s t a l l i t e s i z e o f b o t h r e g i o n s and a l s o t h e s t r e n g t h o f t h e s t r a i n f i e l d . The h i g h e r power d e p o s i t i o n might enhance t h e s i z e and c o n c e n t r a t i o n o f p u r e s i l i c o n i s l a n d , which r e c o v e r s t h e wave v e c t o r c o h e r e n c e o r i n o t h e r words t h e l i f e t i m e o f a n e l e c t r o n i n t h i s r e g i o n and t h u s l e a d t o c r y s t a l l i n e - l i k e ER s p e c t r a . However, w i t h i n t h e e x t e n t o f o u r d e p o s i t i o n c o n d i t i o n s , t h e c r y s t a l l i t e s i z e is s t i l l s o s m a l l t o b e d i r e c t l y d e t e c t e d by X-ray a n a l y s i s . With t h e a i d o f h i g h d e n s i t y o f hydrogen
C4-764 JOURNAL DE PHYSIQUE
r a d i c a l s , we could grow up t h e c r y s t a l l i t e s i z e s u f f i c i e n t l y l a r g e t o t h e regime of m i c r o c r y s t a l l i n e . As was p r e s e n t e d by Tsu e t a l . t h i s m i c r o c r y s t a l l i n e 8 i ~ ~ e x h i b i t s a s i m i l a r ER f e a t u r e t o o u r h i g h e r power d e p o s i t e d Si:H n e a r t h e fundamental edge r e g i o n , and moreover some s t r u c t u r e s a r e found i n t h e h i g h e r energy r e g i o n . T h i s f a c t can b e understood by assuming a f u t h e r recovery of wave v e c t o r coherence which b r i n g about ER s i g n a l a s s o c i a t e d w i t h t h e i n t e r b a n d t r a n s i t i o n . On t h e o t h e r hand, t h e random s t r a i n f i e l d o r i g i n a t e d i n t h e s t r u c t u r a l mismatching a t t h e c r y s t a l l i t e / Si-H i n t e r f a c e would b e weakend by moderately h i g h s u b s t r a t e temperature. T h i s r e s u l t s i n t h e d e c r e a s e o f d i f f u s e d t a i l s t a t e s and t h e s h r i n k i n g o f ER s p e c t r a . The c h a r a c t e r i s t i c f e a t u r e o f ER s p e c t r a i n a-Si:H prepared w i t h v a r i o u s d i f f e r e n t c o n d i t i o n s could b e i n t e r p r e t e d on t h e b a s i s of t h e simple s t r u c t u r a l models on a-Si:H. So f a r a s t h e s e models a r e valued, ER method can be u t i l i z e d a s a good t o o l f o r t h e l o c a l s t r u c t u r a l o r d e r n e s s and t h e s p r e a d i n g of t h e d i f f u s e d t a i l s t a t e s i n a-Si:H.
SUMMARY.-
R e s u l t s o f e l e c t r o r e f l e c t a n c e (ER) s t u d i e s r e v e a l e d t h a t ER s i g n a l depends c r i t i c a l l y on t h e f i l m p r e p a r a t i o n c o n d i t i o n s , rf power and s u b s t r a t e temperature.With i n c r e a s i n g r f power, t h e f e a t u r e o f ER s i g n a l v a r i e s from a s i n g l e broad peak t o a s t r u c t u r a l s i g n a l s i m i l a r t o t h a t seen i n c r y s t a l l i n e semiconductors n e a r t h e M c r i t i c a l p o i n t . T h i s t r e n d might r e f l e c t t h e change o f t h e l o c a l s t r u c t u r a l o r d e r n e s s i n a-Si:H network. 0 While, t h e h i g h e r s u b s t r a t e temperature sharpens t h e ER peaks, which i n d i c a t e s a d e c r e a s e o f t h e l o c a l i z e d t a i l s t a t e s d i f f u s i n g from t h e extended bands. The ER measurment coupled w i t h c o n v e n t i o n a l e l e c t r o n i c and s t r u c - t u r a l s t u d i e s would g i v e a d e t a i l e d i n f o r m a t i o n on t h e s t r u c t u r e o f a-Si:H and i t s e l e c t r o n i c s t a t e s r e l a t i n g t o t h e o p t i c a l t r a n s i t i o n s .
REFERENCES.
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