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TIME RESOLVED SPECTROSCOPY OF THE PHOTOINDUCED ABSORPTION IN
GLOW-DISCHARGE a-Si:H
S. Ray, Z. Vardeny, J. Tauc
To cite this version:
S. Ray, Z. Vardeny, J. Tauc. TIME RESOLVED SPECTROSCOPY OF THE PHOTOINDUCED AB-
SORPTION IN GLOW-DISCHARGE a-Si:H. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-
555-C4-557. �10.1051/jphyscol:19814120�. �jpa-00220738�
JOURNAL DE PHYSIQUE
Colloque C4, suppZ6mennt au nOIO, Tome
42,octobre 1981
TIME RESOLVED SPECTROSCOPY O F THE PHOTOINDUCED ABSORPTION
I N
GLOW-DISCHARGE a-Si:H
S. Ray, Z . Vardeny and J. Tauc
Department of Physics and Division of Engineering, Brown University, Providence, Rhode Island 02912,
U . S.A.A b s t r a c t . - The t i m e and t e m p e r a t u r e dependence o f t h e p h o t o i n d u c e d a b s o r p t i o n s p e c t r a was measured i n glow-discharge a-Si:H i n t h e e n e r g y r a n g e 0 . 7 t o 1.4 eV, t e m p e r a t u r e i n t e r v a l 10 t o 306K and t i m e domain 2 u s t o 2 ms. The r e s u l t s ' a r e c o n s i s t e n t w i t h a model b a s e d on t h e r m a l i z a t i o n o f e x c e s s c a r r i e r s i n t r a p s t a t e s . I n t r o d u c t i o n . - I l l u m i n a t i o n o f a-Si:H i n t h e f u n d a m e n t a l a b s o r p t i o n band p r o d u c e s a midgap a b s o r p t i o n band ( p h o t o i n d u c e d a b s o r p t i o n PA).1-3 I t was shown4 t h a t t h e CW s p e c t r u m o f t h e PA band i s c o n s i s t e n t w i t h p h o t o i o n i z a t i o n o f h o l e s t r a p p e d i n s t a t e s c l o s e t o t h e v a l e n c e band i n t o t h e v a l e n c e band. The s h a p e o f t h e s p e c t r u m c a n b e e x p l a i n e d i f t h e i n i t i a l s t a t e s o f t r a p p e d h o l e s a r e c o n f i n e d t o a n a r r o w e n e r g y i n t e r v a l ( a p p r o x i m a t e d by a d e l t a f u n c t i o n ) and t h e d e n s i t y o f f i n a l s t a t e s i n t h e v a l e n c e band h a s a s q u a r e r o o t dependence. The o r i g i n o f t h e d e l t a - l i k e d i s t r i b u t i o n
o f i n i t i a l s t a t e s was a s c r i b e d t o t h e p e a k i n g o f t h e p r o d u c t ( t r a p d e n s i t y ) X ( h o l e d i s t r i b u t i o n f u n c t i o n ) a t t h e quasi-Fermi l e v e l Epf above t h e v a l e n c e band edge E, assuming t h a t t h e t r a p d e n s i t y d e c r e a s e s s h a r p l y i n t o t h e gap. We have r e c e n t l y s t u d i e d t h e time-dependence o f t h e t o t a l s t r e n g t h o f t h e PA band i n a - ~ i : ~ . ~ , ~ I n t h i s p a p e r we r e p o r t d a t a on t h e t e m p e r a t u r e and time-dependence o f t h e PA s p e c t r a and show t h a t t h e y can b e e x p l a i n e d by t h e model d i s c u s s e d below.
Model of PA spectrum.- The time-dependence i s i n t r o d u c e d i n t o t h e CW model by f o l l o w - i n g t h e i d e a s o f T i e d j e and ~ o s e ~ and O r e n s t e i n and
~ a s t n e r . ~ They c o n s i d e r e d t h e k i n e t i c s o f t h e r e - l a x a t i o n o f t r a p p e d c a r r i e r s p r e c e e d i n g t h e t h e r m a l
t r ~ p p e d e q u i l i b r i u m s t a t e . They i n t r o d u c e d a d e m a r c a t i o n
carriers
e n e r g y Ed which p l a y s t h e same r o l e a s t h e q u a s i - Fermi l e v e l i n e q u i l i b r i u m , namely s e p a r a t e s t h e o c c u p i e d and unoccupied t r a p s . They a l s o assume t h a t t h e t r a p d i s t r i b u t i o n d e c r e a s e s s h a r p l y i n t o t h e g a p s o t h a t t h e h o l e c o n c e n t r a t i o n s t r o n g l y p e a k s i n a n e n e r g y i n t e r v a l c l o s e t o Ed ( F i g . 1 ) . T h i s
" h o l e p a c k e t " moves w i t h t e m p e r a t u r e and t i m e a c c o r d - i n g t o t h e r e l a t i o n
F i g . 1
-
Model f o r photo-induced a b s o r p t i o n . E ~ ( ~ ; T ) = k ~ ~ n ( v t ) ( 1 )
where V i s a v i b r a t i o n a l f r e q u e n c y . The t i m e and t e m p e r a t u r e dependence o f t h e p h o t o i n d u c e d a b s o r p t i o n s p e c t r a s h o u l d f o l l o w t h e e q u a t i o n
Ed from Eq. ( 1 ) f i g u r e s h e r e a s t h e t h r e s h o l d e n e r g y ; we assumed c o n s t a n t m a t r i x e l e m e n t and t i w
>>
kT. Our s p e c t r a l s t u d i e s p r o v i d e f o r t h e f i r s t t i m e a d i r e c t c h e c k o f t h i s model compared t o o t h e r t r a n s p o r t measurements s u c h a s t r a n s i e n t photo- c u r r e n t and t i m e o f f l i g h t . I n o u r c a s e r e c o m b i n a t i o n t a k e s p l a c e i n p a r a l l e l w i t h t h e r m a l i z a t i o n . However, i f t h e t r a p p e d c a r r i e r d i s t r i b u t i o n i s s t r o n g l y peaked a t E d ( t , T ) we e x p e c t t h a t r e c o m b i n a t i o n d o e s n o t change t h e m o t i o n o f t h e peak ( a l t h o u g h i t changes t h e t o t a l c o n c e n t r a t i o n o f h o l e s ) s i n c e t h e r e l e a s e t i m e s o f t r a p p e d c a r - r i e r s on which t h e m o t i o n o f Ed i s b a s e d ( E ~ . ( 1 ) ) d o e s n o t depend on r e c o m b i n a t i o n .Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814120
C4-556 JOURNAL DE PHYSIQUE
Experiment.- The PA band was e x c i t e d by a dye l a s e r pumped w i t h a n i t r o g e n l a s e r w i t h t h e f o l l o w i n g p a r a m e t e r s : photon e n e r g y 2.1 eV, p u l s e d u r a t i o n 10 n s , r e p e t i t i o n r a t e 20 Hz and p u l s e e n e r g y 60115. The sample was amorphous s i l i c o n p r e p a r e d by glow d i s c h a r g e d e p o s i t i o n w i t h 16 a t . % H2 and t h i c k n e s s d = 5 p m . The PA was probed w i t h l i g h t from a t u n g s t e n lamp i n t h e e n e r g y r a n g e o f 0 . 7 t o 1 . 4 eV. The d e t e c t i o n s y s t e m c o n s i s t e d o f a Ge p h o t o d i o d e ( J u d s o n 5-16 LD), a broad band a m p l i f i e r
(PAR 1 1 5 ) f o l l o w e d by a b o x c a r i n t e g r a t o r (PARC 1 6 2 ) and a n X-Y r e c o r d e r . The t i m e r e s o l u t i o n o f t h e e x p e r i m e n t was 0 . 5 V s . The a v e r a g e e n e r g y r e s o l u t i o n o f t h e monochromator p l a c e d ' b e t w e e n t h e sample and t h e d e t e c t o r w a s = 0.03 eV.
F i g . 2
-
T e m p e r a t u r e dependence o f PA s p e c t r a F i g . 3 - Time dependence b f PA a t d e l a y t i m e t = 2 ~ s . Smooth c u r v e s a r e c a l c u - s p e c t r a a t 220 K.l a t e d from Eqs. ( 1 ) and ( 2 ) w i t h V = 1013sec-l.
R e s u l t s and D i s c u s s i o n s . - F i g u r e 2 shows t h e PA s p e c t r u m measured a t f o u r d i f f e r e n t t e m p e r a t u r e s T = 1 0 , 1 2 0 , 220 and 306 K, f o r a f i x e d t i m e t = 2 p s . The s p e c t r a a t high.er t e m p e r a t u r e s h a v e b e e n m u l t i p l i e d by c o n s t a n t s i n d i c a t e d i n t h e f i g u r e f o r b e t t e r comparison of t h e i r s h a p e s . The s t r u c t u r e s which a p p e a r i n t h e c u r v e s a r e t h e r e m n a n t s o f i n t e r f e r e n c e f r i n g e s which c o u l d n o t b e c o m p l e t e l y a v e r a g e d o u t . I n s p i t e o f b e i n g r e l a t i v e l y f e a t u r e l e s s t h e PA specrum shows a d e f i n i t e change of s h a p e w i t h i n c r e a s i n g t e m p e r a t u r e . The e n t i r e PA s p e c t r u m f o r t h e d e l a y t i m e s and t e m p e r a t u r e s u s e d i n t h e e x p e r i m e n t e x t e n d s much below 0 . 7 eV which i s t h e l i m i t o f d e t e c t i o n f o r o u r measurements. The change i n s h a p e o f t h e s p e c t r u m i n t h e measured e n e r g y r a n g e c a n be i n t e r p r e t e d a s d u e t o a s h i f t o f t h e e n t i r e s p e c t r u m t o h i g h e r e n e r g i e s w i t h i n c r e a s i n g T. F i g u r e 3 shows t h e PA s p e c t r u m measured a t f o u r d i f f e r - e n t t i m e s t = ~ U S , 2 0 ~ s , 0 . 2 ms and 2 ms f o r a f i x e d t e m p e r a t u r e T = 220 K , w i t h t h e s p e c t r a a t l o n g e r t i m e s m u l t i p l i e d by a p p r o p r i a t e c o n s t a n t s . We n o t e t h e change i n s h a p e o f t h e s p e c t r u m showing r o u g h l y e q u a l s h i f t s o f t h e s p e c t r u m t o h i g h e r e n e r g i e s when t c h a n g e s by a d e c a d e .
3 5 The s h i f t s o f t h e s p e c t r u m c a n b e s e e n more
3 0 1 0 q u a n t i t a t i v e l y by e x t r a c t i n g t h e a b s o r p t i o n
- -
t h r e s h o l d Ed from t h e measured s p e c t r u m2 2.5 u s i n g Eq. ( 2 ) . F i g u r e 4 shows t h e n o r m a l i z e d
-
P 2.0 o s p e c t r a o f PA p l o t t e d a s ~ I ? ~ A T / T ] ~ v s . T i w0 6
-
P f o r t h e s p e c t r u m a t T = 10 K , t = 2 ~ S and-
p 1.5+=
-
a t T = 306 K , t = 2p S (.ATIT = dAcc). The f i tO 4 2 t o Eq. ( 2 ) i s good o v e r t h e e n e r g y r a n g e o f
21-
l 0 t h e measurement ; t h i s p e r m i t s t h e d e t e r m i n a -0 5 t i o n o f Ed. S i m i l a r p l o t s f o r t h e r e m a i n i n g
0 0 o.o s p e c t r a shown i n F i g . 2 and 3 w e r e a l s o i n
0.0 0.2 0.4 0 6 0 8 1 0 12 1.4 good agreement w i t h Eq. ( 2 ) and y i e l d e d
hY I B V I t h r e s h o l d e n e r g i e s i n t h e r a n g e from 0 . 0 5 eV
t o 0 . 4 4 eV ( F i g . 5 ) . F i g . 4
-
Normalized PA s p e c t r a( a r r o w s show Ed).
F i g u r e 5 ( a ) shows a p l o t o f Ed, o b t a i n e d from t h e
;:v
measured ( t = 2 1.1 S ) . PA s p e c t r a , We s e e t h a t a s a Ed f u n c t i o n depends o f l i n e a r l y T f o r a on f i x e d T i n t0 3
agreement w i t h Eq. ( 1 ) . Using t h e s l o p e o f t h e Ed v s . T c u r v e we o b t a i n v = 1 0 ~ ~ s e c - ~ which i s a r e a s o n a b l e
W 0 2 v a l u e . I n F i g . 5 ( b ) we p l o t t e d Ed a s a f u n c t i o n o f
0 t l o g t f o r a f i x e d T ( T = 220 K ) . The s t r a i g h t l i n e b e h a v i o r i s a l s o i n agreement w i t h Eq. ( l ) . The s h i f t
0 0
.
o 2 0 0 noo of Ed a t 10 K was checked a t t = 2l.1 s and t = 2ms;
w i t h i n o u r e x p e r i m e n t a l l i m i t s o f d e t e r m i n i n g Ed
(L
0.02 eV), Ed d o e s n o t show any s h i f t , i n agreemento 40 w i t h Eq. ( l ) .
T 2 2 0 x
5 03, F i n a l l i n s e r t i n g Eq. ( 1 ) i n Eq. ( 2 ) and u s i n g v = 10Y3sec-l we c a n c a l c u l a t e t h e s h a p e o f t h e PA
0 3 0
.
s p e c t r u m a s a f u n c t i o n o f T and t . The smooth c u r v e s0 Z J , , i n F i g s . 2 and 3 a r e t h e c a l c u l a t e d s p e c t r a f o r t h e
I - 0- 10.' 0 ' NO-' t e m p e r a t u r e s and d e l a y t i m e s o f o u r e x p e r i m e n t ; it i s
t (1lC1
s e e n t h a t t h e f i t t o t h e measured s p e c t r a i s good. The e f f e c t o f t h e s h i f t o f t h e s p e c t r u m w i t h t i m e on t h e F i g . 5
-
T h r e s h o l d e n e r g y d e c a y o f t h e i n t e g r a t e d PA band was c a l c u l a t e d and Ed a s a f u n c t i o n o f found t h a t i t s i n f l u e n c e on t h e power law r e p o r t e d i n t e m p e r a t u r e ( c u r v e a ) and R e f s . 5 and 6 was s m a l l ( t h e e r r o r i n t h e d i s p e r s i o n t i m e ( c u r v e b ) . p a r a m e t e r a i s s m a l l e r t h a n 5% a t 300 K and l e s s a tlower t e m p e r a t u r e s ) .
C o n c l u s i o n s . - We showed t h a t t h e t i m e and t e m p e r a t u r e dependence o f t h e PA s p e c t r a i n GD a-Si:H c a n b e d e s c r i b e d w i t h Eq. ( 1 ) and ( 2 ) w i t h a s i n g l e a d j u s t a b l e p a r a m e t e r v whose v a l u e i s 1013sec-l. T h i s m a t e r i a l i s a p a r t i c u l a r l y s i m p l e c a s e , a p p a r e n t l y b e c a u s e t h e d e n s i t y o f t r a p s t a t e s d e c r e a s e s v e r y s h a r p l y i n t o t h e g a p . I n c o n t r a s t ,
i n s p u t t e r e d a - ~ i : ~ ~ s u c h a s i m p l e a n a l y s i s d o e s n o t a p p e a r t o b e a p p l i c a b l e p r o b a b l y b e c a u s e t h e t r a p d e n s i t y i s n o t s o s h a r p . A d i f f i c u l t y o c c u r s a l s o w i t h a-As2Se3 i n which t h e PA s p e c t r u m c a n n o t b e d e s c r i b e d by Eq. ( 2 ) and t h e d e t e r m i n a t i o n o f Ed from t h e s p e c t r a h a s n o t b e e n f e a s i b l e .
We t h a n k B. A b e l e s f o r k i n d l y p r o v i d i n g u s w i t h samples o f GD a-Si:H and T. R. K i r s t f o r t e c h n i c a l a s s i s t a n c e . The work was s u p p o r t e d i n p a r t by NSF g r a n t DMR-79-09819 and NSF-MRL program a t Brown U n i v e r s i t y .
R e f e r e n c e s .
P. O'Connor and J. Tauc, Phys. Rev. L e t t . 4 3 , 3 1 1 ( 1 9 7 9 ) .
P. O'Connor and J. Tauc, J . Non-Cryst. s o l 7 3 5 and
2,
699 ( 1 9 8 0 ) .M. O l i v e r , J . P e n z i n , A. Chenevas-Paule, J . 6 n - c r y s t . S o l . 21_ and
2,
693 (1980).P. O'Connor and J. Tauc, S o l . S t . Comm.
2,
947 ( 1 9 8 0 ) .2. Vardeny, P. O'Connor, S . Ray and J . Tauc, Phys. Rev. L e t t .
46,
1267 ( 1 9 8 0 ) . S. Ray, 2. Vardeny, J . Tauc, T. Moustakas and B. A b e l e s , I n t . Conf. onT e t r a h e d r a l l y Bonded Amorphous Sem., C a r e f r e e , A r i z o n a , 1981. AIP Conf. P r o c . ( i n p r i n t ) .
T. T i e d j e and A. Rose, S o l . S t . Commun. 3 7 , 49 ( 1 9 8 1 ) .
J. O r e n s t e i n and M. K a s t n e r , Phys. Rev. Ett. ( t o b e p u b l i s h e d ) ; J. O r e n s t e i n Ph.D. T h e s i s , MIT ( 1 9 8 1 ) .
