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ELECTRON IRRADIATION IN AMORPHOUS HYDROGENATED SILICON
R. Navkhandewala, K. Narasimhan, S. Guha
To cite this version:
R. Navkhandewala, K. Narasimhan, S. Guha. ELECTRON IRRADIATION IN AMORPHOUS HYDROGENATED SILICON. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-803-C4-806.
�10.1051/jphyscol:19814176�. �jpa-00220801�
JOURNAL DL PHYSIQUE
C~ZZoque C 4 , suppldment au nOIO, 'l'ome 4.2, octobre 1981 page C4-803
ELECTRON I R R A D I A T I O N I N AMORPHOUS HYDROGENATED S I L I C O N
R . V . Navkhandewala, K . L . Narasirntlan and S. Guha
Tata I n s t i t u t e o j FuizclamentaZ l<c?search, Bombay
-
400005, IndiaAbstract. - T h e d a r k conductivity of amorphous hydrogenated s i l i c o n is found t o i n c r e a s e immediately a f t e r e l e c t r o n i r r a d i a t i o n . The conductivity d e c r e a s e s a s a function of t i m e and s e t t l e s a t a value lower than the original one a f t e r room t e m p e r a t u r e anneal f o r 12 hours. The photoconductivity i s a l s o found t o b e lower. F o r t h e s a m e dose of irradiation, the d e c r e a s e in photoconductivity i s found t o be much l a r g e r in P-doped s a m p l e s in compa- r i s o n t o undoped ones. Moreover, while annealing at 2 0 0 ~ ~ f o r 6 h o u r s r e m o v e s the defects in undoped s a m p l e s , the removal r a t e in P-doped s a m - ples i s slower. P r e s e n c e of Li i n c r e a s e s t h e r e m o v a l r a t e substantially.
T h e r e s u l t s a r e discussed in t e r m s of the vacancies and the vacancy - impurity complexes c r e a t e d by irradiation.
Introduction.
-
Amorphous hydrogenated silicon (a-Si : H ) prepared f r o m glow- d i s c h a r g e decomposition of s i l a n e h a s received a g r e a t deal of attention a s a poten- t i a l s o l a r cell m a t e r i a l (1). E a r l i e r experiments on vacuum deposited amorphous s i l i c o n f i l m s (2,3) showed that the e l e c t r i c a l conductivity of t h e s e m a t e r i a l s is r a t h e r i n s e n s i t i v e t o e l e c t r o n irradiation. T h i s is due t o the l a r g e number of g a p s t a t e s i n t h e s e f i l m s (4) and it h a s been shown ( 5 , 6 ) that in glow-discharge f i l m s which have v e r y few s t a t e s i n the gap, e l e c t r o n i r r a d i a t i o n causes defects which i n c r e a s e t h e spin density and quench t h e photo-luminiscence. We have c a r r i e d out e x p e r i m e n t s t o study t h e effect of e l e c t r o n i r r a d i a t i o n on the d a r k and photoconduc- tivity of a-Si : H films. T h e r e s u l t s f o r undoped f i l m s together with s o m e p r e l i - m i n a r y r e s u l t s on f i l m s doped with P and Li a r e reported in this paper.E x p e r i m e n t a l details, - T h e s a m p l e s w e r e m a d e by d c d i s c h a r g e of 10% SiH4-90%
H2 m i x t u r e a t a p r e s s u r e of 0. 5 T and s u b s t r a t e t e m p e r a t u r e of 3 0 0 ~ ~ . T h e details of sample preparation and character isation a r e discussed elsc.vhere (7,s).
Phosphorus doping is achieved by mixing p H 3 with s i l a n e ; Li doping i s obtained by L i evaporation and diffusion (9). Corning 7059 g l a s s e s with pre-deposited NiCr-Sb contacts, I m m apart, a r e used a s substrates. Typical f i l m thickness i s about l p m . E l e c t r o n i r r a d i a t i o n was c a r r i e d out using a pulsed l i n e a r a c c e l e r a t o r to a t o t a l d o s e of 2. 1015 ~ m - ~ . The s a m p l e was kept in vacuum during i r r a d i a t i o n and a l l m e a s u r e m e n t s w e r e c a r r i e d out i n situ. Before any i r r a d i a t i o n , the s a m p l e s w e r e h e a t dried by annealing a t 2 0 0 ~ ~ f o r 2 hours.
Results.
-
The dependence of d a r k conductivity of a n undoped s a m p l e a s a function of t i m e immediately a f t e r i r r a d i a t i o n i s shown in Fig. 1. We find that immediately a f t e r i r r a d i a t i o n t h e conductivity i s l a r g e r than t h e initial value and slowly s,ettles a t a value below that of original conductivity. Photoconductivity of t h e s a m p l e i s founda able
1 ) t o vary with intensity a sd
= AIv w h e r e 3-
0.8 both b e f o r e andC h
a f t e r irradiation. T h e magnitude of photoconductivity d e c r e a s e s b y about a f a c t o r
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814176
C4-804 JOURNAL DE PHYSIQUE
i
Fig. 1 : Plot of d a r k conductivity after i r r a d i a t i o n a s a function of time.
cha refers
t o the initial conduc-C tivity before irradiation
and'^'
is theU
3 conductivity 1 6 hours a f t e r
-0 c irradiation.
0
0 B-.
Y
n b
,(j10 I I I I I0 10 20 30 40 50 60
Time
(minutes)of 16 after i r r a d i a t i o n Annealing a t 200 0 C f o r about 6 h o u r s i n vacuum r e s t o r e s both the d a r k and photoconductivity t o the original values. We have c a r r i e d out
experiments on a l a r g e number of s a m p l e s and although we find a s c a t t e r in the value of d a r k conductivity immediately after irradiation, the s t a b l e values of both d a r k and photoconductivity a r e found t o be similar.
Table 1. Characterisation of undoped and doped films before and a f t e r irradiation
. . .
Sample No. 168 252A 25 3A 252B 253B
. . .
Dopant Undoped P P L i i P L i + P
-
1 -8bdi
(ohm c m ) 1. 3. 2. 8. 10 2. 4. 2.8.-
1 2. 8. 4. 4. 3. 8. 10-
55hi
(ohm cm) 8. 7. 6.Nomenclature: Gdi = initial d a r k conductivity,
Gphi
= initial photo-conductivity, A t = activation energy, Gdf = d a r k conductivity 2 min. after irradiation,fdf2 = d a r k conductivity 12h after irradiation,
G
hf2 = photoconductivity 12h after irradiation.Yi , vf2
= values of exponent Y i n i t i d l y and 12h after irradiation.Photoconductivity values a r e f o r a n intensity a€ 50 p w / c m 2 incident on t h e sample f r o m Na-lamp.
F o r s a m p l e s v e r y l i g h t l y d o p e d w i t h P, t h e d a r k conductivity c h a n g e s s i m i - l a r l y with t i m e . T h e e x p o n e n t L) i s s l i g h t l y l o w e r ( T a b l e 1). T h e s t a b l e v a l u e of p h o t o c o n d u c t i v i t y a t r o o m t e m p e r a t u r e h o w e v e r i s a n o r d e r of m a g n i t u d e l o w e r . M o r e o v e r , e v e n a f t e r p r o l o n g e d (100h) h e a t t r e a t m e n t a t 200°c, t h e o r i g i n a l p h o t o c o n d u c t i v i t y v a l u e s c a n n o t b e f u l l y r e s t o r e d and a r e l o w e r by a f a c t o r of 4.
If L i i s i n c o r p o r a t e d i n a d d i t i o n t o P ( T a b l e l ) , t h e a n n e a l i n g k i n e t i c s i s q u i t e d i f f e r e n t f r o m o n l y P - d o p e d s a m p l e . T h e d e c r e a s e i n p h o t o c o n d u c t i v i t y is m u c h l o w e r . M o r e o v e r , t h e o r i g i n a l c o n d u c t i v i t y a n d p h o t o c o n d u c t i v i t y c a n now b e r e s t o r e d b y a n n e a l i n g a t 2 0 0 ~ ~ .
D i s c u s s i o n s .
-
E l e c t r o n i r r a d i a t i o n a t h i g h e n e r g y c r e a t e s point d e f e c t s u n i f o r m l y d i s t r i b u t e d in t h e m a t e r i a l (10). In t h e c a s e of undoped h y d r o g e n a t e d s i l i c o n , t h e d e f e c t s c o n s i s t of both m i s s i n g s i l i c o n a n d h y d r o g e n a t o m s . E a c h m i s s i n g s i l i c o n a t o m c r e a t e s f o u r d a n g l i n g b o n d s w h e r e a s a m i s s i n g h y d r o g e n a t o m g i v e s r i s e t o a s i n g l e d a n g l i n g bond. T h e d a n g l i n g b o n d s , i s o l a t e d o r r e - s t r u c t u r e d , w i l l p r o d u c e s t a t e s i n t h e g a p a t d i f f e r e n t l o c a t i o n s i n t h e e n e r g y s p a c e . M o r e o v e r t h e d e f e c t c a n f o r m c o m p l e x e s w i t h t r a c e s of i m p u r i t y t h a t a r e a l w a y s p r e s e n t i n t h e f i l m t o g i v e a d d i t i o n a l s t a t e s i n t h e gap. In a - S i : H, t h e i m p u r i t i e s could b e oxygen.n i t r o g e n o r c a r b o n . A l l t h e s e s t a t e s w i l l h a v e p r o n o u n c e d e f f e c t on t h e d a r k and p h o t o c o n d u c t i v i t y of t h e m a t e r i a l .
With s u c h a c o m p l e x s e t of d e f e c t s c a u s e d b y i r r a d i a t i o n , i t i s not p o s s i b l e t o e x p l a i n t h e e x p e r i m e n t a l d a t a q u a n t i t a t i v e l y a n d o n l y s o m e g e n e r a l o b s e r v a t i o n s w i l l b e m a d e . We f i n d t h a t t h e r e a r e t w o t y p e s of d e f e c t s with d i f f e r e n t a n n e a l i n g b e h a v i o u r . O n e s e t of d e f e c t s w h i c h g e t s a n n e a l e d a t r o o m t e m p e r a t u r e i n c r e a s e s t h e c o n d u c t i v i t y . We s u s p e c t t h a t t h i s d e f e c t is a s s o c i a t e d w i t h o x y g e n s i n c e i n o n e s a m p l e i n w h i c h I . a b s o r p t i o n s h o w e d a m u c h l a r g e r o x y g e n c o n t e n t (1%) t h a n n o r m a l t h e i n i t i a l i n c r e a s e i n c o n d u c t i v i t y w a s m o r e . T h e o t h e r s e t of d e f e c t s i s m o r e s t a b l e . S i n c e t h e e x p o n e n t
3
d o e s not c h a n g e o n i r r a d i a t i o n , f o l l o w i n g R o s e (1 1 ) it a p p e a r s t h a t i r r a d i a t i o n c r e a t e s new s t a t e s b e t w e e n t h e F e r r n i l e v e l a n d t h e q u a s i - F e r m i l e v e l w i t h o u t a n y c h a n g e i n t h e s h a p e of t h e d i s t r i b u t i o n in e n e r g y . S i n c e t h e F e r m i l e v e l is a b o u t 0 . 6 eV b e l o w t h e c o n d u c t i o n b a n d , w e c o n - c l u d e t h a t t h e new s t a t e s a r e l o c a t e d s l i g h t l y a b o v e t h i s e n e r g y . It i s i n t e r e s t i n g t o n o t e t h a t t h e 0. 9 eV l u m i n i s c e n c e i n i r r a d i a t e d a - S i : H h a s b e e n a t t r i b u t e d (1 2) t o a l e v e l a r o u n d t h i s e n e r g y c a u s e d b y d a n g l i n g b o n d s .When P is i n c o r p o r a t e d i n t h e s a m p l e ,
3
d e c r e a s e s s l i g h t l y i n a g r e e m e n t w i t h t h e r e s u l t s o b t a i n e d f r o m o t h e r l a b o r a t o r i e s (13). S i n c e i r r a d i a t i o n c a u s e s a m u c h l a r g e r d e c r e a s e i n p h o t o c o n d u c t i v i t y t h a n i n undoped s a m p l e s , it i s a p p a r e n t t h a t a d d i t i o n a l s t a t e s i n t h e g a p a r e c r e a t e d d u e t o t h e f o r m a t i o n of p h o s p h o r u s - v a c a n c y c o m p l e x . In c r y s t a l l i n e S i doped w i t h P, i t i s known (10) t h a t i r r a d i a t i o n c a u s e s a c c e p t o r c e n t r e s a b o u t 0. 4 eV b e l o w c o n d u c t i o n b a n d (E - C e n t r e ) . In t h e c a s e of a - S i : H, s u c h a l e v e l w i l l a c t a s d e e p e l e c t r o n t r a p and w i t h i l l u m i n a t i o n , as t h e q u a s i - F e r m i l e v e l s w e e p s t h r o u g h i t , w i l l a c t a s a r e c o m b i n a t i o n c e n t r e . A d e c r e a s e i n p h o t o c o n d u c t i v i t y i s t h e r e f o r e e x p e c t e d . In c r y s t a l l i n e s i l i c o n , t h e E - c e n t r e c a n be a n n e a l e d out a t 1 50°c (1 4). T h i s is not t h e c a s e i n a - S i : H and a m o r e c o m p l e x a n n e a l i n g m e c h a n i s m p o s s i b l y e x i s t s .T h e a n n e a l i n g p r o c e s s i s c o n s i d e r a b l y e n h a n c e d i n t h e p r e s e n c e of Li. T h i s is s i m i l a r t o r e s u l t s i n c r y s t a l l i n e S i w h e r e i n t h e p r e s e n c e of L i , t h e i r r a d i a t i o n - i n d u c e d d e f e c t s a r e known (1 5 ) t o a n n e a l out m u c h f a s t e r . In oxygen - f r e e P - d o p e d m a t e r i a l , t h e d e f e c t s a n n e a l out a t r o o m t e m p e r a t u r e i t s e l f w h e r e a s in s a m p l e s i n w h i c h o x y g e n i s p r e s e n t , a n n e a l i n g a t a r o u n d 1 5 0 ° c i s n e c e s s a r y t o r e m o v e t h e d e f e c t s (16). I t i s b e l i e v e d t h a t t h e r e c o v e r y of r a d i a t i o n d a m a g e i s g o v e r n e d by
C4-806 J O U R N A L DE PllYSIQUE
t h e m o t i o n of L i - i o n s w h i c h f o r m c o m p l e x e s w i t h t h e d e f e c t t o r e m o v e t h e s t a t e s r e s p o n s i b l e f o r d e t e r i o r a t i o n of p h o t o v o l t a i c p r o p e r t y . T h e d i f f u s i o n c o n s t a n t of
L i i n o x y g e n - r i c h s i l i c o n i s l o w e r ( 1 7 ) a n d h e n c e a n n e a l i n g a t a h i g h e r t e m p e r a t u r e i s n e c e s s a r y . S i n c e o u r m a t e r i a l c o n t a i n s a b o u t 0. 3 % o x y g e n , a h i g h t e m p e r a t u r e a n n e a l i n g i s n e c e s s a r y t o r e m o v e t h e d e f e c t s .
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