INFRARED AND FAR INFRARED ABSORPTION
OF B- AND P-DOPED AMORPHOUS Si
S. Shen, M. Cardona
To cite this version:
S. Shen, M. Cardona. INFRARED AND FAR INFRARED ABSORPTION OF B- AND PDOPED AMORPHOUS Si. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-349-C4-351.
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JOURNAL DE PHYSIQUE
CoZZoque C4, suppZ6ment au nOIO, Tome 42, octobre 1981
page C4-349
INFRARED AND FAR INFRARED ABSORPTION OF B- AND P-DOPED AMORPHOUS Si
.
S C. Shen and M. cardona*
Shanghai I n s t i t u t e o f TechnicaZ Physics, Academia Sinica, Shanghai, China
*Max-PZanck-Institut
fiir Festkiirperforschung,
7000 S t u t t g a r t 80, F . R . G .
A b s t r a c t . - The i n f r a r e d and f a r i n f r a r e d a b s o r p t i o n s p e c t r a of B- and -pdoped a-Si(H) have been measured. The e f f e c t of a n n e a l i n g on t h e s e s p e c t r a
h a s been s t u d i e d . Three l o c a l m d e s of t h e B-H bond and a band of B-Si have
been i d e n t i f i e d f o r t h e B-doped a-Si (H) The i n t r i n s i c i n f r a r e d a b s o r p t i o n
of t h e Si-Si network i s g r e a t l y enhanced b y t h e presence of t h e dopants.
.
Introduction. - W
e p r e s e n t h e r e measurements of t h e i n f r a r e d and f a r i n f r a r e d abs o r p t i o n s p e c t r a o f B- and P-doped a-Si(H) and t h e e f f e c t of a n n e a l i n g . The i n f r a r e d s p e c t r a of t h e B-doped samples show t h e c h a r a c t e r i s t i c v i b r a t i o n s of t h e B-H
and B-Si bonds a t t h e f r e q u e n c i e s above t h e fundamental v i b r a t i o n s o f t h e S i - S i
network. Upon a n n e a l i n g , a new and weaker band emerges below t h e main band of t h e
B-H bond s t r e t c h i n g band. The wagging band o f t h e Si-H i n t h e weakly B-doped sample
a t 640 cm-I i s swamped i n t h e h e a v i l y doped c a s e s by some broader band which can b e
a t t r i b u t e d t o t h e Si-B and B-H v i b r a t i o n s . A f t e r a n n e a l i n g a t 560 C, t h i s broader
band i s r e s o l v e d i n t o two c l e a r peaks and an a b s o r p t i o n s h o u l d e r , c e n t e r e d a t 620,
845 and 720 cm-l, r e s p e c t i v e l y . Besides t h e s e " l o c a l " modes t h e fundamental absorpt i o n i n t h e r e g i o n between 50 and 550 cm-' i s s t r o n g l y enhanced by t h e doping. From
t h e magnitude of this a b s o r p t i o n enhancement t h e e f f e c t i v e dynamical charges f o r
t h e Si-P and Si-B bonds can be d e r i v e d and compared with t h e p r e d i c t i o n s based on
H a r r i s o n ' s bond charge model.
Experiments and R e s u l t s . - The samples used i n t h e s e measurements were p r e p a r e d on
c-Si s u b s t r a t e s (p > 400 ohm-cm) by glow d i s c h a r g e d e c a n p o s i t i o n o f SiH4-pH3 and
SiH -B H mixtures. Samples 101 t o 103 a r e B-doped, t h e c o n c e n t r a t i o n s o f B a r e
4 2 6
0.01, 0.09 and 0.29, r e s p e c t i v e l y . The samples 1 0 4 t o 106 a r e P-doped, t h e concent r a t i o n s of P being 0.01, 0 . 1 and 0.20, r e s p e c t i v e l y . The measuring methods i n c l u d i n g t h e i n f r a r e d measurement and t h e d e t e r m i n a t i o n of t h e B and P c o n c e n t r a t i o n s
a r e d e s c r i b e d elsewhere ( 5 ) .
The a b s o r p t i o n s p e c t r a o f t h e B-doped samples a r e shown i n Fig. 1 f o r t h e
s p e c t r a l r e g i o n above t h e fundamental a b s o r p t i o n o f t h e S i m a t r i x P 5 5 0 an-'). The
s p e c t r a of t h e samples 101 and 104 a r e b a s i c a l l y t h e same a s t h o s e of undoped
samples ( 6 ) . With i n c r e a s i n g B c o n c e n t r a t i o n , one n o t e s i n F i g . 1 t h e appearance and
growth o f a band a t 2475 cm-'. Upon annealing of t h e s e samples, a n o t h e r weaker band
grows a t 2370 cm-'. F i g u r e 3 shows t h e e v o l u t i o n o f t h i s band during i s o c h r o n a l
a n n e a l i n g . A f t e r a n n e a l i n g a t 510°c, t h e i n t e g r a t e d i n t e n s i t y of this weaker band i s
about one t h i r d of t h a t of t h e main band a t 2475 cm-'. The wagging band of t h e Si-H
bond i s c l e a r l y seen a t 640 cm-' f o r sample 101, which i s r e l a t i v e l y r i c h i n hydrogen and weakly doped w i t h B. I n t h e h e a v i l y B-doped c a s e s , however, it i s swamped
by a b r o a d e r a b s o r p t i o n band. During t h e i s o c h r o n a l annealing, t h e broader absorpA.f t e r annealing a t 560°c, however, it
t i o n f e a t u r e changes l i t t l e up t o 5 1 0 ~ ~
changes d r a m a t i c a l l y and r e s o l v e s c l e a r l y i n t o two peaks and an a b s o r p t i o n shoulder
c e n t e r e d a t 640, 840, and 720 cm-', r e s p e c t i v e l y . The f i r s t of t h e s e peaks c o r r e s ponds t o Si-H wagging and t h a t a t 840 cm-' probably t o B-Si bonds. The shoulder a t
720 cm-I might be due t o a B-H wagging mode, by comparison w i t h t h e vapour, l i q u i d
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981474
JOURNAL DE PHYSIQUE
and c r y s t a l l i n e diborane ( 7 - 8 ) .
We d i d n o t f i n d any f e a t u r e s which could b e a s s o c i a t e d with t h e s t r e t c h i n g of
t h e P-H bonds, although t h e c o n c e n t r a t i o n of P i s a s h i g h a s 20 ( a t ) % . I t should be
p o i n t e d o u t , however, t h a t t h e o s c i l l a t o r s t r e n g t h of t h e P-H bond s t r e t c h i n g between 2100 and 2400 cm-' may b e t o o w a k t o be observed. I n t h e wagging mode r e g i o n
a peak i s seen a f t e r a n n e a l i n g a t 680 cm-'
It probably corresponds t o t h e wagging
of P-H bonds. Fig. 3 shows t h e a b s o r p t i o n s p e c t r a of B-doped a-Si samples i n t h e r e g i o n o f t h e i n t r i n s i c v i b r a t i o n s of t h e S i - S i network. The a b s o r p t i o n s t r e n g t h of
t h e s p e c t r a i n c r e a s e s a s a whole w i t h i n c r e a s i n g doping. T h i s happens a l s o f o r Pdoped samples: f o r sample 106, t h e i n t e g r a t e d s t r e n g t h o f t h e s p e c t r a i s 7 t i m e s
t h a t of sample 104.
.
F i g . 4 shows t h e changes o f t h e i n t e g r a t e d s t r e n g t h s o f some bands d u r i n g
t h e annealing f o r t h e B-doped sample 103. D i f f e r e n t modes have q u i t e d i f f e r e n t ann e a l i n g behaviour. For t h e B-doped sample, t h e 2475 cm-' band and t h e i n t r i n s i c
o p t i c a l phonons c e n t e r e d a t 480 cm-' have almost no change up t o t h e a n n e a l i n g
temperature of 5 1 0 ~
while
~
t h e 1980 an-' band h a s decreased by a f a c t o r of 4 . For
t h e P-doped sample, t h e peaks a t 2000 cm-' and 640 cm-' d e c r e a s e l i k e f o r undoped
a-Si (H) samples ( 4 ) while t h e peak o f 480 cm-I remains v e r y s t r o n g . The r a p i d weakening of a l l bands a t T>550C i s most l i k e l y r e l a t e d t o c r y s t a l l l z a t l o n .
-1
Discussion. - A f t e r a n n e a l i n g up t o 510°C! t h e B-H peak a t 2475 cm
shows no dec r e a s e ( s e e F i g . 4 ) b u t evolves i n t o two peaks c e n t e r e d a t 2475 and 2370 cm-', r e s p e c t i v e l y ( F i g . 3 ) . In viyw o f t h e f a c t t h a t t h e s t r e t c h i n g band of t h e B-H bonds
and 2520 cm-'
( 7 , 8 ) , we t e n t a t i v e l y a s s i g n t h e s e two
i n B2H6 o c c u r s a t 2609 cm
peaks t o B-H bond s t r e t c h i n g mcdes corresponding t o t h e s i n g l e t ( A ~ )and t h e d o u b l e t
(E) of BH3 r a d i c a l s . We can e s t i m a t e t h e e f f e c t i v e charge of t h e B-H bond from t h e
la
integrated absorption
@ ) & of t h i s band i f t h e d e n s i t y N
of B-H bonds i n t h e
B-H
sample i s known. On t h e o t h e r hand, w can a l s o e s t i m a t e t h e d e n s i t y N
of t h e B-H
B-H
bonds by u s i n g t h e f o l l o w i n g e x p r e s s i o n :
NB-H
=
$+
Jla(w)ao
B-H
The e f f e c t i v e charge eG
= 0.2 can ks e s t h a t e d from t h e a b s o r p t i o n s p e c t r a found
i n Ref. 7 , t h u s we obt%E f o r sample 103.
"
2700
2300
1900
800
Wave Number (I
/cm)
Fig. 1: Infrared absorption s p e c t r a
of t h r e e B-doped a-Si(H)
samples i n t h e r e g i o n of
" l o c a l modes "
-a-Si (101) ;
----- a-Si ( 102) ;
- - - a-Si ( 1 0 3 ) .
.
..
Wave
umber (1/cm )
Fig. 2 : F a r i n f r a r e d a b s o r p t i o n
s p e c t r a of t h r e e B-doped
a-Si ( H ) samples.
------ a-Si (101) ;
----- a-Si (102) ;
- a-Si (103)
-. -.
.
N
= 22 a t %, which i s somewhat lower than t h e v a l u e measured from t h e photoelecB-H
t r o n s p e c t r a (29 a t %)
.
F i r s t o r d e r i n f r a r e d a b s o r p t i o n by l a t t i c e modes i s f o r b i d d e n by symmetry i n
c-Si. I t becomes weakly allowed i n a - S i due t o t h e l a c k of long-range o r d e r which
r e l a x e s t h e c r y s t a l momentum and symmetry s e l e c t i o n r u l e s . ~t h a s been shown i n r e f e r e n c e 6 t h a t t h e disorder-induced one phonon a b s o r p t i o n i s suppressed upon hyd r o g e n a t i o n , p a r t l y because of r e l a x a t i o n of i n t e r n a l t e n s i o n s and of t h e a n g u l a r
d i s t o r t i o n s r e s p o n s i b l e f o r t h e d i p o l e moment. However, s t r o n g l y e l e c t r o n e g a t i v e i m p u r i t i e s such a s f l u o r i n e can h i g h l y enhance t h e i n f r a r e d a c t i v i t y of a - S i ( 4 ) . I n
Fig. 2 we can s e e t h a t t h e d i s o r d e r induced one phonon a b s o r p t i o n is remarkably enhanced by t h e dopants P o r B. This enhancement i s only weakly dependent on a n n e a l i n g
and on t h e concomittant e v o l u t i o n of hydrogen from t h e sample a s shown i n F i g . 4.
The P i m p u r i t y w i l l add an i o n i c component t o t h e bonds. The a d d i t i o n a l a b s o r p t i o n
induced by t h e doping can be r e l a t e d t o a dynamical e f f e c t i v e charge f o r t h e Si-P
bond w i t h ( 5 )
Msinc J ~ a ( w ) d w
e*2
Si-P,B
4ll N C ( l - C p )
P P
For t h e samples doped with P we o b t a i n eE.-p = 1.8 which compares favourably w i t h
t h e t r a n s v e r s e e f f e c t i v e charge o b t a i n e d #rom Harrison ' s bond-orbi t a l model (ezi-p=
2 ) . The a n a l y s i s of t h e samples doped with B i s m r e d i f f i c u l t a s t h e atomic mass
of B i s much s m a l l e r t h a n t h a t of S i , a f a c t which l e a d s t o t h e Si-B l o c a l modes a t
=0.8, a value which i s unable t o
840 cm-'. H a r r i s o n ' s model y i e l d s i n t h i s case e*
e x p l a i n t h e observed enhancement o f t h e Si-Si vi&Pions.
2700
2500
2700~ ( c r n - I)
F i g . 3: hVolucion of t h e
2475 cm7. peak a f t e r
a n n e a l i n g a t 5 IOOC
f o r t h e B-doped sample
F i g . 4: N o r m l i z e d i n t e n s i t i e s of t h e 2475, 1890 and
a-Si ( 103)
480 cm7 bands v e r s u s a n n e a l i n g temperature
f o r B-doped sample a-Si ( 1 0 3 ) .
.
References.
-
1. See, f o r i n s t a n c e , M.H. Brodsky ( e d . ) , Amorphous Semiconductors,
( S p r i n g e r Verlag, H e i d e l k r g , 1979)
2 . BRODSKY M . H . , M. CARDONA, and J.J. CUOMO, Phys. Rev.
(1979)
3556.
(1979)
3. LUCOVSKY G . , R . J . NEMANICH, and J.C. KNIGHTS, Phys. Rev.
2064.
4. SHEN S.C., C . J . FANG, and M. CARDONA, Phys. S t a t . S o l . ( b )
(1980) 451.
5. SHEN S.C. and M. CARDONA, Phys. Rev. g ( i n p r e s s ) (1981).
6. SHEN S.C., C . J . FANG, M. CARDONA, and L. GENZEL, Phys. Rev.
(1980) 2913.
7. FREUND I . and R.S. HALFORD, J. Chem. Phys. 43 (1965) 3795.
8 . Q n e l i n ' s Handbuch d e r Pnorganischen Chemie
( S p r i n g e r Verlaglp.6.
9. " ~ a n d o l d t - B o r n s t e i n " T a b l e s ( S p r i n g e r Verlag, Heidelberg, 1951).
101
52