INFRARED AND FAR INFRARED ABSORPTION OF B- AND P-DOPED AMORPHOUS Si

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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 P- DOPED AMORPHOUS Si. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-349-C4-351.

�10.1051/jphyscol:1981474�. �jpa-00220931�

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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 -p- doped 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.

I n t r o d u c t i o n . - We 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 ab- s 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 absorp- t 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 concen- t 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 absorp- t i o n f e a t u r e changes l i t t l e up t o 5 1 0 ~ ~ . A f t e r annealing a t 560°c, however, it 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

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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 P- doped 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 an- n 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 t e m p e r a t u r e o f 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 wea- kening 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 .

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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 de- c 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 i n B2H6 o c c u r s a t 2609 cm 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 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 i n t e g r a t e d a b s o r p t i o n

l a

^@^{) &} 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 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 : B-H

NB-H =

$+

J l a ( w ) a o 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 i n Ref. 7 , t h u s we obt%E f o r sample 103.

"

2700 2 3 0 0

1900

8 0 0

Wave Number

(I

/cm)

F i g . 1 : 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 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 ) .

from t h e a b s o r p t i o n s p e c t r a found

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)

.

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N = 22 a t %, which i s somewhat lower than t h e v a l u e measured from t h e photoelec- t r o n s p e c t r a (29 a t B-H %)

.

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 hy- d 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 i s 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 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* =0.8, a value which i s unable t o 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

a-Si ( 103)

.

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 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.

3. LUCOVSKY G . , R . J . NEMANICH, and J . C . KNIGHTS, Phys. Rev. (1979) 2064.

4. SHEN S.C., C . J . FANG, and M. CARDONA, Phys. S t a t . S o l . ( b )

101

(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

52

( 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).