ANNEALING STUDIES ON LOW OPTICAL ABSORPTION OF GD a-Si:H USING PHOTOACOUSTIC SPECTROSCOPY

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ANNEALING STUDIES ON LOW OPTICAL ABSORPTION OF GD a-Si:H USING PHOTOACOUSTIC SPECTROSCOPY

S. Yamasaki, N. Hata, T. Yoshida, H. Oheda, A. Matsuda, H. Okushi, K.

Tanaka

To cite this version:

S. Yamasaki, N. Hata, T. Yoshida, H. Oheda, A. Matsuda, et al.. ANNEALING STUDIES ON LOW

OPTICAL ABSORPTION OF GD a-Si:H USING PHOTOACOUSTIC SPECTROSCOPY. Journal

de Physique Colloques, 1981, 42 (C4), pp.C4-297-C4-300. �10.1051/jphyscol:1981462�. �jpa-00220919�

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page C4-297

A N N E A L I N G S T U D I E S ON LOW O P T I C A L ABSORPTION OF GD a-Si:H U S I N G PHOTO- ACOUSTIC SPECTROSCOPY

S. Yamasaki, N . Hata, T . Yoshida, H. Oheda, A . Matsuda, H. Okushi and K. Tanaka

EZectrotechnicaZ Laboratory, 1-1-4 Umezono, Sakuramura, Niiharigun, Ibaragi 305, Japan.

A b s t r a c t . - We have performed p h o t o a c o u s t i c spectroscopy (PAS),ESR and ir a b s o r p t i o n measurements on undoped GD a-Si:H f i l m b e f o r e and a f t e r i s o c h r o n a l a n n e a l i n g s , from which a b s o r p t i o n c o e f f i c i e n t (down t o a = 1 cm-I), s p i n den- s i t y (Ns) and bonded H c o n t e n t (CH) were determined. I t h a s been found o u t t h a t t h e e x t r a p o r a t e d spectrum o f s p i n - f r e e a-Si:H shows a long e x p o n e n t i a l t a i l , and t h a t a d d i t i o n a l broad a b s o r p t i o n i s s t r o n g l y c o r r e l a t e d with Ns.

The o r i g i n o f a below E, i s d i s c u s s e d .

I n t r o d u c t i o n . - I n many amorphous compound semiconductors t h e a b s o r p t i o n edge spectrum can be roughly d i v i d e d i n t o t h r e e r e g i o n s ; t h e h i g h a b s o r p t i o n r e g i o n A

(103 < a <

l o 4

cm-I), t h e e x p o n e n t i a l p a r t

B

e x t e n d i n g over 4 o r d e r s of m a c i t u d e o f a and t h e weak a b s o r p t i o n t a i l

g.

There h a s been r e p o r t e d l e s s d a t a on t h e o p t i c a l a b s o r p t i o n o f a-Si:H i n t h e r e g i o n s B a n d C compared with

A,

although an important information on gap s t a t e s n e a r t h e m o b i l i t y edge i s involved i n t h e s e r e g i o n s below t h e o p t i c a l gap. C r a n d a l l and Abeles e t a l . determined independently t h e o p t i c a l a b s o r p t i o n c o e f f i c i e n t down t o a = 10-1 cm-1 u s i n g t h e p h o t o c u r r e n t method [ I ] [2], b u t t h e i r p h o t o e l e c t r i c measurements a r e i n h e r e n t l y accompanied by t h e a m b i g u i t i e s o r i g i n a t e d i n s u r f a c e s t a t e s and/or photon-energy dependence o f VT product. I n e a r l i e r work we have demonstrated t h a t p h o t o a c o u s t i c spectroscopy (PAS) i s a power- f u l t o o l f o r determining t h e low a b s o r p t i o n c o e f f i c i e n t o f a t h i n f i l m below t h e o p t i c a l gap ( r e g i o n

g)

and f r e e from any problems accompanied by t h e t r a n s p o r t phe- nomena o f t h e photo-generated c a r r i e r s [3].

In t h i s r e p o r t we p r e s e n t t h e f i r s t d a t a on t h e below-gap a b s o r p t i o n ( r e g i o n

g)

o f GD a-Si:H a s a f u n c t i o n of a n n e a l i n g temperature, and d i s c u s s them i n r e l a t i o n t o t h e ESR s p i n d e n s i t y and t h e bonded hydrogen c o n t e n t of t h e specimen prepared and annealed under t h e i d e n t i c a l c o n d i t i o n .

Experimental.- Used samples i n t h e p r e s e n t work were d e p o s i t e d i n a g l a s s s u b s t r a t e (Tempax) i n a 13.56-MHz i n d u c t i o n plasma chamber a t a low power d e n s i t y from p u r e SiH4. A flow r a t e o f 5 SCCM, a g a s p r e s s u r e o f 100 mTorr and a s u b s t r a t e tempera- t u r e (Ts) o f 100°C were maintained.

The PAS d a t a a r e c o l l e c t e d a s f o l l o w s : The t h i n f i l m sample t o b e s t u d i e d i s p l a c e d i n s i d e a c l o s e d c e l l c o n t a i n i n g a i r and a s e n s i t i v e microphone, and t h e n i l l u m i n a t e d with a modulated monochromatic l i g h t . The p e r i o d i c h e a t i n g caused by t h e n o n r a d i a t i v e p r o c e s s e s a s s o c i a t e d with o p t i c a l a b s o r p t i o n g i v e s a p e r i o d i c h e a t flow t o t h e a i r g a s and t h e r e s u l t a n t a c o u s t i c s i & n a l i s d e t e c t e d by t h e microphone a s a f u n c t i o n o f t h e photon energy o f t h e i n c i d e n t l i g h t . Actual PAS s i g n a l s o f t h e samples (9

um

i n t h i c k n e s s ) were t r a c e d u s i n g t h e PAR model 6001 p h o t o a c o u s t i c spectrometer. PAS, ESR and i r a b s o r p t i o n measurements were s y s t e m a t i c a l l y c a r r i e d o u t on t h e samples a f t e r each 3-hour i s o c h r o n a l a n n e a l i n g a t d i f f e r e n t t e m p e r a t u r e s e l e v a t e d a t 50°C i n t e r v a l s i n t h e flowing H2, from which a b s o r p t i o n c o e f f i c i e n t (down t o a = 1 cm-I), s p i n d e n s i t y (Ns) and bonded H c o n t e n t (CH) were determined.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981462

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JOURNAL DE PHYSIQUE

Results and d i s c u s s i o n . - Figure 1 shows t h e o p t i c a l absorption s p e c t r a extending down t o a = 1 cm-1 o f t h e undoped a-Si:H (Ts = 100DC) b e f o r e and a f t e r t h e isochro- n a l annealings a t Ta = 300, 450 and 550°C, r e s p e c t i v e l y . The d a t a f o r t h e specimen deposited a t Ts = 300°C a r e a l s o shown i n t h e f i g u r e f o r comparison [3]. These d a t a were obtained from t h e PAS d a t a using t h e exact r e l a t i o n between PAS s i g n a l and a o f a t h i n f i l m sample which we derived from t h e thermal d i f f u s i o n equations t a k i n g i n t o account t h e m u l t i p l e r e f l e c t i o n [ 4 ] . In a low a region t h e normalized PAS s i g n a l q i s approximated a s

where Q(a) i s t h e PAS s i g n a l from t h e sample, QS t h e s a t u r a t e d value o f Q(a) i n high a region, R t h e t h i c k n e s s of t h e sample (9 pm), r12 and rZ2 t h e r e f l e c t i v i t i e s f o r t h e normal incidence o f a l i g h t from t h e sample t o t h e gas and t h e sample t o t h e backing m a t e r i a l , r e s p e c t i v e l y . The d e t a i l e d procedure was described e a r l i e r [3].

A s shown i n t h e f i g u r e , a-Si:H f i l m d e p o s i t e d a t Ts = 300°C shows a long expo- n e n t i a l t a i l i n i t s spectrum, while t h e specimen deposited a t Ts = 100°C h a s an ad- d i t i o n a l o p t i c a l absorption over a wide energy range below 1.7 eV independent trf whether as-deposited o r annealed. But t h e broad o p t i c a l absorption band below 1.7 eV decreases by n e a r l y one o r d e r o f magnitude according a s Ta i n c r e a s e s up t o 300°C, and i n c r e a s e s again f o r Ta > 300°C. This c h a r a c t e r i s t i c behaviour o f low a region caused by annealing i s , t o our knowledge, t h e f i r s t observation i n a-Si:H.

Tsai and F r i t z s c h e have observed i n t h e i r annealing study o f GD a-Si:H t h a t t h e s t r u c t u r e s e n s i t i v e absorption t a i l , i t s o n s e t around a t a = 103 cm-l, does n o t diminish with annealing [5], which i s l i k e l y due t o s c a t t e r i n g l o s s e s o r t h e lack of d a t a accuracy i n h e r e n t t o t h e d i r e c t transmission method i n a t h i n f i l m s t r u c t u r e .

We determined t h e o p t i c a l gap E o using t h e empirical r e l a t i o n

ENERGY ( c V )

Fig. 1 : O p t i c a l absorption s p e c t r a o f GD a-Si:H (Ts = 100°C) b e f o r e and a f t e r each isochronal annealing, which were obtained by analyzing t h e PAS d a t a . The r e s u l t o f a-Si:H (Ts = 300°C) i s a l s o p l o t t e d .

from t h e s p e c t r a i n t h e r a n g e 103 cm-I < a < 104 cm-1 ( r e - gion

A),

and t h e energy Ec c h a r a c t e r i z i n g t h e s l o p e o f t h e exponential p a r t

g

from t h e formula

being f i t t e d t o t h e s p e c t r a i n t h e range o f

l o 2

cm-' <

a < 103 cm-l, r e s p e c t i v e l y . The r e s u l t s a r e shown i n Fig. 2. Eo ( t o p o f t h e f i g - ure) i n c r e a s e s f i r s t up t o Ta

=

200°C and afterwards decreases r a p i d l y according a s t h e bonded hydrogen con- t e n t CH decreases due t o thermal e f f u s i o n (see Fig. 3), being a widely observed tendency [5] [6] [7]. The i n c r e a s e i n Eo with Ta i n low Ta range should mainly be a t t r i b u t e d t o t h e thermal r e c o n s t r u c t i o n o f t h e l o c a l s t r u c t u r e because CH remains constant i n t h i s Ta range (see Fig. 3), which has a l s o been pointed o u t by s e v e r a l

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Fig. 2 : The o p t i c a l gap E, (top) and t h e energy Ec c h a r a c t e r i z i n g t h e s l o p e (bottom) o f a-Si:H

(TS = 100°C) a s f u n c t i o n s o f Ta.

Fig. 3 : Atomic % of bonded H content (CH) i n a-Si:H (Ts = 100°C) a s a f u n c t i o n o f Ta.

groups [5][6][7]. ^On t h e o t h e r hand, d i f f e r e n t from E o , t h e energy Ec a s s o c i a t e d with t h e s l o p e of t h e exponential t a i l does not show a d r a s t i c change over a wide Ta range from 150°C up t o 550°C, a s shown i n t h e bottom o f Fig. 2. The value o f Ec n e a r l y remains a t 0.07 eV, s u r p r i s i n g l y , being c l o s e t o t h o s e of chalcogenide glasses

[ S ] . The specimen deposited a t Ts = 300°C has an Ec s m a l l e r than 0.06 eV (denoted by x i n t h e f i g u r e ) 131.

In o r d e r t o get more d e t a i l e d information on t h e n a t u r e o f t h e broad absorp- t i o n band below E,, we c o r r e l a t e t h e i n t e g r a t e d absorption A with t h e ESR s p i n den- s i t y N,, which i s defined a s

where Aa i s a d i f f e r e n c e between t h e observed value and t h e value determined by t h e f i t t e d exponential r e l a t i o n o f e q . ( 3 ) , i . e . ,

Fig. 4 shows i n t e g r a t e d absorption A and t h e ESR s p i n d e n s i t y Ns a t room tem- p e r a t u r e a s f u n c t i o n s o f Ta. Both q u a n t i t i e s show q u i t e a s i m i l a r behaviour, i . e . , decrease r a p i d l y a s Ta i n c r e a s e s , t a k i n g a minimum around a t Ta = 250

-

300°C, and i n c r e a s e again f o r Ta > 300°C. I t means t h a t t h e broad o p t i c a l a b s o r p t i o n below E o , A defined by eqs.(4) and ( S ) , has a s t r o n g c o r r e l a t i o n with Ns, i n o t h e r words, t h e concentration o f dangling bonds involved i n amorphous Si-H network. The d a t a i n d i - c a t e within experimental e r r o r s t h a t A and N,, t o a f i r s t approximation, a r e con- nected by a l i n e a r r e l a t i o n ,

From eqs. (4), (5) and (6) it i s concluded t h a t t h e absorption spectrum o f s p i n - f r e e a-Si:H should have a long exponential t a i l extending t o low photon e n e r g i e s independent o f E,. This conclusion i s compatible with t h e d a t a f o r a-Si:H d e p o s i t e d a t Ts = 300°C showing t h e long exponential p a r t i n i t s spectrum i n Fig. 1.

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JOURNAL DE PHYSIQUE

One i s tempted t o a t t r i b u t e d i r e c t l y t h e o r i g i n o f t h e broad a b s o r p t i o n A t o dangling bonds, b u t more d e t a i l e d i n f o r m a t i o n w i l l be r e q u i r e d f o r f u r t h e r d i s c u s - s i o n .

Brodsky and h i s coworkers have made an i n i t i a l e f f o r t t o c o r r e l a t e t h e o p t i c a l a b s o r p t i o n with t h e ESR s p i n d e n s i t y i n evaporated amorphous S i f i l m s which a r e non-hydrogenated,

0 and e s t i m a t e d t h e o p t i c a l a b s o r p t i o n E spectrum o f a - S i f r e e from t h e e f -

-

f e c t s of v o i d s and broken bonds by e x t r a p o r a t i n g t h e r e s u l t s t o z e r o s p i n c o n c e n t r a t i o n [9]. According t o t h e i r e s t i m a t i o n t h e e x t r a p o r a t e d spectrum o f s p i n - f r e e a - S i f a l l s s h a r p l y a t 1.8 eV and does n o t obey t h e e x p o n e n t i a l r e l a t i o n , which i s w n o t t h e p r e s e n t c a s e a s mentioned

above. Concerning t h e q u a n t i t a t i v e

-

¹⁰¹⁶

⁵

r e l a t i o n between A and Ns, f o r exam-

Acknowledgments.- We g r a t e f u l l y acknowledge K . Nozaki f o r PAS and M. Yamashita f o r ESR measurements.

.

as

-

^dep.

T

References.

-

[ I ] CRANDALL R. S . , J . Non-Cryst. S o l i d s 35 & 36 (1980) 381.

[2] ABELES B . , WRONSKI C. R . , TIEDJE T. AND CODY G . D., S o l i d S t a t e Commun.

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a p l e t h e r a t i o o f A t o N,, t h e r e ex- i s t s no s u b s t a n t i a l d i f f e r e n c e between t h e p r e s e n t d a t a and t h o s e o f

[3] YAMASAKI S. e t a l . , Proc. o f Topical Conf. on T e t r a h e d r a l l y Bonded Amorphous Semiconductors (Carefree, 1981) ( i n p r e s s ) ,

[4] YAMASAKI S. e t a l . , Jpn. J . Appl. Phys. L e t t . ( t o be p u b l i s h e d ) . [5] TSAI C. C . AND FRITZSCHE H., S o l a r Energy M a t e r i a l s 1 (1979) 29.

[6] TANAKA K . e t a l . , Proc. 12th Conf. on S o l i d S t a t e ~ e v i c e s , (Tokyo,1980) p267.

[7] DENEWILLE A.,BRUYERE J . C . , M I N I A., KAHIL H., DANIELOU R . , AND LIGEON E . , J. Non-Cryst. S o l i d s 35 & 36 (1980) 469.

[8] TAUC J . , Amorphous and Liquid Semiconductors (Plenum P r e s s , London and New York 1974) p159.

[9] BRODSKY M. H . , KAPLAN D.M. AND ZIEGLER J . F . , Proc. 1 1 t h I n t . Conf. Physics o f Semiconductors (Warsaw, 1972) p529.

[lo]

BISHOP S. G . , STROM U., AND TAYLOR P. C., Phys. Rev. L e t t .

34

(1975) 1346.

lo0

^I Brodsky e t a l .

200 400 600

T a ( *C ^Ont h e o t h e r hand, Bishop e t

a l . observed t h e o p t i c a l l y induced ESR and a b s o r p t i o n a t T = 6 K due t o Fig. 4 : Excess o p t i c a l a b s o r p t i o n A l o c a l i z e d paramagnetic s t a t e s i n t h e d e f i n e d i n t h e t e x t and corresponding forbidden gap i n s e v e r a l chalcogen- ESR s p i n d e n s i t y Ns o f a-Si:H (Ts = i d e g l a s s e s [lo]. For A s ~ S e 3 g l a s s , 100°C) a s f u n c t i o n s of Ta. a c c o r d i n g t o t h e i r d a t a , t h e induced

a b s o r p t i o n amounting t o a = 15 cm-l has been observed corresponding t o Ns

-

1018 cm-3. I t should b e n o t e d t h a t t h e r a - t i o of t h e induced a b s o r p t i o n t o Ns t a k e s a v a l u e o f t h e same o r d e r a s t h a t o f A/Ns i n t h e p r e s e n t work, i n s p i t e o f t h e s u b s t a n t i a l d i f f e r e n c e i n t h e s t r u c t u r e o f t h e m a t e r i a l s , a-Si:H v s . amorphous chalcogenides.