EFFECT OF TEMPERATURE ON OPTICAL PROPERTIES OF GLOW DISCHARGE HYDROGENATED AMORPHOUS SILICON FILMS

(1)

HAL Id: jpa-00220765

https://hal.archives-ouvertes.fr/jpa-00220765

Submitted on 1 Jan 1981

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

EFFECT OF TEMPERATURE ON OPTICAL PROPERTIES OF GLOW DISCHARGE

HYDROGENATED AMORPHOUS SILICON FILMS

A. Donnadieu, B. Yous, J. Berger, J. Ferraton, J. Robin

To cite this version:

A. Donnadieu, B. Yous, J. Berger, J. Ferraton, J. Robin. EFFECT OF TEMPERA- TURE ON OPTICAL PROPERTIES OF GLOW DISCHARGE HYDROGENATED AMOR- PHOUS SILICON FILMS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-655-C4-658.

�10.1051/jphyscol:19814144�. �jpa-00220765�

(2)

EFFECT OF TEMPERATURE ON OPTICAL PROPERTIES OF GLOW DISCHARGE HYDROGENATED APiORPHOUS SILICON F I LIIS

A . Donnadieu, B . Y o u s , J.M. B e r g e r , J . P . F e r r a t o n and J . Robin

LaboraLoire d e Speetroscopie 1 1 , Equipe de recherche assoeibe au C.R.R.S., i l n i v e r s i t ~ d e s Sciences e t l'echniques Ou L a n p e d ~ c , 340CO ?do~tpeZ?,ier, France

Abstract* O p t i c a l p r o p e r t i e s of hydrogenated amorphous s i l i c o n f i l m s , prepared by glow d i s c h a r g e o n t o f u s e d q u a r t z s u b s t r a t e s h e l d a t temperature TS v a r y i n g between 50 and 350°C, were determined from n e a r normal s p e c u l a r re- f l e c t a n c e and t r a n s m i t t a n c e measurements i n t h e energy range 0.5 t o 5.5 eV.

The measurements were done a t v a r i o u s temperature Tm i n t h e range 95 t o 750 K.

When Tm becomes h i g h e r t h a n TS, i r r e v e r s i b l e v a r i a t i o n s of o p t i c a l gap appear.

An i n t e r p r e t a t i o n i n terms of an hydrogen e v o l u t i o n and a rearrangement of t h e m a t r i x under a n n e a l i n g i s proposed.

I

-

Introduction0 Many inveo t i g a t i o n s t o determine how t h e hydrogen is inJorporated i n glow d i s c h a r g e amorphous s i l i c o n f i l m s were done d u r i n g t h e l a s t y e a r s . PRITZCIIE, i n a s y n t h e s i s paper [ l ] , i n d i c a t e d t h e d i f f e r e n t r e s u l t s . The l a v temparature f i l m s (TS < 250°C) r e v e a l a l a r g e amount of polymeric (SiB2)nas w e l l a s monohydride;

t h e high temperature f i l m s (TS > 2 5 0 ' ~ ) c o n t a i n predominantly monohydride SiH and o n l y a t r a c e of dihydride. I n o r d e r t o understand t h e bonding c o n f i g u r a t i o n s of hydrogen, same a u t h o r s s t u d i e d , a t room temperature, t h e o p t i c a l p r o p e r t i e s i n f u n c t i o n of H-content of samples and o t h e r s s t u d i e d post-hydrogenated a-Si samples o r hydrogen e v o l u t i o n under s e v e r a l annealings. We determined t h e o p t i c a l p r o p e r t i e s of samples from near-normal measurements of r e f l e c t a n c e and t r a n s m i t t a n c e i n t h e energy range 0.5 t o 5.5 eV. The measurements were performed a t temperature T,va- r y i n g between 95 and 750 K. We found two e f f e c t s : r e v e r s i b l e and i r r e v e r s i b l e va- r i a t i o n s of o p t i c a l gap w i t h Tm. The s i g n of i r r e v e r s i b l e e f f e c t depends on t h e p r e p a r a t i o n temperature Ts.

'I

-

_method*Samples of glow d i s c h a r g e s i l i c o n (GD) were d e p o s i t e d on q u a r t z s u b s t r a t e s by a c a p a c i t i v e l y coupled r f d i s c h a r g e i n a mixture of.50 % argon

-

⁵⁰^X s i l a n e a t 0.a t o r r . The d e p o s i t i o n r a t e i n c r e a s e d from 2 . 5 . ~ / s f o r d e p o s i t i o n a t 350°C t o 5 A / s a t t h e lowest temperature. We can e s t i m a t e t h a t t h e H : S i r a t i o i n c r e a s e s i n t h i s temperature range from below 8 a t X (TS * 350°C) up t o 25-30 a t X (TS

-

50°C). The o p t i c a l l y d e r i v e d t h i c k n e s s e s v a r i e d from 0.9 )1 t o 1.3 fl. The o p t i c a l measurements o f n e a r normal r e f l e c t a n c e and t r a n s m i t t a n c e a t v a r i o u s temperature T m w e r e done i n two a p p a r a t u s which were d e s c r i b e d p r e v i o u s l y

[2,3]. The o p t i c a l gap Eg(Tm) was c a l c u l a t e d from a b s o r p t i o n c o e f f i c i e n t a

according t o t h e e q u a t i o n ( a I I ) l f 2 = y [ ~

-

E ~ ( T , ) ] where E i s t h e photon energy and y t h e s l o p e of t h e s t r a i g t h l i n e .

Each eeasurement a t T,, d i f f e r e n t of room temperature Ta = 295 K , was prece- ded and followed by a m~easurelnent a t room temperature. We c a l l ~ : ( 2 9 5 ~ , Hi) t h e o p t i c a l gap of sample i n M i s t a t e a f t e r t h e pth measurement a t T-

-

²⁹⁵^K. ^The

f o l l o w i n g measurement a t Tm # 295 K can change t h e composition (Rydrogen e v o l u t i o n ) o r t h e form (annealing e f f e c t ) of t h e m a t r i x and produce i r r e v e r s i b l e e f f e c t s . I f

t h e h e a t i n g a t Tm produces r e v e r s i b l e e f f e c t . t h e n Eg(29Srl, Mi)

-

^Eg(295

^,

^Mi) ^;

e l s e i f a t Tm t h e m a t r i x changes, t h e i r r e v e r s i b l e v a r i a t on of o p t i c a l i s

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814144

(3)

C4-656 JOURNAL DE PHYSIQUE

we can e s t i m a t e t h e v a l u e of o p t i c a l gap a t T a s i f t h e m a t r i x h a s n o t changed by m

Eg(Tm, Mi+l) i s t h e measured o p t i c a l gap, Eg(Tm, Hi) t h e c o r r e c t e d o p t i c a l gap.

This formula can b e used only i f t h e change from Hi e t a t e t o Mi+l s t a t e does n o t modify t h e s l o p e of t h e curve E (T Mi) = f(Tm).

g m' I11

-

Experimental r e s u l t s .

Fig. 1 shows t h e v a r i a t i o n of (aE) = f ( ~ ) a t v a r i o u s T, f o r a-Sill t h i n f i l m prepared by GD a t TS = 350.C. We observe a s h i f t of o p t i c a l gap t c u a r d s t h e lower e n e r g i e s when T, i n c r e a s e s v i t h o u t s e n s i b l e m o d i f i c a t i o n of t h e s l o p e y. We found t h e same phenomenon f o r a 1 1 s t u d i e d samples.

Pig. 1

-

V a r i a t i o n of ( a ~ ) l / ~ v e r s u s Fig. 2

-

V a r i a t i o n of o p t i c a l gap Eg(Tm) photon energy E f o r d i f f e r e n t Tm. v e r s u s Tm f o r two GD samples prepared 0 t

5 0 % and 350.C.

Table I

Table I g i v e s t h e v a l u e s o b t a i n e d f o r a e v e r a l samples. Pig. 2 shows t h e v a r i a - t i o n of o p t i c a l gap Eg(Tm) v e r s u s Tm f o r two samples : t h e f i r s t one, w i t h a l w hydrogen c o n t e n t , prepared a t TS = 3 5 0 ' ~ (curve a ) , t h e o t h e r w i t h a h i g h hydrogen c o n t e n t , prepared a t TS

-

^50.C (curbe b ) . We o b r e r v e f o r each sample

(4)

-

a ) I r r e v e r s i b l e e f f e c t s . For high TS f i l m s we observe ( f i g . 2 curve a ) i r r e - v e r s i b l e e f f e c t s above a T m o f 600 t o 650 K. The v a r i a t i o n of t h e o p t i c a l gap AEg(Mi+i, Mi) is n e g a t i v e ; t h e mean v a l u e f o r a l l samples i s -0.05 eV a f t e r t h e h e a t i n g a t 673 K and -0.10 eV a f t e r t h e h e a t i n g a t 723 K. We can a t t r i b u t e t h e s e v a r i a t i o n s t o t h e d e c r e a s e i n t h e H-content when t h e temperature i n c r e a s e s . P e r r i n e t a 1 [4] and BruySre e t a 1 [5] i n d i c a t e d t h a t t h e v a r i a t i o n of o p t i c a l gap w i t h t h e H-content is 0.03 eV ( a t %)-I f o r SiB bonds and 0.1 eV ( a t f o r (Sill2), bonds. I f we use t h e l a s t v a l u e , we f i n d t h a t t h e t o t a l hydrogen which i s r e l e a s e d

i s h i g h e r t h a n t h e hydrogen contained b e f o r e h e a t i n g s . I f we use t h e v a l u e r e l a t i v e t o SiH bonds, we f i n d t h a t H-content d e c r e a s e s of about 1.5 a t . Z a f t e r h e a t i n g a t 673 K and 3 a t . X a f t e r h e a t i n g a t 723 K. The t o t a l v a r i a t i o n of H-content cor- responds t o about 70 X of i n i t i a l r a t i o . These o b s e r v a t i o n s a g r e e very w e l l v i t h r e s u l t s and i n t e r p r e t a t i o n given by Zellama e t a 1 [6] and confirm t h a t h i g h TS f i l m s c o n t a i n predominantly monohydride.

For low TS f ilma we observe ( f i g . 2 curve b ) i r r e v e r s i b l e e f f e c t s from a temperature Tm h i g h e r by 50-C t h a n Ts. But h e r e , t h e v a r i a t i o n of t h e o p t i c a l gap i s p o s i t i v e and e q u a l t o about +0,06 eV. This i n c r e a s e is due t o an a n n e a l i n g e f f e c t which produces a rearrangement o f t h e m a t r i x and a d e c r e a s e i n t h e d e n s i t y of s t a t e s i n t h e forbidden band. I f t h e r e is an hydrogen e v o l u t i o n , i t i s n o t ob- s e r v a b l e . A l l low TS f i l m s d e s i n t e g r a t e a t about T m = 573 K.

b) R e v e r s i b l e e f f e c t s . Equation 2 a l l o w s t o c a l c u l a t e t h e c o r r e c t e d o p t i - c a l gap E (T,, Mi) a t high temperature

T,,,. Fin.

4

shows t h e r e s u l t s o b t a i n e d

x-t\

f g r GD-samples. For comparison, we p l o t -

t e d t h e v a l u e s r e l a t i v e t o a Chemical

' 4 t\

Vapor D e p o s i t i o n (CVD) s i l i c o n sample prepared a t 650.C [3']. For a l l samples t h e e x p e r i m e n t a l v a l u e s a r e l o c a l i z e d between t h e t h e o r e t i c a l curves of

Varshni [7] and Ravindra e t a1 [8]

.

^For ^,^'

e x p e r i m e n t a l curves, i t appears an in- c r e a s e i n t h e s l o p e from CVD curve t o

t h e T = 50.C GD curve when t h e H con- 1.6-T,=350°c G D ^\

t e n t Sncreases. But t h e s l o p e of c u r v e s ^\

r e l a t i v e t o GD samples prepared a t TS

-

50-C and TS

-

280% seems t h e same.

For high TS samples t h e monohydride den- ^1.4-

s i t y i n c r e a s e s and r e a c h e s a c o n s t a n t ~ ~ 6 5 0 ~ c C V D \

v a l u e when TS d e c r e a s e s whereas f o r low ^\

TS samples t h e monohydride d e n s i t y i s ^\_\

c o n s t a n t and t h e amount of polymeric 1.2

i n c r e a s e s . We can t h e n suppose t h a t mo- 0 200 4 0 0 6 0 0 &)T,

nohydride phase i n f l u e n c e s t h e s l o p e of Fig. 3

-

Experimental v a l u e s (+) and o p t i c a l gap c u r v e s w i t h Tm. t h e o r e t i c a l c u r v e s of Varshni (-) and

Ravindra (--) of o p t i c a l gap v e r s u s Tm.

c ) Global e f k c r r . The g l o b a l e f f e c t s when Tm i n c r e a s e s a r e d i f f e r e n t

f o r t h e two k i n d s of samples. They b o t h p r e s e n t a r e v e r s i b l e e f f e c t u n t i l the com- p o s i t i o n o r a r r a n g e e n t of s i l i c o n m a t r i x changes. When i r r e v e r s i b l e e f f e c t appears, i t s s i g n depends on TS. For t h e low TS samples t h e e f f e c t of rearrangement o f m a t r i x ( a l t e r a t i o n of t h e d e n s i t y of s t a t e s i n t h e forbidden band) is more *or- t a n t t h a n t h e hydrogen e v o l u t i o n and g i v e s r i s e t o an i n c r e a s e i n o p t i c a l gap ; b u t t h e s e s a q l e s d e s i n t e g r a t e a t about 573 K. For t h e high TS samples t h e i n v e r s e

(5)

C4-658 JOURNAL DE PHYSIQUE

p h e n o r n o n appears ; the hydrogen evolu- t i o n predominates on t h e arrangement of t h e matri. and t h e o p t i c a l gap decreases.

As f o r CVD samples, f o r a l l t h e m sam- p l e s t h e o p t i c a l gap t e n d s t w a r d s a c-n v a l u e s around 1.30 eV a t 723 K d e n T i n c r e a s e s ( f i g . 4).

Ibh

knowledge of v a r i a t i o n s of t h e e p t i c a l gap with temperature Tm is impor-

t a n t ; w a t l y because of t h e use of t h i s u t e r i a l i n s o l a r energy conversion.

Almowledgement. The a u t h o r s a r e g r a t e f u l t o Prof. G. UEISER and t o J. BEICHLER

T,=~SO"C C V D

from t h e U n i v e r s i t y of MARBWG, RFA, f o r 121

,

providing t h e GD samples and t o Prof. 0 200 400 600

~,k

B.O. SERAPEIN and t o D. BOOTH from t h e

O p t i c a l Sciences Center, U n i v e r s i t y of Fig. 4

-

Co~nparison of measured op- Arizona, f o r providing t h e CVD samples. t i c a l gap of CVD and high TS GD

s i l i c o n f i l m s . Ref erencea

E l ] E. FRITZCHE S o l a r Energy M a t e r i a l s 3 (1980) 447.

[2] J.P. WRRATDN, C. ANCE and A. WNNADIEU Thin S o l i d Films 78 (1981) 207.

[3] ^A. DIVBBCEY, B. YOUS, J.M. BERGER, J.P. FERRATON, J. ROBIN and A. DONNADIEU Thin S o l i d F i l m 78 (1981) 235.

[k] J. PEBBIW, I. SOLOIDN, B. BOURDON, J. FONTENILLE and E. LIGEON Thin S o l i d P i l w 6 2 (1979) 327.

[5] J.C. BRUYERE, A. DENEWILLE, A. M I N I , J. FONTBNILLE and H. DANIEUlU J. Appl.

Phys. 51 (1980) 2199.

161 K. ZELLAM, P. GERMAIN, S. BQUELARD, B. BOURDON, J. FONTENILLE and R. DANIELOU Phys. Rev. t o be published.

[7] Y.P. VARSBNI Physica 34 (1967) 149.

[8] N. RAVINDRA and V.K. SRIVASTAVA J. Phye. Chem. S o l i d s 40 (1979) 791.