A SIMPLIFIED MODEL FOR THE DEPOSITION KINETICS OF GD a-Si : H FILMS

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A SIMPLIFIED MODEL FOR THE DEPOSITION KINETICS OF GD a-Si : H FILMS

A. Matsuda, M. Matsumura, K. Nakagawa, S. Yamasaki, K. Tanaka

To cite this version:

A. Matsuda, M. Matsumura, K. Nakagawa, S. Yamasaki, K. Tanaka. A SIMPLIFIED MODEL FOR

THE DEPOSITION KINETICS OF GD a-Si : H FILMS. Journal de Physique Colloques, 1981, 42

(C4), pp.C4-687-C4-690. �10.1051/jphyscol:19814152�. �jpa-00220775�

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JOURNAL DE PHYSIQUE

L'oL Loque C4, s u p p Liment u u d o 10, T O I ~ 42, nr,tot-rat- 1281

A SIMPLIFIED MODEL FOR THE DEPOSITION KINETICS OF GD a-Si:H FILMS

A. Matsuda, M. Matsumura, K. Nakagawa, S. Yamasaki and K. Tanaka

Abstract. We propose a simplified model for the deposition kinetics of GD a-Si:H as well as a-Si:F:H films taking into account a dissociation energy of a chemical bond of diatomic molecules. The model explains successfully (1) the reason why no film is deposited from the pure SiF4 glow discharge

and ( 2 ) a strong correlation between an emission intensity

ratio of [13]/[SiH) in a plasma and an ir absorption ratio o f dihydride to monohydride modes of a deposited film.

Deposition of qlow-discharge a-Si:H and a-Si:F:H has been reported by many groups, but the details of the growth kinetics of the film, gas- phase reactions as well as surface reactions, have not yet been well understood so far.

In the previous conference we reported the basic techniques of the optical emission plasma spectroscopy (OES) as a powerful tool for plasma diagnosis and discussed the relation- ship between the intensity ratio of the emitted lines and the structural properties of the deposited f ilm(1)

.

Several groups have also tried to characterize the gas plasma usinq either the optical emission spectroscopy or mass spectrometric method with respect to the deposition kinetics(2-41, while Knights has pointed out that the surface reaction is a dominant process in the film deposition ( 5 ) .

In this report we propose a simplified growth model for the deposition of a-Si:H and a-Si:F:H on the basis of experimental results of OES.

Film depositions were tried from four different gas mixtures by the giow-discharge nethod; SiF4, SiF4/t12, SiF4/SiH4 and SiH4, respectively.

, Si, ,Si, ,Si,

,Si, si Sl Sl

,SI S

Fig.1. Schematic model for the film deposition kinetics of a-Si: F:H and a-Si :H.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814152

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C4-688 JOURNAL DE PHYSIQUE

No film was deposited on a glass substrate from a SiF4 glow-discharge against the widely-scanned plasma parameters (power density of 0.01-3

~ / c m 2 , gas pressure of 0.01-lOTorr), while a-Si:H or a-Si:F:H films were obtained from the other three glow-discharges. In parallel, OES were traced in the range from 4000A to 7000A for each glow-discharge plasmas during deposition. Emission lines were mainly observed at 4127A [SiH], 4398A [SiF], 5731A [F2]

,

6021A [H2], 6563A [HI and 6856A

IF]. It should be noted that the SiF4 glow-discharge involves the decomposed species of [SiF], [F2] and [F], nevertheless, giving no film deposition. From a phenomenological point of view, these results indicate that amorphous silicon cannot be formed on the substrate from the plasma in the absence of the excited species such as [HI,

[H2] and [SiH].

Table 1. Dissociation energies of related cornbinations.(6)

In order to interprete the above experimental facts we present a simplified model for the deposition kinetics of a-Si:H and a-Si:F:H, where a binding energy (dissociat-

ion energy of a diatomic molecule) of a chemical bond of the excited species within plasma is a key factor to control the reaction of a

Si-Si 3.10

growing surface

Ne

Dissociation energies of the ^He

diatomic molecules under consider- Ar

ation are listed in Table 1.

Concerning the magnitude of the dissociation energy one can see the following relation in the table;

H-F>Si-F>H-H>Si-Si>Si-H>F-F. 111 In our model the reaction at the

film surface between admolecules and 2100 ^I I I

-

the excited species in gas phase is ^I

determined by a difference in dis-

sociation energy of the relevant

-

²⁰⁸⁰

^-

^Ar

^-

chemical bonds, as shown schematic-

-

ally in Fig.1. At the first step of

2060 He

the film deposition, [SiFI or [SiH] u Ne

-

molecules are adsorbed on the surface

-

of the substrate, being formed as the E 2 0 4 0 -

-

first monolayer. At the second step, 3 for the case of pure SiF4 glow-

discharge, the first monolayer in- 20241

-

volving Si free bonds is completely

passivated by F atoms and a further I I

reaction never takes place because 2000- I I

Si-F bond is the strongest among the SiH4 20 40 60 80 chemical bonds in the plasma accord- VOLUME % OF DlLUENT ing to the inequality [l] (the top

of Fig.1). On the other hand, for ~ i ~ . 2 . OES line intensity ratio the glow-discharge plasma of SiF4 [HJ/[SiHl and ir stretch- diluted by SiH or H (the center of ing band vm of resultant Fig. 1)

,

[HI, [A2] an2 [SiH] radicals films plotted against play an essential role in the volume % of diluent.

Si-F 5.57

F-F 1 - 6 0 Si-H

3.06

P-H 3.05 H-F

5.859

B-H 3.42 H-H

4.478

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The p r e s e n t s i m p l i f i e d model f o r t h e r e a c t i o n p r o c e s s b a s e d on a d i s s o c i a t i o n e n e r g y c a n a l s o e x p l a i n t h e above p h e n o m e n o l o g i c a l r e l a t i o n s h i p between OES and t h e v i b r a t i o n a l p r o p e r t i e s o f t h e f i l m . A s c h e m a t i c model i s shown i n F i g . 4 . Bonding c o n f i g u r a t i o n s o f a d s o r b - e d m o l e c u l e s a t t h e growing s u r f a c e a r e i l l u s t r a t e d a t t h e t o p row, and t h e r e a c t i v e r a d i c a l s , [HI and [ S i H ] , a r e a t t h e l e f t column o f t h e f i g u r e . A c c o r d i n g t o t h e d a t a on t h e d i s s o c i a t i o n e n e r g i e s i n T a b l e 1, t h e f o l l o w i n g r e a c t i o n s a r e e n e r g e t i c a l l y f a v o u r a b l e ; s u r f a c e r e a c t i o n , namely, t h e second

Si-H

+

^[HI ⁺ ^{S i -}

+

H2 ( a d . ) ( g ) ( a d . ) ( g ) and t h e t h i r d S i l a y e r s c a n b e formed 2 100 s u c c e s s i v e l y by t h e f o l l o w i n g r e a c t -

i o n ,

Si-F

+

[Si-HI ^-+ HF

+

S i - S i , [ 2 ] 2080- ( a d . ) ( 9 ) ( 9 ) ( a d . )

s i n c e t h e a b o v e r e a c t i o n is e x o t h e r -

-

m i c , a s i s c l e a r i n T a b l e 1. On t h e FE 2060- same c o n t e x t a-Si:H i s d e p o s i t e d a l s o 0

from t h e p u r e SiH4 plasma ( t h e b o t t o m w o f F i g . 1 ) t h r o u g h a n e n e r g e t i c a l l y

Si-H

+

[Si-HI + S i - S i 4 H2.

( a d . ) ( g ) ( a d . ) ( g )

I I I

0

-

•

0

-

0

C o n s e q u e n t l y , a s shown i n t h e f i g u r e , n i n e d i f f e r e n t c o m b i n a t i o n s o f a d m o l e c u l e s a n d r a d i c a l s r e s u l t i n t e n c a s e s o f new b o n d i n g con- f i g u r a t i o n s o f t h e growing s u r f a c e t h r o u g h t h e above r e a c t i o n s , a l t h o u g h " a p p a r e n t " t e n c a s e s c a n b e r e d u c e d t o t h r e e i n i t i a l bonding c o n f i g u r a t i o n s o f t h e t o p row. I n t h e c o u r s e o f t h e f i l m g r o w t h , d i h y d r i d e ( d e n o t e d by D i n t h e f i g u r e ) and monohydride ( d e n o t e d by MI c o n f i g u r a t i o n s a r e c r e a t e d o n t h e s u r f a c e t h r o u g h t h e r e a c t i o n s w i t h

( [HI [HI ) a n d ( [HI [SiHI )

,

r e s p e c t i v e l y . I t means t h a t t h e number

r e a s o n a b l e r e a c t i o n d e s c r i b e d below; 2040-

-

3 Si-H

+

[Si-HI ^-+S i - S i

+

^{H 2 .} [31 ( a d . ) ( g ) ( a d . ) ( g )

202C N e x t we measured o p t i c a l e m i s s -

i o n i n t e n s i t i e s from t h e e x c i t e d

'0 SiHqIAr 0 SiHqlNe

SiH41H6 A

s p e c i e s , [SiHJ (4127A), [H2] (6021A) A Pure

and [HI (6563A) a s f u n c t i o n s o f f l o w Si H4

r a t e , RF power and g a s p r e s s u r e f o r 2aX)

110 2:O ;O 4:0 0

d i f f e r e n t d i l u e n t s ( H e , Ne and A r )

o f SiH4, and s y s t e m a t i c a l l y compared [ H I I [ S i H I t h e o b t a i n e d d a t a w i t h t h e v i b r a -

t i o n a l p r o p e r t i e s o f a-Si:H f i l m s F i g . 3 . R e l a t i o n s h i p between vm p r e p a r e d u n d e r t h e i d e n t i c a l d e p o s i - o f a-Si:H f i l m s and t i o n p a r a m e t e r s . We h a v e found o u t [ H ] / [ S i H ] o f OES.

a s t r o n g c o r r e l a t i o n between t h e e m i s s i o n i n t e n s i t y r a t i o [HI / [SiH]

of t h e p l a s m a and t h e median wavenumber (v,) o f Si-H s t r e t c h i n g a b s o r p - t i o n band o f t h e d e p o s i t e d f i l m ( v m g i v e s a measure o f t h e i r a b s o r p - t i o n r a t i o o f SiH2 t o SiH modes). The r e s u l t s a r e shown i n F i g . 2 . Both Vm and [ H ] / [ S ~ H ] i n c r e a s e s a s a volume % Of d i l u e n t i n c r e a s e s , namely, Vm i n c r e a s e s w i t h a n i n c r e a s e i n [ H ] / [ S i H ] . T h i s r e l a t i o n a l s o h o l d s f o r t h e d e p o s i t i o n from t h e p u r e SiH4 g l o w - d i s c h a r g e o v e r a wide v a r i a t i o n o f p a r a m e t e r s . F i g u r e 3 shows t h e d a t a p l o t t e d on a vm-vs.-[H]/[SiH] c h a r t , which were c o l l e c t e d from v a r i o u s e x p e r i m e n t a l r e s u l t s i n c l u d i n g t h o s e o f F i g . 2 .

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J O U R N A L DE PliYSlQUE

F i g . 4 . S c h e m a t i c model f o r t h e c r e a t i o n o f SiH m o n o h y d r i d e ( M ) and SiH2 d i h y d r i d e ( D ) .

r a t i o o f SiH2/SiH c o n f i g u r a t i o n s o f t h e d e p o s i t e d f i l m i s c o r r e l a t e d w i t h t h e c o n c e n t r a t i o n r a t i o o f r a d i c a l s [ H ] / [ S i H ] i n t h e p l a s m a , w h i c h i s i d e n t i c a l w i t h t h e o b s e r v e d r e l a t i o n s h i p b e t w e e n OES a n d i r s t r e t c h i n g a b s o r p t i o n shown i n F i g . 3 . I n o u r model we o n l y d e a l w i t h a [SiH] r a d i c a l a n d d o n o t t o u c h upon [ S i H 2 ] a n d [ S i H 3 ] s p e c i e s g i v i n g n o e m i s s i o n . B u t t h e e s s e n t i a l r o l e o f [ S i H ] i n t h e model i s c o m p l e t e l y r e t a i n e d i n t h e r e a c t i o n p r o c e s s e v e n i f [SiH] i s r e p l a c e d by [ S i H 2 ] o r [ S i H 3 ]

.

More d e t a i l e d model i s now i n p r o g r e s s . ( 7 ) R e f e r e n c e s

-

(1) MATSUDA A . e t a l . , J . N o n - c r y s t . S o l i d s , 35&36 ( 1 9 8 0 ) 1 8 3 . ( 2 ) PERRIN J. a n d DELAFOSSE E . , J . P h y s . D: ~ E i F p h ~ s . ,

13

( 1 9 8 0 )

759.

( 3 ) KAMPAS F.J. a n d GRIFFITH R . W . , P r o c . o f T o p i c a l C o n f . o n T e t r a - h e d r a l l y Bonded Amorphous S e m i c o n d u c t o r s , A r i z o n a ( 1 9 8 1 ) .

( 4 ) HIROSE M. e t a l . , R e f . ( 3 ) .

( 5 ) KNIGHTS J. C., J . N o n - c r y s t . S o l i d s , 35&36 ( ; 9 8 0 ) 1 5 9 .

( 6 ) HUBER K . P . , C o n s t a n t s o f D i a t o m i c M o l e c u l e s ; American I n s t i t u t e o f P h y s . Handbook (Ed. GRAY E., McGraw-Hill) ( 1 9 7 2 ) 7-168

( 7 ) MATSUDA A. e t a l . , J p n . J. A p p l . P h y s . ,

2

( 1 9 8 1 ) L183.