SURFACE PROPERTIES OF a-Si : H AND a-Si : F INVESTIGATED BY PHOTOELECTRON SPECTROSCOPY

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SURFACE PROPERTIES OF a-Si : H AND a-Si : F INVESTIGATED BY PHOTOELECTRON

SPECTROSCOPY

L. Ley, H. Richter, R. Kärcher, R. Johnson, J. Reichardt

To cite this version:

L. Ley, H. Richter, R. Kärcher, R. Johnson, J. Reichardt. SURFACE PROPERTIES OF a-Si : H

AND a-Si : F INVESTIGATED BY PHOTOELECTRON SPECTROSCOPY. Journal de Physique

Colloques, 1981, 42 (C4), pp.C4-753-C4-759. �10.1051/jphyscol:19814165�. �jpa-00220789�

CoZZoque

C4,

suppl6rnent au nOIO, Tome 42, octobre 1981 page C4-753

SURFACE PROPERTIES OF a - S i : H AND a - S i : F INVESTIG.4TED BY PHOTOELECTRON SPECTROSCOPY

L. Ley, H . R i c h t e r , R. K x r c h e r , R.L. J o h n s o n a n d J . R e i c h a r d t

Max-PZanck-Institut fur Feslkdrperforschung, Heisenbergstr. i, 7000 Stuttgart 00, F.R. G.

A b s t r a c t .

-

I t i s shown t h a t f i l m s o f a-Si:H and a - S i : F p r e p a r e d a t

room

tem- p e r a t u r e (RT) grow w i t h a s u r f a c e l a y e r o f Si-Hj o r S i - F l a r g e l y i n d e p e n d e n t o f t h e b u l k d i s t r i b u t i o n o f H o r F among t h e p o s s l b l e bor?ding g e o m e t r i e s . T h i s s u g g e s t s a g r o w t h mechanism b a s e d o n a n i n s e r t i o n r e a c t i o n o f SiH (F ) i n t o a n

2

2 S i - H ( F ) bond. Amorphous h y d r o g e n a t e d s i l i c o n f i l m s a r e h i g h l y resistant t o o x i d a t i o n by d r y 0 a s compared t o c - S i . A c t i v a t e d oxygen i s t a k c n u p r e a d i l y a n d t h e o x i d a t i o n ?Lads t o t h e f o r m a t i o n o f a n S i O l a y e r w i t h a n a v e r a g e s t o i c h i o m e t r y x t h a t i s h i g h e r ( l o w e r ) t h a n t h a t o f c - S i f o r R T (T = 2 5 0 ~ ~ ) a-Si:H. The f o r m a t i o n o f a d e p l e t i o n l a y e r i s o b s e r v e d o n t h e h i g h t e m p e r a t u r e D f i l m s .

I . I n t r o d u c t i o n .

-

P h o t o e l e c t r o n s p e c t r o s c o p y (PES) c a n be u s e d t o m e a s u r e t h e e n e r - g y d i s t r i b u t i o n o f e l e c t r o n s i n amorphous s i l i c o n ( a - S i )

.

As s u c h PES p r o v i d e s d i - rect i n f o r m a t i o n a b o u t t h e e l e c t r o n i c s t r u c t u r e of t h i s m a t e r i a l t h a t is r e l a t e d t o a n d o f t e n c o m p l e m e n t a r y t o d a t a o b t a i n e d f r o m o p t i c a l s p e c t r o s c o g y o r e l e c t r i c a l m e a s u r e m e n t s .

T h e v a l e n c e band d e n s i t y o f s t a t e s (VBDOS) o f a - S i , a-Ge a n d t h e i r h y d r o g e n a - t e d a n d f l u o r i n a t e d m o d i f i c a t i o n s h a v e b e e n d e t e r m i n e d b y P E S ' - ~ . C a l c u l a t i o n s o f t h e e l e c t r o n i c s t r u c t u r e o f a-Si:H F may be j u d g e d by t h e q u a l i t y o f t h e i r a g r e e m e n t w i t h t h e s e e x p e r i m e n t s .

:

PES p r o v i d e d t h e f i r s t e v i d e n c e t h a t H a n d F remove n o t o n l y g a p s t a t e s b u t a l s o s t a t e s from t h e t o p o f t h e v a l e n c e b a n d s t h u s i n c r e a s i n g t h e o p t i c a l g a p of a - S i w i t h i n c r e a s i n g H o r F c ~ n c e n t r a t i o n . " ~ T h e i m p l i c a t i o n s o f t h i s e f f e c t on t h e t r a n s p o r t p r o p e r t i e s a r e s t i l l b e i n g e x p l o r e d . 6 '

F e r m i l e v e l s h i f t s i n a c c o r d w i t h t h e v a r i a t i o n i n t h e a c t i v a t i o n e n e r g y o f t h e d a r k c o n d u c t i v i t y c a n b e m e a s u r e d d i r e c t l y i n d o p e d s p e c i m e n s . I n t h i s way m o d e l s o f t h e d o p i n g mechanisms i n a-Si:H have b e e n s u b s t a n t i a t e d . l o S p e c t r o s c o p i c e v i d e n c e t h a t t h e a d d i t i o n o f p h o s p h o r u s i n c r e a s e s t h e d e n s i t y o f s t a t e s i n t h e l o w e r h a l f o f t h e g a p h a s b e e n c o n f i r m e d b y d o p i n g e x p e r i m e n t s o n p a r t l y c o m p e n s a t e d a-Si:H s a m p l e s . l 2

An a s p e c t o f PES t h a t h a s b e e n somewhat n e g l e c t e d i n t h e p a s t is t h e h i g h s u r f a c e s e n s i t i v i t y o f t h e t e c h n i q u e . The a v e r a g e s a m p l i n g d e p t h o f PES i s o n l y a b o u t 2 0 t o 50

8

d e p e n d i n g o n t h e e n e r g y o f t h e e l e c t r o n s . We h a v e u t i l i z e d t h i s a s p e c t i n a d r e s s i n g two q u e s t i o n s . ( i ) What a r e t h e main s u r f a c e s p e c i e s Si-Hn o r S i - F p r e s e n t d u r i n g t h e d e p o s i t i o n o f a m r p h o u s S i f i l m s , a n d how d o t h e y v a r y w i t h n p r e p a r a t i o n c o n d i t i o n s ? ( i i ) How d o t h e i n i t i a l s t a g e s o f o x i d a t i o n p r o c e e d i n a-Si:H compared t o c - S i ? The a n s w e r t o t h e f i r s t q u e s t i o n i s e s s e n t i a l f o r o u r u n d e r s t a n d i n g o f t h e g r o w t h mechanism o f t h e s e f i l m s . The i m ~ o r t a n c e o f t h e n a t i v e s i l i c o n o x i d e i n d e v i c e t e c h n o l o g y i n c l u d i n g MOS p h o t o v o l t a i c e l e m e n t s r e q u i r e s no f u r t h e r e l a b o r a t i o n .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814165

C4-754

JOURNAL D E PHYSIQUE

11. P r i n c i p l e s o f P h o t o e l e c t r o n Spec4xoscopy and

some

Experimental D e t a i l s .

-

The p h o t o e l e c t r o n s p e c t r u m I (E ) o f a specimen i s e x c i t e d by monochromatic r a d i a t i o n of e n e r g y hv, and i t i s measured by means of a n e l e c t r o s t a t i c energy a n a p z e r . Ana- Kin l y z e r and specimen a r e k e p t i n a vacuum e n c l o s u r e a t p r e s s u r e s below 10- t o r r . ; The i n i t i a l o r b i n d i n g e n e r g y E o f t h e e l e c t r o n s i s o b t a i n e d from t h e i r k i n e t i c e n e r g y E t h r o u g h E = hv-EKin -B@ where

0

i s t h e work f u n c t i o n o f t h e a n a l y z e r and t h e

Kin B

z e r o o f EB c o r r e s p o n d s t o t h e Fermi energy E In a s e m i c o n d u c t o r t h e h i g h e s t o c c u p i e d s t a t e s F' a t E ( t h e o n e s w i t h t h e l o w e s t b i n d i n g e n e r g y ) h a v e V t h u s a f i n i t e b i n d i n g e n e r g y E ( t o p o f VB) which c o r r e s p o n d s to t h e d i f f e r e n c e B

r i a t i o n s i n t h e p o s i t i o n o f E r e l a t l v e a r e t h u s r e a d i l y measured i n ~ E S . We s h a l l u s e

" s i 3 ~ "

t h i s f a c t t o d e t e r m i n e band bending induced by t h e o x i d a t i o n of a-Si:H ( s e e s e c t i o n I V )

.

BINMNG ENERGY (eV)

F i g . 1 : Valence band s p e c t r a o f a-Si:H s p u t t e r e d a t RT f o r a r a n g e o f photon e n e r g i e s . The s p e c t r a up t o hv = 2 0 0 eV were o b t a i n e d u s i n g S y n c h r o t r o n r a - d i a t i o n from t h e s t o r a g e r i n g DORIS, Hamburg.

The p h o t o e l e c t r i c c r o s s s e c t i o n depends on t h e p h o t o n e n e r g y . The c r o s s s e c t i o n s f o r t h e H I s and t h e S i 3 s , 3 p v a l e n c e e l e c t r o n s a r e com- pcirable o n l y i n a range of photon e n e r g i e s be- tween SlO a n d 150 eV a s c a n be s e e n in F i g . 1.

I f we s t u d y a-Si:Ii we s h a l l t h e r e f o r e r e s o r t t o u v - e x c i t a t i o n ( h v = 2 1 . 2 o r 40.0 eV) o b t a i n e d from a h e l i u m d i s c h a r g e lamp. A t t h e e n e r g y o f

Al

ka x - r a y s (hv = 1487 eV) hydrogen d e r i v e d s t a t e s g i v e no c o n t r i b u t i o n t o t h e s p c t r u m and t h e e f f e c t o f h y d r q e n o n t h e p u r e l y S i d e r i v e d s t a t e s may b e i n v e s t i g a t e d . l S p e c t r a e x c i t e d by A1 k a r e u s e d

to

i d e n t i f y s p e c i e s o t h e r t h a n S i l i k e a O , P , B e t c . t h r o u g h t h e i r c o r e l e v e l s and t o measure t h e i r c o n c e n t r a t i o n . The s p e c t r a r e p o r t e d h e r e a r e o b t a i n e d from samples p r e p a r e d in s i t u , i . e . under u l t r a h i g h vacuum c o n d i t i o n s in t h e s p e c t r o m e t e r and f r e e o f u n i n t e n t i o n a l c o n t a m i n a n t s .

111. S u r f a c e S p e c i e s o f a-Si:F and a-Si:H.

-

Amorphous f l u o r i n a t e d s i l i c o n p r o v i d e s a p a r t i - c u l a r l y w e l l s u i t e d s y s t e n t o p u r s u e t h i s t o - p l c . 2 ' 1 4 The h i g h l y e l e c t r o n e g a t i v e F i n d u c e s a t r a n s f e r o f e l e c t r o n s from t h e S i t o t h e F when i t r e p l a c e s a n S i - S i bond. T h i s i n c r e a s e

in p o s i t i v e c h a r g e o n t h e S i l e a d s t o a p r c p o r - t i o n a l i n c r e a s e i n t h e b i n d i n g e n e r g y of a l l S i c o r e l e v e l s . The PES s p e c t r u m of t h e S i 2p l e v e l e x h i b i t s t h e r e f o r e a t a i l t o w a r d s h i g h e r b i n d i n g e n e r g i e s ( c o n p a r e F i g . 2 ) t h a t i s w e l l f i t t e d by a s u p e r p o s i t i o n o f 5 e q u i d i s t a n t l i n e s c o r r e s - ponding t o t h e f i v e s i m p l e s t c o n f i g u r a t i o n s i n - v o l v i n g Si-F bonds : Si-Si4-nFn ( n = 0 . . . 4 ) ( f o r s h o r t S i - F n )

.

The i n c r e m e n t a l change i n b i n d i n g e n e r g y p e r a t t a c h e d f l u o r i n e was k e p t c o n s t a n t i n agreement w i t h o b s e r v a t i o n s o n c h l o r i n a t e d m e t h y l s i l a n e s . 2

From f i t s of t h e k i n d shown i n F i g . 2 w c a l c u l a t e t h e p a r t i a l c o n c e n t r a t i o n o f each c o n f i g u r a t i o n c F (Si-F ) a n d t h e t o t a l f l u o r i n e c o n c e n t r a t i o n c ( S i

zp7

a s t h e sum o f t h e l a t t e r The p a r t i a l c o n c e n t r a t i o n s cF (Si-Fn) a r e p l o t t e d F a s a f u n c t i o n of c F ( S i 2p) i n F i g . 3 a f o r a s e r i e s o f f i l m s s p u t t e r e d i n a n Argon/SiF4 mix- t u r e a t room t e m p e r a t u r e . I n F i g . 3b we have

98 100 102 104 98 100 102 104 BINDING ENERGY (eV)

F i g . 2 : S i 2 p c o r e l e v e l s p e c t r a ( d o t s ) and f i t s t o t h e s e s p e c t r a a r e e x p l a i n e d i n t h e t e x t . The i n s e t shows t h e mean s q u a r e d e v i a - t i o n between d a t a a n d f i t a s a f u n c t i o n o f AE, t h e s e p a r a t i o n between t h e component l i n e s . F i g . 3: A b s o l u t e ( a ) and r e l a - t i v e ( b ) F c o n c e n t r a t i o n s i n Si-F u n i t s a s a f u n c t i o n of t h e t o t a f F c o n c e n t r a t i o n . ( c ) i s b a s e d o n a s t a t i s t i c a l model.

r e c a l c u l a t e d t h e r e s u l t s i n t e r m s of t h e r e l a t i v e c o n t r i b u t i o n e a c h S i - F c o n f i g u r a t i o n makes t o t h e

t o t a r F - c o n t e n t . I n t h i s way t h e I I I I

d i s t r i b u t i o n o f F

i s

r e a d i l y corn-

b ^#J 20 30 40

p a r e d w i t h a model c a l c u l a t i o n

CF (Si2p) (at0/*)

( F i g . 3 c ) i n which we assume t h a t S i and F a r e s t a t i s t i c a l l y a t t a c h e d t o t h e f o u r bonds o f a

c e n t r a l S i a t o m s s u c h t h a t a g i v e n F / s ~ r a t i o i s m a i n t a i n e d . The comparison r e v e a l s t h a t t h i s s t a t i s t i c a l model g i v e s a v a l i d d e s c r i p t i o n f o r t h e d i s t r i b u t i o n o f F among t h e most l i k e l y S i - F c o n f i g u r a t i o n w e x c e p t f o r a n overabundance o f S i - F

3 ' When t h e s u r f a c e o f t h e f i l m s is g r a d u a l l y removed by a r g o n i o n s p u t t e r i n g , t h e F c o n c e n t r a t i o n d r o p s r a p i d l y w i t h i n t h e f i r s t 10

8

and l e v e l s o f f t h e r e a f t e r . The F c o n c e n t r a t i o n i n t h i s 10

8

s u r f a c e l a y e r c o r r e s p o n d s t o a n a v e r a g e o f 2 . 7 F a t o m s p e r s u r f a c e S i atom (assuming the d e n s i t y of c - S i f o r a - S i : F ) . The c h a n g e i n t h e S i 2p c o r e l e v e l s p e c t r u m i n d i c a t e s t h a t Si-F3 u n i t s a r e p r e f e r e n t i a l l y r e m v e d w h e r e a s t h e d i s t r i b u t i o n o f t h e o t h e r s p e c i e s r e p r e s e n t b u l k v a l u e s . We t a k e t h i s a s e v i d e n c e t h a t t h e s u r f a c e o f t h e f i l m s is s a t u r a t e d w i t h Si-F3 u n i t s i n d e p e d e n t

C4-756 JOURNAL

DE PHYSIQUE

of t h e bulk d i s t r i b u t i o n of f l u o r i n e among t h e p o s s i b l e bonding geometries.

A s i m i l a r l y d e t a i l e d a n a l y s i s of t h e hydrogen c o n f i g u r a t i o n s i n a-Si:H by PES i s n o t p o s s i b l e . The e l e c t r o n e g a t i v i t y d i f f e r e n c e betweef5H and S i i s t o o small t o induce any s i z a b l e chemical s h i f t i n t h e S i c o r e l e v e l s . We have t o r e s o r t t o t h e v a l e n c e band s p e c t r a i n t h i s c a s e . T m bonding forms o f H i n a-Si:H a r e d i s t i n g u i s h - a b l e through t h e i r v a l e n c e band spectrum (compare Fig. 4 ) . T k polyhydride configu- r a t i o n c h a r a c t e r i z e d by t h e two hydrogen induced peaks A and B s e p a r a t e d by 4.5 eV and t h e monohydride (Si-H) form with two peaks C and D , 2 . 1 eV a p a r t t h a t s t r a d d l e t h e p o s i t i o n of A. The i d e n t i f i c a t i o n of t h e two s p e c t r a was based on a a m p a r i s o n w i t h PES s p e c t r a of H on c-Si and through a number of c a l c u l a t i o n s . 4 t 5 t 1 6 The iden- t i f i c a t i o n of peaks C and D with a Si-H c o n f i g u r a t i o n

is

unique. A l l o t h e r s p e c i e s such a s Si-H ( n > l ) o r (Si-H 2 ) n g i v e r i s e t o very s i m i l a r s p e c t r a of t h e form A-B and t h e y m u s t n t h e r e f o r e b e lumped t c q e t h e r under t h e l a b e l "polyhy- d r i d e s " .

We have s t u d i e d t h e v a l e n c e band s p e c t r a o f numerous f i l m s p r e p a r e d by s p u t - t e r i n g i n A r / H m i x t u r e s and by t h e glow-discharge decomposition o f SiH4, non-dilu- t e d and d i l u t e 8 i n v a r i o u s amounts of A r , H and He. W have s t u d i e d anodic and c a t h o d i c f i l m s . A l l t h e s e p r e p a r a t i o n condiEions a r e known t o g i v e a v a r i e t y o f hydrogen c o n c e n t r a t i o n s and c o n f i g u r a t i o n s . 1 7 ' 1 8 I n a l l c a s e s we f i n d t h a t polyhy- d r i d e i s t h e dominant s u r f a c e s p e c i e s f o r f i l m s prepared a t room temperature. As an example, c o n s i d e r t h e s e r i e s o f s p e c t r a ( F i g . 5 ) o b t a i n e d f o r f i l m s d e p o s i t e d on t h e cathode i n a d i s c h a r g e o f SiH4 d i l u t e d w i t h i n c r e a s i n g amounts of He. Peaks A and B dominate t h e s p e c t r a . Even w i t h high n e g a t i v e b i a s which l e a d s t o heavy ion bombard-

0 2 4 6 8 1 0 1 2 1 6 1 6 1 E ) BINDING ENERGY (&I

Fig. 4 : Valence band s p e c t r a o f two gd F i g . 5: Valence band s p e c t r a of a-Si:H a-Si:H samples. The t h e o r e t i c a l curve p r e p a r e d i n a dc d i s c h a r g e from an SiH4/

i s

from Ref. 4. He mixture. V

i s

t h e s u b s t r a t e b i a s . B

i n t h e c o n c e n t r a t i o n of t h e polyhydride conEiguration but no i n d i c a t i o n of substan- t i a l amounts of Si-H s p e c i e s . We a r e t h e r e f o r e l e d t o b e l i e v e t h a t p o l y h y d r i d e s and presumably Si-H3 ( i n analogy t o Si-F ) a r e e s s e n t i a l f o r t h e growth of a-Si:H.

3

What a r e t h e consequences of t h e s e o b s e r v a t i o n s f o r t h e growth of a-Si:H and a-Si:F? S i n c e t h e p r e s e n c e of Si-X ( X = H,F) i s independent o f t h e d i s t r i b u t i o n of X among t h e d i f f e r e n t conf iqurai?ions i n t h e

bulk

of t h e f i l m s we conclude t h a t f i l m growth and t h e i n c o r p o r a t i o n of X a r e l a r g e l y independent of each o t h e r . We v i s u a l i z e a f r o n t of Si-X a t t h e s u r f a c e t h a t i s pushed forward by an i n s e r t i o n r e a c t i o n whereby an S i - x Z 3 r a d i c a l f r a n t h e g a s phase i n s e r t s i t s e l f i n t o an Si-X bond c r e a t i n g a new Si-Si-X3 u n i t . l 9 Such a mechanism could l e a d , i n p r i n c i p l e , t o a hydrogen- o r f l u o r i n e - f r e e network e x c e p t f o r t h e s u r f a c e l a y e r of Si-X3 u n i t s .

But what about t h e high temperature (TD

$

3 0 0 ~ ~ ) a-Si : H f i l m s ? Here we c l e a r - l y f i n d SF-H u n i t s . Do t h e s e f i l m s t h e r e f o r e grow by a d i f f e r e n t mechanism o r a r e t h e Si-H s p e c i e s t h e r e s u l t of a p o s t - d e p o s i t i o n a n n e a l i n g t h a t o c c u r s while t h e f i l m c o o l s down? The answer could come from t h e f l u o r i n a t e d f i l m s . I n f r a r e d and g a s e v o l u t i o n s t u d i e s i n d i c a t e t h a t Si-F s p e c i e s a r e s t a b l e a t l e a s t up t o %35o0cZ0 whereas Si-H

(n>l)

s t a r t s t o break u; a t * 2 5 0 ~ ~ . ~ ' The s u r f a c e composition of a-Si:F f i l m s remain2 unchanged f o r d e p o s i t i o n t e m p e r a t u r e s up t o 3 5 0 ~ ~ i n c o n t r a s t t o a-Si:H. P o s t - d e p o s i t i o n a n n e a l i n g i s t h e r e f o r e l i k e l y t o be r e s p o n s i b l e f o r t h e mo- nohydride s i g n a t u r e of h i g h t e m p e r a t u r e a-Si:H f i l m s . This p o i n t , b w e v e r , needs f u r t h e r i n v e s t i g a t i o n .

F i g . 6: Oxygen coverage a s a f u n c t i o n o f Fig. 7 : Oxygen coverage ( a ) and r e l a t i v e 0 exposure.

2 amount o f oxidized s i l i c o n (b) a s a func-

t i o n o f oxygen exposure.

I V . The Oxidation of a-Si:H.

-

The i n i t i a l s t a g e s i n t h e o x i d a t i o n of a-Si:H were s t u d i e d by exposing gd a-Si:H f i l m s prepared i n s i t u t o i n c r e a s i n g amounts of oxygen and monitoring a f t e r e a c h s t e p he

i n t e n s i t y , shape, and p o s i t i o n of t h e S i 2p and 0 1 s c o r e l e v e l s . For com-

p a r i s o n , samples of c-Si were s u b j e c t e d ^a3' t o t h e i d e n t i c a l t r e a t m e n t and a n a l y s i s .

-

The c-Si samples (p-type, 2-5

IdZ

cm) ^N a

were f i l e d i n vacuo t o expose c l e a n s u r f a c e s of d i f f e r e n t o r i e n t a t i o n . T m 3; ^{E a 2 -}

-

k i n d s of a-Si:H were s t u d i e d : high ^X temperature f i l m s which e x h i b i t mainly _{z a l -}

c -

3 -

2 -

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J O U R N A L

DE PHYSIQUE

Si-H s u r f a c e s p e c i e s and f i l m s p r e p a r e d a t room t e m p e r a t u r e w i t h p o l y h y d r i d e s p e c i e s a t t h e s u r f a c e .

F i g . 6 shows t h e 0 l s / S i 2p i n t e n s i t y r a t i o a s a f u n c t i o n of e x p o s u r e t o d r y m o l e c u l a r 0 2 . The c-Si sample t a k e s 0 up r a p i d l y t o a s a t u r a t i o n c o v e r a g e which we d e f i n e a s a monolayer (1 ML) o f a d s o r k d oxygen f o l l o w i n g Ref. 22. On t h e a-Si:H f i l m s n o oxy e n s i g n a l c o u l d b e d e t e c t e d f o r e x p o s u r e s up t o 1 0 6 ~ 0 (11, = 1 Lang- muir = IxlO-' t o r i

.

s e c ) . When t h e a-Si:H s a m p l e s were exposed t o a c t i v a t e d oxygen

( i . e . 0 i n t h e p r e s e n c e o f a h o t f i l a m e n t ( F I ~ C O O C ) ) we o b s e r v e an u p t a k e of oxy- gen a s shown i n F i g . 7 a . A p l a t e a u i n t h e oxygen c o v e r a g e a s i n c - S i 2 i s a b s e n t , how- e v e r , and t h e c o v e r a g e p r o c e e d s s t e a d i l y beyond t h e monolayer p o i n t . The low tem- p e r a t u r e a-Si:H f i l m a c c u m u l a t e s a b o u t t w i c e a s much oxygen a s t h e h i g h t e m p e r a t u r e f i l m by t h e t i m e a n e x p o s u r e o f lo6

L

o f a c t i v a t e d oxygen i s r e a c h e d .

A s soon a s t h c oxygen bonds t o a S i atom a chemical s h i f t i s i n d u c e d on t h e S i 2 p l e v e l . The i n t e n s i t y of t h e s e s h i f t e d components r e l a t i v e t o t h a t o f t h e un- s h i f t e d S i 2p l e v e l i s p l o t t e d i n F i g . 7b and t h e i r s p e c t r a l d i s t r i b u t i o n s i n F i g . 8.

The o x i d a t i o n o f t h e a-Si:H f i l m s p r o c e e d s from t h e v e r y b e g i n n i n g o f t h e oxygen up- t a k e ; a n i n i t i a l s t a g e o f s o l e l y p h y s i - s o r b e d oxygen ( i . e . o n e t h a t d o e s n o t i n - d u c e a n y c h e m i c a l s h i f t ) is n o t o b s e r v e d . The d i f f e r e n c e i n oxygen c o v e r a g e between t h e low- and t h e hi* t e m p e r a t u r e f i l m d o e s n o t l e a d t o a s i m i l a r d i f f e r e n c e i n t h e number of a c t u a l l y o x i d i z e d S i atoms, how- e v e r . I n F i g . 9 we show tb r a t i o of t h e oxygen s i g n a l I ( O 1 s ) t o t h e i n t e n s i t y of t h e c h e m i c a l l y s h i f t e d S i 2 p l e v e l s I ( S i 2p Ox)

.

T h i s r a t i o h a s been t r a n s -

I

310a 10'

w'

10' lo6

0 1 2 3 4 5 6

Exposure to activated 0 ~ [ 1 0 ~ t o r r . s e c ]

F i g . 8: D i f f e r e n c e s p e c t r a between c l e a n F i g . 9 : The a v e r a g e s t o i c h i o m e t r y a n d o x i d i z e d a-Si:H f i l m s . The s p e c t r a a r e X ( S i 0 ) o f t h c o x i d i z e d s u r f a c e of a d j u s t e d i n a m p l i t u d e . The r e l a t i v e i n t e n - t h r e e X s i samples.

s i t i e s SiO / S i a r e g i v e n as a p a r a m e t e r .

u s i n g a f u l l y o x i d i z e d sample a s a s t a n d a r d . To a p p F e c i a t e t h e s i g n i f i c a n c e o f this f i g u r e , o n e h a s t o remember t h a t a f t e r a n e x p o s u r e o f 1 0 6 ~ b o t h amorphous f i l m s con- t a i n more S i a t o m s t h a t a r e o x i d i z e d t h a n t h e c - S i s a m p l e s . Yet t h e s e a t m s have on a v e r a g e

a lower

s t o i c h i o m e t r y

than

c - S i i n t h e c a s e o f t h e h i g h t e m p e r a t u r e f i l m , w h e r e a s t h e a v e r a g e s t o i c h i o m e t r y of t h e low t e m p e a a t u r e f i l m e x c e e d s t h a t of c-Si beyond a n e x p o s u r e o f ~ 1 0The a p p a r e n t e x c e s s s t o i c h i o m e t r y o f ~ ~ . the low tcmpera- t u r e f i l m s c o u l d r e f l e c t t h e p r e s e n c e o f oxygen n o t d i r e c t l y bound t o S i ( e . g . Si-H- 0 ) . A f u l l e r u n d e r s t a n d i n g o f t h i s and t h e " s u b s t o i c h i m e t r y " o f t h e high-tempera- t u r e f i l m s must a w a i t a d e t a i l e d a n a l y s i s of t h e s p e c t r a s u c h a s F i g . 8 .

F i n a l l y , we o b s e r v e f o r t h c h i g h - t e m p e r a t u r e a-Si:H f i l m a band-bending of 0 . 1 5 eV c o r r e s p o n d i n g t o a d e p l e t i o n l a y e r f o r n - t y p e m t e r i a l . The d e p l e t i o n l a y e r f o r m s j e t w e e n l o 4 and 1 0 6 ~ oxygen e x p s u r e . The i n t e r f a c e c h a r g e s r e s p o n s i b l e a r e most l i k e l y OH- g r o u p s . No band b e n d i n g was f o u n d i n c - S i a n d t h e low-temperature f i l m s were t o o r e s i s t i v e t o measure t h e band b e n d i n g . It s h o u l d b e s t r e s s e d t h a t t h e o x i d e l a y e r t h a t l e a d s t o t h e Fermi l e v e l s h i f t is l e s s t h a n 1 0

2

t h i c k .

C o n c l u s i o n .

-

We h a v e i n v e s t i g a t e d t w a s p e c t s o f t h e s u r f a c e p r o p e r t i e s o f

a - S i : H ( F ) : ( i ) t h e s u r f a c e c h e m i s t r y o f s p e c i m e n s r i g h t a f t e r t h e i r p r e p a r a t i o n w i t h - o u t any e x p o s u r e t o a t m o s p h e r e a n d ( i i ) t h e i n i t i a l s t a g e s i n t h e o x i d a t i o n o f a-Si:H. I t a p p e a r s from t h e f i r s t i n v e s t i g a t i o n t h a t t h e growth o f t h e f i l m s p r o - c e e d s m a i n l y t h r o u g h t h e f o r m a t i o n a n d t h e advancement o f a f r o n t o f Si-X s p e c i e s

( X = H , F ) . The i n c o r p o r a t i o n o f X i s t o a c e r t a i n d e g r e e d e c o u p l e d from

dis

^growth

mechanism. We have s u r f a c e s r i c h i n Si-X3 even when t h e bulk c o n t a i n s m a i n l y Si-X o r Si-X2 s p e c i e s . The o x i d a t i o n e x p e r i m e n t s r e v e a l a r e n a r k a b l e r e s i s t e n c e o f a-Si:H a g a i n s t t h e a d s o r p t i o n o f d r y m o l e c u l a r oxygcn. E x c i t e d o r i o n i z e d oxygen l c a d s t o t h e f o r m a t i o n o f a s u b o x i d e l a y e r S i O v e r y much l i k c i n c - S i . T h i s is due t o a s a t u r a t i o n o f a l l s u r f a c e bonds by h d r o g e n and t h e e x t r a e n e r g y needed to b r e a k up t h e s e bonds must come from t h e e x c i t e d oxygen. The o b s e r v a t i o n o f band b e n d i n g i n t h e i n i t i a l s t a g e s o f o x i d a t i o n i s i n c o n t r a s t t o r e s u l t s on f u l l y o x i d i z e d s p e c i - m e n s . 9 ' 2 3 T h i s p o i n t must t h e r e f o r e a w a i t f u r t h e r i n v e s t i g a t i o n .

R e f e r e n c e s .

-

1. VON ROEDERN B., L. LEY, M. CARDONA, Phys. Rev. L e t t .

2

(1977) 15.76.

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3 . GRUNTZ K . J . , L. LEY, M . CARDONA, R.L. JOHNSON, G. HARBEKE, B. VON ROEDEIIN, J . Non-Cryst. S o l i d s

35+36

(1950) 453.

4. e . g . CHING W . Y . , D . J .

LRM,

C.C. LIN, Phys. R w . B

21

( 1 9 8 0 ) 2378.

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fi

(1980) 43.

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(1980) 55.

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12

(1979) 2209 10. SPEAR W. E., P.G. LECOMBER, P h i l . Mag.

2

( 1 9 7 6 ) 935.

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(1979) 4 3 3 . 12. LECOMBER P.G. ( p r i v a t e communication)

.

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s u p p l . A (1980) 1245.

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(1975) 1728.

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,

R . W. GRIE'FITH, i n R e f . 18.

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(1980) 6140.

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(1977) 561.

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