THE LOCAL STRUCTURE OF a-SiOx : H FILMS

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THE LOCAL STRUCTURE OF a-SiOx : H FILMS

G. Greaves, X. Jiang, R. Jenkins, E. Holzenkampfer, S. Kalbitzer

To cite this version:

G. Greaves, X. Jiang, R. Jenkins, E. Holzenkampfer, S. Kalbitzer. THE LOCAL STRUC- TURE OF a-SiOx : H FILMS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-853-C8-856.

�10.1051/jphyscol:19868164�. �jpa-00226068�

JOURNAL DE PHYSIQUE

Colloque C8, suppl6ment au n o 12, Tome 47, decembre 1986

THE LOCAL STRUCTURE OF a - S i 0 x : H FILMS

G.N. GREAVES, X.L. JIANG, R.N. JENKINS, E. HOLZENKAMPFER" and S. KALBITZER*

SERC, Daresbury Laboratory, GB-Warrington W A 4 4AD, Great-Britain

" ~ e p a r t m e n t of Physics, University of Marburg, 0 - 3 5 5 0 Marburg, F.R.G.

ax-~lanck-~nstitut fiir Kernphysik, 0 - 6 9 0 0 Heidelberg, F.R.G.

A b s t r a c t . - The S i K-edge

of t h i n f i l m s of amorphous hydrogenated s i l i c o n oxide r a n g i n g from S i t o S i O l e 7 have been measured. A n a l y s i s o f the S i - S i and Si-0 p a i r d i s t r i b u t i o n f u n c t i o n s r e v e a l s s i g n i f i c a n t bond l e n g t h changes w i t h oxygen c o n c e n t r a t i o n . Films of i n t e r m e d i a t e composition ( e . g . SiO) d i s p l a y c o n s i d e r a b l e d i s o r d e r i n t h e n e a r e s t neighbour S i environment.

Both f a c t s g i v e c l e a r evidence f o r random bonding i n t h e s e amorphous s i l i c o n o x i d e s .

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

The amorphous s i l i c o n oxide system, a-SiO , i s p a r t i c u l a r l y i n t e r e s t i n g in t h a t it

X

o f f e r s a v a r i a b l e bandgap semiconductor ranging from 1.7 eV ( a - S i ) t o 9 eV (a-Si02).

Moreover it models t h e i n t e r f a c e between c r y s t a l l i n e s i l i c o n and amorphous s i l i c o n d i o x i d e i n MOST devices. The o p t i c a l p r o p e r t i e s of evaporated a-SiO f i l m s were

X

o r i g i n a l l y i n v e s t i g a t e d by P h i l i p p ( 1 ) . These r e s u l t s were confirmed by

Holzenkampfer e t a l . ( 2 ) who s t u d i e d them i n c o n j u n c t i o n w i t h t h e ESR p r o p e r t i e s . These a u t h o r s have shown t h a t t h e e l e c t r o n i c and d e f e c t p r o p e r t i e s of a-SiO f i l m s

X

can be i n t e r p r e t e d i n terms of s t r u c t u r e s which a r e randomly bonded. I n t h i s t y p e of model s i l i c o n atoms can adopt t h e t e t r a h e d r a l c o n f i g u r a t i o n s S i ( S i 3 0 ) , S i ( S i 2 0 2 ) and S i ( S i 0 3

a s w e l l a s t h e chemically ordered s i t e s Si(Si,,

and Si(O,,) c h a r a c t e r - i s t i c of s i l i c o n and s i l i c a . By c o n t r a s t t h e i n t e r m e d i a t e oxide SiO has been well- s t u d i e d by a wide v a r i e t y of techniques - n o t a b l y X-ray ( 3 ) and neutron (4) d i f f r a c - t i o n , (MAS )NMR ( 5 ) and XPS and XANES ( 6 ) . For s i l i c o n m n o x i d e a mixture model has been proposed comprising 5 t o 10 a r e g i o n s of o r d e r e d s i l i c o n and s i l i c a ( 3 ) . Bulk SiO, however, i s commonly used, and a s t h i s i s p r e p a r e d by s i n t e r i n g s i l i c o n and s i l i c a it might be expected t o be microphase s e p a r a t e d . I n any event p r i o r t o t h e p r e s e n t s t u d y c o n c l u s i v e evidence f o r random bonding i n a-SiO f i l m s i n c l u d i n g SiO

X

was n o t forthcoming.

Measurements and A n a l y s i s

I n an e f f o r t t o r e s o l v e t h e controversy between t h e random bonding and mixture

models we have measured t h e S i K-edge EXAFS of t h i n f i l m s of hydrogenated s i l i c o n

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868164

C8-85 4 JOURNAL DE PHYSIQUE

I . . . . I . .

1-

a, U C

n

b n

u

0

Fig.

XANES of a-SiOx:H f i l m s

Fig. 2

EXAFS of s i l i c o n monoxide:

experiment

and theory

oxide. The f i l m s were prepared by r . f . glow discharge from mixtures of s i l a n e and n i t r o u s oxide and deposited on s u b s t r a t e s of Be f o i l held a t 2700C. Sputtered amorphous s i l i c o n and s i l i c a were used a s model compounds. The chemical composition of t h e samples were measured using Rutherford b a c k s c a t t e r i n g (0 c o n t e n t ) and y-ray a n a l y s i s of t h e

+ p r e a c t i o n (H c o n t e n t )

7 ) . Room temperature S i K-edge EXAFS s p e c t r a were obtained i n txansmission a t

u s i n g t h e ACO s t o r a g e r i n g and a KAP monochromator. The r a p i d curved wave theory was used f o r a n a l y s i s ( 8 ) with phase s h i f t s taken from t h e SRS EXAFS Data Bank (9). These were s l i g h t l y adjusted t o p r e c i s e l y f i t t h e t e t r a h e d r a l coordination and bond l e n g t h s of t h e model s t r u c t u r e s v i z S i - 0 1.62 a and Si-Si 2.35 8 . The non-structural EXAFS parameters included a quantum e f f i c i e n c y f a c t o r of 0.8 and a photoelectron imaginary self-energy of 2.7 eV. To account f o r t h e considerable d i f f e r e n c e s i n t h e e l e c t r o n i c s t r u c t u r e of Si-O and Si-Si bonds, two energy zeros were used - 16 eV f o r Si-O d i s t r i b u t i o n s and 7 eV f o r Si-Si a s t r i b u t i o n s . These were chosen to match t h e d i f f e r e n c e i n energy gap between s i l i c a and s i l i c o n . Figure

shows t h e XANES f o r t h r e e of t h e f i l m s c l e a r l y i n d i c a t i n g t h e s i z a b l e chemical s h i f t across t h e s i l i c o n oxide system. I n t h e same vein, two c e n t r a l atom phase s h i f t s were employed - one f o r s i l i c a ( c o n t a i n i n g a Madelung c o r r e c t i o n ) and one f o r s i l i c o n (comprising n e u t r a l atoms).

The Si-O and Si-Si p a i r d i s t r i b u t i o n f u n c t i o n s f o r t h e f i l m s were analysed s e p a r a t e -

l y from F o u r i e r - f i l t e r e d spectra. Results f o r intermediate oxides were checked by

analysing t h e complete n e a r e s t neighbour r a d i a l d i s t r i b u t i o n function of s i l i c o n ;

t a k i n g values f o r c e n t r a l atom phase s h i f t s and energy zeros by weighting t h o s e of

S i and Si02 according t o composition. Figure 2 shows t h e comparison between experi-

ment and theory f o r a f i l m of s i l i c o n mnoxide. Both approaches yielded almost

i d e n t i c a l r e s u l t s .

Results and Discussion

W h i l s t s p e c t r a f o r hydrogenated and s p u t t e r e d amorphous s i l i c o n were almost t h e same, t h e smaller coordination numbers of t h e hydrogenated f i l m (3.9 compared t o 4.0) meant t h e s p u t t e r e d f i l m was s l i g h t l y more disordered. For convenience t h e EXAFS ~ebye-Waller f a c t o r s f o r t h e s i l i c o n r i c h and oxygen r i c h ends of t h e system were f i x e d a t t h e common value of 0.0053 f12. The coordination numbers f o r oxygen and s i l i c o n were taken from t h e chemical a n a l y s i s . Hydrogen d e f e c t s were presumed t o be mnohydrides r a t h e r than dihydrides which is expected f o r t h e s u b s t r a t e temperature employed i n deposition

10

.

W e found oxygen and s i l i c o n bond lengths v a r i e d w i t h t h e concentration of oxygen,

T h i s i s shown i n f i g . 3 ( a ) where S i 4 bonds decrease from 1.69 fl t o

1.62 fl and Si-Si bonds i n c r e a s e from 2.35 t o 2.49 fl a s x i n c r e a s e s . This i s p r e c i s e l y t h e behaviour expected i f t h e s t r u c t u r e s a r e randomly bonded. I f on t h e o t h e r hand t h e s t r u c t u r e s were comprised simply of mixtures of s i l i c o n and s i l i c a , no change would be expected. The gradual incorporation of oxygen i n t o t h e n e a r e s t neighbour environment of s i l i c o n w i l l i n c r e a s e t h e charge on s i l i c o n s causing them t o r e p e l one another but i n c r e a s i n g t h e i r a t t r a c t i o n t o oxygens. Suppose P i s t h e

n p r o b a b i l i t y of f i n d i n g t h e c o n f i g u r a t i o n S i ( S i 0

i n a randomly bonded s t r u c t u r e

n 4-n

- n running from 0 t o 4. I f R and r a r e t h e i n d i v i d u a l bond lengths of Si-O and n

Si-Si r e s p e c t i v e l y f o r t h a t c o n f i g u r a t i o n , t h e s e can be deduced from t h e measured average bond lengths and ; given i n f i g . 3 ( a ) , viz.

2 (4-n) PnRn - C n P n r n -

R =

r =

2x 4 ( 1 - ^x/2)

S i - S i

S i - 0

( a ) ( b )

Fig. 3

S i a and Si-Si bond lengths ( a ) and Debye-Waller f a c t o r s ( b )

Values f o r oxygen and S i l i c o n bond l e n g t h s f o r t h e f i v e p o s s i b l e configurations a r e

given i n Table 1.

JOURNAL DE PHYSIQUE

Table 1

Si-O(R) and Si-Si(r) bond lengths for the random bonding configurations Si(Sin04-n) deduced from measured average bond lengths given in fig.3(a).

Configuration Si(O,+) Si( Si03

Si(Si202) Si(Si30) Si( Si+

Finally the amplitudes of EXAFS oscillation for intermediate oxides were found to be considerably reduced compared to those for silica or silicon. Whilst a fall

Rn (A) 1.61 1.63 1.66 1.71

in total coordination number cannot be ruled out, the most likely explanation is an rn (A)

2.45 2.44 2.39 2.35

increase in disorder. Indeed we would expect this to be particularly pronounced for a randomly bonded structure where configurations accommodating differing proportions of oxygen and silicon were juxtaposed. The Si-0 and Si-Si EXAFS Debye-Waller factors for a-SiOx:H films are presented in fig.3(b). These are deduced on the basis of tetrahedral coordination and the measured chemical composition of the films. Note the incongruent behaviour as the oxygen content is varied. The

disorder in Si-0 bonds decreases with increasing oxygen whilst the disorder in Si-Si bonds increases. This effect is not predicted by a mixture model where order in the silica and silicon regions will to a first approximation remain constant. It is expected, however, if the structure is randomly bonded. In this case with increas- ing oxygen disorder should decrease for oxygen ligands and increase for silicon ligands as the configurations Si(Si30), si(Si202) and Si(Sio3) become successively dominant.

Acknowledgements

The authors are indebted to the SERC, CNRS and 13ax Planck Institute for provision of facilities and to the British Council for financial assistance.

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