REFLECTION SAXS INVESTIGATION OF AMORPHOUS THIN FILMS

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REFLECTION SAXS INVESTIGATION OF AMORPHOUS THIN FILMS

R. Manaila

To cite this version:

R. Manaila. REFLECTION SAXS INVESTIGATION OF AMORPHOUS THIN FILMS. Journal de

Physique Colloques, 1981, 42 (C4), pp.C4-213-C4-216. �10.1051/jphyscol:1981444�. �jpa-00220901�

JOURNAL DE PHYSIQUE

CoZZoque C4, suppze'ment au nO1O, Tome 42, octobre 1981 page C4-213

REFLECTION SAXS INVESTIGATION OF AMORPHOUS THIN FILMS

R. Manaila

I n s t i t u t e of Physics and TechnoZogy o f Materials, Bucharest, P.O. Box MG7, Romania

A b s t r a c t . - The p r e s e n t work p o i n t s o u t t h e advantages of a r e - f l e c t i o n v a r i a n t of small-angle X-ray scattering(RSAXS) i n t h e study of amorphous t h i n films.The method i s a p p l i e d t o amorph- ous

Max

(a-Ge) l-x f i l m s (0.07<x*O. 32) ,allowing t h e d e t e r m i n a t i o n of t h e s i z e d i s t r i b u t i o n and magnitude o f t h e compositional

f l u c t u a t i o n s .

I n t r o d u c t i o n . - The p r e s e n t i n c r e a s i n g i n t e r e s t i n t h e m i c r o s t r u c t u r e o f t h e amorphous f i l m s [I] r e q u i r e s adequate s t r u c t u r a l methods t o as-

s e s s q u a n t i t a t i v e l y t h e e x i s t i n g f l u c t u a t i o n s i n d e n s i t y and/or com- nosition.On t h e o t h e r h a n d , t h i n f i l m s a r e u s u a l l y d i f f i c u l t t o be r e - moved from t h e s u b s t r a t e , i n o r d e r t o be submitted t o conventional t r a n s m i s s i o n small-angle X-ray i n v e s t i g a t i o n s .

A r e f l e c t i o n SA S method (RSAXS) i s p a r t i c u l a r l y s u i t a b l e i n t h e

'd

study of f i l m s 103-10

8

t h i c k , a s t h e g l a n c i n g i n c i d e n c e a m p l i f i e s t h e beam p a t & i n t h e m a t e r i a l by a f a c t o r l / s i n 0 . A l s o , t h e d i f f r a c t e d i n t e n s i t y i s b e t t e r averaged,due t o t h e l a r g e i r r a d i a t e d area.For t h i n n e r f i l m s o r low a b s o r p t i o n c o e f f i c i e n t s an a b s o r p t i o n c o r r e c t i o n may be r e q u i r e d .

A s very l i t t l e i s known nowadays about t h e s t r u c t u r e o f amorph- ous s u r f a c e s and underlying layers,RSAXS could a l s o open new i n v e s t i - g a t i o n p o s s i b i l i t i e s i n t h i s f i e l d . S u r f a c e mechanical i m p e r f e c t i o n s and oxide l a y e r s can a l s o g i v e r i s e t o s c a t t e r i n g e f f e c t s l2] , b u t t h e y a r e u s u a l l y c o n c e n t r a t e d i n t h e range o f very small k = 4 l l s i n e / ~ .

When used with a p a r a f o c u s i n g qeometry,RSAXS i s s e n s i t i v e only t o t h e f l u c t u a t i o n component which i s normal t o t h e f i l m surface.The- r e f o r e , i t i s expected t o give u s e f u l information i n t h e c a s e of a n i - s o t r o p i c v o i d s t r e p o r t e d i n evaporated and s p u t t e r e d a-Ge f i l m s [3].

Experimental technique.- RSAXS r e l i e s on t h e s t r o n g l i m i t a t i o n of t h e e q u a t o r i a l divergence i n a Bragg-Brentano d i f f r a c t o m e t r i c set-up.Our experiments were performed with a small-angle device mounted on t h e sample h o l d e r o f a conventional Siemens d i f f r a c tometer. The e q u a t o r i - a l divergence was l i m i t e d by means o f a Au-plated e d g e , p a r a l l e l t o t h e sample s u r f a c e a t a v a r i a b l e d i s t a n c e - T h e divergence of t h e beam was about 2. 1om3rad,which corresponds t o a r e s o l u t i o n Ak of 2.10'~8-'

f o r t h e CuK, wavelength which was used.This i s t o be compared w i t h a t y p i c a l e q u a t o r i a l divergence of 1.10-4rad i n conventional SAXS cameras.

T h u s , r e f l e c t i o n SAXS can be u s e f u l mainly i n problems which do n o t r e q u i r e a hiqh r e s o l u t i o n . Also, i n o u r experiments t h e measurable k range extended downwards t o about 6 . 1 0 ' ~ g'l(2B=0.8°) ,due t o t h e

l i m i t e d l e n g t h of t h e sample-This l i m i t a t i o n i s a l s o r e l a t e d t o t h e proximity of t h e t o t a l r e f l e c t i o n i n c i d e n c e a n g l e .

RSAXS e f f e c t s a r e superposed o n t o t h e sample volume s c a t t e r i n g . I n a F r e s n e l formalism a p p l i e d t o t h e d i f f r a c t o m e t e r geometry,a sam-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981444

C4-2 14 JOURNAL DE PHYSIQUE

p l e of uniform e l e c t r o n d e n s i t y w i l l s c a t t e r a s m a l l - a n g l e i n t e n - s i t y ( k ) ~ k - ~ , a l t h o u g h t h i s Sguendence can b e e a s i l y d i s t o d d by an i m p e r f e c t p l a n e i t y o f t e samule s u r f a c e . 4 n y d e n s i t y f l u c t u a t i - ons i n t h e s i z e range 1 0 - l o 2 w i l l modulate t h i s a v e r a g e s c a t t e r i n g and t h i s e f f e c t can be t r e a t e d i n t h e F r a u n h o f e r a p p r o x i m a t i o n , due t o t h e i n v e s t i g a t e d d i s t a n c e s b e i n g s m a l l i n comparison w i t h t h e sample l e n g t h .

Inhomo e n e i t i e s i n c e r m e t - t e Mo (a-Ge)l-, t h i n f i l m s . - The p r e s e n t work r z p o r t RSAXS r e s u l t s o%zaine$ G m o r p h o u s cermet Mo,(a-Ge)l-, t h i n f i l m s w i t h x=0.07, 0.22 and 0.32.

The f i l m s were o b t a i n e d by c o - e v a p o r a t i o n o n t o f u s e d s i l i c a s u b s t r a t e s and had t h i c k n e s s i n t h e range (2-3)

. l o 3

g . ~ e t a i l e d r e - s u l t s on t h e i r s t r u c t u r e and e l e c t r i c a l p r o p e r t i e s were r e p o r t e d i n C5J.

The s c a t t e r e d i n t e n s i t y o b t a i n e d by RSAXS on t h e s e f i l m s i s shown i n f i g . 1 . The cermet f i l m s d i s p l a y s t r o n g i n t e n s i t y m o d u l a t i o n s , s u p e r p o s e d o n t o a smoothly waryinq volume s c a t t e r i n q . The m o d u l a t i o n s _{- - - -} .-

i n c r e a s e i n a m p l i t u d e w i t h t h e

f log l b b .

units.) m e t a l c o n c e n t r a t i o n x and s o does t h e g e n e r a l l e v e l o f t h e s c a t t e r e d i n t e n s i t y i n t h e s m a l l - a n g l e ran- ge.For x = 0 . 0 7 ( f i g . l , s a m p l e M),the o s c i l l a t i o n s a r e b a r e l y v i s i b l e .

T h i s e f f e c t i s r e m i n i s c e n t o f t h e SAXS on q u a s i - s p h e r i c a l p a r - t i c l e s and must be a s c r i b e d t o zones o f c o m p o s i t i o n a l f l u c t u a t i - on, where t h e Mo c o n c e n t r a t i o n i s above t h e a v e r a g e v a l u e .

I n f o r m a t i o n a b o u t t h e s i z e and d e n s i t y f l u c t u a t i o n i n t h e s e zo- n e s can be i n f e r r e d from t h e high- f r e q u e n c y o s c i l l a t o r y p a r t o f t h e RSAXS i n t e n s i t y . T h i s i n v o l v e s se- p a r a t i n g t h e low-frequency, mono- tonous i n t e n s i t y component I v ( k )

,

which i s due t o sample volume e f - f e c t s , from t h e t o t a l s c a t t e r e d i n t e n s i t y I ( k ) . T h e s e p a r a t i o n com- n r i s e s two s t e p s : s u b s t r a c t i o n o f and a v e r a q e power f u n c t i o n i n k and s u b s e q u e n t F o u r i e r f i l t e r i n g o f t h e remaining low-f requency component.

- __L_)

-22 ^-08

^-0.6 Fiq.1.RSAXS i n t e n s i t y on a-MoX(a-

C R ) ~ - ~ f i l m s w i t h x = 0 . 0 7 ( ~ ) , 0.22

109 k(A-')

⁽ a ) and 0.32 ( b )

.

The r e s u l t i n g o s c i l l a t o r y p a r t k . i ( k ) o f t h e RSAXS i n t e n s i t y i s shown i n f i g . 2 f o r t h e samples w i t h x=0.22 and 0.32 : i ( k ) = ( I ( k ) / I y ( k ) ) - 1 . M u l t i p l i c a t i o n by k f a v o u r a b l y r e d u c e s t h e w e i g h t o f t h e low a n g l e r a n g e r w h i c h i s most e f f e c t e d by t h e f i n i t e l e n g t h o f t h e sample.

A s can b e s e e n i n f i g . 2 , t h e o s c i l l a t o r y component i s r a p i d l y damped a t h i g h e r k,which p o i n t s t o a l i m i t e d s i z e r a n g e o f t h e i n v e s - t i g a t e d c o m p o s i t i o n a l f l u c t u a t i o n s

.

A F o u r i e r t r a n s f o r m o f k . i ( k ) y i e l d s t h e e l e c t r o n d e n s i t y f l u c - t u a t i o n A@=@-povs.the d i s t a n c e R from an a v e r a q e atom i n t h e sample

( f i g . 3 ) .The a v e r a g e e l e c t r o n d e n s i t y p depends on t h e composition x.

I t i s t o b e n o t i c e d t h a t f o r x=0?22 t h e r e i s a s i n g l e maximum a 1 R=125

2.

For x=0.32,a s e r i e s o f a l m o s t e q u a l l y s p a c e d peaks ap-

p e a r s , i n d i c a t i v e o f a h i g h e r number o f c l o e r packed zones. The d e t a i l s below 80

4

a r e r e s i d u e s o f t h e low-frequency component

.

The A p ( R ) dependence can be i n t e r - p r e t e d i n t h e frame o f a model ,con- s i s t i n q i n q u a s i - s p h e r i c a l Mo-rich z o n e s , d i s p e r s e d i n a m a t r i x w i t h a Mo c o n c e n t r a t i o n below t h e a v e r a g e x.

A t b o t h Mo c o n c e n t r a t i o n s , t h e Mo-Mo p a i r d i s t a n c e s between zones a r e ex- q e c t e d t o a p p e a r a s maxima i n t h e d i s - t r i b u t i o n o f t h e d e n s i t y f l u c t u a t i o n s . The a r e a o f a maximum,i.e.the number o f e x c e d e n t a r y e l e c t r o n p a i r s , i s ex- p e c t e d t o be ( A p ) 2 . ~ n t h e o t h e r hand, t h e d i s t r i b u t i o n o f t h e Gc-Mo p a i r d i s t a n c e s between t h e m a t r i x and t h e zones s h o u l d b e r a t h e r f e a t u r e l e s s .

A d d i t i o n a l s u g g e s t i o n a b o u t t h e p a c k i n g of t h e Mo-rich zones comes from e l e c t r o n microscopy o b s e r v a t i o n s F i g . 2 . 0 s c i l l a t o r y RSAXS component f o r samples w i t h x=0.22 ( a ) and 0.32 ( b )

.

(P .B .Barna ,A.Barna,L.Toth,R.Manaila,unpublished work) .They r e v e a l f o r x-0.32 a s u p e r s t r u c t u r e o f darker,Mo-rich c i r c u l a r z o n e s , s u p e r p o s e d o n t o t h e f i n e - q r a i n e d p a t t e r n , c h a r c t e r i s t i c f o r homogeneous amor- nhous f i l m s . These z o n e s l a b o u t 100

$,

i n d i a m e t e r l a r e packed r a t h e r c l o s e l y i n l a r g e r Mo-rich r e q i o n s

and a r e b a r e l y v i s i b l e i n t h e r e s t o f t h e m a t r i x . F o r x=0.22, t h i s s e g r e g a t i o n o f t h e m e t a l i s much l e s s marked.

F o r a c l o s e p a c k i n g o f h a r d s p h e r e s w i t h t h e d i a m e t e r D,the p r e v a i l i n g p a i r d i s t a n c e s a r e D,1.63 D,1.95 D , e t c .

.of

c o u r s e r t h e p a c k i n g o f t h e zones

20-

i s n o t r e a l l y c l o s e and t h e y may d i f f e r by t h e i r Mo concen- t r a t i o n , s o t h a t t h e above p a i r d i s t a n c e s have o n l y a h i n t va- 1 u e . B ~ assuming f o r t h e d i s - t a n c e o f c l o s e s t approach D t h e v a l u e 96 W,correspondinq t o t h e F i g . 3 . E l e c t r o n d e n s i t y f l u c t u a - t i o n vs.k i n samples w i t h

x=0.22 ( a ) and 0.32 ( b )

. _{LO 80 120 160 R(d)}

^&

f i r s t maximum, t h e i n t e r - z o n e d i s t a n c e s p r e d i c t e d f o r a c l o s e p a c k i n g a r e 157 and 187 2 , i n rough agreement w i t h t h e p o s i t i o n s o f t h e p e a k s on f i g . 3 .

The d a t a i n f i g . 3 b can a l s o b e used t o e s t i m a t e t h e magnitude d f t h e e l e c t r o n d e n s i t y f l u c t u a t i o r t A D f o r x=0.32.Taking i n t o a c c o u n t t h e a r e a A o f t h e main peak a t 96 and t h e p r o b a b i l i t y p o f f i n d i n g a Mo-rich zone a t a g i v e n d i s t a n c e R from a n o t h e r zone,Ap= &ID.

A rouqh e s t i m a t e , h a s e d on t h e e l e c t r o n m i r o s c o y o b s e r v a t i o n s , g i v e s p 0.1 , s o t h a t bo i s found t o h e 2 2. 1 0 - P e l . ~ - g . T h i s i s t o be

JOURNAL DE PHYSIQUE

compared w i t h t h e a v e r a g e e l e c t r o n d e n s i t y Q,=8.10-l e l

-8-'

c o n t r i b u - t e d by t h e M o atoms w i t h t h e a v e r a g e c o n c e n t r a t i o n o f 0.32 , p o i n t i n g t o r a t h e r h i g h l o c a l v a r i a t i o n s i n t h e x v a l u e .

T h u s , e l e c t r o n d e n s i t y v a r i a t i o n s , d u e t o d e n s i t y or c o m p o s i t i o n f l u c t u a t i o n s i n t h i n f i l m s , c a n be q u a n t i t a t i v e l y a s s e s s e d by RSAXS, p r o v i d e d t h e s i z e o f t h e f l u c t u a t i o n s f a l l s i n t o a c o n v e n i e n t r a n g e .

REFERENCES

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(1980) 1 9 1 .

2 . ROTH M., J . A p p l . C r y s t . 10 (1977) 172

3. BARNA A., BAFWA P.B., ~ ~ N O C Z I G . , SUGAWARA H. a n d THOMAS P . , P r o c . l n t . T o p i c a 1 C o n f - S t r u c t u r e and E x c i t a t i o n s i n Amorph.Solids, W i l l i a m s b u r q (USA),march 1976,p.199

4 . WARREN B.E., "X-ray D i f f r a c t i o n " , Addison-Wesley,Reading,1969, p. 142

5. BELU A . , DEVENYI A . , MANAILA R., M I U L., RUSU C., BARNA A.,

BAFWA P.B., RADNOCZI G. a n d TOTH L . , A c t a P h y s i c a H u n q a r i c a ( u n d e r p r e s s )

6. SADOC J.F., DIXMIER J. a n d G U I N X E R A . , J . N o n - C r y s t a l l i n e S o l i d s 1 2 (1973) 4 6 .