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page Cl0-387. STRUCTURES AND OPTICAL BEHAVIOR OF OVERCOATED GRANULAR FILMS. S. Damaskinosr, F.E. Girouard, A. Pinard and V.V. Truong.
JOURNAL DE PHYSIQUE

Colloque CIO, suppl6rnent a u n012, Tome 44, decembre 1983

page Cl0-387

STRUCTURES AND O P T I C A L BEHAVIOR OF OVERCOATED GRANULAR F I L M S S. Damaskinosr, F.E. Girouard, A . Pinard and V.V. Truong

De'parternent de Physique, Universite' de Moncton, Moncton, N . B., Canada E1A 3E9 Resume: Des f i l m s g r a n u l a i r e s d e c u i v r e e t d ' a r g e n t d @ p o s e s s u r du v e r r e o n t 6 t & r e v e t u s d ' u n e c o u c h e e p a i s s e de m e t a l s e m b l a b l e o u d i f f s r e n t . Le c o m p o r t e m e n t o p t i q u e d e c e s f i l m s a @ t & etudie e n m e s u r a n t l a r e f l e c t i v i t e du c d t P d u v e r r e d a n s l a r e g i o n s p e c t r a l e s l B t e n d a n t de 0 . 3 Z 1 . 0 micron. Les r e s u l t a t s i n d i q u e n t que l e s f i l m s g r a n u l a i r e s r e c o u v e r t s c o n s e r v e n t l e u r s t r u c t u r e d 1 a g r 6 g a t s , m6me a p r s s l e r e v e t e m e n t . Les F c h a n t i l l o n s o n t a u s s i 6 t @ soumis a des t r a i t e m e n t s t h e r m i q u e s , e t d e s changements dans E n gGn6ra1, bien q u ' u n l e s proprigtes optiques ont e t 6 Ptudies. d g p l a c e m e n t d e s s t r u c t u r e s d ' a b s o r p t i o n s o i t o b s e r v 6 , l e comportement o p t i q u e g l o b a l des f i l m s r e c o u v e r t s r e s t e s e m b l a b l e . L ' a n a l y s e d e s p r o p r i e t e s o p t i q u e s a @ t F f a i t e ?I 1 ' a i d e d ' u n e a p p r o c h e t h 6 o r i q u e d e champ moyen. Abstract: G r a n u l a r s i l v e r and c o p p e r f i l m s d e p o s i t e d o n g l a s s have been o v e r c o a t e d w i t h a t h i c k l a y e r e i t h e r o f s i m i l a r o r dissimilar metal. The o p t i c a l b e h a v i o r o f t h e s e f i l m s was t h e n s t u d i e d by m e a s u r i n g t h e r e f l e c t a n c e f r o m t h e g l a s s s i d e i n t h e Pesul t s i n d i c a t e d t h a t s p e c t r a l r e g i o n from 0 . 3 t o 1 . 0 m i c r o n . the overcoated granular films preserved t h e i r aggregate s t r u c t u r e , even a f t e r o v e r c o a t i n g . The s a m p l e s w e r e a1 s o s u b j e c t e d t o p r o l o n g e d h e a t t r e a t m e n t s and c h a n g e s i n t h e o p t i c a l p r o p e r t i e s were s t u d i e d . In g e n e r a l , a l t h o u g h a s h i f t i n a b s o r p t i o n s t r u c t u r e s h a s been o b s e r v e d , t h e o v e r a l l o p t i c a l b e h a v i o r o f t h e o v e r c o a t e d f i l m s remained s i m i l a r . Analysis o f o p t i c a l propert i e s h a s been made u s i n g a m e a n - f i e l d t y p e t h e o r y a p p r o a c h .

I n t h e l a s t t e n y e a r s , t h e r e h a s been a c o n s i d e r a b l e i n c r e a s e i n i n t e r e s t r e g a r d i n g t h e e l e c t r i c a l and o p t i c a l p r o p e r t i e s o f d i s c o n t i n u o u s m a t e r i a l s d u e t o t h e i r many p o s s i b l e a p p l i c a t i o n s 1 . I n p a r t i c u l a r t h i n g r a n u l a r metal f i l m s exhi b i t s e l e c t i v e a b s o r p t i o n i n t h e v i s i b l e spectrum. The f i l m p r o p e r t i e s d e p e n d l a r g e l y on t h e s i z e , s h a p e and d i s t r i b u t i o n o f t h e p a r t i c l e s on t h e s u b s t r a t e 2 . In t h e p r e s e n t w o r k , Ag and Cu granular films overcoated with a By e x a m i n i n g t h e o p t i c a l t h i c k m e t a l l a y e r h a v e been i n v e s t i g a t e d . to 1 .O b e h a v i o r o f t h e s e f i l m s i n t h e s p e c t r a l r a n g e from 0 . 3 micron, one--is a b l e t o draw q u a l i t a t i v e c o n c l u s i o n s a b o u t t h e s t r u c ture of the overcoated films.

I - EXPERIMENTAL The c o p p e r a n d s i l v e r g r a n u l a r f i l m s w e r e p r e p a r e d i n a vacuum of t o r r by t h e r m a l e v a p o r a t i o n o f t h e m e t a l o n t o g l a s s 'present

address : Department of Physics, Waterloo University, Waterloo, Ontario, Canada N2L 3GT

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831079

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s u b s t r a t e s preheated t o 300'~. The s a m p l e was t h e n a n n e a l e d f o r two After hours a t 3 0 0 ' ~ t o promote t h e a g g r e g a t i o n o f t h e p a r t i c l e s . a l l o w i n g t h e f i l m t o c o o l t o room t e m p e r a t u r e s , a t h i c k l a y e r o f m e t a l was d e p o s i t e d o v e r t h e g r a n u l a r f i l m . The m e t a l u s e d f o r o v e r c o a t i n g was e i t h e r a l u m i n i u m o r s i l v e r a n d t h e t h i c k n e s s o f t h i s s e c o n d l a y e r s o t h a t t h e f i n a l s y s t e m c o u l d be was s u f f i c i e n t l y l a r g e ( % 1 0 0 nm) c o n s i d e r e d opaque. The r e f l e c t i v i t y o f t h e g r a n u l a r f i l m a t n e a r normal i n c i d e n c e was measured on t h e g l a s s s i d e o f t h e sample i n t h e s p e c t r a l r e g i o n from A f t e r i n i t i a l m e a s u r e m e n t s , t h e s a m p l e s were 0.3 t o 1 .0 micron. then subjected t o d i f f e r e n t periods o f annealing. By f o l l o w i n g t h e changes in o p t i c a l p r o p e r t i e s o f t h e s e samples with heat t r e a t m e n t , c o n c l u s i o n s c o u l d be drawn on t h e e v o l u t i o n o f t h e f i l m s t r u c t u r e . The m e t a l d e p o s i t i o n i n a l l c a s e s was d o n e s i m u l t a n e o u s l y on two s u b s t r a t e s , with one being kept f o r t h i c k n e s s measurements u s i n g t h e T o l a n s k y m e t h o d 3 . The o p t i c a l t h i c k n e s s o f t h e c o p p e r g r a n u l a r f i l m s v a r i e d from 2 . 5 n m t o 1 4 . 4 n m whereas t h e s i l v e r g r a n u l a r f i l m s Flectron micrographs had t h i c k n e s s e s i n t h e 1 3 . 0 - 2 1 . 0 nm r a n g e . o f t h e s e f i l m s h a v e shown an a g g r e g a t e d s t r u c t u r e s i m i l a r t o t h e o n e i n Truong and S c o t t ' s w o r k 2 . For c o p p e r f i l m s w i t h t h i c k n e s s above t h e n o m i n a l s i z e o f p a r t i c l e s was a b o u t t w i c e t h e v a l u e o f 10.0 n m , the optical thickness. The f i l l i n g f a c t o r , a s e s t i m a t e d f r o m e l e c t r o n m i c r o g r a p h s , was u s u a l l y l e s s t h a n 0 . 6 f o r t h e c o p p e r f i l m s . In t h e c a s e o f s i l v e r , t h e s i z e o f t h e p a r t i c l e s forming t h e f i l m s did not e x c e e d 6 0 . 0 n m and t h e f i l l i n g f a c t o r f o r a l l f i l m s was e s t i m a t e d t o v a r y from 0 . 3 to 0.5.

II

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RESULTS A N D DISCIJSSION

The r e f l e c t i v i t y a t n e a r - n o r m a l i n c i d e n c e h a s b e e n m e a s u r e d f o r s i x samples o f copper g r a n u l a r f i l m s overcoated with aluminium. Figure 1 shows t y p i c a l e x a m p l e s o f t h e r e f l e c t i v i t y o f f r e s h l y d e p o s i t e d f i l m s . F i l m s I , I 1 and 1 1 1 c o r r e s p o n d t o s a m p l e s w i t h c o p p e r g r a n u l a r f i l m s a n d 8 . 5 nm r e s p e c t i v e l y . h a v i n g o p t i c a l t h i c k n e s s e s o f 1 4 . 4 , 13.1

F i g . 1 - N e a r - n o r m a l r e f 1 e c t i vi t y f o r t h r e e c o p p e r / a l u m i n i u m sampl e s a s a function o f wavelength. One c a n o b s e r v e on a l l c u r v e s two minima and a p e a k n e a r 0 . 6 pm. The minimum a t 0 . 3 5 pm s e e m s t o h a p p e n a t t h e same w a v e l e n g t h r e g a r d l e s s o f t h e f i l m t h i c k n e s s , w h e r e a s t h e o n e i n t h e s p e c t r a l r e g i o n from

0.75 t o 0 . 9 0 um i s d e p e n d e n t upon f i l m s t r u c t u r e . With i n c r e a s i n g f i l m t h i c k n e s s , t h e r e i s a s h i f t i n t h e l a t t e r minimum t o l o n g e r wave1 e n g t h s . A p r e v i o u s s t u d y by T r u o n g a n d S c o t t 2 on g r a n u l a r c o p p e r f i l m s d e p o s i t e d on g l a s s s u b s t r a t e s h a s shown a s i m i l a r s h i f t o f t h e a b s o r p t i o n peak t o l o n g e r w a v e l e n g t h s a s t h e f i l m t h i c k n e s s i n c r e a s e d . The a b s o r p t i o n i n g r a n u l a r f i l m s was a t t r i b u t e d t o p l a s m a r e s o n a n c e i n t h e metal1 i c p a r t i c l e s . The a b s o r p t i o n p e a k s f o r c o p p e r a g g r e g a t e d f i l m s having s i m i l a r t h i c k n e s s e s t o t h o s e i n t h e p r e s e n t s t u d y were l o c a t e d A p p a r e n t l y t h e a b s o r p t i o n found i n t h e r e g i o n from 0 . 6 t o 0 . 7 pm. in aggregated films alone a l s o manifests i t s e l f in t h e overcoated g r a n u l a r f i l m s o f t h e p r e s e n t work. The a l u m i n i u m c o a t i n g c a n o f c o u r s e i n f l u e n c e g r e a t l y t h e l o c a t i o n o f t h e a b s o r p t i o n peak a s t h i s l o c a t i o n d e p e n d s on many p a r a m e t e r s , s u c h a s t h e f i l l i n g f a c t o r , t h e f i l m t h i c k n e s s and t h e d i e l e c t r i c c o n s t a n t o f t h e m a t e r i a l s u r r o u n d i n g The s t r u c t u r e i n t h e r e f l e c t i v i t y n e a r t h e metal1 i c p a r t i c l e s . 0 . 3 5 urn ( F i g . 1 ) may be a t t r i b u t e d t o i n t e r b a n d t r a n s i t i o n s i n c o p p e r . Such a r e s u l t i s i n a g r e e m e n t w i t h w o r k s by J o h n s o n and C h r i s t y ' and W i l l i a m s e t a1 who h a v e l o c a t e d an i n t e r b a n d t r a n s i t i o n n e a r 0 . 3 2 um.

A t long w a v e l e n g t h s , one can n o t i c e a s h a r p r i s e i n r e f l e c t i v i t y , e s p e c i a l l y i n t h e c a s e o f f i l m 1 1 1 . T h i s r i s e would be a s s o c i a t e d w i t h t h e p r e s e n c e o f t h e a1 uminium o v e r c o a t i n g w h i c h i s a v e r y good r e f 1 e c t o r in t h i s spectral region. Changes o c c u r i n g a f t e r a n n e a l i n g s have a1 s o been i n v e s t i g a t e d . Film I r e p r e s e n t s a t y p i c a l e x a m p l e when t h e f i l m i s a n n e a l e d a t 3 0 0 ' ~ f o r s u c c e s s i v e p e r i o d s o f 2 , 2 5 0 and 750 h . After each s t e p o f annealing, m e a s u r e m e n t s o f r e f l e c t i v i t y have been made. The r e s u l t s a r e shown i n Fig. 2. The c h a n g e s o b s e r v e d would be c a u s e d by r e a r r a n g i n g o f s t r u c t u r e s o f t h e g r a n u l a r f i l m and d j f f u s i o n o f a l u m i n i u m i n t o c o p p e r .

F i g . 2 - E f f e c t s o f a n n e a l i n g on sample I . The c u r v e s 1 , 2 , 3 and 4 c o r r e s p o n d t o p e r i o d s o f a n n e a l i n g o f 0 , 2 , 250 and 750 h o u r s , respective1 y. In o r d e r t o u n d e r s t a n d t h e p r o p e r t i e s o f o v e r c o a t e d g r a n u l a r f i l m s , t h e M a x w e l l - G a r n e t t a p p r o a c h c a n be u s e d 6 . The p a r t i c l e s f o r m i n g t h e g r a n u l a r f i l m a r e c o n s i d e r e d s p h e r i c a l and v e r y s m a l l w i t h r e s p e c t t o t h e wavelength o f t h e i n c i d e n t 1 i g h t . For simp1 i c a t i o n p u r p o s e , t h e medium s u r r o u n d i n g t h e p a r t i c l e s c a n be a s s u m e d t o be a i r , a l t h o u g h t h e r e may a l s o have b e e n a l u m i n i u m o x i d e o r a l u m i n i u m i t s e l f , i f t h e

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v o i d s between copper p a r t i c l e s were p a r t i a l l y f i l l e d i n a f t e r t h e overcoating.

A f u r t h e r s i m p l i c a t i o n assumes t h a t t h e c o p p e r p a r t i c l e s a r e d i s t r i b u t e d i n a square a r r a y on t h e glass substrate. By t a k i n g t h e f i l m t h i c k n e s s as t w i c e t h e r a d i u s o f t h e p a r t i c l e s , examples o f c a l c u l a t i o n s o f t h e r e f l e c t i v i t y u s i n g t h i s model can be f o u n d i n F i g . 3 . The c u r v e s show a s i m i l a r b e h a v i o r t o t h e o n e o b s e r v e d w i t h s a m p l e s i n The F i g . 1 , w i t h a maximum i n t h e r e g i o n f r o m 0 . 6 t o about 0 . 7 pm. l o c a t i o n o f t h e minimum a t l o n g - w a v e l e n g t h s depends o n t h e s i z e o f a g g r e g a t e s i n a w a y c o n s i s t e n t w i t h e x p e r i m e n t a l m e a s u r e m e n t s ( ~ i g 1. ) .

F i g . 3 - C a l c u l a t e d r e f l e c t i v i t y f o r two g r a n u l a r copper f i l m s o v e r coated w i t h aluminium. Curve 1 : c o p p e r p a r t i c l e r a d i u s = 2 0 . 0 nm, curve 2: c o p p e r p a r t i c l e r a d i u s z 2 5 . 0 nm, f i l l i n g f a c t o r = 0.6; O p t i c a l c o n s t a n t s f o r c o p p e r and a l u m i n i u m a r e f i l l i n g f a c t o r = 0.6. f r o m R e f . 7. The a b o v e c o m p u t a t i o n s , a l t h o u g h o b t a i n e d w i t h f a i r l y c r u d e a s s u m p t i o n s about t h e g r a n u l a r l a y e r , can account q u a l i t a t i v e l y f o r t h e feat u r e s observed i n t h e experiment. They c o n f i r m t h a t t h e g r a n u l a r f i l m s have i n d e e d p r e s e r v e d p a r t i a l l y t h e i r aggregated s t r u c t u r e a f t e r overcoating. After successive annealings, bulk properties o f the m e t a l l i c p a r t i c l e s a r e n e c e s s a r i l y a l t e r e d and u t i l i s a t i o n o f b u l k c o p p e r o p t i c a l c o n s t a n t s i s no l o n g e r p o s s i b l e . Meaningful comparis o n s b e t w e e n t h e o r y a n d e x p e r i m e n t c a n b e made o n l y w h e n o p t i c a l c o n s t a n t s o f t h e formed a l l o y a r e d e t e r m i n e d and used i n c o m p u t a t i o n s . Measurements o f r e f l e c t i v i t y f o r o v e r c o a t e d g r a n u l a r s i l v e r f i l m s have a l s o b e e n made. These f i l m s were o v e r c o a t e d w i t h a t h i c k l a y e r o f e i t h e r aluminium o r s i l v e r . I n b o t h cases, r e s u l t s s i m i l a r l y suggest e d t h a t t h e a g g r e g a t e n a t u r e o f t h e s i l v e r f i l m s has been p r e s e r v e d a f t e r overcoating. Computations u s i n g t h e Maxwell - G a r n e t t approach As a n c a n qua1 i t a t i v e l y e x p l a i n t h e e x p e r i m e n t a l a b s o r p t i o n f e a t u r e s . e x a m p l e o f a t y p i c a l b e h a v i o r , F i g . 4 shows t h e m e a s u r e d r e f l e c t i v i t y o f a g r a n u l a r s i l v e r f i l m 2 0 . 4 nm t h i c k , o v e r c o a t e d w i t h a 1 0 0 . 0 nm aluminium l a y e r . I n t h a t c a s e , i t was a l s o n o t e d t h a t t h e m i n i m u m a t 0 . 9 um i s v e r y m u c h d e p e n d e n t o n t h e s i l v e r f i l m s t r u c t u r e s .

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Fig. 4 Near-normal r e f l e c t i v i t y o f a g r a n u l a r s i l v e r f i l m o v e r c o a t e d w i t h aluminium. Curve 1 : f r e s h l y d e p o s i t e d s a m p l e ; c u r v e 2 : sample subjected t o 2 h o f annealing.

I11

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CONCLUSION

T h i s p a p e r h a s shown t h a t o v e r c o a t e d g r a n u l a r f i l m s i n g e n e r a l p r e serve t h e i r aggregate s t r u c t u r e s . The o p t i c a l p r o p e r t i e s c a n be a c c o u n t e d f o r q u a l i t a t i v e l y by t h e M a x w e l l - G a r n e t t a p p r o a c h . The e v o l u t i o n o f t h e f i l m s s u b j e c t e d t o a n n e a l i n g i s q u i t e i n t e r e s t i n g , denot i n g in c e r t a i n cases a considerable increase in absorption i n the v i s i b l e region.

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S e e , f o r e x a m p l e , R . L a n d a u e r i n E l e c t r i c a l T r a n s p o r t and O p t i c a l P r o p e r t i e s o f I n h o m o g e n e o u s M e d i a , e d . by J . C . G a r l a n d a n d D . B . Tanner (AIP C o n f e r e n c e P r o c e e d i n g s , No. 4 0 , 1 9 7 8 ) p . 2 . S e e , f o r e x a m p l e , V . V . T r u o n g and G . D . S c o t t , J . O p t . S o c . A m . 67 ( 1 9 7 7 ) 5 0 2 . S . T o l a n s k y , M u l t i p l e - R e a m I n t e r f e r o m e t r y o f S u r f a c e and F i l m s (Clarendon, Oxford, 1 9 4 8 ) . P . 5 . J o h n s o n a n d R . W . C h r i s t y , P h y s . Rev. 86 ( 1 9 7 2 ) 4 3 7 0 A . R . W i l l i a m s , J . F. Tanak, and V . L . Moruzzi, Phys. 9ev. L e t t e r s 28 (1 9 7 2 ) 6 7 1 . G . B o s i , F . E . G i r o u a r d , and V . V . T r u o n g , J . Appl . P h y s . 53 (1982) 648. P h y s i c s D a t a , O p t i c a l P r o p e r t i e s o f M e t a l s P t . 1 1 , e d . by J . H . W e a v e r , C . K r a f k a , D . W . Lynch and E . E . ~ o c h c h i n f o r m a t i o n s zentrum E n e r g i e , P h y s i k , Mathematik, 1 9 8 1 ) .