ELLIPSOMETRIC STUDIES OF OXYGEN ADSORBED ON SILVER FILMS

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ELLIPSOMETRIC STUDIES OF OXYGEN ADSORBED ON SILVER FILMS

M. Watanabe, P. Wissmann

To cite this version:

M. Watanabe, P. Wissmann. ELLIPSOMETRIC STUDIES OF OXYGEN ADSORBED ON SILVER FILMS. Journal de Physique Colloques, 1983, 44 (C10), pp.C10-459-C10-462.

�10.1051/jphyscol:19831090�. �jpa-00223548�

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JOURNAL DE PHYSIQUE

Colloque CIO, supplement au n°12, Tome 44, decembre 1983 page C10-459

E L L I P S O M E T R I C S T U D I E S OF OXYGEN A D S O R B E D ON SILVER F I L M S M. Watanabe and P. Wissmann*

Research Institute for Catalysis, Hokkaido University, Sapporo, 060 Japan

*Institut fttr Physikalisehe und Theoretische Chemie der Universititt Erlangen-Mirnberg, Egerlandstrasse S, 8520 Erlangen, F.R.G.

Résumé - L'adsorption d'oxygène sur des couches minces d'Argent déposées par ëvaporation a été étudiée à 295 et 77 K par ellipsomëtri:e et par mesures de résistance. La constante diélectrique obtenue pour l'oxygène adsorbé sous forme atomique, à 1152 nm, est ëj = - 1,8 - 3,5i à 295 K et - 2,0 - 4,0i à 77 K ; ces valeurs sont bien interprétées par la théorie de Bennett et Penn qui décrit le couplage entre l'oxygène adsorbé et les électrons de conduction.

Les valeurs 6A et Sii observées à A = 633 nm indiquent l'existence d'une cou- che d'interface induite. La constante diélectrique de l'oxygène adsorbé sous forme moléculaire à 77 K est réelle et égale à ei = 1,8.

Abstract - Adsorption of oxygen on evaporated siIver films is studied at 295 and 77 K by means of ellipsometry and resistance measurements. The dielectric constant of atomically adsorbed oxygen obtained at X = 1152 nm is ei = -1.8 - 3.5i at 295 K and -2.0 - 4.0i at 77 K, which are well understood by the Bennett and Penn theory describing coupling between adsorbed oxygen and conduction electrons. "Observed <5A and 5I(J values at X = 633 nm indicate the existence of an induced interface layer. The dielectric constant of molecularly adsorbed oxygen at 77 K is found to be real as ei = 1.8.

Oxygen is atomically adsorbed on polycrystalline silver surfaces at room temperature while at 77 K it seems to be adsorbed atomically at early stage of adsorption and then weakly and molecularly |1-3|. We intend in this paper to compare ellipsometric respon- se of adsorbed oxygen at 295 K with that at 77 K. A few other groups have studied this system at room temperature by means of ellipsometry using light wavelengths of 633 nm or shorter [4-6|. Our light source was a He-Ne laser of A = 1152 nm, where the metal dielectric constant can be described by an extended Drude equation including the ano- malous skin effect. As well we used another He-Ne laser of 633 nm to compare our results with literature work. Evaporated thin films were considered to be advantageous for determining the life time of conduction electrons and Fuch's specularity parameter from resistivity measurements.We measured changes in the ellipsometric angles 6A and Sty as a function of oxygen coverage, which were converted into the complex dielectric constants of adsorbed oxygen. They were found to be different from those of liquid or gas phase oxygen.

I - EXPERIMENTS

Experimental details are given elsewhere |7,8[. A null method ellipsometer (Rudolph 43603-200E) was used in the PSCA arrangement. Samples were polycrystalline and continuous films deposited on glass substrates.in UHV conditions. Spectrally pure silver was evaporated from a tungsten helix heater onto the substrate which was kept at am- bient temperature. The thickness D of the films was in the range of 25 to 50 nm, which was determined by atom absorption spectroscopy. The resistance of the films was measured by a so-called four electrode method during evaporation, annealing and gas admis- sions. The gas dose was provided by breaking the seals of ampoules filled with 02 gas of 99.999 % purity. The adsorbed amount of O2 was determined by monitoring increments of the resistance using calibration curves obtained for spherical glass cells |8|.

Since variations in A andfdue to O2 adsorption were small,we evaluated them with the help of computer analysis. Details of the method are given elsewhere |9|.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831090

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JOURNAL DE PHYSIQUE

I 1

-

RESULTS

Electronmicrographs show t h a t a continuous f i l m s t r u c t u r e w i t h a p r e f e r r e d (111) o r i - e n t a t i o n i s formed f o r D 2 15 nm. The f i l m s become smoother w i t h i n c r e a s i n g D. Obser- ved

A

and $ values o f t h e f i l m s w i t h o u t adsorbate a r e converted i n t o d i e l e c t r i c con- s t a n t 2 2 by computer c a l c u l a t i o n s o f a two l a y e r system i n c l u d i n g t h e metal f i l m s and the glass s u b s t r a t e w i t h r e f r a c t i v e index o f 1.48. Obtained d i e l e c t r i c constant i s

7

⁼^{( -}⁷⁵

⁺

⁴⁾

^-

^(2.5

⁺

0 . 5 ) i a t X = 1152 nm w h i l e t h e Drude equation gives Z2 =

-

2.5

-

2.341 w i t h ~b = 2.5 being t h e core e l e c t r o n s c o n t r i b u t i o n 1101. At X = 633 nm we o b t a i n e d 22 = (-22 c 2)

-

(0.4

+

0 . 2 ) i w h i l e the Drude value i s Z2 = -19.5

-

0.48i Thus t h e observed E"2 values are well' described by the Drude theory.

S o l i d curves i n Fig. l a show t h e coverage dependence o f t h e e l l i p s o m e t r i c a l parame- t e r s 6A and 6$ d e f i n e d as

6A = A(O=o)

-

A (O), €i$ = $(o=o) - Q(O), ( 1

where 0 i s the degree o f oxygen coverage. I t was ascertained t h a t 6A and 6$ a r e i n - dependent o f f i l m thickness D when i t i s l a r g e r than 25 nm. I f an a d l a y e r i s t r a n s - p a r e n t f o r an i n c i d e n t 1 ig h t , a simple s t r a t i f i e d l a y e r model p r e d i c t s 111

I

^:

where a i s the atomic p o l a r i z a b i l i t y o f t h e adsorbate and C(A) i s a f u n c t i o n o f A, i n c i d e n t angle of t h e l i g h t and adsorbent o p t i c a l p r o p e r t i e s . Obviously t h e r e s u l t s o f F i g . l a a r e v e r y i n c o n s i s t e n t w i t h eq. (2),so t h a t we may need an induced i n t e r - face l a y e r and/or a complex d i e l e c t r i c constant o f adsorbed oxygen t o e x p l a i n t h e data. The 6.4 % increase i n t h e r e s i s t a n c e seems t o i n d i c a t e t h a t oxygen atoms a r e n o t adsorbed by s i l v e r f o r 02 pressures s m a l l e r than 1.3 Pa used i n t h e present experiments (see Fig. l b ) .

A t 7 7 K observed changes i n 6A and 69 values a r e 6A = 0.28O, 6$ = 0.07O a t X = 1152 nm and 6A = l . O O , )16 = -0.2Z0 a t 633 nm f o r t h e coverage o f Nad = 4.0~1014 molecules/cm2. A t low coverages, v a r i a t i o n s i n

6A, 6$ and 6R a r e s i m i l a r t o those i n Fig. 1 _for_(633nm) for (1152nm) though 614 a t 633 nm i s l a r g e l y negative. A t

h i g h coverages 6A i s almost 1 in e a r and 6$ i s degree a ) constant. A C h a r a c t e r i s t i c f e a t u r e i s t h a t

6~ does n o t tend t o s a t u r a t e a t h i g h covera-

-

^0.4

ges u n l i k e t h a t a t 295 K, p o s s i b l y due t o

weak adsorption o f molecular oxygen. The ma- 0.6- ximum change i n t h e r e s i s t a n c e dR/R a t 7 7 K

i s 7 . 2 . % f o r a Ag f i l m o f D = 42 nm.

I 1 1 DISCUSSION

The surfaces o f t h e evaporated Ag f i l m s a r e t r e a t e d t o be f l a t , w h i c h i s an admissible assumption because t h e i r bumpiness i s much

s m a l l e r than A. Furthermore, we now take 02 adsorbed ( 1 0 ' ~ / c m ~ ) i n t o account t h a t t h e glass s u b s t r a t e o n l y

i n f l u e n c e s the a b s o l u t e values o f A and Q b u t i s i n e f f e c t i v e f o r 6A and 614 when t h e t h i c k n e s s o f Ag f i l m s i s l a r g e r than the s k i n depth hs = 21 nm. Applying t h e model o f Fig. 2b, 6A and 6$ a r e expressed i n simply a n a l y t i c a l forms by t h e l i n e a r approxima- t i o n under the c o n d i t i o n o f dad/X << 1.

Non-classical expressions o f 6A and )16 which i n c l u d e t h e non-local e f f e c t s were given i n a previous paper 1101:

Fig. I Changes in ellipsometric angles 8 A ,

By

at 1152 nm (0, A) and 6 3 3 nm (e

,

A) and resist-

ance of Ag film 8 R plotted with the coverage of 0.0 0.1 0 . 2 0.3 0.4 adsorbed oxygen a t 295 U. Dotted curve shows 0 2 adsorbed (10" molecules/cm2) calculated 8 i which arises from the p parameter

effect. Simulations of 8A and Bg at 1152 nm are given by chained curves.

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where d e f i n i t i o n s o f v a r i a b l e s and d i e l e c t r i c constants a r e given i n Fig. 2. (KR), i s t h e normal component o f l o n g i t u d i - n a l wave v e c t o r 112,131. When y i s small

,

then 6A and )16 r e - duce t o the c l a s s i c a l forms.

Eqs. 3 a r e t h e expressions a t O = 1. Since the observed 6A and 6Q values a r e n o n - l i n e a r w i t h coverage, t h e coverage curves are analyzed w i t h t h e h e l p o f Lorentz-Lorenz equa- t i o n by s u b s t i t u t i n g E l i n eqs. 4 and 5 by E l ( @ ) accor- d ~ n g t o 1101

vacuum

I

^dad-

vacuum adlayer

before adsorption after adsorption

Fig. 2 Stratified layer model of oxygen adsorption on Ag films and definitions of variables.

The d i e l e c t r i c constant o f Ag i s given by t h e extended Drude equation i n c l u d i n g ano- malous s k i n e f f e c t s (10,141 as

w i t h

where w and wp a r e frequencies o f i n c i d e n t l i g h t and t h e plasmon, r e s p e c t i v e l y , 6s i s t h e s k i n depth, p i s t h e Fuchs s p e c u l a r i t y parameter, rot.= v / l o p i s t h e o p t i c a l l i f e time o f conduction e l e c t r o n s , ~ , t i s the s t a t i c l i f e lme c a l c u l a t e d from observed r e s i s t a n c e o f Ag f i l m s and Tee i s t h e l i f e time determined from e l e c t r o n - e l e c t r o n c o l l i s i o n s 1151 which are ~ m p o r t a n t when w i s l a r g e and t h e Fermi surface i s non-spherical o r surface s c a t t e r i n g s o f conduction e l e c t r o n s a r e e f f e c t i v e . Since the f i l m s a r e assumed t o be f l a t , t h e r e s i s t a n c e i s analyzed by the Fuchs- Sondheimer theory. Observed r e s i s t a n c e changes a r e approximately expressed by 1101:

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C 10-462 JOURNAL DE PHYSIQUE

from which we c a l c u l a t e d 6p by using observed (6RIR) values. Dl1 i s 1.4 a t 295 K, thus we o b t a i n 6p = 0.36. A f t e r numerical c a l c u l a t i o n s we found t h a t t h i s leads t o 6J, = 0.05q which i s i l l u s t r a t e d by the d o t t e d curve i n F i g . 1. The c a l c u l a t i o n r e - veals t h a t 6p influences o n l y l i t t l e 6A. For the 77 K r e s u l t s , s i m i l a r conclusions hold.

F o l l o w i n g t h e above procedure we simulated observed 6A and 6$ curves w i t h parametriz- i n g E l and dad, b u t dad i s f i x e d around 0.2 nm f o r atomic oxygen and around 0.35 nm f o r molecular oxygen. A t 295 K adsorbed oxygen i s atomic. S i m u l a t i o n curves of 6A and 614 a r e shown i n Fig. 1 a t 1152 nm w i t h b e s t f i t t e d 21 value being Z1 = -1.8

-

3 . 5 i . On t h e o t h e r hand, a t 633 nm none o f E l values can i n t e r p r e t the coverage curves, i n p a r t i c u l a r w i t h r e s p e c t t o t h e steep r i s e o f 6A a t t h e e a r l y coverage. Therefore, we conclude t h a t a t 633 nm t h e formation o f an induced i n t e r f a c e l a y e r must be taken i n t o account, i . e . model ( c ) r a t h e r than model (b) should be a p p l i e d . This conclusion im- p l i e s t h a t d i e l e c t r i c constant o f the i n t e r f a c e l a y e r depends s t r o n g l y on

X

l i k e E2 o f Ag. I n model ( c ) , determination o f Z1 i s impossible because o f many unknown parameters.

Along t h e same l i n e s we analyzed our low coverage data a t 77 K. We simulated coverage curves o f 6A and 61) and a r r i v e d a t the same conclusion t h a t model ( b ) e x p l a i n s w e l l t h e r e s u l t a t 1152 nm w i t h s i m u l a t i o n parameters o f E -2.0

-

4.0i and dad = 0.2 nm, b u t does n o t e x p l a i n a t a l l t h e r e s u l t a t 633 nm, whi!e=model ( c ) does. Observed changes i n e l l i p s o m e t r i c angles i n t h e h i g h coverage range a r e 6A ⁼0.150, S$ = -0.020 a t 1152 nm and 6A = 0.400, 61) = 0.000 a t 633 nm, which can be i n t e r p r e t e d by weakly adsorbed molecular oxygen w i t h 2 1 = 1.8

-

O.Oi and dad = 0.35 nm.

I V CONCLUSIONS

D i e l e c t r i c constants o f atomic oxygen on s i l v e r a t 1152 nm are 21 = -1.8

-

3.5i a t 295 K and El = -2.0

-

4.0i a t 77 K. Such complex d i e l e c t r i c constants w i t h n e g a t i v e r e a l p a r t s were p r e d i c t e d by t h e Bennett and Penn theory (161 d e s c r i b i n g p o s s i b l e r e - sonant bonds between adsorbates and conduction e l e c t r o n s . UPS spectra o f adsorbed oxygen on s i l v e r 13,171 show two peaks a t 8.0 and 12 eV from Ef which l i e below t h e conduction band, and a peak a t 3.2 eV i n t h e sp conduction band. D i e l e c t r i c p r o p e r t i e s o f adsorbed oxygen may r e f l e c t t h e c o v a l e n t nature o f e l e c t r o n s a t 8.0 eV and t h e m e t a l l i c n a t u r e o f e l e c t r o n s a t 3.2 eV. The existence o f induced i n t e r f a c e l a y e r depends on

X

o f i n c i d e n t l i g h t . Non-local effects i n t h e l i n e a r approximation a r e as small as 6 % a t h = 633 nm and n e g l i g i b l e a t 1152 nm.

We would l i k e t o thank Prof. G. Wedler f o r h i s continuous encouragement. The present work was f i n a n c i a l l y supported by t h e fund from Japanese Government and the Deutsche Forschungsgemeinschaft.

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