THE EFFECT OF SUBSTRATE TEMPERATURE ON THE BEHAVIOUR OF GOLD AND SILVER ON THE 100 TUNGSTEN PLANE

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THE EFFECT OF SUBSTRATE TEMPERATURE ON THE BEHAVIOUR OF GOLD AND SILVER ON THE

100 TUNGSTEN PLANE

D. Joag, J. Jones

To cite this version:

D. Joag, J. Jones. THE EFFECT OF SUBSTRATE TEMPERATURE ON THE BEHAVIOUR OF

GOLD AND SILVER ON THE 100 TUNGSTEN PLANE. Journal de Physique Colloques, 1984, 45

(C9), pp.C9-59-C9-64. �10.1051/jphyscol:1984911�. �jpa-00224389�

JOURNAL DE PHYSIQUE

Colloque C9, supplkment au n012, Tome 45, dkcembre 1984 page C9-59

THE EFFECT OF SUBSTRATE TEMPERATURE ON THE BEHAVIOUR OF GOLD AND S I L V E R ON THE 100 TUNGSTEN PLANE

D.S. Jose and J.P. Jones

School o f Electronic Engineering Science, University College of North Wales, Dean S t r e e t , Bangor, Gwynedd, LL57 lUT, U.K.

*Department o f Physics, University of Poona, Pune, 411007, India

RQsumG - Pour reconcilier les caractgristiques travail de sortie (a)/ couver- ture

obtenues par Jones et Roberts

et celles de Richter et Gomer

nous avons dtudib les caractgristiques 0 -

en fonction des doses et de la tempdrature. Le traitement thermique est un facteur important. Pour relier ce travail P celui de Bauer et al.[l], de Sidorski et al. 161, nous avons aussi 6tudid la variation de 9 introduite par la condensation d'or et d'ar- gent sur W(100) P haute tempbrature. Nous mettons ainsi en Qvidence la for- mation d'alliage en surface.

Abstract - In an effort to reconcile the work function (Q) and coverage (0) characteristics obtained by Jones and Roberts [3] with those of Richter and Gomer [2] we have studied the dependence of the Q - O characteristic on dose and temperature, and find thermal treatment to be the determining factor.

To make contact with similar work by Bauer et a1 [I] and Sidorski et a1 [6]

we have also studied changes in

induced when gold and silver are condensed on W(100) at high temperature, and find some evidence for formation of surf ace alloys.

1 - INTRODUCTION

Adsorption of sub monolayer and multilayer amounts of group lb metals on tungsten single crystal surfaces has been studied by several techniques. Despite the similar atomic radii of Au and Ag LEEDIAuger has shown that they exhibit quite different behaviour on the W(100) surface [l 1. The field emission microscope

(FEM) has also been used to study Au and Ag on the W(100) surface [Z-61 and there appears to be some disagreement on the

-

characteristic between different workers. The gradual increase in

with increasing

obtained by Jones and Roberts [3] for Au on W(100) is consistently different from the abrupt

transformation detected by Richter and Gomer [Z]. Moreover, FEM studies of Ag on W(100) in the low coverage range [5,6] shows that Q can either increase, decrease or remain constant, and it is difficult to reconcile these observations with the observation that silver behaves like an electropositive adsorbate on low index tungsten planes.

We therefore decided to investigate the

- O relationship for both Au and Ag adsorbed on W(100) at various temperatures and in rigorously clean conditions in an attempt to remove the existing anomalies.

2 - EXPERIMENTAL

All reported measurements of work function were made on a (100)-oriented tungsten field emitter in a probe hole microscope which has been described [7]. Au and Ag sources were prepared from specpure metals by melting short lengths of 1 mm diameter wires in pre-outgassed tungsten spirals. The sources were thoroughly degassed in ultra high vacuum. As judged by the stability of field emission current the level of contaminant gas in the tube was less than 10-l2 torr. Values of Q were computed from the ten-point Fowler Nordheim plots using least-squares

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984911

C9-60 JOURNAL DE PHYSIQUE

a n a l y s i s and changes i n t h e p r e - e x p o n e n t i a l term were a l s o c a l c u l a t e d and a r e e x p r e s s e d by B where,

where Ao i s t h e p r e - e x p o n e n t i a l term f o r t h e c l e a n s u r f a c e and A t h a t f o r t h e s u r f a c e w i t h a d s o r b a t e . Coverages a r e quoted a s numbers of e q u i v a l e n t doses. The work f u n c t i o n of W(100) i s t a k e n t o be 4.80 + 0.01 e V .

3 - ^{RESULTS AND} DISCUSSION

.o

J

d o s e s o t g o l d

Fig.1 - ( a ) Change i n work function,A@, produced by condensing e q u a l i n c r e m e n t s of gold d i r e c t l y o n t o W(100) a t 78K.

( b ) Corresponding changes i n term B.

F i g u r e l ( a ) shows an almost l i n e a r r i s e i n A@ w i t h dose t o a p l a t e a u a t 0.65eV b u t w i t h two " s p i k e s " b e f o r e t h e p l a t e a u . T h e r e a f t e r A@ jumps a b r u p t l y by about 0.15eV a s observed by R i c h t e r and Gomer [ 2 , 8 ] u s i n g FEM and Sargood [9 ] u s i n g RHEED. By d e p o s i t i n g Au a t 78K on a macroscopic W(100) c r y s t a l s u r f a c e and a n n e a l i n g a t 350K Sargood was a b l e t o i d e n t i f y t h e r i s e i n @ w i t h development of a

~ ( 2 x 1 ) s t r u c t u r e and t h e s h a r p jump i n @ w i t h f o r m a t i o n of an Au(100) s t r u c t u r e compressed by about 17%.

The s p i k e s i n f i g u r e l ( a ) a r e thought t o r e s u l t from f o r m a t i o n of s m a l l a r e a s of t h e compressed Au l a y e r which a r e n o t s t a b l e and d i s s o l v e i n t o t h e ~ ( 2 x 1 ) s t r u c t u r e . A s t h e coverage i n c r e a s e s beyond a monolayer t h e compressed Au l a y e r p e r s i s t s u n t i l a t 2 monolayers a d e c r e a s e i n

marks t h e f o r m a t i o n of an

uncompressed Au s t r u c t u r e which i s t h e n s u b s t a n t i a l l y independent of dose. The

unchanging v a l u e of B a t low coverage i s somewhat s u r p r i s i n g , f i g u r e l ( b ) , but

t h e r e a f t e r B d e c r e a s e s a s observed by R i c h t e r and Gomer [Z].

Annealing each dose condensed at 78K for 30sec at 330K prevents formation of the compressed Au layer. This is shown in figure 2(a) where, after the initial rise A@ remains constant at 0.5eV except for a brief excursion to 0.6eV which may be attributed to transient formation of the compressed structure. The increase in B, figure 2(b), cannot be quite understood although B becomes negative at later stages.

1 .o

max e r r o r I 0-8- a

0-6

-

5 10 15 20

I

doses o t g o l d

Fig.2 - (a) Change in work function

with dose for gold condensed at 78K and annealed for 30 sec. at 330K.

(b) Corresponding changes in term B.

The subsequent sharp decrease in

coincides with the appearance of W(100) of a bright island like growth in the field emission pattern, and the following increase in

to its final coverage-independent value accompanies removal of the island by dissolution or expansion. Based on Sargood's observations we believe that the final state is an unstrained Au(100) layer and that its high work function (5.82 eV) compared to that of macroscopic Au(100) (5.47 eV) [lo] may result from a decrease in the local field enhancement factor due to development of an extensive flat Au(100) surface. The effect of substrate temperature during condensation of gold on the

characteristic is shown in figure 3(a).

With increasing temperature the expected initial rise in

at 300K gives way to a

decrease which is slight at 750K but more pronounced at 825K and 900K. This

behaviour agrees with that reported by Bauer et a1 [l] and Sidorski [ll]. The

lowering of

has been attributed to microfacetting for both Pb on W(100) (121 and

Au on W(100) [I]. Changes in B also appear to be sensitive to condensation

temperature becoming positive at higher substrate temperatures; this is

consistent with a reduction in effective emitting area by microfacetting.

JOURNAL DE PHYSIQUE

I rnax e r r o r

, 0-6

-

d o s e s o f g o l d

Fig.3 - ( a ) Change i n work f u n c t i o n A@ produced by condensing each dose on t h e s u b s t r a t e h e l d a t t h e i n d i c a t e d temperature.

( b ) Corresponding changes i n term B.

(ii) Ag on W(100)

The @-dose c h a r a c t e r i s t i c f o r Ag on W(100) shows l e s s dependence on t e m p e r a t u r e t h a n t h a t of g o l d , f i g u r e 4 ( a ) .

:\

1 , '

2 -0

5 10 15

d o s e s o f s i l v e r

Fig.4 - ( a ) Changes i n work f u n c t i o n w i t h dose f o r s i l v e r condensed on t h e t i p a t t h e i n d i c a t e d t e m p e r a t u r e .

( b ) Corresponding changes i n term B.

Accepting t h a t t h e r e i s some l o s s of s i l v e r from t h e e m i t t i n g a r e a by downshank d i f f u s i o n we c o n s i d e r t h a t , w i t h t h e p o s s i b l e e x c e p t i o n of 650 K, t h e s t a t e s achieved a t h i g h coverage a r e e s s e n t i a l l y t h e same a t a l l t e m p e r a t u r e s and t h i s behaviour s u b s t a n t i a l l y a g r e e s w i t h t h e f i n d i n g s of Bauer e t a l . [I], e x c e p t t h a t t h e p r e s e n t v a l u e s of A@ a r e much l a r e r (0.7 - l.OeV) t h a n t h o s e r e p o r t e d by Bauer e t a1. (0.3 eV) and S i d o r s k i [67, (0.25 eV). Following Bauer e t a l . [I ] we c o n s i d e r t h a t t h e low coverage d e c r e a s e s i n

r e s u l t s from f o r m a t i o n of a ~ ( 2 x 1 ) s t r u c t u r e which, a t t e m p e r a t u r e s above 370K does n o t form s o r e a d i l y due t o d i f f u s i v e l o s s of s i l v e r from t h e t i p apex. At 830K Bauer e t a l . observed f o r m a t i o n of ~ ( 2 x 2 ) followed by ~ ( 1 x 1 ) and t h e l a t t e r s t r u c t u r e was observed t o have a h i g h e r work f u n c t i o n (4.1 e V ) t h a n ~ ( 2 x 1 ) (3.98 eV). It seems q u i t e p o s s i b l e t h a t t h e observed i n c r e a s e i n f i n a l work f u n c t i o n a t 650K, f i g u r e 4 ( a ) , can be a t t r i b u t e d t o t h e same cause. Unlike Bauer e t ax. [ l ] and S i d o r s k i [ 6 ] we do n o t f i n d t h e i n i t i a l s l o p e of t h e @-dose c h a r a c t e r i s t i c t o be v e r y s e n s i t i v e t o s u b s t r a t e t e m p e r a t u r e d u r i n g condensation.

I n view of t h e much l a r g e r d e c r e a s e i n

which we observe i t i s i n t e r e s t i n g t o c a l c u l a t e po t h e z e r o coverage d i p o l e moment from t h e Helmholtz e q u a t i o n u s i n g t h e coverage s c a l e r e p o r t e d by Bauer e t al. 111 and we o b t a i n

=

4.25 x 10-3 C.m

which a g r e e s w e l l w i t h t h e c l a s s i c a l d i p o l e moment pc d e r i v e d from t h e r e l a t i o n

=

2 r i e

where r i i s t h e i o n i c r a d i u s of t h e adatom and e t h e e l e c t r o n i c c h a r g e , which g i v e s

u,

4.03 x 10-~Oc.m

, b

Thus s i l v e r behaves a s an e l e c t r o p o s i t i v e adatom when adsorbed on W(100). On a s i m p l e p i c t u r e e l e c t r o p o s i t i v e s i l v e r is expected t o i n c r e a s e t h e e f f e c t i v e e m i t t i n g a r e a and t h u s t o y i e l d n e g a t i v e v a l u e s of B. However f i g u r e 4 ( b ) shows t h a t t h i s i s n o t t h e c a s e and a t a l l t e m p e r a t u r e s B remains p o s i t i v e .

4 - ^SUMMARY

( a ) D i f e r e n c e s i n t h e -

c h a r a c t e r i s t i c f o r g o l d between R i c h t e r and Gomer and J o n e s and Roberts r e s u l t from d i f f e r e n e s i n t r e a t m e n t of t h e d e p o s i t . The h i g h work f u n c t i o n compressed Au (100) l a y e r does n o t form i f t h e Au d e p o s i t i s a n n e a l e d due t o l o s s of Au by d i f f u s i o n o u t of t h e W(100) plane.

(b) The f i n a l s t r u c t u r e formed a t t h i c k n e s s e s g r e a t e r than 3 monolayers i s thought t o be Au(100) and t h e r e l a t i v e l y high work f u n c t i o n 5.82 eV compared w i t h t h e macroscopic v a l u e 5.47 eV

r e s u l t from a d e c r e a s e i n t h e l o c a l f i e l d enhancement f a c t o r .

( c ) At h i g h e r Au d e p o s i t i o n t e m p e r a t u r e s t h e i n i t i a l d e c r e a s e i n

i s i n d i c a t i v e of t h e two-dimensional a l l o y i n g o r m i c r o f a c e t t i n g of t h e W(100) s u r f a c e . ( d ) S i l v e r atoms a d s o r b a s p o s i t i v e d i p o l e s of mement po

4.25 x C.m a t

z e r o coverage. Work f u n c t i o n changes i n d i c a t e t h a t t h e f i n a l s t r u c t u r e adopted below 650K d i f f e r s from t h a t formed a t , o r above, t h i s t e m p e r a t u r e i n broad agreement w i t h t h e f i n d i n g s of Bauer e t a l .

( e ) The l i n e a r changes i n

i n t h e low coverage r e g i o n s when Au and Ag a r e condensed on W(100) p l a n e i n d i c a t e t h a t t h e growth proceeds by i s l a n d f o r m a t i o n .

Acknowledgement

One of u s , DSJ, thanks t h e A s s o c i a t i o n of Commonwealth U n i v e r s i t i e s , U.K. f o r a f e l l o w s h i p and t h e U n i v e r s i t y of Poona, I n d i a f o r t h e l e a v e t o a v a i l of t h e same.

References

[ l ] ^Bauer E., Poppa, H., Todd, G and Davies, P.R., J.Appl.Phys. 48, (1977) 357.

[ 2 ]

R i c h t e r , L. and Gomer, R, Phys.Rev.Letters, 37, (1976) 763.

[ 3 ] J o n e s , J.P. and R o b e r t s , E.W., Thin S o l i d F i l m s , 48, (1978) 215.

C9-64 JOURNAL DE PHYSIQUE

[4] Jones, J.P. and Jones, N.T., Thin Solid Films, 35, (1976) 83.

[5] Kolaczkiewicz, J. and Sidorski, Z., Surf .Sci. 63, (1977) 501.

[6] Sidorski, Z., Szelwicki, T. and Dworecki, Z., Thin Solid Films, 61, ⁽¹⁹⁷⁹⁾

203.

[7] Jones, J.P. and Roberts, E.W., Surf .Sci,, 69, (1977) 185.

[8] Richter, L. and Gomer, R., Surf.Sci., 83, (1979) 93.

[9] Sargood, A.J., PH.D. Thesis, University of Southampton, 1969.

[lo] Michelson, H.B., J.Appl.Phys. 48, (1977) 4729.

[ll] Sidorski, Z., Appl.Phys. A, 2 (1984) 213.

[12] Bauer, E., Poppa, H. and Todd, G., Thin Solid Films, 28, (1975) 19.