THERMAL ROUGHENING OF TUNGSTEN SURFACES STUDIED BY FIELD EMISSION

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THERMAL ROUGHENING OF TUNGSTEN SURFACES STUDIED BY FIELD EMISSION

Y. Gong, R. Gomer

To cite this version:

Y. Gong, R. Gomer. THERMAL ROUGHENING OF TUNGSTEN SURFACES STUD- IED BY FIELD EMISSION. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-15-C6-20.

�10.1051/jphyscol:1987603�. �jpa-00226806�

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J O U R N A L DE P H Y S I Q U E

Colloque C6, supplgment au nO1l, T o m e 48, novembre 1987

THERMAL R O U G H E N I N G OF T U N G S T E N S U R F A C E S STUDIED B Y FIELD EMISSION

Y.M. ~ o n ~ * a n d R. Gomer

The James Franck Institute, The University of Chicago, Chicago, IL 60637, U.S.A.

Abstract.- Surface self-diffusion measurements of tungsten atoms and other defects were carried out by the field emission fluctuation method at various positions along the zones (011)-(112) and (011)-(001). In the first zone, consisting of (01 1 ) terraces and closepacked steps 2-dimensional diffusion attributed to atohis on terraces and 1-dimensional diffusion attributed to kinks diffusing along steps was found. At 875 K kink diffusion decreases sharply, then increases again and become 2-dimensional at 950 K, suggesting disorder and eventual disappearance of steps. This is accompanied by a decrease in atom diffusion and eventual merging of both regimes above 1000 K .

On the zone (011)-(001) steps are not closepacked and only 1 -dimensional was seen, although two regimes were also found. The lack of 2.dimensional diffusion is attributed to the inability of atoms to climb none closepasked steps. Evidence for roughening was also found in most locations of this second zone but at much lower temperatures than along (011)-(112).

I - INTRODUCTION

Roughening transitions on stepped metal surfaces are of theoretical interest but have received relatively little experimental attention. Engel and coworkers have recently look at stepped Ni surfaces and found evidence of roughening from He di- ffraction studies/l/. We show here that changes in the surface self-diffusion of

W atoms and other defects can give an indication of roughening. The regions cho- sen for investigation lie in the zone (011)-(112) and the zone (011)-(001). Fig.

(1) gives the probed planes in two zones. A very preliminary report of this work has been given/2/ and a detailed account is to be published elsewhere/3/.

I1 - PRINCIPLE OF METHOD

The method used here is an extension of the field emission fluctuation technique, described in detail previously/4,5/. As normally used a field emitter is unifor- mly covered with adsorbate and current from a small region, 50-100 Angstroms in radius is measured by means of a probehole. At temperatures where the adsorbate is mobile there are fluctuations in the number of adsorbed particles in the probe and this gives rise to fluctuations in emission current. The time autocorrela- tion function of these fluctuations is nearly proportional to that of the parti- cle number, fn(t)

*permanent addressl Department of Radi~~Electronics, Peking University, Beijing, The PeopleEs Republic of China.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987603

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JOURNAL DE PHYSIQUE

Fig. ( 1 ) - Schematic diagram o f f i e l d emission p a t t e r n , i n d i c a t i n g r e l e v a n t d i r e c t i o n s and probe p o s i t i o n s : #I--(257) ; #2--(123) ;

#3--(235) ; #4--(023) ; #5--(012); #6-y(013) ; #7--(017).

where < (6n) 2 > i s the mean square number f l u c t u a t i o n i n t h e probed area P., D the d i f f u s i o n c o e f f i c i e n t , t t h e time and y t h e d i m e n s i o n a l i t y o f d i f f u s i o n . I t can be seen from Eq.(l) t h a t f n ( t ) at-? f o r 2-dimensional and ^c~t" / 2 f o r 1-dimensional d i f f u s i o n a t long times. I t can be shown141 t h a t f n ( t ) can be expressed as a f u n - c t i o n t/b where

where ro i s the r a d i u s of t h e probed area A. So t h a t comparison o f t h e experimen- t a l and t h e o r e t i c a l curves y i e l d s and hence D.

I n t h e present case the d i f f u s i n q e n t i t i e s are n o t absorbed species a t f i x e d ave- rage c o n c e n t r a t i o n b u t "1oose" W atoms and o t h e r defects, created t h e r m a l l y . I t can be shown131 t h a t t h i s need n o t a f f e c t t h e form of the c o r r e l a t i o n f u n c t i o n i f the l i f e t i m e of t h e d i f f u s i n g species i s comparable t o r,,.

I I 1 - .,EXPERIMENTAL PROCEDURE

The d e t a i l o f t h e experimental procedure have been given previously/5,6/. The f i - nal pressure i n t h e system was 3-4x10-ll t o r r , The experiments c o n s i s t e d o f f i n - d i n g t h e f i e l d emission c u r r e n t c o r r e l a t i o n f u n c t i o n f i ( t ) f o r a number o f probe- h o l e l o c a t i o n s along t h e zone (011)-(112) and along t h e zone (011)-(001), u s i n g t h e e l e c t r o s t a t i c d e f l e c t i o n e l e c t r o d e s t o move t h e emission p a t t e r n . The e m i t t e r r a d i u s was r* = 1880 Angstroms and t h e probe r a d i u s was ro-% 60 Angstroms. Since

~ o w l e r - ~ o r d h b i m parameters f o r W atoms and o t h e r d e f e c t s a r e n o t known t h e e x p e r i - mental c o r r e l a t i o n f u n c t i o n i s then compared t o a t h e o r e t i c a l one based on Eq.(l) couched i n terms o f tit,. The a c t i v a t i o n energy E f o r s u r f a c e s e l f - d i f f u s i o n can be g o t from t h e p l o t s o f l o g D vs. 1/T. t h i s i s based on t h e assumed form

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Fig. (2) -. Work f u n c t i o n @ vs. T f o r (257), (123) and (235).

Some work f u n c t i o n measurements were a l s o made. Fig.(2) g i v e s work f u n c t i o n vs.

temperature.

I V - RESULTS AND DISCUSSION

The regions examined were stepped surfaces along t h e zone (011)-(112) and along t h e zone (01 1)-(001). I n t h e f i r s t case t e r r a c e s and steps have < 110 > o r i e n t a t i o n , steps are closepacked and r u n along < I 1 1 > d i r e c t i o n s . I n t h e second zone t e r r a c e s a r e i n i t i a l l y < 110 > and steps < 100 >, w i t h t e r r a c e s becoming < 100> and steps

< 110> as (001) i s approached. The steps a r e n o t closepacked and r u n along < 100>

d i r e c t i o n s .

For t h e zone (011)-(112) r e s u l t s were very s i m i l a r on a l l stepped surfaces and showed two d i s t i n c t types o f d i f f u s i o n . This c o u l d be seen by u s i n g d i f f e r e n t time scales f o r t h e c o r r e l a t i o n . The o r e r a p i d l y decaying f u n c t i o n showed t'l behavior a t l o n g times, t h e slower t"" behavior a t l o n g times. This suggests t h a t t h e f a s t regime corresponds t o atoms d i f f u s i n g on and across terraces; t h i s r e q u i r e s atoms t o c l i m b and descend steps. The slower 1-dimensional process suggests d i f f u s i o n along steps, and probably corresponds t o d i f f u s i o n of n e g a t i v e k i n k s , o r p o s i t i v e - n e g a t i v e k i n k combinations. Fig.(3) shows l o g D vs. 1/T f o r b o t h types f o r a t y p i c a l case, Evidence f o r a roughening t r a n s i t i o n comes from t h e f o l l o w i n g observations: A t 875 K. D(s1ow) decreases s h a r p l y and then i n c r e a - ses a t 925 K. Up t o t h i s temperature slow d i f f u s i o n i s s t i l l 1-dimentional b u t a t 950 K becomes Z d i m e n s i o n a l . T h i s suggests t h a t steps c o n t a i n some k i n k s which can d i f f u s e , b u t t h a t a t T >875 K steps become s u f f i c i e n t l y disordered t o impede k i n k d i f f u s i o n and a t 950 K become so d i s o r d e r e d t h a t k i n k s r u n i n every d i r e c t i o n and d i f f u s i o n i s Zzdimensional, The f a s t , s i n g l e a t o m . d i f f u s i o n increases up t o

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Fig. (3) - ^{l o g}^Dvs, 1/T f o r slow and f a s t d i f f u s i o n regimes on (257).

Values o f a c t i v a t i o n energies E and p r e f a c t o r s Doare i n d i c a t e d on t h e f i g u r e . The i n i t i a l l y f a s t e r d i f f u s i o n i s 2-dimensional, t h e i n i t i a - l l y slow regime l-dimensional below 950 K.

900 K and then decreases, because t h e increased step d i s o r d e r and decreasing t e r - race w i d t h s impede t h i s process. Above 1000 K o n l y a s i n g l e d i f f u s i o n regime i s d i s c e r n i b l e . Thus we conclude t h a t on surfaces c o n t a i n i n g closepacked steps t h e r e i s a gradual b u t w e l l defined increase i n d i s o r d e r l e a d i n g e v e n t u a l l y t o enough roughening f o r steps t o l o s e t h e i r i d e n t i t y .

On t h e zone (071)-(0011 where steps a r e n o t closepacked and t e r r a c e s s w i t c h o r i e n - t a t i o n t h e s i t u a t i o n seems more complicated. Two d i f f u s i o n regimes a r e seen on (023) and (01 7), which c o n s i s t o f (01 1) and (001 ) t e r r a c e s r e s p e c t i v e l y , b u t b o t h regimes a r e 1-dimensional. On (023) ( F i g . ( 4 ) ) t h e f a s t d i f f u s i o n decreases sharp- l y a t 650 K and merges w i t h t h e slow one a t 714 K; above t h i s temperature D i n c - reases again b u t become P-dimensional o n l y a t 900 K. However, no sharp decrease i n t h e slow regime, presumably corresponding t o k i n k d i f f u s i o n i s seen. On (017) the f a s t d i f f u s i o n drops a t 650 K and merges w i t h t h e slow d i f f u s i o n a t 770 K;

the l a t t e r increases w i t h o n l y a change i n s l o p e a t 700 K ( F i g . ( 5 ) ) . D i f f u s i o n becomes 2-dimensional a t 900.'K. On (013) no r e a l evidence f o r a t r a n s i t i o n i s seen, w h i l e (012) i s s i m i l a r t o (013). Thus roughening seems l e s s w e l l defined, o r a t l e a s t i s l e s s c l e a r l y seen from d i f f u s i o n changes, when steps a r e n o t c l o - sepacked. I t seems c l e a r , however, t h a t some roughening occurs b u t a t much lower temperatures than along t h e zone (01 1 ) - ( I 12).

Experiments on ( O l l ) , (112) and (111) were a l s o made. On (011) no s i g n i f i c a n t s i g n a l was seen and i t shows t h a t t h e p o p u l a t i o n o f f r e e atoms t h e r e i s n e g l i g i - b l e . On (112) a s i n g l e d i f f u s i o n regime was seen and t h e observed d i f f u s i o n i s e s s e n t i a l l y ZTdimensional. On (111) a s i n g l e 2-dimensional regime was seen b u t a t T = 750.K D decreased, then increased again, i n d i c a t i n g a surface roughening.

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F i g , (4)

-

¹

Both regimes

og D vs. 1/T f o r (023), ( x ) f a s t , ( 0 ) slow

a r e l d i m e n s i o n a l below 900 K . ^regime.

Fig. ( 5 ) - l o g D vs. 1/T f o r (017). (x) f a s t , ( 0 ) slow regime.

Both regimes are 1 -dimensional below 900 K.

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It is necessary to point that in the experimental procedures the pre- sence of the electric field does not cause the thermal-field build up of W atoms in the range of the temperatures.

V - CONCLUSION

The experimental results have indicated that self-diffusion on stepped surface can be used to investigate roughening by the field emission fluctuation method. It is rather clear that roughening on well defined stepped surfaces proceed via partial disordering of steps, followed by more global disorder.

VI - ACKNOWLEDGMENT

This work was supported by NSF Grant 86-00320. We have also benefi- ted from the MRL of the National Science Foundation at the Universi- ty of Chicago.

REFERENCES

/1/ E. H. Conrad, R. A. Aten, D. S. Kaufman, L. R. Allen, and T.

Engel, J. Chem. Phys. 84, 1015 (1986) ; ibid. 85, 4756, (1986).

/2/ Y. M. Gong and R. Gomer, Scanning Electron Microscopy, in press.

/3/ Y. M. Gong and R. Gomer, J. Chem. Phys. submitted.

/4/ R.Gomer, Surf. Sci. 38, 373 (1973).

/5/ S. C. Wang and R. Gomer, J. Chem. Phys. 83, 4193 (1985).

/6/ R. Difoggio and R. Gomer, Phys. Rev. B25, 3490 (1982).