Diffusion studies of adsorbates on tungsten (110)

Abstracts of 25th International Field Emission Symposium

1.

2.

A. R. Waugh, 22nd IFES, Atlanta, 1975; A. R. Waugh, J. Phys. E: Sci. Inst. ii, 49 (1978) . A. R. Waugh, 24th IFES, Oxford, 1977; A. R. Waugh, M. J. Southon, S u r f a c e Sci. 68, 79 (1977) .

D E T E R M I N A T I O N OF A T O M I C J U M P L E N G T H DISTRIBUTIONS FOR SURFACE MIGraTION OF S I N G L E A D A T O M S P. L. Cowan*,

T.T.

Tsong **%

~Bell Laboratories, Murray Hill, New Jersey 07974; ~*Department of Physics Pennsylvania State University, Univ e r s i t y Park, P e n n s y l v a n i a 16802

S u r f a c e m i g r a t i o n of s i n g l e a d a t o m s on a p e r f e c t c r y s t a l l o g r a p h i c p l a n e occurs by a r a n d o m s e q u e n c e of a t o m i c jumps. However, atomic jumps are not n e c e s s a r i l y r e s t r i c t e d to n e a r e s t neighbor sites, as is c o ~ o n l y assumed. A n a l y s i s of FIM o b s e r v a t i o n s of m i g r a t ing a d a t o m s is u n i q u e l y c a p a b l e of yielding the d i s t r i b u t i o n of jump l e n g t h s and d i r e c t i o n s . E x i s t i n g data i n d i c a t e s that W adatoms on W(ll0) jump p r e d o m i n antly a l o n g d i r e c t i o n s in the plane, but can jump v a r i o u s d i s t a n c e s in these d i r e c t i o n s . -~This r e s e a r c h done at the P e n n s y l v a n i a State U n i v e r s i t y was s u p p o r t e d by NSF Grant No. DMR 76-i1418.

A STUDY OF C H E M I S O R B E D N I T R O G E N AND O X Y G E N ON G R O W T H OF THIN GOLD FILMS ON T U N G S T E N C. P a t e l University C o l l e g e of N o r t h W a l e s , Department of E l e c t r o n i c E n g i n e e r i n g Science, Dean Street, Bangor, G w y n e d d , N. W a l e s , U n i t e d K i n g d o m

W h e n c o n d e n s e d on t u n g s t e n in ultrahigh vacuum, m e t a l s are k n o w n to form m o n o l a y e r and m u l t i l a y e r films. The p r e s e n c e of s u b - m o n o l a y e r a m o u n t s of p r e a d s o r b e d gas can, h o w e v e r , p r e v e n t such films f r o m forming. In an a t t e m p t to e s t a b l i s h the roles of c h e m i s o r b e d gases, d e t a i l e d studies have b e e n made of the e f f e c t s of small q u a n t i t i e s of n i t r o g e n and o x y g e n as c o n t a m i n a n t gases on the s t r u c t u r e of thin g o l d film on t u n g s t e n by o b s e r v i n g the change in the w o r k - f u n c t i o n w i t h c o v e r a g e and t e m p e r a t u r e using p r o b e h o l e and simple field e m i s s i o n m i c r o s copy. A d s o r p t i o n of n i t r o g e n on t u n g s t e n

37]

f o l l o w e d by gold shows n o t a b l e changes in the w o r k - f u n c t i o n w h e n c o m p a r e d w i t h the 'clean' g o l d b e h a v i o r and c o n s e q u e n tly r e s u l t s in c r y s t a l i i t e g r o w t h at a g o l d c o v e r a g e g r e a t e r than 28. Heating the c o m p o s i t e layers at a t e m p e r a t u r e T > 800 K causes c r y s t a l l i t e s to dissoTve and gold reverts to 'clean' b e h a v ior. O x y g e n a d s o r p t i o n on a g o l d - b e a r i n g t u n g s t e n s u r f a c e p r o d u c e s a d e c r e a s e in of : -0.45 eV for s p r e a d i n g t e m p e r a tures of 600-750 K. This d e c r e a s e in seems u n u s u a l since the o x y g e n s t i c k i n g p r o b a b i l i t y on g o l d is v e r y low for the t e m p e r a t u r e used. B e c a u s e the o x y g e n a t t a c k s the t u n g s t e n s u b s t r a t e m o r e s e v e r e l y than nitrogen, the gold atoms a g g r e g a t e much more r e a d i l y into crysta l l i t e s than in the case of nitrogen. The s t e p w i s e h e a t i n g of the f o l l o w i n g systems W-Au, W - N 2, W - N 2 - A u , W-O2, W - A u - O 2 - A u , W - O 2 - A u , have b e e n e x a m i n e d for the d e s o r p t i o n c h a r a c t e r i s t i c s of the c h e m i s o r b e d gases on m o r p h o l o g y of gold overlayers. In b o t h n i t r o g e n and o x y g e n cases the o r d e r of t h e i r a d s o r p tion p l a y s an i m p o r t a n t part, and this is found to be n o t i c e a b l e w h e n the stepw i s e h e a t i n g of the f o l l o w i n g systems was p e r f o r m e d : W - O 2 - A u Case 1 and W - A u - O 2 - A u Case 2. In case i, ¢ inc r e a s e s v e r y s h a r p l y b e t w e e n T = 500750 K and b e c o m e s steady in the temperature region T = 900-1400 K. Case 2 shows the o p p o s i t e e f f e c t in the T = 500-750 K t e m p e r a t u r e region. This b e h a v i o r is t h o u g h t to be due to the i n v e r s i o n of dipoles, and w i l l be desc r i b e d in detail. C o - a d s o r p t i o n of (N2-Au) has b e e n s t u d i e d on the W(100), (iii) and (211) planes. R e s u l t s w i l l be d e s c r i b e d and c o m p a r e d w i t h k n o w n data of the (N2-W) and (W-Au) s y s t e m on these planes.

D I F F U S I O N S T U D I E S OF A D S O R B A T E S ON T U N G S T E N (ii0) J i a n n - R u e y Chert, R o b e r t G o m e r Departments of P h y s i c s a n d C h e m i s t r y a n d the J a m e s F r a n c k I n s t i t u t e , The University of C h i c a g o , C h i c a g o , Illinois 60637

The c o r r e l a t i o n of f u n c t i o n s of field e m i s s i o n c u r r e n t f l u c t u a t i o n s f r o m the (i!0) p l a n e of t u n g s t e n field emitters covered with submonoiayer amounts of oxygen, c a r b o n m o n o x i d e and xenon, have b e e n measured. At t e m p e r a tures above the o n s e t of s u r f a c e diffusion, the e x p e r i m e n t a l c o r r e l a t i o n f u n c t i o n s agree w i t h a t h e o r e t i c a l m o d e l b a s e d on s i n g l e p a r t i c l e d i f f u -

372

Abstracts of 25th International Field Emission Symposium

sion, I and yield d i f f u s i o n c o e f f i c i e n t s and a c t i v a t i o n energies of d i f f u s i o n on this plane. For oxygen, a c t i v a t i o n energy of d i f f u s i o n rises from 14 k cal/ mole for O - W < 0.2 to 22 k cal/mole for O/W = 0.56. For v i r g i n CO, no d i f f u s i o n could be u n e q u i v o c a l l y e s t a b l i s h e d at any coveraaes, a l t h o u g h a weak signal set in at 250°K. For b e t a CO, d i f f u s i o n w i t h an a c t i v a t i o n energy 23 k ca!/mole is found, s u g g e s t i n g O is the o b s e r v e d species diffusing. For xenon, an activation energy of ~ 3 k cal/mole was found at % = 0.5 and ~ = 0.9. At high t e m p e r a t u r e s the decay of the c o r r e l a tion function is slower than p r e d i c t e d Dy theory. It is s u g g e s t e d that this results from d y n a m i c a l correlations, or m u l t i - p a r t i c l e d e n s i t y fluctuations, w h i c h can occur w h e n several a d s o r b a t e particles are s i m u l t a n e o u s l y in motion. At low t e m p e r a t u r e s , e x p o n e n t i a l decay of the c o r r e l a t i o n function is o b s e r v e d and e x p l a i n e d as a p r e d i f f u s i v e flipflop of a d s o r b a t e p a r t i c l e s b e t w e e n b i n d i n g locations. The t e m p e r a t u r e d e p e n d e n c e of the mean square f l u c t u a tions in the d i f f u s i v e regime is disc u s s e d in terms of a d s o r b a t e - a d s o r b a t e interactions. i.

R. Gomer, (1973> .

FIELD

Surface

EMISSION

Sci.

AND

30,

ION SOURCES

BASIC W A V E - M E C H A N I C A L T H E O R Y FIELD-ION ENERGY DISTRIBUTIONS

A

Richard

373

Currently, the theory is in the formal stage of development, but the f o l l o w i n g results are already clear: i. A proof can be given that the usual q u a s i - c l a s s i c a l t r e a t m e n t of f i e l d - i o n i m a g i n g should be a v a l i d first a p p r o x i m a t i o n , if the e l e c t r o n t r a n s i t i o n r a t e - c o n s t a n t is given by an e x p r e s s i o n i n v o l v i n g an e l e c t r o n i c overlap integral. 2. The usual q u a s i - c l a s s i c a l treatments of energy d i s t r i b u t i o n s should be valid a p p r o x i m a t i o n s , with the following exceptions: (a) The shape of the h i g h - e n e r g y edge of the main peak; (b) D i s c u s s i o n of s u r f a c e - p l a s m o n a s s i s t e d field ionization. In the e x c e p t i o n a l cases the influence of o v e r l a p factors b e t w e e n v i b r a t i o n a l w a v e - f u n c t i o n s needs to be taken into account. For s u r f a c e - p l a s m o n creation the p r e s e n c e of an a d d i t i o n a l term in the e x p r e s s i o n for the energy distribution could p r o v e d e c i s i v e in r e j e c t i n g this m e c h a n i s m of the J a s o n e f f e c t on p u r e l y t h e o r e t i c a l grounds. These results are not e x a c t l y unexpected. However, the author b e l i e v e s that the p r e s e n t t r e a t m e n t of field ionization, w h i c h has long been r e c o g n i z e d as a "difficult" subject, has been able to avoid logical errors that appear in p r e v i o u s treatments. Very few, if any~ p r e v i o u s t r e a t m e n t s are s t r i c t l y comp a t i b l e w i t h the b a s i c axioms of quantum mechanics. 1.

OF

R. Gomer, Phys. 38,

I~. W. Swanson, 1613 (1963).

J. Chem.

G. Forbes

Department of P h y s i c s , U n i v e r s i t y Aston, Gosta Green, Birmingham, United Kingdom

of

This p a p e r p r e s e n t s the o u t l i n e of a new w a v e - m e c h a n i c a l theory of field ionization and f i e l d - i o n energy d i s t r i b u tions. The theory is a g e n e r a l i z a t i o n of the a p p r o a c h used by Gomer and Swanson I, and avoids the use of the L a n d a u - Z e n e r approximation. The theory may be r e g a r d e d as a direct fermion a n a l o g to the theory of r a d i a t i v e emission as d e v e l o p e d by Dirac. The e s s e n t i a l feature of the new theory is that it avoids the "double infin i t y of final states" p r o b l e m by e n c l o s i n g the d e p a r t i n g ion in a hypothetical o n e - d i m e n s i o n a l s l o p i n g - s i d e d box for w h i c h the w a v e - f u n c t i o n s can be determined. The rest of the analysis then p r o c e e d s by analogy with the well e s t a b l i s h e d theory of M o l e c u l a r Spectroscopy.

FIELD E M I S S I O N AREA DISPLAYS Jules

SOURCES

FOR LARGE

D. Levine

RCA L a b o r a t o r i e s , Jersey 08540

Princeton,

New

C o m p o n e n t and systems aspects of field e m i t t e r s as e l e c t r o n sources have been s t u d i e d for large area displays: 106 display elements, 104 cm 2 d i s p l a y area, and a d e s i g n cost for under $i00. We first i n v e s t i g a t e d U O 2 / W e u t e c t i c s p r o v i d e d by P r o f e s s o r A. T. Chapman. Later, a new field e m i t t e r t e c h n o l o g y was d e v e l o p e d u s i n g c o p p e r and glass fused sheets. I This was more suited to mass production. By m e c h a n i c a l or phot o l i t h o g r a p h i c means, Cu tips were formed i0 mils apart on a square array. A tip, when a p p r o p r i a t e l y t r e a t e d and fused w i t h a glass insulator and a copper extractor, y i e l d e d 10-100 ~A at 400-1000 V. The design value was