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FIELD ION EMISSION FROM ADSORBED FILMS - HYDROGEN AND XENON
P. Schwoebel, G. Hanson
To cite this version:
P. Schwoebel, G. Hanson. FIELD ION EMISSION FROM ADSORBED FILMS - HY- DROGEN AND XENON. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-121-C2-131.
�10.1051/jphyscol:1986218�. �jpa-00225651�
JOURNAL D E PHYSIQUE
Colloque C 2 , suppl6ment au n 0 3 , Tome 47, mars 1986 page c2-121
FIELD ION EMISSION FROM ADSORBED FILMS - HYDROGEN AND XENON
P.R. SCHWOEBEL and G.R. HANSON
School of Applied and Engineering Physics and
The National Research and Resource Facility for Submicron Structures, Knight Laboratory, Cornell University, Ithaca, NY 14853, U.S.A.
A b s t r a c t - L i q u i d hydrogen a d l a y e r s a r e o b s e r v e d i n f i e l d i o n e m i s s i o n a t 0 . 6 V / # and 5 . O K . An a n t e c e d e n t t o t h e l i q u i d f o r m a t i o n i s t h e a p p e a r e n c e o f s t a b l e , condensed d r o p l e t s o f a c r i t i c a l s i z e i n t h e i o n e m i s s i o n p a t t e r n a s t h e e m i t t e r t e m p e r a t u r e i s d e c r e a s e d . F o r p u r e hydrogen g a s , t h e d r o p l e t s form i n t h e r e g i o n s o f h i g h e s t f i e l d f o r a n n e a l e d < I l O > e m i t t e r s and a p p e a r a s s t a t i o n a r y , low c u r r e n t e m i s s i o n r e g i o n s p r o d u c i n g z l p A . I n t h e p r e s e n c e o f h e l i u m background g a s , t h e d r o p l e t s a r e m o b i l e . These d r o p l e t s form on t h e lower f i e l d r e g i o n s , p r i m a r i l y t h e c r y s t a l f a c e t s of 2 1 1 , and s p r e a d a c r o s s t h e e m i t t e r s u r f a c e b e f o r e t h e l i q u i d l a y e r a p p e a r s i n t h e i o n e m i s s i o n . Apparent l i q u i d and s o l i d e f f e c t s o c c u r f o r xenon i o n e m i s s i o n a t x 5 3 K .
Introduction
The mechanisms o f l i q u i d f o r m a t i o n and l i q u i d l a y e r r e s p o n s e i n l a r g e e l e c t r o s t a t i c f i e l d s a r e f a c t o r s t o be c o n s i d e r e d i n t h e o p e r a t i o n of g a s e o u s f i e l d i o n s o u r c e s . E f f e c t s of i n c r e a s e d i o n c u r r e n t and t h e l o w e r i n g of t h e t h r e s h o l d i o n i z a t i o n f i e l d s have been r e l a t e d t o t h e p r e s e n c e of p h y s i s o r b e d f i l m s l a b on t h e f i e l d e m i t t e r s u r f a c e . D i e l e c t r i c c u s p s have been i n v e s t i g a t e d a s an i o n s o u r c e mechanism ,2 and f i e l d c o n d e c j a t i o n o f some s p e c i e s h a s been s t u d i e d mass s p e c t r o m e t r i c a l l y . 3 I n a d d i t i o n , f i e l d e l e c t r o n s t u d i e s o f a d l a y e r e f f e c t s 4 have shown f i e l d c o n d e n s a t i o n phenomena, however, i o n imaging of t h e l i q u i d d i e l e c t r i c / v a c u u m i n t e r f a c e h a s n o t been w i d e l y d i s c u s s e d i n t h e l i t e r a t u r e . I o n e m i s s i o n d i s t r i b u t i o n s which a r e c o n t r o l l e d by hydrogen and xenon m u l t i l a y e r c o n d e n s a t i o n show a number of p h y s i c a l e f f e c t s ; c r i t i c a l phenomena, d i e l e c t r i c s u r f a c e i n s t a b i l i t i e s , and j e t t i n g .
Experimental
The e x p e r i m e n t a l a p p a r a t u s h a s been d e s c r i b e d p r e v i o u s l y . 5 F i e l d e m i t t e r s were zone r e f i n e d <110> t u n g s t e n , f l a s h a n n e a l e d t o -2800K p r o d u c i n g r e l a t i v e l y smooth, rounded endforms. F i g u r e 1 shows a xenon i o n image (56K. l V / A ) of s u c h an e m i t t e r . Image g a s e s were r e s e a r c h g r a d e p u r i t y and t h e i r chamber p r e s s u r e s were measured a t room t e m p e r a t u r e . The q u o t e d f i e l d s t r e n g t h s were o b t a i n e d from Fowler-Nordheim d a t a a n d / o r BIF v a l u e s f o r hydrogen and h e l i u m . For t h e e x p e r i m e n t s w i t h h y d r o g e n , t h e e m i t t e r b a s e t e m p e r a t u r e was 5.OK a s d e t e r m i n e d by
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986218
J O U R N A L DE PHYSIQUE
s u b s t i t u t i o n o f a c a r b o n r e s i s t o r thermometer f o r t h e t i p l o o p a s s e m b l y . With xenon, t i p t e m p e r a t u r e s were r e d u c e d below77K by pumping o n t h e l i q u i d n i t r o g e n i n t h e c r y o s t a t and q u o t e d v a l u e s were assumed t o be i d e n t i c a l t o t h e t e m p e r a t u r e i n d i c a t e d by a c o p p e r - c o n s t a n t a n thermocouple immersed i n t h e c r y o s t a t . The t e m p e r a t u r e o f t h e m u l t i l a y e r f i l m s is n o t measured, b u t would be e x p e c t e d t o be s l i g h t l y g r e a t e r t h a n t h e s u b s t r a t e t e m p e r a t u r e .
Results
Hydrogen A d s o r p t i o n
One t e s t f o r t h e p r e s e n c e o f a m u l t i l a y e r f i l m was d e m o n s t r a t e d by H a l p e r n and
~ o m e r . ~ These a u t h o r s showed h y s t e r e s i s i n t h e f i e l d e l e c t r o n c u r r e n t which i s i n t e r p r e t a b l e i n t e r m s of e l e c t r o n s t i m u l a t e d d e s o r p t i o n o f a d i e l e c t r i c , m u l t i l a y e r hydrogen f i l m a r o u n d t h e e m i t t e r apex. The magnitude of t h e f i e l d e l e c t r o n c u r r e n t i n c r e a s e upon f i l m d e s o r p t i o n i s a measure o f r e l a t i v e f i l m t h i c k n e s s e s . One t e s t of t h e p r e s e n t i n s t r u m e n t a l c a p a b i l i t i e s was t h e d e t e c t i o n of t h i s e f f e c t and it was t h u s a p p a r e n t t h a t s i g n i f i c a n t l y " h e a v i e r " f i l m s t h a n t h o s e r e p o r t e d by t h e above a u t h o r s were o b t a i n a b l e . Hydrogen f i l m d e s o r p t i o n c h a r a c t e r i z a t i o n by f i e l d e l e c t r o n c u r r e n t i n c r e a s e s was, however, l i m i t e d by r a p i d c a t h o d e bombardment i n i t i a t e d s u r f a c e p r o c e s s e s when t h e open f i l m e l e c t r o n e m i s s i o n r e a c h e d v a l v e s of l o 2 t o 1 0 % ~ .
F i g u r e 2 i s an i o n e m i s s i o n p a t t e r n w i t h a n a p p l i e d f i e l d of 0.6V/A, a hydrogen chamber p r e s s u r e of 5 x t o r r , a n e m i t t e r t e m p e r a t u r e o f 5.OK, and a n e m i s s i o n c u r r e n t o f -1pA. The a b s e n c e o f s u b s t r a t e d e t a i l i n a d d i t i o n t o t h e d i f f u s e w a v e - l i k e p a t t e r n s u g g e s t s t h a t a c o n t i n u o u s m u l t i l a y e r hydrogen f i l m is p r e s e n t . The wave c h a r a c t e r and i t s sudden movement i n t h e v i s u a l p a t t e r n i n d i c a t e t h a t t h e d i e l e c t r i c l a y e r i s e l e c t r o h y d r o d y n a m i c a l l y u n s t a b l e 6 w i t h a R a y l e i g h - T a y l o r w a v e l e n g t h o f 12008. I n c r e a s e s i n t h e s y s t e m hydrogen g a s p r e s s u r e i n i t i a t e d e s o r p t i o n o f t h e l i q u i d l a y e r which is s e e n a s a l a r g e f l a s h i n t h e i o n p a t t e r n . The r e f o r m a t i o n o f t h e l i q u i d phenomena t e r m i n a t e s a g a i n by a p p a r e n t j e t t i n g a f t e r a t i m e p e r i o d which d e c r e a s e s w i t h i n c r e a s i n g g a s s u p p l y r a t e . I f t h e f o r m a t i o n o f t h e m u l t i l a y e r hydrogen f i l m is slowed by c a u s i n g t h e e m i t t e r t e m p e r a t u r e t o c o o l from a h i g h e r t e m p e r a t u r e , i . e . , room t e m p e r a t u r e t o 5 K , w i t h a c h a r a c t e r i s t i c t i m e p e r i o d o f 2 t o 3 m i n u t e s , d e t a i l i n t h e i o n p a t t e r n a t 0.6V;A a p p e a r s f i r s t a s i o n e m i s s i o n from a s e r i e s o f d i f f u s e s p o t s 51508 i n d i a m e t e r . Each s p o t f o r m s i n d e p e n d e n t l y a n d a b r u p t l y u n t i l a u n i f o r m p a t t e r n is produced f i l l i n g t h e a n n u l a r r e g i o n a r o u n d t h e a x i a l ( 1 1 0 ) f a c e t . With c o n t i n u e d a d s o r p t i o n t h e f o r e m e n t i o n e d s p a c i a l - w a v e p a t t e r n o f F i g u r e 2 a p p e a r s .
When hydrogen g a s t o r r ) i s a chamber background component i n h e l i u m t o r r ) , hydrogen f i l m c o n d e n s a t i o n i s observed w i t h c h a r a c t e r i s t i c s s i m i l a r t o but more s t r i k i n g t h a n t h o s e f o r p u r e hydrogen. F i g u r e 3a shows hydrogen e m i s s i o n s p o t s which a r e more w e l l d e f i n e d t h a n t h o s e o b s e r v e d w i t h p u r e hydrogen. These e m i s s i o n s p o t s n u c l e a t e p r i m a r i l y on o n e o f t h e 21 1 f a c e t s , and a s c o n d e n s a t i o n c o n t i n u e s , p r e v i o u s l y n u c l e a t e d e m i s s i o n s p o t s s p r e a d a c r o s s t h e e m i t t e r a s i f t h e y were b e i n g 'pushed away' by t h e n u c l e a t i n g s p o t , F i g u r e 3 b , 3 c and 3d. The s p r e a d i n g o c c u r s away from t h e 21 1 r e g i o n s i n a n a n n u l u s around (1 1 0 ) . Hydrogen i o n c u r r e n t s a r e o r d e r of 1pA d u r i n g t h e He/H2 p r o c e s s .
An i n t e r p r e t a t i o n of t h e s e o b s e r v a t i o n s t o be p r e s e n t e d h e r e i s a n a l o g o u s t o t h e c r i t i c a l l i q u i d d r o p l e t n u c l e a t i o n phenomena7 i n a g a s . The d i f f e r e n c e s between t h e r e s p o n s e o f t h e hydrogen d r o p l e t s w i t h and w i t h o u t h e l i u m p r e s e n t w i l l n o t be a d d r e s s e d ; t h e s e e f f e c t s must c o n c e r n e i t h e r t h e p h a s e c h a r a c t e r i s t i c s o f t h e He/H2 m i x t u r e a t =5K o r t h e h e a t i n g of t h e hydrogen a d l a y e r . Only t h e v e r y o b v i o u s s t a b i l i t y o f t h e i n d i v i d u a l d r o p l e t s w i l l be d i s c u s s e d i n t e r m s of i o n i z a t i o n r a t e , v a p o r p r e s s u r e , and d i p o l e i n t e r a c t i o n a r g u m e n t s .
Xenon Adsorption
C i r c u l a r i o n e m i s s i o n s p o t s a r e a l s o a s s o c i a t e d w i t h m u l t i l a y e r f i l m s when xenon is a d m i t t e d a s t h e sample g a s . Xenon i o n e m i s s i o n from s p o t s =200A i n d i a m e t e r becomes v i s i b l e i n a f i e l d of 0.4V/A when t h e e m i t t e r t e m p e r a t u r e is reduced t o 53K, F i g u r e 4a (5.8kV). The e m i s s i o n s p o t s a p p e a r p r e f e r e n t i a l l y i n a r e a s of h i g h r e l a t i v e f i e l d on t h e e m i t t e r s u r f a c e , t h e r e g i o n s s u r r o u n d i n g t h e 110 and 211 c r y s t a l f a c e t s . For t h e r a n g e of t e m p e r a t u r e s (TL53K) and p r e s s u r e s ( ~ > ~ 5 x 1 0 - 5 t o r r ) i n v e s t i g a t e d , d r o p l e t f o r m a t i o n was n o t o b s e r v e d t o c o n t i n u e with t h e e v e n t u a l c o a l e s c e n c e i n t o a c o n t i n u o u s m u l t i l a y e r f i l m a s w i t h H2 a t 5K.
However, when a p p l i e d f i e l d is d e c r e a s e d , e x i s t i n g e m i s s i o n s p o t s j o i n w i t h n e a r e s t n e i g h b o r s , F i g u r e 4b (QkV), producing l i n e a r a s opposed t o c i r c u l a r a r e a s of e m i s s i o n . Upon r a i s i n g t h e v o l t a g e t o i t s i n i t i a l v a l u e , t h e l i n e s of i o n e m i s s i o n decouple i n t o c i r c u l a r e m i s s i o n a r e a s a g a i n w i t h t h e r e s u l t i n g e m i s s i o n d i s t r i b u t i o n b e i n g n e a r l y i n d i s t i n g u i s h a b l e from t h e o r i g i n a l , s e e F i g u r e 4c (5.8kV), c f . F i g u r e 4a. G r e a t e r r e d u c t i o n s i n v o l t a g e l e a d t o g r e a t e r changes.
D e c r e a s i n g t h e a p p l i e d v o l t a g e t o 4.2kV, F i g u r e 4d, produces an emission p a t t e r n s i m i l a r t o t h a t of 4b (4kV). A f t e r r e d u c i n g t h e v o l t a g e t o 3.5kV, no change i n t h e e m i s s i o n p a t t e r n is observed f o r =I s e c . Then t h e l i n e s of e m i s s i o n shown i n F i g u r e 4d e l o n g a t e and move a c r o s s t h e e m i t t e r s u r f a c e , e v e n t u a l l y m e e t i n g up w i t h o t h e r s p r e a d i n g l i n e s of e m i s s i o n . The r e s u l t i n g e m i s s i o n p a t t e r n i s shown i n F i g u r e 4e a t 4.6kV; no e f f e c t beyond a n i n c r e a s e i n t h e i o n c u r r e n t was observed i n t h e p a t t e r n upon i n c r e a s i n g t h e v o l t a g e from 3.5kV t o 4.6kV f o r t h e photograph.
' L i q u i d - l i k e ' b e h a v i o r can be o b s e r v e d i n i o n e m i s s i o n from m u l t i l a y e r Xe, b u t t h e s e e f f e c t s a r e c o m p l i c a t e d by s o l i d i f i c a t i o n o f t h e a d l a y e r i n t h e p r e s e n t experiment. Very deep s o l i d l a y e r s may be formed by r e p e a t i n g t h e above procedure. Beginning w i t h a p a t t e r n s u c h a s i n F i g u r e 4 a , t h e a p p l i e d v o l t a g e is reduced t o 2.5kV (=0.2V/A). A t f i r s t no i o n e m i s s i o n is s e e n , y e t a f t e r 1 minute a t 6 x l 0 - ~ t o r r and 53K, Xe i o n e m i s s i o n becomes v i s i b l e , F i g u r e 5 a . I f t h e v o l t a g e is now i n c r e a s e d t o 5.2kV (-0.4V/A), s o l i d Xe i s imaged on t h e e m i t t e r s u r f a c e , F i g u r e 5b. T h a t s u c h an i o n image is indeed from s o l i d Xe is evidenced by t h e p r e s e n c e o f a Xe c r y s t a l f a c e t w i t h d i s c e r n i b l e n e t p l a n e s a t t h e l e f t hand edge of F i g u r e 5b. The random e m i s s i o n d i s t r i b u t i o n on t h e r i g h t i s l i k e l y a r e s u l t of f i e l d s t r e s s induced s o l i d l o s s .
Discussion
Common t o both t h e c o n d e n s a t i o n o f hydrogen and xenon on t u n g s t e n is t h e a b r u p t a p p e a r a n c e of s t a b l e , independent d r o p l e t s i n t h e i o n e m i s s i o n p a t t e r n . The i o n e m i s s i o n d i s t r i b u t i o n d e l i n e a t e s t h e n u c l e a t i o n of t h e l i q u i d phase on t h e e m i t t e r s u r f a c e i n t h e p r e s e n c e of an a p p l i e d e l e c t r o s t a t i c f i e l d under t h e c o n d i t i o n of a d e c r e a s i n g e m i t t e r t e m p e r a t u r e . A s t h e t e m p e r a t u r e d e c r e a s e s t h e f i e l d enhanced p r e s s u r e i n c r e a s e s , d r i v i n g t h e c o n d e n s a t i o n . The mechanism which p e r m i t s i o n i z a t i o n s u f f i c i e n t t o o b s e r v e t h e d r o p l e t must be t h e enhanced f i e l d above a curved d i e l e c t r i c - v a c u u m i n t e r f a c e .
he s t a b i l i t y o f t h e d r o p l e t a f t e r its appearance on t h e s u r f a c e is e x p l a i n a b l e e i t h e r i n t e r m s of a f r e e energy minimum f o r some c r i t i c a l r a d i u s o r i n t e r m s of a k i n e t i c c o n d i t i o n r e l a t e d t o t h e e m i s s i o n of n e u t r a l s and/or i o n s . The f r e e energy of d r o p l e t n u c l e a t i o n i n a g a s i s a sum of s u r f a c e and volume t e r m s , 7 having no e n e r g y minimum.
where AG is t h e change i n Gibbs f r e e e n e r g y , r t h e d r o p l e t r a d i u s , Y t h e s u r f a c e t e n s i o n , T t h e t e m p e r a t u r e , P t h e p r e s s u r e , and Peq t h e e q u i l i b r i u m vapor p r e s s u r e of t h e bulk l i q u i d . Equation 1 shows t h e e x i s t e n c e o f a c r i t i c a l r a d i u s above which t h e d r o p l e t grows and below which i t e v a p o r a t e s . A c o n s t a n t e l e c t r i c
JOURNAL DE PHYSIQUE
f i e l d a p p l i e d t o a n e u t r a l d i e l e c t r i c c o n s t i t u t e s an a d d i t i o n a l f r e e energy volume term, FOPo ( 4 n r 3 / 3 ) , where Fo is t h e a p p l i e d f i e l d i n t h e absence of t h e l i q u i d phase, and Po i s t h e l i q u i d p o l a r i z a t i o n . This p o l a r i z a t i o n term i s dependent upon t h e shape of t h e d r o p l e t and t h e d e t a i l e d phase c h a r a c t e r of t h e l i q u i d i n t h e f i e l d . Given t h a t i d e n t i f i a b l e , mobile emission s p o t s e x i s t i n t h e p a t t e r n preceeding t h e formation of an extended, m u l t i l a y e r f i l m , i t i s suggested t h a t t h i s is a c r i t i c a l phenomena governed by Equation 1 . The c o n t r i b u t i o n t o Equation 1 due t o t h e presence of the e l e c t r i c f i e l d i s a volume term and t h e f u n c t i o n a l form of t h e f r e e energy e q u a t i o n is t h e r e f o r e not changed. The d r o p l e t should continue t o grow i n s i z e a f t e r n u c l e a t i o n u n l e s s k i n e t i c a l l y l i m i t e d by t h e emission of n e u t r a l s o r i o n s .
The c u r v a t u r e of t h e d r o p l e t enhances t h e l o c a l e l e c t r o s t a t i c f i e l d 8 t o a maximum value a t t h e apex. Taking t h e value of t h e f i e l d a t t h e apex t o be Fa and % t o be t h e f i e l d enhancement f a c t o r ,
where E i s the d i e l e c t r i c constant of t h e l i q u i d . In a d d i t i o n t o t h e i o n i z a t i o n of d r o p l e t m a t e r i a l , t h e e l e c t r i c f i e l d s t r e s s e s t h e drop c a u s i n g a v e r t i c a l deformation.8 An i n i t i a l l y hemispherical d i e l e c t r i c drop when placed i n an e l e c t r i c f i e l d w i l l become p r o l a t e . The f i e l d enhancement f a c t o r a t t h e a p e x , % , is dependent upon t h e d r o p l e t shape8 and i n c r e a s e s a s t h e drop becomes p r o l a t e . Figure 6 shows t h e apex f i e l d j u s t o u t s i d e t h e vacuum/dielectric i n t e r f a c e f o r a d i e l e c t r i c e l l i p s o i d a s a f u n c t i o n of t h e r a t i o of i t s axes.8
~ e c k e y 3 h a s given t h e form of t h e Kelvin equation when a c o n s t a n t e l e c t r i c f i e l d is p r e s e n t :
where E and El a r e t h e p o l a r i z a t i o n e n e r g i e s of a molecule i n t h e gas and l i q u i d $bases r e s p e c t i v i l y , p l is t h e l i q u i d d e n s i t y , and M t h e molecular weight. Equation 3 shows t h a t a s t h e d r o p l e t becomes p r o l a t e , t h e d e c r e a s i n g l o c a l r a d i u s of c u r v a t u r e and t h e i n c r e a s i n g apex e l e c t r i c f i e l d work t o g e t h e r (Eg>E1) t o r a i s e t h e e f f e c t i v e vapor p r e s s u r e i n t h e apex r e g i o n of t h e drop.
The dominant term f o r r=1008 is t h e p o l a r i z a t i o n energy term i n t h e e x p o n e n t i a l . Assuming t h a t t h e p o l a r i z a t i o n energy of a molecule i n t h e gas phase a t t h e drop apex is its vacuum v a l u e ,
a being t h e molecular p o l a r i z a b i l i t y . To c a l c u l a t e E l , t h e l o c a l e l e c t r i c f i e l d a t a molecule i n t h e d i e l e c t r i c , FmO1, must be e s t i m a t e d . Equation 5 g i v e s t h e Lorentz r e l a t i o n 9 which i s a p p r o p r i a t e f o r f l u i d s ,
Fd i s t h e macroscopic f i e l d i n t h e d i e l e c t r i c (Fd=BFo/z) a t t h e d r o p l e t apex and Po is the magnitude of t h e p o l a r i z a t i o n v e c t o r . Using t h e d e f i n i t i o n of t h e e l e c t r i c s u s c e p t a b i l i t y he,Po=XeFd and c=1+4nXe, equation 5 can be w r i t t e n
From e q u a t i o n s 3 and 6 ,
To e s t i m a t e t h i s q u a n t i t y t h e c o n s t a n t s t a b u l a t e d below have been used t o c a l c u l a t e t h e vapor p r e s s u r e i n c r e a s e a s a f u n c t i o n of 5 , s e e F i g u r e 7 .
T a b l e I
( " A t 5K, i n a f i e l d of 0.6V/A, t h e hydrogen molecule w i l l be a h i n d e r e d r o t a t o r . The p o l a r i z a b i l i t y i n t h i s c a s e is 0.9383 10. For a l i q u i d d e n s i t y of
. 0 7 ~ / c m 3 , t h e C l a s s i u s - M o s s o t t i r e l a t i o n 9 y i e l d s ~ = 1 . 2 7 )
The e n e r g e t i c s o f d r o p l e t f o r m a t i o n a r e s u c h t h a t i n c r e a s e s i n t h e i n t e r n a l f i e l d d u e t o s t r e t c h i n g i n t o a more p r o l a t e form a c t t o s t a b i l i z e t h e l i q u i d phase r e l a t i v e t o t h e g a s p h a s e i n t h e g e n e r a l e n v i r o n s o f t h e e m i t t e r . However, b o t h t h e n e u t r a l d e n s i t y above t h e d r o p l e t and t h e i o n i z a t i o n r a t e a t t h e a p e x i n c r e a s e when t h i s d e f o r m a t i o n o c c u r s . The d r o p l e t s i z e and s h a p e depend upon t h e n e u t r a l g a s s u p p l y r a t e s i n t o i t s s u r f a c e , t h e s u r f a c e f i e l d i n which i t i s s i t u a t e d , and t h e d e g r e e o f i o n - n e u t r a l c o l l i s i o n a l c o u p l i n g a c t i n g t o remove n e u t r a l s from t h e enhanced gas p r e s s u r e r e g i o n of t h e apex. E m i s s i o n s p o t i n t e n s i t y and s i z e v a r i a t i o n s can be s e e n i n m i c r o g r a p h s 3 a - 3 c where t h e l a r g e s t and most i n t e n s e d r o p l e t s a p p e a r o n t h e 211 and 110 f a c e t s . To r e a c h t h r e s h o l d i o n i z a t i o n f i e l d s i n t h e low f i e l d r e g i o n s o f t h e e m i t t e r s u r f a c e , t h e d r o p l e t must be l a r g e r and more p r o l a t e . Because l a r g e r d r o p l e t s u r f a c e a r e a s imply a g r e a t e r n e t s u p p l y of n e u t r a l s from t h e vapor p h a s e , t h e v a r i a t i o n o f i n t e n s i t y i s c o n s i s t e n t w i t h i o n e m i s s i o n b e i n g t h e d r o p l e t s t a b i l i z i n g f a c t o r . The i n d e p e n d e n c e of e a c h d r o p l e t and t h e i r t e n d e n c y t o move l i k e s o a p b u b b l e s on a s u r f a c e must be e x p l a i n a b l e i n t e r m s of t h e r e p u l s i v e i n t e r a c t i o n of t h e i r p o l a r i z a t i o n v e c t o r s c o u n t e r a c t i n g t h e a t t r a c t i v e c a p i l l a r y f o r c e s . The t o t a l e m i s s i o n o f i o n s and n e u t r a l s from t h e e n t i r e a r r a y of hydrogen d r o p l e t s d o e s n o t e x c e e d t h e n e u t r a l t r a n s p o r t from t h e g a s a t Fo=O t o t h e emi t t e r f o r t h e s e e x p e r i m e n t a l c o n d i t i o n s b e c a u s e c o n d e n s a t i o n c o n t i n u e s w i t h t h e e v e n t u a l f o r m a t i o n o f a c o n t i n u o u s m u l t i l a y e r f i l m and t h e n j e t t i n g . T h i s is n o t t h e c a s e f o r xenon, where t h e d r o p l e t i o n c u r r e n t s a r e n o t i c e a b l y h i g h e r and t h e n e u t r a l s u p p l y r a t e s from t h e s o u r c e r a d i a t i o n s h i e l d s a r e l e s s due t o x e n o n ' s low vapor p r e s s u r e n e a r 53K.
The dependence o f t h e vapor p r e s s u r e on t h e e l e c t r o s t a t i c f i e l d p r e s e n t i s a f a c t o r c o n t r i b u t i n g t o t h e g e n e r a t i o n of n e u t r a l m o l e c u l e s i n l i q u i d m e t a l i o n The magnitude o f t h e a p e x f i e l d is a c r u c i a l p a r a m e t e r i n d e t e r m i n i n g t h e vapor p r e s s u r e enhancement a s i s shown by E q u a t i o n 3. For a n example, c o n s i d e r g o l d w i t h a p o l a r i z a b i l i t y of a=lO.783 l 2 and a vapor p r e s s u r e of
= - 3 x 1 0 - ~ t o r r ~ 3 a t i t s m e l t i n g p o i n t of 1336K. The e f f e c t i v e vapor p r e s s u r e i n a n i o n i z i n g f i e l d o f 1V/8 is - 8 x l 0 - ~ t o r r and i n an a p p l i e d f i e l d o f 2V/A, - 1 O t o r r . For m e t a l s h a v i n g a r e l a t i v e l y l a r g e m o l e c u l a r p o l a r i z a b i l i t y i n c o n j u n c t i o n w i t h a s i g n i f i c a n t v a p o r p r e s s u r e a t t h e i r m e l t i n g p o i n t , t h e vapor p r e s s u r e i n c r e a s e i n t h e h i g h f i e l d r e g i o n is a f a c t o r a f f e c t i n g t h e i o n e n e r g y d i s t r i b u t i o n and mass s p e c i e s b e c a u s e mean f r e e p a t h s become s u f f i c i e n t l y s h o r t t h a t i o n - n e u t r a l i n t e r a c t i o n s o c c u r b e f o r e t h e i o n s r e a c h r e g i o n s of lower n e u t r a l d e n s i t y .
Conclusion
I o n imaging d u r i n g t h e c o n d e n s a t i o n of hydrogen and xenon shows t h e n u c l e a t i o n of s t a b l e , i n d e p e n d e n t d r o p l e t s . A p r o t o m o r p h i c t h e o r y s u g g e s t s t h a t t h e
JOURNAL DE PHYSIQUE
e n e r g e t i c a l l y f a v o r a b l e m i n i m i z a t i o n of t h e d r o p l e t f r e e e n e r g y by c o n t i n u e d growth is b a l a n c e d k i n e t i c a l l y by i o n and n e u t r a l e m i s s i o n from t h e a p e x of t h e d r o p l e t . F u r t h e r i n s i g h t i n t o t h e n u c l e a t i o n phenomena o f m u l t i l a y e r a d s o r b a t e s i n l a r g e e l e c t r o s t a t i c f i e l d s can be o b t a i n e d by d e t a i l e d i n v e s t i g a t i o n s as a f u n c t i o n o f e m i t t e r t e m p e r a t u r e and r e l a t i v e g a s p r e s s u r e . I n a d d i t i o n , i o n e n e r g y a n a l y s i s would p r o v i d e d i r e c t i n f o r m a t i o n o n f i l m t h i c k n e s s and i o n i z a t i o n mechanisms.
Acknowledgements
T h i s r e s e a r c h i s d i r e c t e d t o t h e development o f g a s e o u s f i e l d e m i s s i o n under a program o f t h e N a t i o n a l R e s e a r c h and R e s o u r c e F a c i l i t y f o r Submicron S t r u c t u r e s , NSF G r a n t ECS-8200312.
The c o n t r i b u t i o n s o f i n d i v i d u a l s i n MSC, NRRFSS, and LASSP on t h e t e c h n i c a l s u p p o r t s t a f f a r e g r a t e f u l l y acknowledge.
R e f e r e n c e s
1 a . R . Comer, F i e l d Emission and F i e l d I o n i z a t i o n , Harvard U n i v e r s i t y P r e s s (1961 ) .
1 b . R. Comer, F i e l d Emission a n d F i e l d I o n i z a t i o n i n Condensed P h a s e s , Aces.
Chem. Res. 5, 41 (1 971 ) .
2 P. S u d r a u d , J. Van d e W a l l e , C . C o l l i e x , and R. C a s t a i n g , C o n t r i b u t i o n o f F i e l d E f f e c t s t o t h e Achievement o f Higher B r i g h t n e s s Ion S o u r c e s , S u r f . S c i . 70, 392 ( 1 9 7 8 ) .
-
3 Hans-D. Beckey, F i e l d I o n i z a t i o n Mass S p e c t r o m e t r y , Pergamon P r e s s (1971 ) . 4 B. H a l p e r n a n d R . Gomer, F i e l d E m i s s i o n Through Hydrogen and Helium L a y e r s ,
J. Chem. Phys. 21, 3042 ( 1 9 6 9 ) .
5 P.R. Schwoebel and G . R . Hanson, Beam C u r r e n t S t a b i l i t y from L o c a l i z e d Emission S i t e s i n a F i e l d I o n S o u r c e , J. Vac. S c i . Tech. 3, 214 ( 1 9 8 5 ) .
6 J.R. M e l c h e r , E l e c t r o h y d r o d y n a m i c and Magnetohydrodynamic S u r f a c e Waves and I n s t a b i l i t i e s , Phys. F l u i d s 2, 1348 ( 1 9 6 1 ) .
7 A.B. P i p p a r d , C l a s s i c a l Thermodynamics, Cambridge U n i v e r s i t y P r e s s ( 1 9 6 4 ) . 8 L.D. Landau and E.M. L i f s h i t z , E l e c t r o d y n a m i c s o f C o n t i n u o u s Media, Pergamon
P r e s s ( 1 9 6 0 ) .
9 J . D . J a c k s o n , C l a s s i c a l E l e c t r o d y n a m i c s , Wiley ( 1 9 7 5 ) .
10 J. H i r s c h f e l d e r , C . C u r t i s s , and R. B i r d , M o l e c u l a r Theory o f Gases and L i q u i d s , Wiley (1967).
11 A. Wagner, T. V e n k a t e s a n , P.M. P e t r o f f , and D. B a r r , D r o p l e t Emission i n L i q u i d Metal I o n S o u r c e s , J. Vac. S c i . Tech. 2, 1186 ( 1 9 8 1 ) .
1 2 J. T h o r h a l l s s o n , C. F i s k , and S . F r a g a , Use o f Atomic SCF F u n c t i o n s w i t h Minimal B a s i s S e t s , J . Chem. Phys. 3, 1987 (1 9 6 8 ) .
1 3 R.E. Honig, Vapor P r e s s u r e Data f o r t h e S o l i d and L i q u i d E l e m e n t s , RCA Rev.
2 3 , 567 ( 1 9 6 2 ) .
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Figures
Figure 1. R e f e r e n c e t o t h e t y p i c a l f i e l d o f v i e w a n d m a g n i f i c a t i o n . F l a s h a n n e a l e d W <110> imaged a t 56K and 1V/A w i t h Xe.
Figure 2. M u l t i l a y e r h y d r o g e n f i l m which i s a p p a r e n t l y e l e c t r o h y d r o d y n a m i c a l l y u n s t a b l e due t o t h e l a r g e v e r t i c a l f i e l d s t r e s s p r e s e n t ; n o t e t h e -2008 w a v l e n g t h s e e n t h r o u g h o u t t h e i o n p a t t e r n . T=5. O K , PH =5x1 ~ - ~ t o r r , FO=O. 6V/A.
2
JOURNAL DE PHYSIQUE
Figure 3. ( a ) , ( b ) , and ( c ) H, d r o p l e t f o r m a t i o n p r i m a r i l y on t h e 211 c r y s t a l f a c e t s f o l l o w e d by t h e i r s u b s e q u e n t s p r e a d i n g a c r o s s t h e s u r f a c e a s t h e e m i t t e r t e m p e r a t u r e a p p r o a c h e s 5.OK; ( d l a p p e a r a n c e a f t e r c o a l e s c e n c e i n t o a c o n t i n u o u s m u l t i l a y e r f i l m : Fo=0.6V/A, P H ~ = ~ ~ - ~ t o r r , ~ ~ , = l o - ~ t o r r .
F i g u r e 4. ( a ) Xe d r o p l e t condensation a s t h e e m i t t e r temperature d e c r e a s e s t o 53K. Fo=0.4V/R (5800V), pXe=6xl ~ - ~ t o r r ; ( b ) d r o p l e t s i n ( a ) a f t e r reducing v o l t a g e t o 4000V; ( c ) appearance of ( b ) a f t e r r e t u r n i n g t o 5800V; ( d ) d r o p l e t s i n ( c ) a f t e r reducing t h e v o l t a g e t o 4200V; ( e ) e f f e c t on ( d ) by reducing t h e v o l t a g e t o 3500V.
c2-130 J O U R N A L DE PHYSIQUE
Figure 5. ( a ) Xe i o n e m i s s i o n a t 2500V(=0.2V/A); ( b ) a p p e a r a n c e o f s o l i d xenon on t h e e m i t t e r s u r f a c e . T h i s was s e e n a s a r e s u l t of r a i s i n g t h e f i e l d i n ( a ) t o 5200V (=0.4V/A). pXe=6x1 ~ - ~ t o r r , T=53K.
RATIO OF THE ELLIPSOIDAL AXES
Figure 6. The f i e l d enhancement f a c t o r 0 a t t h e d r o p l e t a p e x a s a f u n c t i o n of t h e r a t i o o f t h e d r o p l e t ' s major t o minor a x i e s ( e l l i p s o i d a l g e o m e t r y ) f o r Xe and H
1 <B & E , E b e i n g t h e l i q u i d ' s d i e l e c t r i c c o n s t a n t . 2 '
Figure 7. The v a p o r p r e s s u r e , n o r m a l i z e d t o i t s e q u i l i b r i u m b u l k l i q u i d z e r o f i e l d v a l u e a t t h e d r o p l e t a p e x , a s a f u n c t i o n o f t h e f i e l d e n h a n c e m e n t f a c t o r shown i n F i g u r e 6.
