Colloque C7, supplement au no 11, Tome 47, Novembre 1986. HOT-ELECTRON EMISSION FROM COMPOSITE METAL-INSULATOR MICROEMITTERS.
JOURNAL DE PHYSIQUE Colloque C7, supplement au n o 11, Tome 47, Novembre 1986
HOT-ELECTRON EMISSION FROM COMPOSITE METAL-INSULATOR MICROEMITTERS
M.S. MOUSA and R.V. LATHAM' M u ' t a h U n i v e r s i t y , A l - K a r a k , Jordan s st on U n i v e r s i t y , G B - B i r m i n g h a m B 4 7 E T , G r e a t - B r i t a i n
A b s t r a c t : F i e l d e m i s s i o n m i c r o s c o p y a n d e l e c t r o n s p e c t r o s c o p y have b e e n used t o s t u d y t h e e m i s s i o n c h a r a c t e r i s t i c s o f s e v e r a l t y p e s o f m e t a l i n s u l a t o r rriicmernitters: t h e s e i n c l u d e r e s i n - , alumina-, and h y d r o c a r b o n - c o a t e d t u n g s t e n , and g l a s s - c o a t e d s i l v e r s t r u c t u r e s . D e t a i l s a r e given o f t h e t h r e s h o l d switch-on phenomnon, t h e r e v e r s i b l e current-voltage characteristic, t h e high-field saturated missiorl durrent, t h e emission i m g e , and t h e field-dependence of t h e s p e c t r a l s h i f t and FWHM. C o n s i d e r a t i o n is a l s o g i v e n t o t h e t e c h n o l o g i c a l s i g n i f i c a n c e of these findings. 1.
INTRODUCTION
E x t e n s i v e e x p e r i m e n t a l s t u d i e s by Latham a n d co-workers [ I - 5 1 , a n d mre r e c e n t l y by F i s c h e r a r d h i s c o - w o r k e r s [ 6 , 7 1 , h a v e shown t h a t t h e e l e c t r o n e m i s s i o n r e s p o n s i b l e f o r t h e prebreakdown c u r r e n t s t h a t f l o w between v a c u u m i n s u l a t e d h i g h v o l t a g e e l e c t r o d e s o r i g i n a t e s f r o m i s o l a t e d p a r t i c u l a t e m i c m s t r u c t u r e s randomly l o c a t e d o n a n e l e c t r o d e s u r f a c e . The phenomenon h a s b e e n a n a l y s e d t h e o r e t i c a l l y i n t e r n o f a f i e l d - i n d u c e d h o t - e l e c t r o n m i s s i o n (FIHEE) model i n v o l v i n g a m e t a l i n s u l a t o r e m i s s i o n r e g i m e 18,91, a n d t h i s h a s b e e n shown t o p r e d i c t t h e c o r r e c t f i e l d - d e p e n d e n c e o f b o t h t h e e m i t t e d c u r r e n t a n d s p e c t r a l h a l f - w i d t h (FWM) o f i n d i v i d u a l s i t e s . With t h e d u a l a i m o f t e s t i n g t h e v a l i d i t y o f t h i s model, a n d e s t a b l i s h i n g w h e t h e r t h e mechanism c o u l d b e r e p r o d u c e d u n d e r c o n t r o l l e d l a b o r a t o r y c o n d i t i o n s , s t u d i e s have b e e n m d e o f t h e e m i s s i o n c h a r a c t e r i s t i c s o f a v a r i e t y of c o n p o s i t e m e t a l - i n s u l a t o r m i c r o e m i t t e r s t h a t a t t e n p t e d t o s i r m l a t e t h e assumed s t r u c t u r e o f n a t u r a l l y o c c u r i n g m i c r o s c o p i c e m i s s i o n sites. The f i r s t s y s t e m t o b e s t u d i e d c o n s i s t e d o f a s t a n d a r d e l e c t m l y t i c a l l y e t c h e d t u n g s t e n m i c r o w n i t t e r c o a t e d w i t h a 0.01-0.2pm t h i c k l a y e r o f epoxy r e s i n . A s r e p o r t e d e l s e w h e r e ~ 1 0 , 1 1 1 , t h e s e e m i t t e r s w e r e g e n e r a l l y c h a r a c t e r i s e d by e x h i b i t i n g a n i n i t i a l t h r e s h o l d s w i t c h - o n phenomenon, a n d a s u b s e q u e n t r e v e r s i b l e current-voltage c h a r a c t e r i s t i c t h a t s a t u r a t e d a t high f i e l d s . Occasionally however, a s d e s c r i b e d i n t h i s p a p e r , d e v i e n t b e h a v i o u r a l c h a r a c t e r i s t i c s w e r e o b s e r v e d , s u c h a s m u l t i p l e s w i t c h - o n e v e n t s , which p r o v i d e a n i n p o r t a n t i n s i g h t i n t o t h e p h y s i c a l n a t u r e o f t h e mechanism r e s p o n s i b l e f o r t h e e m i s s i o n . Subsequently, o t h e r composite s t r u c t u r e s were s t u d i e d I l l ] , and t h i s paper w i l l r e p o r t on t h r e e of t h e s e , narrely a l u m i n a - o n - t u n g s t e n , ' h y d m c a r b o n - o n - t u n g s t e n , a n d P a r t i c u l a r consideration w i l l be given t o t h e technological glass-on-silver. inportance of t h e s e s t r u c t u r e s a s p o t e n t i a l e l e c t r o n sources.
2. 2.1
EXPERIMENTAL Emitter Fabrication
Examples o f t h e f o u r t y p e s o f c o m p o s i t e mitter t o b e c o n s i d e r e d a r e shown i n t h e m i c r o g r a p h s o f F i g u r e 1. T h r e e o f t h e s e , namely ( a ) t o ( c ) , a r e b a s e d upon a t u n g s t e n m i c r o p o i n t p r o d u c e d by t h e w e l l known e l e c t r o l y t i c e t c h i n g t e c h n i q u e o r i g i n a l l y d e s c r i b e d by N u l l e r 112 I. I n t h e c a s e o f ( a ) , a f r e s h l y p r e p a r e d a n d c l e a n e d W-tip is c o a t e d by a t h i n l a y e r of e p o x y l i t e r e s i n u s i n g t h e recorm-ended
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986725
C7-140
J O U R N A L DE PHYSIQUE
Fig. 1 : Micrographs of four types of composite microemitter (ai r e s i n on W, fbl hydrocarbon on W, ( e l A Z 2 0 3 on W and ( d l glass on Eiectrodag. [I01 procedure f o r producing b i o - m d i c a l micro-electrodes I10,111, and photographed i n a TEM using BOkeV e l e c t r o n s : t h e W-tip r a d i u s and r e s i n l a y e r t h i c k n e s s a r e e s t i r r a t e d t o be -30 and -150nm r e s p e c t i v e l y . The hydrocarbon c o a t i n g on e m i t t e r ( b ) was produced by exposing a f r e s h l y prepared t i p t o t h e prolonged borrbardment of BOkeV e l e c t r o n s i n t h e sarre TEM r e f e r r e d t o above, whose o p e r a t i n g p r e s s u r e was o n l y -10-~mbar [I 1 1. Under t h e s e c o n d i t i o n s , electron-impacted s u r f a c e s [e.g. a p e r t u r e s ) becorre contawinated by a "coke-like" d e p o s i t formed by t h e polymerisation o f purrp o i l m l e c u l e s . The e s t i m t e d s u b s t r a t e t i p r a d i u s and f i l m t h i c k n e s s a r e 40 nm and 130 nm r e s p e c t i v e l y . The alumina-on-tungsten e m i t t e r shown i n t h e scanning e l e c t r o n micrograph of F i g u r e I c was prepared by f i r s t c o a t i n g a W-tip w i t h a l a y e r of aluminium, and t h e n a n o d i s i n g t h i s i n h y d m f l o u r o b o r i c a c i d [ I 1 I t o an A1203 l a y e r having an e s t i m t e d t h i c k n e s s of '4pm. The l a s t t y p e of e m i t t e r t o be s t u d i e d , n a m l y t h e g l a s s - o n - s i l v e r , was f a b r i c a t e d by a n e n t i r e l y d i f f e r e n t technique [ I l l . Here, t h e i n n e r w a l l of a g l a s s c a p i l l i a r y t u b e was f i r s t coated w i t h a t h i c k l a y e r of E l e c t r o d a g 915 ( a suspension o f m e t a l l i c s i l v e r ) . The t u b e was then extruded using a c o m r c i a l " p u l l i n g " d e v i c e F i n a l l y , t h e excess g l a s s a t t h e extruded t i p was c a r e f u l l y u n t i l it necked-off. etched away u n t i l o n l y a t h i n l a y e r of g l a s s covered t h e i n n e r conducting c core"^ i . e . a s shown i n t h e o p t i c a l micrograph o f Figure Id. 2.2
Analytical F a c i l i t y
The corrposite m i c r o e m i t t e r s d e s c r i b e d above were c h a r a c t e r i s e d using t h e experimental f a c i l i t y shown i n F i g u r e 2. T h i s corrbines a f i e l d emission microscope with a Van Oostrum r e t a r d i n g p o t e n t i a l a n a l y s e r , w i t h t h e l a t t e r i n c o r p o r a t i n g a n automatic e l e c t m n i c d i f f e r a t i o n c a p a b i l i t y [10,111. As w i l l be seen, t h e i n p u t anode of t h e s p e c t r o m e t e r is phosphored and "doubles" a s t h e s c r e e n f o r r e c o r d i n g emission images. The o r i e n t a t i o n of a n irrage on t h e s c r e e n can be a d j u s t e d by a d j u s t i n g t h e t i p p o s i t i o n v i a a bellows-linked x-y-z m n i p u l a t o r . I n o r d e r t o r e f e r e n c e recorded s p e c t r a t o t h e Fermi l e v e l of t h e s u b s t r a t e cathode. t h e s p e c t r o m e t e r was i n i t i a l l y c a l i b r a t e d using a c l e a n t u n g s t e n t i p [Ill.
Fig. 2 : A schematic of the analytical f a c i l i t y , incorporating a field emission microscope and an electron spectrometer. [ 10 1
2.3
Emission C h a r a c t e r i s t i c s
Although t h e r e were found t o b e s t a t i s t i c a l v a r i a t i o n s i n t h e b e h a v i o u r o f t h e f o u r t y p e s o f e m i t t e r s t u d i e s , and indeed a m n g e m i t t e r s of any g i v e n t y p e [Ill, t h e g e n e r a l i s e d f o r m o f t h e e m i s s i o n c h a r a c t e r i s t i c of a l l c a n p o s i t e m e t a l - i n s u l a t o r m i c r o s t r u c t u r e s c a n be r e p r e s e n t e d s c h e r r a t i c a l l y a s shown i n F i g u r e 3.
Fig. 3 : fie generalised form of the current-vo ltage characteristic of a metalinsulator composite emitter. [ 10 1
-
CSI
.
Thus, f o r t h e p o i n t - p l a n e e l e c t m d e geometry o f F i g u r e 1, a v i r g i n e m i t t e r is s e e n t o e x h i b i t a n i n i t i a l t h r e s h o l d switch-on phenomena a t some v o l t a g e V Subsequently, a n e m i t t e r h a s a ;.eversible I-V c h a r a c t e r i s t i c which t y 6 Y c a l l y g i v e s a s a t u r a t e d e m i s s i o n c u r r e n t o f 2-5pA f o r a p p l i e d v o l t a g e s > Vsat. F o r t h e c a s e of r e s i n - o n - t u n g s t e n e m i t t e r s [101, where t h e t i p f i e l d ET c a n be approximated by € ~ = V / k r , w i t h r b e i n g t h e t i p r a d i u s and k a c o n s t a n t -5, it was found t h a t t h e s w i t c h i n g f i e l d Esw, and t h e f i e l d ESat r e q u i r e d f o r s a t u r a t e d emission, v a r i e d w i t h t h e r e s i n l a y e r t h i c k n e s s Ad a s shown r e s p e c t i v e l y i n F i g u r e s 4a and 46.
JOURNAL DE PHYSIQUE
Fig. 4 : Illustrating how the switch-on fieM Esw and saturated emission field Es,t depend on the insulating lager thickness. These p l o t s t h e r e f o r e i n d i c a t e t h a t t h e r e is a n optirmm i n s u l a t o r l a y e r t h i c k n e s s of -0.15-0.2~m f o r o b t a i n i n g a low-field e m i s s i o n response. I n t h i s c o n t e x t , it is a l s o i r r p o r t a n t t o n o t e t h a t t h e e m i s s i o n t h r e s h o l d o f t h e un-coated e m i t t e r shown i n F i g u r e l a was found t o be s i g n i f i c a n t l y h i g h e r t h a n w i t h t h e c o a t e d e m i t t e r 1101. A t y p i c a l e m i s s i o n i m g e of a corrposite e m i t t e r a p p e a r s a s shown i n F i g u r e 5a, and is s e e n t o c o n s i s t o f a s i n g l e b r i g h t s p o t whose d i a m t e r is f i e l d - d e p e n d e n t , b u t is u s u a l l y 5 t o 6 t i r r e s srraller t h a n t h a t produced by a n i d e n t i c a l l y p o s i t i o n e d
la1
Fig. 5 : Typical emission images la and c ) and electron spectra (b and d ) obtained respectively from single- and multi-channel emitters.
Ib1
(dl
W-tip d e l i v e r i n g a n e q u i v a l e n t e m i s s i o n c u r r e n t I101. A s i l l u s t r a t e d i n F i g u r e 5b, f o r t h e c a s e of a r e s i n - o n - t u n g s t e n e m i t t e r 1101, t h e s h i f t of t h e e n e r g y spectrum o f t h e e m i t t e d e l e c t r o n s from t h e s u b s t r a t e Fermi l e v e l , and t o a l e s s e r e x t e n t t h e FWHN, a r e a l s o found t o be f i e l d - d e p e n d e n t . O c c a s i o n a l l y , one o b s e r v e s secondary switch-on p r o c e s s e s d u r i n g t h e i n i t i a l a p p l i c a t i o n of t h e a p p l i e d f i e l d 110,111. I n s u c h c a s e s , t h e e m i s s i o n i m g e is found t o c o n s i s t of s e v e r a l d i s c r e t e s p o t s such a s shown i n F i g u r e 5c. Cormspondi n g l y , i f t h e e l e c t r o n energy s p e c t r a of t h e i n d i v i d u a l s p o t s a r e r e c o r d e d , t h e y a l l g i v e s i n g l e - p e a k d i s t r i b u t i o n s , b u t randomly p o s i t i o n e d on t h e e n e r g y a x i s , a s i l l u s t r a t e d i n F i g u r e M. If t h e f i e l d is changed, each s p e c t r u m v a r i e s independe n t l y , b u t f o l l o w s t h e same t r e n d a s t h a t shown i n F i g u r e 5b f o r t h e s i n g l e - s p o t
irrage.
3.
DISCUSSION
The e x p e r i r r e n t a l f i n d i n g s r e p o r t e d b o t h h e r e and elsewhere [10,111, c l e a r l y d e n o n s t r a t e t h a t field-,induced e l e c t m n e m i s s i o n c a n be pmmoted by o v e r - l a y i n g a m t a l l i c m i c r o e m i t t e r w i t h a sub-micron t h i c k i n s u l a t i n g medium. On t h e b a s i s of t h e e l e c t r o n s p e c t r a l and e m i s s i o n i m g e d a t a p r e s e n t e d i n F i g u r e 5 , which b e a r s a s t r i k i n g r e s e n b l e n c e t o t h a t r e c o r d e d f r o m o t h e r m e t a l - i n s u l a t o r s t r u c t u r e s [3,51, it is b e l i e v e d t h a t t h e e m i s s i o n f r o m t h e p r e s e n t c o n p o s i t e e m i t t e r s o r i g i n a t e s from a h o t - e l e c t r o n based mchanism. According t o t h i s rrodel [8,91, t h e e m i s s i o n m c h a n i s m is i n i t i a t e d when a narrow c h a n n e l i n t h e i n s u l a t i n g rredium s w i t c h e d i n t o a c o n d u c t i n g state^ i.e. a s i l l u s t r a t e d i n F i g u r e 6. T h i s p m c e s s t h e n f a c i l i t a t e s
Fig. 6 : a ) A schematic representation of a switched-on single charnel emitter. Multi-spot imzges are formed by the presence of additional non-axial channels (shown d o t t e d ) . b ) A band diagram representation of a switched on conducting channel [8-101. t h e t u n n e l l i n g o f e l e c t r o n s f m m t h e s u b s t m t e m e t a l i n t o t h e c o n d u c t i o n band o f t h e i n s u l a t o r , and t h e i r s u b s e q u e n t h e a t i n g by t h e p e n e t r a t i n g e l e c t r i c f i e l d t o a n e n e r g y t h a t e n a b l e s them t o b e e m i t t e d i n t o t h e vacuum q u a s i t h e m i o n i c a l l y . The l a t e s t v e r s i o n o f t h i s model [ 9 1 t a k e s i n t o a c c o u n t t h e i n f l u e n c e of e l e c t r o n t r a p s s i t u a t e d i n t h e c o n d u c t i n g channel, and is c o n s e q u e n t l y a b l e t o p r o v i d e a q u a n t i t i v e e x p l a n a t i o n of both t h e I - V c h a r a c t e r i s t i c and t h e f i e l d - d e p e n d e n c e o f t h e s p e c t r a l s h i f t and FWHM. According t o t h i s model, t h e s a t u r a t e d errSssion c u r r e n t observed w i t h t h e p r e s e n t c o n p o s i t e e m i t t e r s is a m n i f e s t a t i o n of b u l k - l i m i t e d conduction. The d e v i e n t b e h a v i o u r i l l u s t r a t e d i n F i g u r e s 5c and d is t h o u g h t t o be a s s o c i a t e d w i t h t h e f o m t i o n , o r s w i t c h i n g on of secondary, non-axial c h a n n e l s , s u c h a s shown d o t t e d i n F i g u r e 6. I t h a s been found t h a t t h e i n c i d e n c e o f t h i s e f f e c t c a n be minimised by r e d u c i n g t h e f i e l d d i v e r g e n c e a t t h e t i p , e.g. by u s i n g a r e l a t i v e l y l a r g e t i p - s c r e e n d i s t a n c e when i n i t i a l l y switching-on a n e m i t t e r . From t h e t e c h n o l o g i c a l p o i n t o f view, t h i s new t y p e o f c o n p o s i t e e m i t t e r o f f e r s s e v e r a l p o t e n t i a l a d v a n t a g e s o v e r t h e mre c o n v e n t i o ~ lm e t a l l i c t u n g s t e n e m i t t e r . F i r s t l y , t h e y c a n o p e r a t e i n p o o r e r vacuum c o n d i t i o n s [ < l ~ - ~ r b r )secondly, , they c o u l d p r o v i d e a b r i g h t e r e l e c t m n o p t i c a l s o u r c e , t h i r d l y , t h e i r emission c u r r e n t is much l e s s s u s c e p t a b l e t o v o l t a g e f l u c t u a t i o n s i f t h e e m i t t e r is o p e r a t e d under s a t u r a t e d e m i s s i o n c u r r e n t c o n d i t i o n s . On t h e o t h e r hand, t h e s e e m i t t e r s c u r r e n t l y s u f f e r f m m t h e s e r i o u s p r a c t i c a l l i m i t a t i o n o f b e i n g l i a b l e t o randomly s w i t c h on and o f f w i t h t i m . However, t h i s phenomenon, which is a p p a r e n t l y an i n h e r e n t f e a t u r e of t h e u n d e r l y i n g e m i s s i o n mechanism, c a n t o s o m e x t e n t be c o n t r o l l e d by i n s i t u t h e m 1 p r o c e s s i n g [ I l l .
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4.
CONCLUSION
I t h a s been e x p e r i m e n t a l l y d e n o n s t r a t e d t h a t c o n p o s i t e m t a l - i n s u l a t o r m i c m s t r u c t u r e s can p r o m t e a f i e l d - i n d u c e d h o t - e l e c t r o n e m i s s i o n (FIHEE) m c h a n i s m t h a t is of c o n s i d e r a b l e i n t e r e s t b o t h from t h e p h y s i c a l and t e c h n o l o g i c a l p o i n t s o f view. However, t o a c h i e v e a g r e a t e r c o n t m l on t h e mchanism, it w i l l b e n e c e s s a r y i n f u t u r e s t u d i e s t o i n t m d u c e a g r e a t e r c o n t m l on t h e r m t e r i a l p r e p a r a t i o n of emitters. REFERENCES 1.
A l l e n N.K.
2.
A t h ~ a l C.S.
3.
A t h a l C.S. and Latham R.V.,
4.
Latham R.V., "High Voltage Vacuum I n s u l a t i o n : The P h y s i c a l Basis", Academic P r e s s , Ldn/NY, 1981.
5.
O a y l i s s K.H.
6.
N i e d e m n n Ph.,
Sankarmrran N.,
7.
N i e d e m n n Ph.,
PhD T h e s i s , Univ. o f Geneva, 1986.
8.
Latham R.V.,
Vacuum, 32,137-140,
9,
B a y l i s s K.H.
and Latham R.V.,
10.
Latham R.V.
and Mousa M.S.,
11.
Rousa M.S.,
PhD T h e s i s , Univ. o f Aston, 1984.
12.
M u l l e r E.W.,
and Latham R.V.,
J.Phys.D.
a n d Latham R.V.,
11,L55-L57,
P h y s i c a , 104C, 46-49,
Vacuum, 36,211-217,
1981.
1985.
Noer R.J. and F i s c h e r O.,
J.Appl.Phys.,59,1986.
1982.
Proc.Roy.Soc., J.Phys.D:
106,541-550,
1978.
1981.
P h y s i c a , 1@4C, 189-195,
and Latham R.V.,
Z.Phys.,
Appl. Phys.,
A403.285-311,
Appl.Phys.,
1937.
1986.
19,699-713,
1986.
