I. FIELD-INDUCED HOT-ELECTRON EMISSION (FIHEE) FROM MIM MICROSTRUCTURES

HAL Id: jpa-00225641

https://hal.archives-ouvertes.fr/jpa-00225641

Submitted on 1 Jan 1986

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

I. FIELD-INDUCED HOT-ELECTRON EMISSION (FIHEE) FROM MIM MICROSTRUCTURES

N. Xu, R. Latham

To cite this version:

N. Xu, R. Latham. I. FIELD-INDUCED HOT-ELECTRON EMISSION (FIHEE) FROM MIM MICROSTRUCTURES. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-67-C2-78.

�10.1051/jphyscol:1986210�. �jpa-00225641�

JOURNAL DE PHYSIQUE

Colloque C2, supplkment au n03, Tome 47, mars 1986 page

c2-67

I . FIELD-INDUCED HOT-ELECTRON EMISSION ( F I H E E ) FROM M I M MICROSTRUCTURES

N.S. XU and R.V. LATHAM

Department of Mathematics and Physics, University of Aston, Gosta Green, GB-Birmingham B 4

7 E T ,

Great Britain

h b s t r a c t - The f i e l d - i n d u c e d prebreakdown c u r r e n t s t h a t f l o w between vacuum- i n s u l a t e d high v o l t a g e e l e c t r o d e s i n t h e f i e l d range 10-30 MV.mdl r e s u l t from a "nor-metallic" e l e c t r o n emission mechanism which i s l o c a l i s e d a t micron- s i z e d anomalous s u r f a c e i n c l u s i o n s . The p r e s e n t paper r e p o r t s on r e c e n t experimental evidence, i n c l u d i n g emission image and spatially-resolved energy a n a l y s i s d a t a , which s u g g e s t s t h a t t h e emission i s d e r i v e d from a f i e l d - i n d u - ced h o t - e l e c t r o n mechanism (FIHEE) a s s o c i a t e d with m e t a l - i n s u l a t o r - m e t a l ( M I M ) m i c r o s t r u c t u r e s .

1. INTRODUCTION

The h i g h v o l t a g e hold-off c a p a b i l i t y of broad-area vacuum-insulated e l e c t r o d e s i s limiter' by 3 f i e l d - i n d u c e d e l e c t r o n emission p r o c e s s t h a t o c c u r s a t i s o l a t e d micro- sco$>ic s i t e s on t h e cathode s u r f a c e [ l ] . These prebreakdown c u r r e n t s were o r i g i n a l l y a t t r i b u t e d t o a Fowler-Nordheitr: e l e c t r o n t u n n e l l i n g [ 2 ] from i s o l a t e d m e t a l l i c whis- Icers, o r m i c r o p r o t r u s i o n s , where t h e macroscopic gap f i e l d i s g e o m e t r i c a l l y enhanced from t y p i c a l o p e r a t i n g v a l u e s of 10-30 MV.m-I t o a t h r e s h o l d microscopic v a l u e of -3 X 109

v.m-l

^[3-51, More r e c e n t l y however, t h e f i n d i n g s from a range of d e d i c a t e d a n a l y t i c a l s t u d i e s have been shown t o b e i n s t a r k c o n f l i c t with t h i s model. Thus, t h e e l e c t r o n s p e c t r a of t h e p r o c e s s have "non-metallic" c h a r a c t e r i s t i c s 16-81, t h e emission o f e l e c t r o l u m i n e s c e n t o p t i c a l photons [ 9 , 1 0 ] , and l a s t l y , anode probe tech- n i q u e s have shown t h e emission t o o r i g i n a t e from anomalous i n c l u s i o n s t h a t a r e insu- l a t e d from t h e e l e c t r o d e s u r f a c e [ l 1-131

.

On t h e b a s i s of t h i s evidence Latham and co-workers 114-171 have proposed a f i e l d - i n d u c e d h o t - e l e c t r o n emission FIHEE mechanism, o p e r a t i n g i n a m e t a l - i n s u l a t o r - vacuum ( H I V ) regime, t o e x p l a i n t h e observed phenomena. T h i s assumes t h a t under ap- p r o p r i a t e i n t e r f a c e c o n d i t i o n s , e l e c t r o n s may t u n n e l from t h e metal s u b s t r a t e i n t o t h e conduction band of t h e d i e l e c t r i c i n c l u s i o n , where t h e y a r e t h e n h e a t e d through 1-3ev by t h e p e n e t r a t i n g e l e c t r i c a l f i e l d and e m i t t e d o v e r t h e s u r f a c e p o t e n t i a l b a r r i e r i n a q u a s i thermionic manner. This model, p a r t i c u l a r l y i n i t s most devel- oped form [ 1 7 ] , h a s s u c c e s s f u l l y provided a q u a n t i t a t i v e i n t e r p r e t a t i o n o f b o t h t h e l i n e a r form of t h e Fowler-Nordheim (FN) p l o t of t h e c u r r e n t - v o l t a g e c h a r a c t e r i s t i c

( i . e . f,g1/v2 v e r s u s 1 / v ) , and t h e field-dependence of t h e s p e c t r a l half-width (FWI:M) and s p e c t r a l s h i f t AE,. Furthermore, i t provided a q u a l i t a t i v e e x p l a n a t i o n of t h e a s s o c i a t e d e l e c t r o l u m i n e s c e n c e e f f e c t .

However, t h e r e i s c o n s i d e r a b l e e x p e r i m e n t a l evidence, which h a s l e d Latham 118, 191 t o s u g g e s t t h a t t h e r e a l s o a r e a s i g n i f i c a n t number of s i t e s where t h e emission o r i g i n a t e s from a metal-insulator-metal m l c r o s t r u c t u r e . For example, anode probe s t u d i e s [7,13] have shown t h e s u p e r f i c i a l s t r u c t u r e o f s i t e s i s e l e c t r i c a l l y i n s u l - a t e d from t h e s u b s t r a t e e l e c t r o d e and f r e q u e n t l y c o n t a i n s m e t a l l i c elements. Second- l y , it h a s been noted t h a t i f a t e s t e l e c t r o d e s u f f e r s a f l a s h - o v e r , t h e r e i s a dram- a t i c i n c r r a s e i n t h e s u r f a c e d e n s i t y of emission s i t e s , which a r e a s s o c i a t e d w i t h

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986210

C2-68

JOURNAL DE PHYSIQUE

"splashed" d r o p l e t s o f m e t a l l i c anode m a t e r i a l s [211, a p p a r e n t l y i n s u l a t e d from t h e s u b s t r a t e metal cathode by t h e s u r f a c e oxide. T h i r d l y , experiments i n i t i a t e d by Latham [l81 and co-workers [l91 e s t a b l i s h e d t h a t when carbon, i n g r a p h i t i c form, i s d e p o s i t e d on an e l e c t r o d e s u r f a c e , it c r e a t e s " a r t i f i c i a l " s i t e s t h a t have i d e n t i c a l e n i s s i o n c h a r a c t e r i s t i c s t o " n a t u r a l " s i t e s . This paper r e p o r t s on f u r t h e r s t u d i e s of t h e s e M I M s i t e s , and p r e s e n t s a q u a n t i t a t i v e model o f t h e emission mechanism.

2. EXPERIMENTAL SYSTEMS

The b a s i c f a c i l i t y used f o r s t u d y i n g t h e emission c h a r a c t e r i s t i c s o f t h e s e micro- s c o p i c a l l y l o c a l i z e d s i t e s was developed by Latham and co-workers [8,221 and i s sche- m a t i c a l l y i l l u s t r a t e d i n F i g u r e 1.

Rotary to

z Md

conwtor

Orw E I K U I ~ for y motor

F i g u r e 1 ( a ) The e l e c t r o n s p e c t r o m e t e r f a c i l i t y and a s s o c i a t e d systems f o r s t u d y i n g t h e emission c h a r a c t e r i s t i c s of i n d i v i d u a l s i t e s .

I t i s c e n t r e d upon a UHV ((10-1°mbar) h i g h - r e s o l u t i o n h e m i s p h e r i c a l e l e c t r o n s p e c t r o - meter [23] which i s i n t e r f a c e d w i t h a p l a n e - p a r a l l e l high v o l t a g e t e s t gap c o n s i s t i n g of a cathode and an anode with an a x i a l probe h o l e . To l o c a t e t h e p o s i t i o n s o f ind- i v i d u a l emission s i t e s , t h e specimen i s s y s t e m a t i c a l l y searched by scanning it i n a s ? i r a l p a t t e r n i n f r o n t of t h e anode probe h o l e . When a s i t e comes on-axis, e l e c t r - ons p a s s i n g through t h e h o l e a r e d e t e c t e d by t h e s p e c t r o m e t e r , and t h e i r s i g n a l i s used t o b r i g h t - u p a s t o r a g e o s c i l l o s c o p e whose e l e c t r o n beam i s synchronously scan- ned. This f a c i l i t y t h e r e f o r e p r o v i d e s a h i g h - r e s o l u t i o n map showing n o t only t h e g e n e r a l s i t e d i s t r i b u t i o n on t h e specimen c a t h o d e , b u t a l s o t h e s u b - s t r u c t u r e of t h e i n d i v i d u a l s i t e s [151. Such a map may subsequently b s used t o r e l o c a t e any given s i t e on-axis where i t s I - V c h a r a c t e r i s t i c can b e measured by r e c o r d i n g t h e e l e c t r o n c u r r e n t c o l l e c t e d by t h e i n t e r f a c e l e n s a f t e r t r a n s m i s s i o n through t h e anode probe h o l e . A l t e r n a t i v e l y , t h e s e same e l e c t r o n s can be focused Fnto t h e e n t r a n c e a p e r t u r e of t h e a n a l y z e r f o r r e c o r d i n g t h e energy spectrum of t h e s i t e , which f r e q u e n t l y rev- e a l e d a complex multi-peak s t r u c t u r e [161.

To i n v e s t i g a t e t h e p h y s i c a l o r i g i n of t h i s s p e c t r a l s u b - s t r u c t u r e , t h e i n t e r f a c e l e n s h a s been s u c c e s s i v e l y modified [8,24] t o i n c l u d e a phosphor s c r e e n a t t h e e n t r - ance a p e r t u r e of t h e i n p u t l e n s , and a s l o t i n t h e l e n s e l e c t r o d e t o view t h e s c r e e n

( s e e F i g u r e 1 ) . Thus, t h e specimen, t h e anode, t h e l e n s and t h e s c r e e n form an e l e - c t r o n o p t i c a l emission imaging system, which h a s comparable a n a l y t i c a l c a p a b i l i t y t o t h e c o n v e n t i o n a l f i e l d emission microscope. For example, a p a r t from making d i r e c t o b s e r v a t i o n s of t h e v i s u a l c h a r a c t e r i s t i c s of t h e emission image of s i t e s , it i s a l s o p o s s i b l e , by qaking micro-adjustments t o t h e cathode p o s i t i o n and l e n s v o l t a g e , t o c e n t r e any d e s i r e d r e g i o n of an imaqe over t h e probe h o l e and measure i t s energy spectrum.

3. EXPERIMENTAL DATA

D e t a i l e d s t u d i e s o f emission s i t e s on a range mechanically and c h e m i c a l l y p o l i s h -

ed cathode m a t e r i a l s (e.g. Cu, A l , M O ) , have shown,/that t h e i r g e n e r a l c h a r a c t e r i s t i c s a r e n o t s i g n i f i c a n t l y i n f l u e n c e d by t h e c h o i c e o f ' e l e c t r o d e m a t e r i a l o r s u r f a c e pro- c e s s j n g . Accordingly, t h e p r o p e r t i e s t o be d e s c r i b e d i n t h e f o l l o w i n g s e c t i o n s a r e t o be seen a s t y p i c a l of a p r o c e s s t h a t can occur on any e l e c t r o d e m a t e r i a l

111.

3.1. Emission image c h a r a c t e r i s t i c s

The emission images of a l l s i t e s studked can be b r o a d l y d i v i d e d i n t o two t y p e s . The f i r s t , which accounts f o r 70-80% of s i t e s , i s i l l u s t r a t e d by t h e example of Fig- u r e 2 a , and i s seen t o c o n s i s t of an a p p a r e n t l y random d i s t r i b u t i o n of d i f f u s e s p o t s :

F i g u r e 2 ( a ) and (b) C h a r a c t e r i s t i c emission images o f n a t u r a l l y o c c u r r i n g s i t e s on broad-area e l e c t r o d e s .

( C ) A n image o b t a i n e d from an a r t i f i c i a l carbon s i t e .

f u l l d e t a i l s of t h i s t y p e of p r o c e s s can b e found elsewhere [ 8 ,171. The second t y p e of image, i l l u s t r a t e d i n F i g u r e 2 ( b ) , i s seen t o have a more g e o m e t r i c a l form, con- s i s t i n g of s e v e r a l "new-moon" o r "orange" segments, whose outer-most edges a r e r e l a - t i v e l y well-defined. Another important p r o p e r t y of t h i s t y p e o f image, i s t h a t i t s o v e r a l l s i z e and, i n p a r t i c u l a r , t h e mean r a d i i of i t s c o n s t i t u e n t segments, i n c r e a s e g r a d u a l l y with i n c r e a s i n g a p p l i e d f i e l d .

3.2. E l e c t r o n s p e c t r a l c h a r a c t e r i s t i c s

Two t y p e s o f measurement a r e r e p o r t e d . The f i r s t , r e p r e s e n t e d by t h e sequence of Figure 3a, measures t h e field-dependence of t h e s p e c t r a l response from a f i x e d p o i n t i n t h e image; i n t h i s c a s e t h e c e n t r e of t h e i n n e r segment, l a b e l l e d 5 on Fig- u r e 2b. Following a procedure d e t a i l e d elsewhere [ 1 7 ] , such a sequence may b e used t o c o n s t r u c t a F-N p l o t of t h e emission, s i n c e t h e a r e a of each spectrum i s p r o p o r t - i o n a l t o t h e c u r r e n t d e n s i t y a t a given f i e l d .

F i g u r e 3 ( a ) A sequence of e l e c t r o n energy s p e c t r a recorded from segment 5 of Figure 2b i n t h e f i e l d range of 7.4

-

8.8 ~ ~ m - l .

( b ) S p e c t r a r e c o r d e d from t h e i n d i v i d u a l segments of F i g u r e 3b a t a c o n s t a n t f i e l d of 8 t?Vm-l.

The p l o t corresponding t o t h i s s p e c t r a l sequence i s p r e s e n t e d i n Figure 4 a , and rev- e a l s t h e t y p i c a l c h a r a c t e r i s t i c s of a l o w - f i e l d ( $ 10 MV.m-1) l i n e a r r e g i o n with field-enhancement f a c t o r s o f 200

<

⁶ ( 1000, [ l ] , and a non-linear h i g h - f i e l d r e g i o n .

JOURNAL

DE PHYSIQUE

An e q u a l , i f n o t more important f e a t u r e o f t h e evidence of F i g u r e 3a, i s t h a t it r e v e a l s f i r s t l y how t h e p o s i t i o n of t h e s p e c t r a l peak s h i f t s t o lower e n e r g i e s with i n c r e a s i n g f i e l d , and secondly how t h e s p e c t r a l half-width (Fi;IIE,!) a l s o i n c r e a s e s w i t h i n c r e a s i n g f i e l d . These e f f e c t s a r e i l l u s t r a t e d g r a p h i c a l l y i n Figure 4b and show

Figure 4 ( a ) Fowler-Nordheim p l o t of t h e emission from segment 5 of F i g u r e 3b.

( b ) Field-dependence o f t h e s p e c t r a l s h i f t and FWHM b u i l t up from sequences such a s F i g u r e 4a.

how t h e s h i f t AEs i n c r e a s e s slowly a t low f i e l d s

( S

8 M V . m - l ) , b u t very much more r a p i d l y a t h i g h f i e l d s . I n c o n t r a s t , it w i l l b e seen t h a t t h e FWHM e x h i b i t s a con- v e r s e field-dependence.

The second t y p e of c o n s t a n t - g a i n s p e c t r a l measurement, which i s i l l u s t r a t e d i n F i g u r e 3b, i n v e s t i g a t e s how t h e s p e c t r a l responsed v a r i e s from segment t o segment.

Thus, with r e f e r e n c e a l s o t o F i g u r e 2b, it w i l l b e seen t h a t t h e r e i s a marked i n c - r e a s e i n t h e emission c u r r e n t d e n s i t y between t h e o u t e r and innermost segment, coup- l e d with a small b u t s i g n i f i c a n t s h i f t of t h e s p e c t r a maxima towards low e n e r g i e s . However, it i s i m p o r t a n t t o n o t e t h a t t h e magnitude of t h e s h i f t involved i n t h i s l a t t e r e f f e c t is v e r y much l e s s t h a n t h a t observed between t h e inclividual s p o t s of t h e image shown i n F i g u r e 2a [81.

4. DISCUSSION

The concept of a h o t - e l e c t r o n emission mechanism o c c u r r i n g a t M I M m i c r o s t r u c t u r - e s was f i r s t suggested by Latham [l81 f o l l o w i n g s t u d i e s of a r t i f i c i a l carbon s i t e s on broad-area e l e c t r o d e s [191. More r e c e n t l y , Xu and Latham C251 have f u r t h e r sub- s t a n t i a t e d t h i s p r o p o s a l by i d e n t i f y i n g c l e a r s i m i l a r i t i e s between t h e emission imag- e s o b t a i n e d from carbon s i t e s and t h o s e o b t a i n e d from e a r l i e r s t u d i e s on purpose- f a b r i c a t e d M I M s t r u c t u r e s [261. From a conparison of t h e emission images p r e s e n t e d i n F i g u r e 2b and c , it can be concluded from t h e p r e s e n t i n v e s t i g a t i o n t h a t a s i m i l a r type o f emission mechanism a l s o o p e r a t e s a t -25% of t h e emission s i t e s t h a t occur n a t u r a l l y on broad-area e l e c t r o d e s . I n t h i s c o n t e x t , it i s s i g n i f i c a n t t o n o t e t h a t from a s t a t i s t i c a l a n a l y s i s of t h e e l e m e n t a l composition of n a t u r a l l y o c c u r r i n g s i t e s on niobium [271, -20% were a s s o c i a t e d w i t h carbon c o n c e n t r a t i o n s .

The e l e c t r o n s p e c t r a l s e r i e s o f F i g u r e 3a and t h e d e r i v e d d a t a of F i g u r e 4 a l s o r e v e a l s s i m i l a r emission c h a r a c t e r i s t i c s t o t h o s e found f o r a r t i f i c i a l carbon s i t e s

[19,251. I n p a r t i c u l a r , t h e half-width (FWHM)and s h i f t (AE,) d a t a of Figure 4b, ex- h i b i t s t h e same l i n e a r l o w - f i e l d dependence found f o r carbon s i t e s [ 1 9 ] , and expected on t h e o r e t i c a l r e a s o n i n g f o r a h o t - e l e c t r o n emission mechanism [14]. C l e a r p a r a l l e l s a l s o e x i s t between t h e s p e c t r a l d a t a of F i g u r e 3b and t h a t found f o r carbon s i t e s

[ X I ; namely, t h a t t h e r e i s o n l y a s m a l l d i f f e r e n c e i n t h e e l e c t r o n energy d i s t r i b - u t i o n o b t a i n e d from t h e c o n s t i t u e n t segments of an emission image, A s w i l l now b e d i s c u s s e d , t h i s l a t t e r important o b s e r v a t i o n f u r t h e r s u p p o r t s t h e concept of h o t - e l e c t r o n emission from a M I M regime i n which e l e c t r o n s a r e c o h e r e n t l y s c a t t e r e d from t h e t o p metal e l e c t r o d e .

I n o r d e r t o o b t a i n t h e r e v e r s i b l e , s t e a d y - s t a t e emission c o n d i t i o n s d e s c r i b e d i n t h i s p a p e r , it i s n e c e s s a r y t o assume t h a t t h e i n s u l a t i n g f i l m undergoes a channel- forming p r o c e s s 116,171 s i m i l a r t o t h a t observed b o t h i n p u r p o s e - f a b r i c a t e d M I M dev- i c e s [28] and a r t i f i c i a l carbon s i t e s [191. According t o t h i s model, it i s f i r s t l y assumed t h a t t h e i n t e r f a c e between t h e m e t a l l i c s u h s t r a t e and t h e i n s u l a t i n g l a y e r l o c a l l y forms a b l o c k i n g - c o n t a c t [ i 7 ] , s o t h a t when a n e l e c t r i c a l f i e l d i s i n i t i a l l y a p p l i e d t o t h e gap, a v o l t a g e droh w i l l b e e s t a b l i s h e d a c r o s s t h e M I M s t r u c t u r e . A t some t h r e s h o l d f i e l d , however, e l e c t r o n s w i l l be a b l e t o t u n n e l from t h e s u b s t r a t e cathode i n t o t h e conduction band of t h e i n s u l a t o r , a s i n d i c a t e d i n F i g u r e 5a. Some of t h e electsrons w i l l b e t r a p p e d by l o c a l i z e d c e n t r e s w h i l s t t h e o t h e r s w i l l b e ac- c e l e r a t e d towards t h e t o p m e t a l l i c l a y e r and a t t e m p t t o escape i n t o t h e vacuum. How- e v e r , a s d i s c u s s e d by Simmons [ 2 6 ] , t h e y w i l l b e c o h e r e n t l y s c a t t e r e d i n t h e t o p met- a l l i c l a y e r and t h a t u n l e s s t h e y have s u f f i c i e n t energy, determined by t h e Bragg equ- a t i o n , w i l l b e b a c k - s c a t t e r e d i n t o i n s u l a t o r . I n f a c t , even when t h e e l e c t r i c f i e l d h a s been i n c r e a s e d t o t h e p o i n t where forward s c a t t e r i n g i s p o s s i b l e , an e l e c t r o n w i l l n o t b e a b l e t o escape i n t o t h e vacuum i f i t s p a t h l e n g t h i n t h e t o p e l e c t r o d e i s l o n g e r t h a n i t s mean f r e e p a t h ; i . e . a s f o r d i r e c t i o n 1 of F i g u r e 5b. However, a s d i s c u s s e d by Simmons [261, t h i s r e s t r i c t i o n can be r e l a x e d i f a channel t e r m i n a t e s a t a p o s i t i o n near t o a t r i p l e - j u n c t i o n r e g i o n such a s shown i n F i g u r e 5b.

FL-

-

N,

F i g u r e 5 ( a ) Energy band c o n f i g u r a t i o n o f an M I M s t r u c t u r e i n i t s " e m i t t i n g s t a t e " . (b) A schematic r e p r e s e n t a t i o n o f a p o s s i b l e g e o m e t r i c a l s t r u c t u r e f o r

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

Thus, e l e c t r o n s d i f f r a c t e d i n d i r e c t i o n 2, and having a v e l o c i t y v e c t o r p a r a l l e l t o t h e i n s u l a t o r - m e t a l i n t e r f a c e which i s b i g enough f o r them t o surmount t h e s u r f a c e p o t e n t i a l b a r r i e r of t h e m e t a l , w i l l be e m i t t e d i n t o t h e vacuum and form a "new moon"

segment such a s shown i n F i g u r e 2b. Experimental evidence s u g g e s t s t h a t i n many c a s e s , c h a n n e l s form i n c l u s t e r s i n t h e t r i p l e - j u n c t i o n r e g i o n , s o t h a t an image such a s shown i n F i g u r e 3b w i l l c o n s i s t of s e v e r a l segments. For t h e i d e a l s i t u a t i o n where t h e p r o p e r t i e s of t h e s e channels and t h e i r a s s o c i a t e d g e o m e t r i c a l f i e l d enhan- cement a r e t h e same, e l e c t r o n s a s s o c i a t e d with d i f f e r e n t segments i n an image would have t h e same energy d i s t r i b u t i o n . I n p r a c t i c e , it i s u s u a l l y i m p o s s i b l e t o a c h i e v e t h e above c o n d i t i o n s s o t h a t t h e measured t o t a l energy d i s t r i b u t i o n s from t h e sep- a r a t e segments w i l l b e s l i g h t l y d i f f e r e n t , a s shown i n F i g u r e 3b. Furthermore, i f t h e e l e c t r o n c u r r e n t i s monochramatic, it would form a s h a r p "new moon" segment, b u t s i n c e , i n f a c t , t h e e l e c t r o n s have a f i n i t e energy d i s t r i b u t i o n , t h e segments u s u a l l y appear t o be d i f f u s e d with a n i l l - d e f i n e d t r a i l i n g edge.

I t f o l l o w s from t h i s r e a s o n i n g t h a t with an i n c r e a s e i n t h e e l e c t r i c a l f i e l d , more and more e l e c t r o n s w i l l l h a v e enough energy t o emerge from t h e s u r f a c e . Conseq- u e n t l y , t h e o v e r a l l s i z e of a segment w i l l i n c r e a s e , and a c c o r d i n g l y i t s s p e c t r a l half-width w i l l be broadened, which i s e q u i v a l e n t t o s a y i n g t h a t t h e e f f e c t i v e e l e c - t r o n temperature Tei h a s i n c r e a s e d . However, t h e energy gained by e l e c t r o n s from t h e p e n e t r a t i n g f i e l d i n t h e i n s u l a t o r i s n o t f u l l y used t o i n c r e a s e t h e i r k i n e t i c energy, s i n c e some i s i n e v i t a b l y l o s t t o t h e i n s u l a t o r through t h e c a r r i e r s c a t t e r i n g

JOURNAL

DE

PHYSIQUE

p r o c e s s e s [29] t h a t w i l l occur a t t h e r e l a t i v e l y high e l e c t r i c a l f i e l d s e x i s t i n g i n t h e i n s u l a t o r when t h e above mechanism i s o p e r a t i n g . For example, e l e c t r o n s w i l l n o t be i n d e f i n i t e l y a c c e l e r a t e d s i n c e t h e y w i l l l o s e energy by e i t h e r i n t e r a c t i n g with phonons of t h e c r y s t a l l i n e l a t t i c e o r themselves e m i t t i n g phonons t o t h e l a t - t i c e . Thus, t h e i n c r e a s i n g s p e c t r a l s h i f t with i n c r e a s i n g a p p l i e d e l e c t r i c a l f i e l d i s a m a n i f e s t a t i o n of such e l e c t r o n energy l o s s mechanisms. Correspondingly, t h e broadening of t h e s p e c t r a l half-width i s evidence of an i n c r e a s i n g k i n e t i c energy of t h e e l e c t r o n , i . e .

- ³

k Te: t h e s e e f f e c t s a r e i l l u s t r a t e d q u a l i t a t i v e l y i n Figure 5a. I n g e n e r a l , ther;fore, t h e s p e c t r a l s h i f t and half-width vary conversely with t h e e l e c t r i c a l f i e l d , which i n a p h y s i c a l s e n s e , r e f l e c t s t h e energy c o n s e r v a t i o n law.

5. CONCLUSION

Complementary experimental evidence i n t h e form of e l e c t r o n emission images and s e l e c t i v e a r e a e l e c t r o n s p e c t r a l d a t a h a s been p r e s e n t e d which s u p p o r t s t h e concept of a f i e l d - i n d u c e d h o t - e l e c t r o n emission mechanism based on a M I M m i c r o s t r u c t u r e . A q u a l i t a t i v e model h a s a l s o been p r e s e n t e d which f i r s t l y i s a b l e t o e x p l a i n t h e char- a c t e r i s t i c segmented appearance of emission images i n terms of c o h e r e n t e l e c t r o n

s c a t t e r i n g w i t h i n t h e upper metal e l e c t r o d e . Secondly, it p r o v i d e s a c o n s i s t e n t e x p l a n a t i o n f o r t h e s p a t i a l and field-dependent p r o p e r t i e s of t h e e l e c t r o n s p e c t r a .

ACKNOWLEDGEMENT

One of t h e a u t h o r s ( N S Xu) wishes t o g r a t e f u l l y thank t h e Government of t h e P e o p l e ' s Republic of China f o r f i n a n c i a l l y s u p p o r t i n g him while c a r r y i n g o u t t h i s work.

REFERENCES

1. 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 as is", Academic P r e s s , London and New York (1981).

2. Fowler R H and Nordheim L, Proc. Roy. Soc.

S,

( 1 9 2 8 ) , 173.

3. Vibrans G E, Tech. Rep. 353, Lin. Lab, MIT, (NTIS, ESDTDR, 64-173), 1964.

4. Chatherton P A , Proc. Phys. Soc. (London),

88,

(1966), 231.

5. Swanson L W, Crouser L C and Charbonnier F M, Phys. Rev.

151,

(19663, 327.

6. Allen N K and Latham R V , J . Phys. D: Appl. Phys.,

G,

( 1 9 7 8 ) , L55.

7. Athwal C S and Latham R V , Physica,

W ,

( 1 9 8 1 ) , 189.

8. B a y l i s s K H and Latham R V , Vacuum,

2,

( 1 9 8 5 ) , 211.

9. Hurley R E and Dooley P J , J. Phys. D : Appl. Phys.,

19,

( 1 9 7 7 ) , L195.

10. Hurley R E, J. Phys. D: Appl. Phys.,

12,

( 1 9 7 9 ) , 2229.

11. Cox B M, J. Phys. D : Appl. Phys.,

8,

( 1 9 7 5 ) , 2065.

12. Athwal C S and Latham R V, Physica,

E,

( 1 9 8 1 ) , 46.

13. Niederman Ph, Sarkurraman N and F i s h e r $I, 2nd Workshop on RF S u p e r c o n d u c t i v i t y ~ Geneva, (1984)

,

583.

14. Latham R V, Vacuum,

2,

(1982), 137.

15. Latham R V , IEEE Trans. Elec. I n s u l . EI-18, ( 1 9 8 3 ) , 194.

16. Athwal C S and Latham R V , J. Phys. D: Appl. Phps.,

17,

( 1 9 8 4 ) , 1029.

17. B a y l i s s K H and Latham R V , Submitted f o r p u b l i c a t i o n (1985).

18. Latham R V , P r i v a t e Communication CERN (Geneva), (May 1983).

19. Athwal C S , B a y l i s s K H , Calder R and Latham R V ,

IEEE Trans. Plasma S c i . , (19851, t o be p u b l i s h e d . 20. Latham R V , 2nd Workshop on RF S u p e r c o n d u c t i v i t y , Geneva, (19841, 533.

21. B a y l i s s K H , Ph.D. T h e s i s , Aston U n i v e r s i t y , Birmingham, UK.

22. Allen N K , Athwal C S and Latham R V , Vacuum,

2,

( 1 9 8 2 ) , 325.

23. Braun E , e t a l , J. Phys. E: S c i . Instrum.,

g,

(19781, 222.

24. Xu N S , I n t e r n a l Report, Aston U n i v e r s i t y , (1984).

25. Xu N S and Latham R V , submitted f o r publication (1985).

26. Simmons J G , P h y s i c a l Rev. L e t t . , Vol. 17, No. 13, ( 1 9 6 6 ) , 675.

27. Niederman Ph, P r i v a t e Communication, (1984).

28. Yu B Yankelevitch, Vacuum, Vol. 30, No. 3, (1979), 97.

29. T h e i s T N , e t a l , P h y s i c a l Rev. L e t t . , Vol. 52, No. 16, ( 1 9 8 4 ) , 1445.