II. THERMAL AND PHOTO-ASSISTED HOT-ELECTRON EMISSION FROM MIM MICRO-STRUCTURES

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II. THERMAL AND PHOTO-ASSISTED HOT-ELECTRON EMISSION FROM MIM

MICRO-STRUCTURES

N. Xu, R. Latham

To cite this version:

N. Xu, R. Latham. II. THERMAL AND PHOTO-ASSISTED HOT-ELECTRON EMISSION FROM MIM MICRO-STRUCTURES. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-73-C2-77.

�10.1051/jphyscol:1986211�. �jpa-00225642�

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STRUCTURES

N . S . XU and R . V . LATHAM

D e p a r t m e n t

of

M a t h e m a t i c s

and

P h y s i c s ,

University of

A s t o n , GB-Birmingham B4 7 E T , G r e a t B r i t a i n

Abstract - A high resolution electron spectrometer has been used to demon- strate experimentally that both thermal (T varying from -300-700 K) and photo

(A-350nm) stimulation can enhance the field-induced hot-electron emission from a metal-insulator-metal (MIM) microstructure. A detailed analysis of electron energy spectra has provided relationships between temperature and such emission parameters as current, spectral half-width and spectral shift.

These studies have also provided valuable information about electron trans- port in the MIM structure and the electron energy distribution in the metal- insulator interface in immediate proximity to the vacuum.

1 , INTRODUCTION

In an accompanying paper [ I ] , detalls were given of an investigation into the room temperature properties of the naturally occurring field-induced electron emission sites found on broad area high voltage electrodes. Emission image and spatially resolved electron spectral data were shown to be consistent with a field-induced hot- electron emission (FIHEE) mechanism occurring at metal-insulator-metal (MIM) micro- structures, in which the emitted electrons were coherently scattered from the top metal electrode. In this paper, we report on further studies of this type of emission regime in which the effects of temperature and external photon stimulation are investigated. It will be shown that the e:ectron spectral response to these two parameters under constant field conditions is in qualitative agreement with the predictions of the above MIM emission model.

2, EXPERIMENTAL SYSTEMS, PROCEDURES AND FINDINGS

The measurements to be described below were made in the same basic electron spectrometer facility described in the accompanying paper [ I ] and elsewhere [ Z ] . Use was also made of a heated specimen stage [3] with an associated thermocouple

(see Flgure 1 ) which provided the capability of recording emission spectra in the temperature range 30C-700 K. In addition, a simple a-c bridge circuit was used to monitor the capacitance and hence the separation of the planar high voltage gap,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986211

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JOURNAL

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PHYSIQUE

H a t e r

Insulating Spherical

mlrror

shield

F i g u r e 1 The'experimental set-up f o r h e a t i n g a specimen and f o c u s i n g UV r a d i a t i o n on t o t h e cathode e l e c t r o d e .

2 - 1 , Temperature s t i m u l a t e d emission

I n o r d e r t o s t u d y t h e temperature-dependence of s p e c t r a under c o n s t a n t f i e l d c o n d i t i o n s , it i s f i r s t n e c e s s a r y t o f o l l o w t h e room-temperature procedure d e s c r i b e d i n t h e accompanying paper f o r l o c a t i n g a chosen s i t e on-axis o p p o s i t e t h e anode probe h o l e . The specimen i s now slowly h e a t e d t o t h e maximum r e q u i r e d temperature of

-

⁷⁰⁰^Kwith s u f f i c i e n t time b e i n g allowed f o r out-gassing and t h e chamber p r e s - s u r e t o r e t u r n t o

<

10-lombar. Having s e t t h e gap t o i t s r e q u i r e d v a l u e ( u s u a l l y 0.5 mm), t h e f i e l d i s slowly a p p l i e d u n t i l t h e emission image of t h e a x i a l l y l o c a t e d

EL.

w

0.2 e V l D i v

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Figure 2 S p e c t r a r e c o r d e d a t t e m p e r a t u r e s of 420 K and 630 K a t a f i e l d of 12.4 I4V.m-1.

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and hence t h e emission i n c r e a s e s with temperature. T h i s e f f e c t i s f u r t h e r i l l u s t r - a t e d i n F i g u r e 3 ( a ) , which p r e s e n t s t h e temperature dependence of t h e emission cur- r e n t under c o n s t a n t f i e l d c o n d i t i o n s , a s measured d i r e c t l y from t h e complete s p e c t r a l sequence [31, Secondly, t h e s p e c t r a of Figure 2 a l s o show t h a t tk:ere i s an appctrent s h i f t i n t h e peak towards t h e Fermi l e v e l of t h e m e t a l l i c s u b s t r a t e a s t h e temperat- u r e i s i n c r e a s e d . L a s t l y , it i s e v i d e n t t h a t t h e shape of t h e s p e c t r a v a r i e s with i n c r e a s i n g temperature; i n p a r t i c u l a r , t h e s p e c t r a l FWHM i s broadened and t h e s p e c t r a become more symmetrical. A s p r e v i o u s l y , t h e complete s p e c t r a l sequence h a s been used t o compile t h e p l o t s of F i g u r e 3 ( b ) which shows t h e temperature-dependence of t h e s p e c t r a l s h i f t and FWHM. From t h e s e p l o t s , it should be noted t h a t t h e s h i f t and FWHM do n o t change i n t h e temperature range -300 K

-

-400 K.

3W 400 5M bal 703 Temperature I K I

F i g u r e 3 ( a ) Temperature-dependence of an emission s i t e c u r r e n t a t a c o n s t a n t f i e l d of 12.4

~ v . m - l .

( b ) Temperature-dependence of t h e s p e c t r a l s h i f t AEs, and half-width FWHM, a t a c o n s t a n t f i e l d of 12.4 M V . ~ - ~ .

2.2. Photon-stimulated emission

To i n v e s t i g a t e t h e e f f e c t s of photon s t i m u l a t i o n on t h e emission mechanism, t h e modified experimental set-up shown i n F i g u r e 1 was employed. Here, o p t i c a l photons from a mercury d i s c h a r g e lamp e n t e r t h e chamber through a s p e c i a l q u a r t z window

( t r a n s p a r e n t t o UV) and a r e d i r e c t e d towards a concave aluminium m i r r o r ( r e p l a c i n g t h e p r e v i o u s phosphor s c r e e n ) which f o c u s e s them o n t o t h e cathode s u r f a c e through t h e 0.5 mm diameter anode probe h o l e . A s p r e v i o u s l y , t h e experimental procedure f i r s t l y i n v o l v e s using t h e specimen scanning system t o l o c a t e an emission s i t e op- p o s i t e t h e probe h o l e , and t h e n , by microadjustments of t h e specimen p o s i t i o n , t o

" b l i n d l y " l o c a t e a r e g i o n of t h e emission image over t h e probe h o l e i n t h e aluminium m i r r o r t h a t g i v e s a s t a b l e single-peak spectrum. Having recorded t h i s on a s t o r a g e o s c i l l o s c o p e under "dark" c o n d i t i o n s , t h e q u a r t z window i s t h e n opened t o W r a d i a - t i o n and t h e spectrum a g a i n recorded under i d e n t i c a l f i e l d c o n d i t i o n s , Thus, F i g u r e 4 p r e s e n t s t y p i c a l s p e c t r a which were measured r e s p e c t i v e l y with and without u l t r a - v i o l e t r a d i a t i o n under a c o n s t a n t f i e l d of 11 MV.m-l. From i n s p e c t i o n of t h i s F i g u r e , it i s e v i d e n t t h a t t h e emission c u r r e n t h a s been s i g n i f i c a n t l y i n c r e a s e d by t h e UV r a d i a t i o n .

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JOURNAL

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F i g u r e 4 S p e c t r a recorded with and without UV r a d i a t i o n a t a c o n s t a n t f i e l d of 11 w . m - l .

3. DISCUSSION

A s d i s c u s s e d i n an accompanying paper [ I ] and elsewhere [ 4 , 5 ]

,

it i s assumed t h a t t h e emission c u r r e n t o r i g i n a t e s from a s i t e a s s o c i a t e d w i t h a m e t a l - i n s u l a t o r - metal m i c r o s t r u c t u r e , a t which a f i e l d - l n d u c e d h o t e l e c t r o n emission mechanism

o p e r a t e s . According t o t h i s model, e l e c t r o n s f i r s t l y 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 medium. They a r e t h e n a c c e l e r a t e d toward t h e t o p m e t a l l i c l a y e r , and subsequently e m i t t e d i n t o t h e vacuum by a c o h e r e n t s c a t - t e r i n g p r o c e s s . T h i s same m o d e l w i l l now be used t o e x p l a i n t h e experimental r e s u l t s d e s c r i b e d i n t h i s paper.

Considering f i r s t t h e thermal s t i m u l a t i o n of t h e emission c u r r e n t , it i s ap- p a r e n t t h a t a t a c o n s t a n t f i e l d , more e l e c t r o n s w i l l have t h e p o s s i b i l i t y of being e m i t t e d through t h e p o t e n t i a l b a r r i e r s a t both t h e M I i n t e r f a c e between t h e metal s u b s t r a t e and t h e i n s u l a t o r and t h e MV i n t e r f a c e between t h e t o p metal e l e c t r o d e and t h e vacuum ( i . e . due t o Richardson-Schottky e f f e c t ) when t h e specimen i s h e a t e d above room temperature. A s a f u r t h e r consequence of t h e i n c r e a s e of temperature, t h e c a r - r i e r c o n c e n t r a t i o n i n t h e i n s u l a t o r w i l l i n c r e a s e s i n c e more e l e c t r o n s w i l l a c q u i r e t h e n e c e s s a r y energy t o escape from t h e t r a p c e n t r e s i n t o t h e conduction band. T h i s w i l l r e s u l t i n a d e c r e a s e i n t h e v o l t a g e drop a c r o s s t h e i n s u l a t o r , and a modific- a t i o n of t h e M I b a r r i e r u n t i l an e q u i l i b r i u m s t a t e i s achieved, The e x t e r n a l r e s u l t of t h e s e a d j u s t m e n t s w i l l be an i n c r e a s e i n t h e emission c u r r e n t , i . e . a s i s found e x p e r i m e n t a l l y .

These same p r o c e s s e s w i l l a l s o l e a d t o t h e o t h e r v a r i a t i o n s i n t h e e l e c t r o n en- e r g y s p e c t r a shown i n F i g u r e 2. F i r s t l y , t h e peak of a spectrum w i l l b e s h i f t e d t o - wards t h e Fermi l e v e l a s a r e s u l t of t h e d e c r e a s e i n voltage-drop a c r o s s t h e i n s u l a t - o r . I t w i l l however be noted t h a t i n t h e range of temperature from 300 K t o about 400 K , t h e s p e c t r a shows no obvious s h i f t ; i , e . a p p a r e n t l y i n d i c a t i n g t h a t t h e r e a r e no t r a p c e n t r e s i n t h e energy r e g i o n between t h e bottom of conduction band and 0.1 eV below it. Secondly, t h e spectrum w i l l become more symmetrical and t h e FWHM broadened a s t h e cathode temperature i n c r e a s e s t h e d e n s i t y of s t a t e s f u n c t i o n o p p o s i t e t h e t o p of t h e vacuum p o t e n t i a l b a r r i e r , i . e . a s o c c u r s i n c o n v e n t i o n a l thermionic emission.

We s h a l l now c o n s i d e r t h r e e p o s s i b l e p r o c e s s e s by which photons, under c o n s t a n t f i e l d c o n d i t i o n s , could s t i m u l a t e t h e b a s i c emission mechanism. ( a ) I f t h e photons only p e n e t r a t e a s h o r t d i s t a n c e i n t o t h e t o p metal l a y e r , t h e y may be absorbed by e l e c t r o n s which w i l l t h e n have enough energy t o surmount t h e s u r f a c e p o t e n t i a l bar- r i e r and s o enhance t h e emission. I t f o l l o w s t h a t t h i s i n t e r a c t i o n w i l l t e n d t o charge t h e t o p metal e l e c t r o d e p o s i t i v e l y , t h e r e b y i n c r e a s i n g t h e f i e l d a c r o s s t h e i n s u l a t o r and hence t h e f l u x of e l e c t r o n s t u n n e l l i n g from t h e metal s u b s t r a t e . ( b ) I f t h e photons c a n p e n e t r a t e t o t h e bulk i n s u l a t o r r e g i o n , t h e y may s t i m u l a t e t h e e l e c t r o n s from t r a p c e n t r e s . However, u n l e s s t h i s p r o c e s s o c c u r s i n t h e v i c i n i t y of t h e conduction channel, it i s u n l i k e l y t h a t s t i m u l a t e d e l e c t r o n s w i l l s i g n i f i c a n t l y i n f l u e n c e t h e emission c u r r e n t . ( c ) I f t h e photons can p e n e t r a t e through t h e i n s -

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t e d from s i t e s on broad-area high v o l t a g e e l e c t r o d e s v a r i e s with t e m p e r a t u r e , have been shown t o be i n accordance with 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 o p e r a t i n g a t M I M m i c r o s t r u c t u r e s . T h i s same model h a s a l s o been shown t o p r o v i d e a q u a l i t a t i v e e x p l a n a t i o n of how t h e emission may be s t i m u l a t e d by U.V. photons. The f i n d i n g s have f u r t h e r indicated t h a t t h e r e a r e a s i g n i f i c a n t number of t r a p c e n t r e s e x i s t i n g i n t h e energy band gap of t h e i n s u l a t i n g medium, and t h a t t h e s e p l a y an important r o l e i n t h e emission p r o c e s s .

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 supporting hiw. while c a r r y i n g o u t t h i s work.

REFERENCES

1. N S Xu and R V Latham, Proc. 32nd IFES, P i t t s b u r g h (USA), (1985).

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

35,

( 1 9 8 5 ) , 211.

3. K H B a y l i s s , Ph.D. T h e s i s , Aston U n i v e r s i t y , Birmingham, UK, (1984).

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

,

( 1983)

.

5. R V Latham, 2nd Workshop on RF S u p e r c o n d u c t i v i t y , CERN ( ~ e n e v a ) , ( 1 9 8 4 ) , P533.