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EFFECT OF LACOMIT FILMS ON COLD-CATHODE HOT-ELECTRON EMISSION
M. Mousa
To cite this version:
M. Mousa. EFFECT OF LACOMIT FILMS ON COLD-CATHODE HOT-ELECTRON EMISSION.
Journal de Physique Colloques, 1988, 49 (C6), pp.C6-237-C6-242. �10.1051/jphyscol:1988640�. �jpa-
00228137�
EFFECT OF LACOMIT FILMS ON COLD-CATHODE HOT-ELECTRON EMISSION
M.S. MOUSA
Department of Natural Sciences, PO Box 7 , Mu'tah University, Al-Karak.
Jordan
Abstract
The effect of lacomit dielectric films on the emission properties of microemitters has been studied
.
It is observed that such films enhance field electron emission from metal-insulator-vacuum (M-I-V) microstructures. It is found that this cathode switches- on at low applied fields and acquires a subsequent reversible I-V characteristics and reasonably stable emission currents. Energy distribution measurements performed with a retarding potential energy analyser and with a high resolution electrostatic deflection electron energy analyser show that the distribution is broadened and shifted significantly to lower energy.These characteristics have been qualitatively explained in terms of a hot-electron emission mechanism.
1. Introduction
The use of electron emitting cathode devices operating in the field emission mode has seen growing interest during the past decade,more than electron emitters operatingin thermal mode
.
A primary advantage that can be obtained by operating in the field mode[l,2] is the absence of a heater which simplifies the tube technology and allows instant operation, i.e.no warming up period is required
.
The technological advantages of field emission elec- tron sources,and its superior performance to the thermionic electron sources are well known[3].Original experimental studies of field emitter materials consisted of tungsten [4,51, lan- thanum hexaboride [6,7] and carbon fibres [8,91.
Recent experimental work [ID-131 has shown rising interest in the development of compo- site field emitters. It is well established [14,15] that some dielectric materials such as carbon, in certain forms, e.g. graphite, can promote the field emission of electrons at low fields (<20 ~ ~ m - l ) when deposited on the surface of a planar electrode [16,17]. Such compos- ite electrodes which consist of metal-insulator (M-I) or metal-insulator-metal (M-I-M) microstructures, promotes electron emission by a1'non-metallic" field-induced hot-electron emission (FIHEE) mechanism [17,18]. Latham [19], and Latham et a1 L201 proposed that this mechanism is responsible for the naturally occuring process that gives rise to localised electron emission from broad-area vacuunl-insulated high voltage electrodes. Experimental studies by Niedermann et a1 L211 have similarly shown that the emission of electrons originate
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988640
C6-238 JOURNAL DE PHYSIQUE
from isolated microstructures on cathode surfaces.
In a recent simulation study [1,22], M-I type hot-electron emission was carried out by prod- ucing and testing composite microemitters which consist of resin-coated electrolytically etched tungsten microemitters. This study showed that sub-micron thick dielectric layers can promote en- hanced electron emission. In this paper the influence of sub-micron layer of lacomit dielectric films on the emission properties of the microemitters has been investigated. The experimental data obtained is compared with resin-coated tungsten microemitters [l]. Analysis of the experimental results suggest that the emission is derived from a FIHEE mechanism associated with M-I-V micro- structures.
2. Experimental Work
Measurements on the emission characteristics of the M-I systems were conducted using two ultra high vacuum systems. The first one is a retarding potential analyser that incorporates an automatic electronic differentiation capability to obtain an energy spectrum output. The anode of the analy- ser has a phosphored screen for recording emission images. Moreover, it provides the possibility of measuring emission current versus applied voltage. The second system is a high resolution elec- trostatic deflection electron energy spectrometer. to obtain the total energy spectra. In this spectrometer, electrons passing through a probe hole are focused by an interfacing elevtrostatic lens into the entrance aperture of the input lens of an energy analyser and are counted by an elec- tron multiplier. The resulting signal is amplified and displayed on a storage oscilloscope before being photograhped by a polaroid camera.
Microstructures are fabricated using tungsten tips that are electrochemically etched from a fine wire that have an
-
0.1 mm diameter in a 2 M solution of sodium hydroxide. These tips were coated with lacomit which is a petrolium product having good insulating properties and used by electron microscopists to isolate their specimens from ground. This has been done by dipping the tungsten tip into the lacomit solution then removing it slowly thus forming a film of thickness,4 0.1 pm (thin) or 0.15
-
0.5 pm (medium) using electron microscopy techniques. The experiments in this investigation were carried out under background pressure of about 10-l0 mbar. The pressure was maintained at about 10-* mbar during testing.The first step in taking data from freshly prepared "virgin" composite microemitters is to slowly increase the applied voltage, V, until a switch-on process is initiated. This voltage VSW , will cause an instantaneous rise in the emission current from an effective zero-value to a saturated value ISAT
.
The saturation current, ISAT, then extends down from VSW to a lower voltage VSAT.
For voltages lower than VSAT, the emission current falls smoothly to zero as the value for the applied voltage is reduced to a threshold value VTH
.
Subsequent recycling through the same voltage range will show a smooth dependence of current on voltage but with a significant built-in hysteres~s between the increasing and decreasing values.3. Results
3.1 Current-Voltage Characteristics
the I-V plots of eight similarly prepared tips having thin and medium thicknesses of lacomit coatings.
The emitters were heat treated between 100°C and 240°C
.
Thus, from noting the data in Table 1 it- is possible to note that : for low temperature baking (i.e. 4 1 4 0 'C ) the VSW , VSAT and VTH have typically higher values (-twice) and lower ISAT values than those of the emitters baked at higher temperatures ( i.e. 200 or 240°C).
The threshold voltage can be as low as one fifth of VSW.
Figure 1 illustrates the typical I-V characteristics of a tungsten micropoint emitter coated with a layer of lacomit that have thickness(0.5 p. It is characterised by a high conventional switch-on voltage VSWl of 2300 volts, giving a saturation current, ISAT , value of 4(nfl. On lowering the voltage, ISATextends down to 1100 volts then falls smoothly to zero as the voltage is decreased to a threshold value VTH of
-
100 volts. On recycling the voltage, switch-on is again observed, although at lower voltage VSW2 =l100 volts, producing same value of ISAT.
But, in this case there was no extended saturation region, rather the emission current showed a smooth decrease with applied voltage. on recycling the voltage again, the current increases gradually (doted curve in Figure l) with applied voltage before it switches-on again at VSW = 1100 volts and ISAT to 4 pA.Then the current will also follows the same (single arrowed) curve of Figure 1 down to zero value.
After this, the emitter characteristic will follow the double-arrowed broken curves exhibiting a hysteres'is effect.
Figure 2 illustrates a comparison between the F-N plots of the uncoated tungsten microemitter (curve A) and that obtained from the low-field region I-V data of the coated emitter (curve B).
From table 1 it can be seen that the lacomit insulating coating results in a decrease in the slope of these F-N plots from mW = 9782 to mu+,- = 3215 , i.e. by a factor of 3 to 1. The decrease is mainly due to the decrease in the work function due to presence of the insulator. The high field region of the I-V characteristic departs linearity in F-N plot of Figure 2.
Volt 1
Fig. 1 The I-V characteristics of tungsten Fig. 2 Fowler-Nordheim plots of the uncoated microemitters coated with insulating (curve A) and lacomit coated (curve B)
lacomit
.
microemitter.
C6-240 JOURNAL DE PHYSIQUE
3.2 Emission Image Characteristics
The basic concept behind this type of experiment is to use a phosphored anode screen that is ,capable of recording the spatial distribution of the impinging electrons
.
Figure 3(A) presents a typical emission image of a tungsten microemitter coated with a layer of lacomit coating that have a thickness less than 0.5 (nm
.
This image was photographed from the phosphored screen of the retarding potential analyser, that have 1 mm diameter probe hole. The image is seen to be characterised by a random distribution of sub-emission centres. The sub- structure of this image appeared to flicker. The flickering is caused by individual sub-emission centres randomly switching on and off.
Another important characteristic of these images is that the number of spots is field dependent. As shown from the sequence of images of Figure 3 , when the applied voltage is reduced from 2000 volts ( Figure 3 A ) to 1100 volts ( Figure 3 C ) the number of sub-emission centres as well as the image spot size was found to decrease.
Fig.3 A sequence of emission m g e s of tungsten microemitters coated with lacomit obtained at applied voltage V ;
( A ) V = 2000 volts ( B ) V = 1600 volts ( C ) V = 1100 volts
.
3.3 Electron Spectral Characteristics
The measurement of energy distribution was carried out fy use of the high resolution elec- trostatic deflection electron energy analyser. Figure 4 shows a typical spectrum of the coated emitter that was found to be : (a) displaced towards lower energies b y - 0.75 eV with respect to Fermi level (FL) of the metallic cathode and (b) has a significantly larger FWHM ; i.e. approximat- ely equal to 0.75 eV and (c) is more symmetrical than the tungsten spectrum
.
The single peaked spectrum shown in Figure 4 was obtained from one of the bright spots that composes the multi-spot emission image shown in Figure 3
.
The electron energy spectra provides valuable information about the electron emission mechanism associated with the composite micropoint emitters.
3.4 The E f f e c t s of i n Situ, UHV Baking
The influence of in s i t u thermal processing of t h e tungsten microemitters coated with a l a y e r of lacomit is s i g n i f i c a n t . I n Table 1 it is c l e a r l y i l l u s t r a t e d t h a t baking a t low temper-
atures, i.e. 100°C or 140°C did not produce a significant effect on the emission properties. When the emitter was baked to high temperatures , e.g. 240 OC, the following general trends are noticed:
(a) performance is not significantly distinguished compared to that of the emitters baked to 200°C,
but is distinguished from those emitters baked to temperatures below 140°C , (b) Emission image and current appeared to be more stable, (c) the value of VSW i s markedly decreased, and (d) the 'SAT is increased
.
However, a bakeout at-
350°C destroyed the switching characteristics of the composite microemitter.
This is apparently due to the change in the conduction properties of the conducting channel that will be formed in the insulator when the switch-on process occurs.4. Discussion
The new tupe of composite microemitters reported here had the following effects : (a) a switch-on mechanism ; (b) after switching , the saturated current extends over a wide voltage range;
(c) a reversible I-V characteristic with a hysteresis effect; (d) linear F-N plots in the low- field region; (e) the emission image is characterised by random distribution of sub-emi~ssion centres; (f) the number of spots is field dependent; (g) electron energy spectra was found to be shifted towards lower energies with respect to FL of the underlying tungsten field emitter.
From a comparison of t h e s e emission p r o p e r t i e s with those obtained elsewhere [1,20,22], it can be cocluded from t h e present i n v e s t i g a t i o n t h a t c o a t i n g a m e t a l l i c microemitter with sub-micron t h i c k l a y e r of i n s u l a t o r promotes field-induced hot-electron emission. Clear s i m i l a r i t i e s b e t w e n t h e emission inages and e l e c t r o n s p e c t r a l d a t a obtained here, with those recorded from o t h e r M-I [18,23] and M-I-M 1171 structures, seem to be consistent with the belief that the emission process is a hot-electron emission process. This model [1,19,20] assumes the formation of a narrow cond- uction channel in the dielectric layer. The electrons tunnel from the metal substrate into the conduction band of the dielectric medium and are heated by an avalanche process to an energy that enables them to be emitted into the vacuum quasi thermionically. According to this model, the
C6-242 JOURNAL DE PHYSIQUE
saturated emission c u r r e n t observed w i t h t h e present composite emitters, except f o r those baked a t 240°C , i s a m a n i f e s t a t i o n o f b u l k - l i m i t e d conduction.
This ccanposite micropoint emitter may provide a base for f u r t h e r i n v e s t i g a t i o n t o develop t h i s e m i t t e r a s a p o t e n t i a l e l e c t r o n source. A min problem is t o get rid of t h e multi-spot emission image and t o produce a s t a b l e s i n g l e spot.
The experimsntal results presented here have indicated t h a t composite M-I microstructures made w i t h l a c o m i t coatings agrees w i t h t h e concept o f a FIHEE mechanism. The new type o f composite e m i t t e r i s a p o t e n t i a l e l e c t r o n source.
I n s i t u UHV thermal t r e a t m n t of microemitters h a s been found t o inprove their p e r f o m c e . Table 1 mcission d a t a obtained from tungsten emitters coated with d i e l e c t r i c laccanit.
I n s u l a t o ~ thickness Baking temp. of v S
emitter ( *C)
vEt
Thin 100 2400 1 2100 1900 1200 9863 3039
-
140 2300 1 2100 1800 1100 10795 3560- -
200 1300 3 1200 800 300 10218 3464- -
240 1100 4 0000 650 100 9782 3215M i u m 100 4100 2 3800 2500 1200 9436 3084
-
140 3800 2 3600 2400 900 10180 3288-
200 2500 7 2000 1300 200 10137 3432- -
240 2400 9 1900 1200 75 9916 3341i. R V Latham and M S busa, J.Phys.D : Appl.Phys.
19,
699, 1986.2. D Stevmrt ,P D Wilson, R V Latham and N K Allen, J . Materials Sci.,
16,
111,1981.3. L W Swanson and A E B e l l , Adv. Electron., 2 , 193 , 1973.
4. A V Crem, M Isacson and L, Johnson, Rev. Sci. I n s t m . , 2, 241,1969.
5 . E G Shirokov, Sov. Phys.-Tech. Phys., 2, 1134,1970.
6. F H Plomp, Proc. 5 t h European Congress on e l e c t r o n microscopy, 2 , 1972.
7. T Mulvey, J. de Phys., G , C2, 149,1984.
8. C Lea , J.Phys. D : Appl. Phys., 6 , 1105, 1973.
9. R V Latharn and M A %dim , J. ~ h 5 . E : Sci. Instnnn. , 20 , 181 , 1987.
10.N A Cade, G H Cross, R A Lee, S Bajic and R V Latham, J.Phys. D : Appl. Phys.,
1,
148,1988.11.N S Xu and R V Latham , J. de Phys. , 3 , C2 , 67 , 1986.
12.M S Mousa , Vacuum , in p r e s s , 1988.
13.M S busa and R V Latham , J. de Phys. , 47 , C2 , 139 ,1986.
14.S B a j i c and R V Latham , J. Phys. D : Appl. Phys. ,
21,
200 ,1988.15.M.S.Mousa , J. de Phys., 48 , C6 , 109 , 1987.
16.C S Athwal, K H Bayliss, R Calder and R V Latham, IEEE Trans. Plasma Sci.,PS-G, 225,1985.
17.N S Xu and R V Latham, J. Phys. D : Appl. Phys.,
19,
477 ,1986.1 8 . K H Bayliss and R V Latham , Vacum ,
35,
6 , 211 ,1985.19.R V Latham , Vacuum , 2, 3 , 137 ,1982.
20.K H Bayliss and R V Latham , Proc. R. Soc. Lond.
m,
285, 1986.21.Ph Niedenmnn, N Sankarranan, R J Noer and 0 Fischer , J. Appl. Phys. , 59, 892 ,1986.
22.M S busa , J . de Phys. , 48 , C6 , 115 1987
.
23.C S Athwal and R V Latham T ~ h y s i c a
