Electron emission from electroformed carbon films

V~'cuum/vo?ume38/number

1/pages 31 to 35/1988

0042-207X/88S3.00 + .00 Pergamon Journals Ltd

Printed in Great Britain

Electron emission f r o m e l e c t r o f o r m e d carbon films H Araki and T H a n a w a , Electron Beam Laboratory, Faculty of Engineering, Osaka University, 2- 1 yamada-oka,

Suita, Osaka 565, Japan received 10 November 1986

The angular and energy resolved electron spectral data is presented. The spectra have two kinds of peak: a high energy peak (HEP) being nearly independent of the beam deflection angle, and a low energy peak (LEP) shifting towards lower energy with the angle. The HEP is explained by an emission mechanism that field-induced hot electrons are coherently scattered from the metal islands presented in the insulator, and it is inferred that the LEP electrons may be generated by the ionizing collision possibly caused in the insulator.

1. Introduction From discontinuous carbon films, electron emission (EE) and anomalies such as voltage controlled negative resistance (VCNR) have been found ~. They are extremely similar to the characteristics of electroformed metal/insulator/metal structure and it is suggested that the two types of device have an identical microscopic structure. Models to interpret those phenomena were proposed: one postulated the impurity levels in the bands of bulk insulators ~'3 and the other referred to the existence of conducting filaments which could be ruptured due to the Joule heating and regenerated a. The second of these two mechanisms explained most of the experimental data well. Recently, however, the high resistance {OFF) state of the films has been found to arise even under no Joule heating and it is suggested its occurrence is due to a pure electronic process ~. Detailed information on the transit mechanism of electrons in the insulator can be supplied with the EE phenomena. F r o m the electron energy spectral data, it was found that electrons emitted in low voltage range (Vy < 8 V) distributed above the cathode Fermi level 6"s. Such a low voltage anomalous emission is not necessarily related to V C N R , because the anomalous emission was detected even for the devices with a monotonic conduction current characteristic 9. Present study does not concern the low voltage EE. Emission in the high voltage range (V~.>8 V) was inferred to be field emission from metal islands 8. In present films, however, as the typical emission image obtained at a fluorescent screen is not formed as expected from the field emission, but formed in the well-defined edge of an arc with solid segment, it was attempted to obtain some information from the angular resolved electron spectrum.

2.

Experimental

The carbon film device has coplanar structure as inserted in Figure 1. Device preparation was described elsewhere ~. The thickness of the deposited carbon film was 100 nm. After forming, the film had a discontinuous film region of 10 ,am width parallel to

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Figure 1. Measuring system. S 1 and S2-Specimens; A-slit; C-collector; G l-first grid; G z and G3-retarding grids; G.~-shielding grid; F. C.-Faraday cup. The inset is schematic geometry of device structure. the electrodes. Both ends of the region correspond to the virtual electrodes. The electron energy spectrum was measured by a retarding field analyser as shown in Figure 1. The specimen was set on a position S 1 or S z and could be rotated around an axis as shown in the figure. The position S 2 was at the centre of a hemispherical collector C of radius 47 mm and was used to measure the angular dependence of emission currents. The position S~ was at a distance of 6 mm from a circular slit A of radius 0.4 mm and was used for the measurements of the electron energy spectra or for the emission angle dependence. This analyser had an energy resolution of about 0.1%. The collector voltage Vc which was enough to collect the electrons emitted from positions S ~ and $2 was 140 and 1000 V respectively. In the present energy analyses, V,= 140 V was used. When the device was rotated at the position $1, a change in the relative position between the device electrodes and the collector was made. The trajectory of emitted electrons may be only slightly influenced by such a change, since those electrons travel only a short length. The retarding field analyser had four grids (Gt-G,~) provided by gold mesh with a transmission of 80% 31

H Araki and T Hanawa. Electron emission from electroformed carbon films

each. The spacing between them was AG t = 5 mm, G ~G_, = 9 mm, G z G 3 = 2 mm and G 3 G . ~ = 6 m m . G~ and G,~ were biased at 238 V. A retarding field was provided by Gz and G~. The slow dc sweep voltage V, was modulated with a small ac voltage (0.5 V rms, 1.5 kHz}. Transmitted currents were detected with a lock-in amplifier and recorded on an xy recorder. The energy spectrum was measured as a function of the electron deflection angle ~ or of the bias voltage applied on the films V.c, where ~ is defined as the glancing angle to the slit measured from the macroscopic field direction in the films at the emission point. Angular dependence of the currents I~(~1 emitted from the films set on S t is shown in Figure 2. Electron beam flux is thin in a plane perpendicular to the edge of discontinuous film and is long in a plane parallel to it as illustrated in the figure.

3.

Vf=lSV HEP

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Results

The energy spectrum measured at a fixed film voltage V~.= 15 V with ~ as a parameter is shown in Figure 3, in which the Fermi level position of the virtual cathode corresponds to eV, = 4'Au, where 4',~o=4.8 eV is the gold work function. There are two peaks: a high energy peak (HEP) shifting towards lower energy only at a slow rate with = and low energy. peak (LEP) shifting at a fast rate with ~. The LEP level is lowered down to the level of V,= V~-at the highest ~. The half width ofthe H E P slightly increases with ~, while that of the LEP decreases with c~ at ~ > 6 8 ~, i.e. at the LEP distributing in V,> 12.5 V. The H E P intensity is largest at the ~ which gives I,(7) peak and exponentially decreases initially with increasing .~. The H E P level position is at 0.9+0.3 eV below the cathode Fermi level and the high energy tail of the H E P spectrum extends to 1.5-1.6 eV above the cathode Fermi level. This excess energy seems to correspond to the anomalies detected in the low voltage EE 7-9. The lower energy tail of the largest H E P spectrum extends down to I,;= 8.5 V, that is 3.7 eV below the cathode Fermi level. These electrons must gain an energy either higher than or equal to ~b~,, from a bias field, so the potential through which the electron has fallen at the emission point will be greater than 3.7 V + qS~,~/c= 8.5 V. Electrons giving the largest H E P must gain an energy greater than 7.6 eV from a bias field. The energy spectrum for various film voltages V~- is shown in

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Figure 4. In Figure 4(a), the deflection angle ~ was set at the value giving le(~) peak. The LEP is totally absent for various I,). The H E P shifts only a little towards lower energy with increasing I/~,. These peaks have a half width of 2-3 eV increasing with V~..The values of the peak position and the half width are comparable with the data of Blessing et al ~. In Figure 4(b), = was was set at a fixed angle 70 °. The LEP widely shifts towards lower energy with increasing V~-,while the H E P does only a little. The Vc value used in the measurements of I,(:~) as shown in Figure 2 was too small to see the emission angle ~o, because in the free space just outside the intensive emission point the strong electric field nearly parallel to the bias field direction in the films must exist. To deduce ~o, a specimen was set on S: and the angular dependence of the emission currents le(:~) was measured with Vc as a parameter. The angle of le(~) peak approached 90-" with increasing V, as shown in Figure 5 and this tendency was independent of the magnitude of V,~. This means that the electrons with the largest intensity are emitted at approximately right angles to the film surface. The data described in this section were independent of the polarity of the bias voltage. Therefore, nonuniformity of insulator material in the electric field direction will be unimportant in the present data. 4.

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Discussion

It is apparent from Figure 5 that the angular resolved electron energy spectrum includes the effects of the lateral electric field in the free space over the films. If /~, ~o and the kinetic energy of emitted electron are constant, electrons emitted from a point closer to the cathode will be captured by the collector at a lower angle ~. Field emission from the metal islands is hardly caused at emission angles as large as ~o = 90~- It was decided to attend to the

H Araki and T Hanawa. Electron emission from electroformed carbon films HEP

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they gain an energy of more than E i in further transport is where 1 - I = with E > E~ are subjected to either collision per unit travelling distance and l~is the mean free path for the ionizing collision. The number of electrons with E > E~ will decay depending on the factor of e x p { - ( E E~)/eFl} with increasing E. This decay factor is also expressed as e x p { - (x-x~)/l}, where x = E/eF is the travelling distance along the field direction. Intensity of the H E P spectrum as shown in Figure 3 is nearly proportional to the H E P height. If a relation x-xi=K(:z-~) with a constant K holds, the logarithm of the peak height is expected to obey a linear function of ~t. Such a dependence is linear but results in two slopes as shown in Figure 6. As the equipotential lines over the films show, there are two areas where either an accelerating field or a retarding field prevails. Therefore, the K value for the electrons escaping the former area will be larger than that for the latter area. The fact that the H E P shifts towards lower energy with increasing ~t seems to reflect that

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Figure 5. Angular dependence of l~ at 1,'/= 16 V for various collector voltage 1/~setting the specimen on the position S 2. behaviour of electrons in the M I M microstructure which was applied at a high electric field. We postulate that electrons are injected from the virtual cathode into the conduction band of the insulator by tunnelling. In the transporting process, a fraction of injected electrons must be subjected to ionizing collision, since the H E P electrons gained an energy of more than 7.6 eV. The threshold energy for the ionizing collision E~ is equivalent to (3/21E~-~3.7 eV ~°, where Ea is the band gap of the insulator ( -'- 2.5 eV). Injected electrons will attain E~ when they travel the distance x~ = E~/eFalongthe field direction under the electric field F. We assume that injected electrons interact only with optical phonons before they gain the energy E~, then the probability that they gain E~ is p(E~} = e x p ( - EjeFl,)= e x p ( - x~/I,), where l, is the mean free path for optical phonon scattering. The probability that

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H Araki and 1 Hanawa: Electron emission from electroformed carbon films

the electrons travelling through the conduction band undergo more collisions with phonons with increasing x. The phonon scattering process in the conduction band must be a random sequence of many events ~t, hence we can also expect that the slight increase in the H E P halfwidth with ~t (or x) is brought about as a result of the phonon scattering. The LEP spectrum will be discussed next. If the LEP electrons start from the virtual cathode, the LEP half width ought to increase with increasing ct, but in Figure 3, it decreased with ~. Hence, the LEP electrons must start from the sources within the insulator. Two kinds of source are postulated. The first is generated by the ionizing collision and the second is by the presence of the metal islands which may be brought from the metal contacts in the forming process ~'" ~3 The first postulation: conservation of energy as well as m o m e n t u m holds between a primary particle and the three secondary particles ~°. When a primary electron having an energy E~ generates an electron-hole pair, each of the three secondary particles has the energy of E~/9 in the field direction: the energy level difference between the primary and the secondary electron is A E = (8/9)E~. lfa primary electron has E~ ( = 3.75 eV), the energy level position of the secondary electron is A E = 3.3 eV lower than the cathode Fermi level. This electron will be accelerated to the LEP emission point with a certain energy loss AE'(0-0.9 eV), then the highest LEP position will be found at V , = ( d p A ~ + A E + A E ' ) / e = 8 . 1 - 9 . 0 V. This V, value is slightly larger than that for the highest LEP position (Figure 3, ~t=62~). E~ may take a value smaller than (3,/2)E~. If we use E~ equivalent to E~ as is the case of Si, we get V,=7.0-7.9 V coinciding with the highest LEP position. The LEP intensity at the position of a constant V, will exponentially decrease with = as is the case of H E P and result in the background level, because the LEP intensity is not so intensive as the H E P intensity. The LEP electrons generated at a higher energy level may also yield the LEP electrons at a lower energy level by the ionization process shown in Figure 7. In such a process, the LEP intensity will decrease with lowering of the energy level. Free holes produced as a result of the secondary ionization will transit towards the cathode. Some of them will be trapped at localized states and may provide EL centres, and others may travel near the interface and cause space charges to bt.ild up there. Their space charges will produce a narrow high field region in the M - I interface and increase the probability of electrons to tunnel from the cathode into the insulator. Evidence of the strong high I

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Figure 7. A band diagram representation of the emission regime. 34

field region near the cathode has been found ~~. As the electric field increases, the ionizing point will approach the cathode, the space charge density near the interface will increase and an inversion like layer will build up there. Since the holes in this layer are adjacent to the cathode, they may capture the electrons within the metal in the process 1 as shown in Figure 7. Then, in Auger processes 2 and 3, excited electrons will be produced. If the parameters at the M.-I interface are given as E,.- E / = 1.05 eV and E~- - E,. = 1.45 eV, the maximum energy ofelectrons excited in the process 3 is 1.45 eV higher than the cathode Fermi level. This value agrees with the maximum excess energy in the H E P distribution of Figure 3. An Auger neutralization process has been suggested -'.6.r. The second postulation: tunnelling of electrons may take place from the each Fermi level of the metal islands into the insulator conduction band and accelerating to the LEP emission point follows. In this case, the LEP half width will reflect the transit distance, and the LEP intensity will depend on both the transit distance and the active area in tunnelling. The ionization process will be caused also iv. this hypotheses. The dependences of 6 and AE,,, on V.r for the H E P in Figure 4(a) will now be discussed, where we denote the H E P half width by 6 and the energy difference between the cathode Fermi level and the H E P level by AE,,,. The tunnelling process from the cathode to the insulator and the travelling process in the conduction band will be also important for the dependences. The dependences for the former process are given by 6=0.693d and A E r , = d 1~, where d = heE/2(2mqb)~"'-t, dp is the barrier height in tunnelling t " 1J and t is the image force correction factor ( -~ 1). Both of them increase in proportion to F. It is difficult to tind the F v a l u e consistent with the present result from these relations. In the latter process. phonon scattering is important. If the H E P electron energy is assumed to be independent of V~, the relevant relations can be expressed by f i T z x ~ ' ~ F - ,;z and A E ~ , ~ x T z F - t since the energy loss is nearly proportional to x. These dependences are also inconsistent with the present data. Only the combined process can possibly explain them. From Figure 4(b), it is suggested that the emission point of the LEP electrons shifts towards the anode with increasing I,). The value of ~ for the LEP electrons emitted from a given point decreases with increasing Vy. Hence, in order that the LEP electrons may be collected with a constant ~, the LEP emission point must shift towards the anode with increasing Vy. It has been noted that ~0 of the H E P electrons ranged to over 90:. Hot electrons may be deflected from the direction of a macroscopic field in the films according to the following situations: (a) coherent scattering of hot electrons caused at the edge of the polycrystalline metal tilms ~6-~ ~; (b~ focussing of hot electrons to an impurity zone with a high dielectric constant ~9; (c) near-spherical character of the momentum-space distribution of hot electrons 20--z,,. The deflection mechanism favourable to the H E P emission is (a). If hot electrons originate from the impurity fragments, a number of point focussed electron beams should be observed. The H E P spectrum with a broad lower energy tail differs from Maxwellian type as expected in (c). The emission image has a feature similar to that of(a). We assume that the metal islands are essentially polycrystalline, and that hot electrons are diffracted at either A g ( l l l ) or C(002), which are the most intensely diffracting planes. It is shown in the Appendix that hot electrons can eject from the films only when they arrive at the metal islands with a gain higher than eV~,,, which is estimated to be 8.0 eV for Ag(l I 1 ) and 6.6 eV for C(002), and that the emission

H .4raki andoT Hanawa. Electron emission from electroformed carbon films

angle at the eV~, is equal to % = 9 0 ~. T h e eV~, for Ag(l I 1) agrees well with the gain (>_7.6eV) for the largest H E P . T h e H E P electrons gained an e n e r g y h i g h e r t h a n the eV~, w h i c h will be e m i t t e d at an angle =o lower t h a n 90 ~'. H o w e v e r , as their e m i s s i o n points shift t o w a r d s the a n o d e , they will be collected at a r a t h e r higher angle :~, t h a n that for the t h r e s h o l d electrons. T h e relation of :% vs V~ in the A p p e n d i x is a p p l i c a b l e also to the metal islands which are p r e s e n t within the i n s u l a t o r surface. T h e L E P e l e c t r o n s at V , = 15 V o f Figure 3 can only get an energy nearly equal to the i n s u l a t o r w o r k function, w h i c h is lower than the eV~,. therefore they d o n o t e x p e r i e n c e the diffraction process (a). This s t a t e m e n t applies to the L E P e l e c t r o n s in the range of I / , > 4 L ~ , ', , . e + ( V ; - V ~ , )--- I I . S V . In this range, the process (cl may be d o m i n a n t .

5.

Summary and remarks

(l) T h e EE processes in the high voltage range (V.f>_ 11 V) include the hot electron g e n e r a t i o n process t h r o u g h the i n s u l a t o r conduction band. (2) Electrons injected from the c a t h o d e into the i n s u l a t o r form the H E P s p e c t r u m . E l e c t r o n s g e n e r a t e d by the ionizing collision in the i n s u l a t o r m a y p a r t i c i p a t e in the L E P s p e c t r u m . (3) T h e excited e l e c t r o n s g e n e r a t e d by A u g e r p r o c e s s in the c a t h o d e metal m a y p r o v i d e the a n o m a l o u s e l e c t r o n s w h i c h are d i s t r i b u t e d in the H E P . (4) Large deflection of h o t e l e c t r o n s from the m a c r o s c o p i c field direction to surface n o r m a l m a y take place at the silver islands in the i n s u l a t o r by the c o h e r e n t scattering process. If hot electrons e n c o u n t e r a metal island, a large fraction o f them will be a b s o r b e d in it. H e n c e , the presence o f n u m e r o u s metal islands m a y c o n t r o l ionizing collision a n d s u p p r e s s avalanche breakdown.

;" J G Simmons and R R Verderber, Appl Phys Lett, 10, 197 (1967). ~~ J G Simmons. R R Verderber, J Lytollis and R Lomax, Phys. Rev. Lett, 26, 655 (1966). a~ N S Xu and R V Latham, J Phys D: appl Phys, 19, 477 (1986). ~o j p Vigouroux, J P Duraud, C Le Gressus, G Petite, P Agostini and C Boiziau, Scanning Electron Microscopy, I, 179 (1985). 2o C Bulucea, Solid State Electron. 18, 363 (1975). 2~ G G P Van Gorkom and A M E Hoeberechts, J appl Phys, 51, 3780 (1980). -'-' R V Bellau and A E Widdowson, J Phys D: Appl Phys, 5, 656 (1972).

Appendix We postulate that electrons, which are injected from the cathode into the conduction band of the insulator by tunnelling and are then accelerated towards the anode, arrive at a metal island with the energy equal to eVb+ q, where Vo is the potential drop across the region between the cathode and the metal site and r/is the Fermi energy of the metal. If those electrons suffer coherent scattering at a diffraction plane (h,k,l) of the polycrystalline metal, the diffraction angle 20 is given by sin0= h,"2d~{2m(e V~+ q)} ;'~-,where dhk~is the spacing of adjacent planes 1~'1 ~'. If the scattered electrons are emitted from the metal surface which is parallel to the electric field (Figure 8), the deflection angle "0 of the emitted electrons can be expressed by /eVn+~\t,2 f

cos:xo=lkeV~_~ }

l1

h2

4mdh.t.~-Va+r/i },

,1,

where ~bis the barrier height at the interface. Only electrons arriving at the surface satisfy the condition I > cos(n/2 - 20) > {(~b+ ~/),/(eV~,+ r/)};/2 can escape over the barrier. From this, we obtain h eV~>~-~.

(2)

~ 4md~

The spacing of the most intensively diffracting plane is d ~ t = 2.36 A for silver and do0 ., = 3.37 .h, for graphite. % is shown in Figure 8 as a function of eV~,, in which ~t=5.48eV, q~=4.5eV for silver and ~=0.022eV, ~b= 4.0 eV for graphite were used. Threshold value of Vb is V~,= 8.0 V for Agfl 11 ) and I/0,= 6.6 V for C(002).

References

; H Araki, O Hirabaru and T Hanawa, Shinku, 26, 22 (1983). 2 T W Hickmott, J appl Phys 35, 2118 (1964); 35, 2679 (1964). 3 J G Simmons and R R Verderber, Proc Roy Soc, A, 301, 77 (1967). •t G Dearnaley, A M Stoneham and D V Morgan, Rep Prog Phys,33, 1129 (1970). 5 H Araki and T Hanawa, Thin Solid Films, 121, 17 (1984). 6 T W Hickmott, J appl Phys. 36, 1885 (1965). ; T W Hickmott, Thin Solid Films, 9, 431 (1972). 8 R Blessing and H Pagnia, Phys Star Sol, (b) 110, 537 (1982). 9 R Grakh, Radio En.q Electron Phys, 22, 83 (1977). ~o j L Moll, Physics of Semiconductor~. McGraw-Hill, New York (1964). ~1 R M Handy, Jappl Ph.vs. 37, 4620 (1966). ~ H Araki and T Hanawa, submitted to Thin Solid Films. ~~ H Araki and T Hanawa, submitted to Thin Solid Films. a'~ S A Nepijko and V I Styopkin, Poverkhnost'.fiz. Khim. mekh, No. i, 51 (1983). ~5 R D Young, Phys Rev, 113, 110 (1959).

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35