Field emission characteristics of nanostructured thin film carbon materials

Field emission characteristics of nanostructured thin film carbon materials

Applied Surface Science 215 (2003) 214–221 Field emission characteristics of nanostructured thin film carbon materials A.N. Obraztsova,*, A.P. Volkov...

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Applied Surface Science 215 (2003) 214–221

Field emission characteristics of nanostructured thin film carbon materials A.N. Obraztsova,*, A.P. Volkova, Al.A. Zakhidova, D.A. Lyashenkoa, Yu.V. Petrushenkob, O.P. Satanovskayab a

Department of Physics, Moscow State University, Leninskie gory, 1-2, Moscow 119992, Russia b SRPC ‘‘Platan’’, Fryasino 141120, Russia Received 15 November 2002; accepted 17 January 2003

Abstract Nanostructured carbon (nC) thin film materials were obtained by chemical vapor deposition (CVD) in dc discharge activated hydrogen–methane gas mixture. Film structure, surface morphology and phase composition was studied by Raman, electron microscopy and electron spectroscopy methods. A highly efficient cold electron emission was found for the films composed from graphite-like nano-structures including carbon nanotubes (CNT) and graphite nano-crystallites. Electron emission tests and analysis exhibit non-classical behavior of the nano-carbon cold cathodes. The statistical analysis of the cold emission show normal distribution of parameters of separate emission centers with narrow standard deviation. The possible mechanism of the cold emission is discussed. The highest efficiency of the nano structured carbon cathodes is demonstrated in the prototypes of vacuum cathodoluminescent lighting devices. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Field emission; Cathodoluminescence; Carbon nanotubes; CVD

1. Introduction Numerous experimental studies have shown ‘‘unusually’’ low-threshold field emission (FE) of electrons from various carbon materials. Two different approaches have been proposed to explain this phenomenon for carbon with ‘‘diamond-like’’ [1] and ‘‘graphitelike’’ [2] structures and electronic properties. However, the models, based on these approaches do not allow explanation of all experimentallyobserveddetailsforFE and accompanying effects. Some of these observa*

Corresponding author. Tel.: þ7-095-9394126; fax: þ7-095-9392988. E-mail address: [email protected] (A.N. Obraztsov).

tions indicate on non-metallic and hot-electron origination of low-field emission from graphite-like carbons [3]. On the other hand electron emission from dielectric diamond, having no free electrons, looks like FE from non-diamond carbons [4]. In our recent publications (see for example, [5–7]) we found experimentally evidences of general nature of FE from different carbon materials, including polycrystalline and nanocrystalline diamond films with non-diamond graphite-like inclusions, carbon nanotubes (CNT), carbon fibers, bulky and powdered graphite, etc. In accordance with the proposed model the particular property of carbon materials to emit electrons at low fields is a result of well-ordered structures of nano-sized carbon species like CNT

0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00293-9

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and nano-crystallites of graphite. At the same time these nanostructured carbon (nC) materials must emit electrons under fields corresponding to normal Fowler–Nordheim FE like usual metals. In this paper, we present results of low-field electron emission study for nC thin film material grown by chemical vapor deposition (CVD) in dc discharge plasma activated gas mixture of hydrogen and methane. The fundamental results are illustrated by the prototypes of cathodoluminescent lamps with nC cold cathodes.

2. CVD carbon film cathode fabrication The film nC material fabrication was performed using CVD in a hydrogen–methane gas mixture activated by a direct current discharge. The original facility of the dc CVD system is described in detail elsewhere [8]. In brief, the nC films were deposited on Si or Ni substrates usually. The maximal size of the substrates is 50 mm. A static pressure of approximately 100 Torr of a H2/CH4 gas mixture was maintained during the deposition. The dc discharge was activated in the CVD reactor between the substrate, located on a water cooled anode, and a tungsten cathode. The substrate temperature was maintained at approximately 1000 8C. The dc voltage applied between the electrodes was approximately 800 V and the discharge current density was in range of 0.25–0.75 A/cm2. The nC film morphology and composition were tested by using electron microscopy, Raman spectroscopy and other methods [5–7]. A typical scanning electron microscopy (SEM) image of the nC film surface is shown in Fig. 1. This nC film was deposited on 10 mm  10 mm Si wafer and has the usual morphology of our nC cathodes with graphite nano-sized flake-like crystallites (GNC) and carbon nanotubes (CNT). The characteristic sizes of the sharp edges of GNC and CNT are in range of 10–50 nm. These nC species form on substrate surface homogeneous and rather porous layers with thickness of 2–4 mm dependent on deposition time of 45–120 min. Some peculiar CNTs and GNCs have a length exceeding average film thickness by 1 or 2 mm. The aspect ratios of the nC species (ratios of the species height to the edge size— H/r) are in range of 40–600 as estimated from the electron microscopy observations.

Fig. 1. SEM image of typical surface morphology of the nanocarbon CVD film cold cathode. The arrows show a few CNT species.

3. Field emission test measurements The FE tests were performed in a vacuum diode configuration with a flat parallel anode and cathode. The anode was a glass plate coated with a transparent, conducting indium and tin oxide (ITO) film. This schema provided a very easy yet adequate method of determining the macroscopic values of electric field strength (F) and FE current density in contrast to any tip-like or ball-like anode probe configurations (see e.g. [9]). The conducting anode film was covered with a phosphor layer allowing an image of FE site distribution over the cathode surface to be obtained. The typical images of distribution of FE sites taken at voltages of 300, 400, and 1000 V applied between parallel 10 mm  10 mm cathode and anode separated by a gap of 200 mm are shown in Fig. 2. While the macroscopic value of field (F) is in range of 1–10 V/mm in the test measurements the local field value (E) on FE site surface may be estimated as E ¼ bF where factor b is the same order as the aspect ratios H/r. Some reduction of the field enhancement is possible due to field screening effect because of rather dense growth of CNTs and GNCs in the porous nC layer. The opposite effect of the local field enhancement factor increase is possible in case

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Fig. 2. Images of FE site distribution obtained for 10 mm  10 mm square nano-carbon cold cathode with macroscopic field (F) 1.5 V/mm (1), 2 V/mm (2), and 5 V/mm (3). The phosphor screen luminescence along the cathode periphery is due to electron emission from the sample side surfaces.

if actual size of FE sites is less than their geometrical shape evaluated from electron microscopy observations. The detailed discussion on this matter was published in our papers [5–7]. But in any case for purpose of application of the nC cold cathodes it is important that the very efficient FE provides high intensity electron beam and remains to be very stable at moderate applied voltages.

And the normal distribution pffiffiffiffiffiffiffiffiffiffiof the FE site radius is given by nðrÞ ¼ ðN= 2ps2 exp½ðr  r0 Þ2 =2s2 , where r0 is an average size of radius. By having the total number of emission sitesRon the cathode equal to N, it may be written R that N R¼ nðrÞ dr. Thus, the total FE current is I ¼ J dS ¼ Jpr 2 nðrÞ dr, by assuming that emission occurs from all surface area (S ¼ pr 2 ) of each emission site.

4. Statistical analysis of nC cathode characteristics and FE mechanism The typical example of current–voltage (I–V) dependence for our nC cathode is shown in Fig. 3. The distinctive features of the FE characteristics are: (i) well linearization of the I–V curve in Fowler–Nordheim (FN) coordinates in a wide range of voltages and currents; (ii) substantial curvature of the FN I–V plot at the lowest voltages and currents; (iii) homogeneity of the FE site distribution over the film surface. The linear character of the FN plot corresponds to classical electron tunneling mechanism of FE [10,11] while its substantial curvature may be explained by statistical distribution of the emitting sites geometrical, structural and electronic characteristics [12]. Although FE site characteristics have complex physical origins, a normal (Gaussian) distribution can be a good first approximation to the actual distribution. The FE image homogeneity at moderate electric field (see Fig. 2) is an evidence of a narrow width of the normal distribution with a small standard deviation s. To simplify further consideration we will believe that the subject for the statistical distribution is an effective size (radius r) of emission sites only.

Fig. 3. Typical I–V experimental dependence plotted in FN coordinates (dots) and fitted curve calculated by using formula (3) with s ¼ 0:1 r0 (line).

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In accordance with classical FN theory the density of FE current is expressed by equation   A j3=2 J¼ E2 exp B YðyÞ ; (1) jtðyÞ E where E is a local electric field on emission site surface, j the work function of cathode material; constants A and B are the combinations of electron charge (e), mass (m)pffiffiffiffiffiffi and Planck’s p constant (h): A ¼ e3 =8ph, B ¼ ffiffiffiffiffiffi 8p 2m=3he; y ¼ e eE =j is a parameter of function tðyÞ ¼ YðyÞ  ð2y=3ÞðdYðyÞ=dyÞ. The function Y(y) values are known in a tabulated form and for low field (E < 104 V/mm) may be expressed as YðyÞ ¼ 0:95 1:03y2 [11]. At the same low-field approximation tðyÞ ¼ 1 and Eq. (1) may be rewritten by using tabular data as J¼

A 2 E exp½1:03Be3 j1=2 exp½0:95Bj3=2 E1 ; j (2) 2

for the units of current density (J), A/m , electric field (E), V/m, work function (j), eV. The constants values are: A ¼ 1:5414  106 A eV V2, B ¼ 6:8309 109 eV3/2 V m1, and 1:03Be3 ¼ 10:1 eV. The most common method to interpret local field value E relationships with applied voltage V is to assume their linear conversion: E ¼ ðH=rÞF ¼ ðH=rÞðV=dÞ where F is the macroscopic field in planar electrode configuration with the interelectrode distance d and aspect ratio for a separate FE site equal to ratio of its height (H) over the substrate surface to radius which may be estimated to the same order as radius of emission site area, r. We will assume in our qualitative consideration the same height H for all the emission sites. In this case for total emission current we will have formula "   # Dr0 s2 D 2 2 I ¼ CNF exp  þ ; (3) F 2 F where constants   ApH 2 10:1 C¼ exp pffiffiffiffi ; j j

D ¼ 0:95B

j3=2 : H

Last formula (3) for I–V dependence differs from classical expression deduced from FN theory for FE from metals by the second term in the exponent. Fig. 2 illustrates the good agreement of experimental results

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with the calculation performed by using Eq. (3) for s ¼ 0:1r0 . The second term causes excess current and upward curvature in the FN plot. This term vanishes when all emission sites have the same geometrical characteristics since s ¼ 0. But the term increases at lower values of macroscopic field (or applied voltage) if s 6¼ 0. The inputs of both terms in (3) become ˚ when we assume comparable at s in range of 0.5–2 A r ¼ 5 nm, H ¼ 1 mm, j ¼ 5 eV and F in range of 1– 10 V/mm, correspondingly. This numerical estimation is in good agreement with our initial assumption about narrow width of the normal distribution with a small standard deviation s. The FN plot curvature increases with s but for linear part of the plot we can believe s and second term in (3) equal to zero. Comparison of the theoretical results (formula (3)) and the experimental I–V curve (see Fig. 3) allows us to obtain empirical relationships for two parameters— work function j and field enhancement factor b ¼ H=r: j3=2 =b ¼ 7:4  103 eV3/2. This relationship is shown in Fig. 4 graphically. One can see that expected range for work function of 4–5 eV corresponds to field enhancement factor of about 103. This value of b contradicts our SEM and TEM observations of geometrical shapes of CNT and GNC species on the carbon cathode surface. By having the sharpness edge size of about 10 nm these carbon structures have height up to 5 mm and corresponding geometrical field enhancement factor may be in range of 50–500 (see Fig. 1 and [5–7]). Another contradiction relates to the estimation of number of emission sites N. This estimation may be made for linear part of FN plot from an intercept by assuming that all emission sites have the same surface area S ¼ pr 2 . Thus, for the particular case of FN plot in Fig. 3 we have N 106 cm2 that is in good agreement with our observations by using phosphor screen (see Fig. 2 and [5–7]). However, for lowvoltage curved part of the plot our estimation gives us the value N 1013 cm2. The emission site density decrease with voltage increase may be explained by the screening effect leading to suppression of emission from the sites having smaller field enhancement factors. But even in the case of the screening the number N 1013 cm2 is possible, if emission sites have atomically small sizes of their top edges only. The alternative explanation consists in the suggestion that emission occurs from an area exceeding the

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Fig. 4. Empirical relationships for work function j and field enhancement factor b obtained from comparison of theoretical I–V dependence and experimental curve.

top edge of the nano-carbon species and including some part of their lateral surface also. In this case estimated number of emission sites will be decreased by two or three orders without extreme decrease of the characteristic sizes of emitted carbon species. This explanation agrees with our previous publications [5– 7] as well as with other observations showing origination of field emission from CNT lateral walls [13,14]. A mechanism providing such lateral emission is nonmetallic behavior of nano-carbon emitters and electron tunneling through potential barrier reduced in vicinity of defects of sp2 coordinated graphite-like carbon atoms network [5–7,15]. These defects have sp3 hybridization in well-ordered carbon materials and provide partly localized electron states on the emitter surface. It is important to note that the sizes of FE sites are in atomic scale. This implies that the main physical assumptions made in the classical FN theory of FE are not valid for these emitters. Correspondingly, the results of this theory cannot be used for any numerical estimation but only for qualitative speculations. In such speculations, we should take into account the atomic structure of the nano-carbon emitters in contrast to that of the free electron metal. Fig. 5 illustrates this approach schematically, showing the energy barrier on a cathode-vacuum interface for normal graphite and for nC graphite-like material with an sp3 cluster on the surface (Fig. 5a and b,

respectively). The quantum wells related to the carbon atoms’ positions are shown in the figure instead of the usual energy bands. The upper electron levels lie near the top of the wells in the case of graphite, providing zero band-gap and semimetallic properties. The barrier height on the surface is equal to the work function of about 5 eV for graphite. The non-graphite (sp3) carbon cluster is represented in the energy diagram by two bands separated by a gap between the highest occupied states (HOS) and the lowest unoccupied states (LUS). It may be assumed that this HOS-LUS gap varies for clusters of different sizes and configurations and may reach a value of about 4 eV, and that the gap center is located near the Fermi level of the graphite part. It may also be assumed that the vacuum level lies near the LUS, similar to that of diamond with sp3 atomic configuration [16]. An additional downward shift of the HOS and LUS may be due to polarization of the ‘‘dielectric’’ cluster when an external electric field is applied to the cathode surface. As a result, electrons from the graphite part of the emitter may penetrate into vacuum due to tunneling through the two thin barriers more easily than in the case of ordinary graphite or/and metals. This mechanism is very similar to so-called solid-state field-controlled electron emission (SSE) [17], except that instead of an ultra-thin wide bandgap semiconductor layer on a metal, we have two kinds of carbon materials (sp2 and sp3) connected to each

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Fig. 5. Schematic energy diagram presentation of vacuum-cathode interface for (a) normal graphite and (b) nC graphite-like material (GNC and CNT) with an sp3 cluster on the surface. The separate carbon atoms are shown by quantum wells (grey) separated by energy barriers (white). The energy bands related to the sp3 cluster are shown by black. Fermi level (EF), vacuum level (Evac) and electron potential at applied field (F ¼ V/d) are shown by straight lines. The dashed line shows electron potential without consideration of polarization effect providing additional reduction of barrier high with applied field.

other. This last circumstance is probably a unique property of carbon. An interface between sp2 and sp3 is very sharp and very thin providing increase in electron tunnel probability and FE efficiency. In fact, the proposed non-classical mechanism of FE from nC materials is similar to hot electron emission. Since atomically small thickness of the ‘‘dielectric’’ sp3 carbon layer electron energy lost during transport through the layer should be very small also. But, in case of high density of current, total energy liberation in the small area may be a cause of cathode material destruction and deviation of I–V curves in FN plots from linear behavior at highest currents (see [5– 7] e.g.). Nevertheless, experimentally observed density of stable emission current up to 1 A/cm2 is quite enough for many device applications of nC cathodes. 5. Lighting element prototype with nC cold cathodes The main advantage of cold cathodes consists in the possibility to obtain electron beam without heating

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power consumption. In combination with high efficiency of cathodoluminescent (CL) process it may provide record characteristics of CL light emission devices (lamps) using cold cathodes. But standard (and most efficient) CL phosphors require rather high accelerating voltages for the electron beam. While FE cathode is made from usual metals with high-field emission threshold this requirements are not very difficult to be satisfied because of necessity to apply rather high voltages. To meet both requirements simultaneously, namely providing low-field electron emission and high accelerating voltages, we use a cylindrical diode configuration for our lamps similar to that proposed in [18] and modified for CNT cathode in [19]. The nC film was deposited on cylindrical 1 mm diameter Ni wire and fixed along an axis of 20 mm cylindrical glass tube having Al anode deposited onto its side wall and CL phosphor layer above the anode (see Fig. 6) [20]. An important advantage of the cylindrical diode configuration in comparison with planar one is that for the same applied voltage V, the macroscopic electric field is given by F ¼ V/ [r ln(R/r)], compared to F ¼ V/d for two planar electrodes with an interelectrode distance d. Practically, this means that the macroscopic field at the cathode surface F(r) will be much higher in the cylindrical case for identical d ¼ R  r and V. This makes the realization of a field emission diode possible, working at suitable voltages applied between electrodes separated by a rather large distance. For example, our experimental configuration for the sealed cylindrical diode lamps shows very high intensity of light emission (up to 100,000 cd/m2) with applied voltage 10 kV.1,2 This level of brightness corresponds to total luminous flux from the lamp of about 150 200 lm. Taking into account that power consumption of the lamp is about 1 W the lamp power efficiency is up to 30%.3

1 CL phosphor type P-53 Y3(Al,Ga)5O12:Tb supplied by Nichia Corp. (Japan) was used in these lamps. 2 Minolta LS-100 Luminance-Meter was used in these measurements. 3 These are very rough estimations based on simplest measurements and allowing evaluation of a principal advantage of our lamps. More careful studies are in progress now and will be published latter.

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Fig. 6. Schema and photograph of sealed cylindrical diode CL lamp with nC cold cathode.

6. Conclusions Electron emission from nano-carbon cold cathodes obtained by CVD method has been found experimentally to be very efficient with low-voltage switch-on threshold and high density emission sites. The numerical estimations on base of classical FN approach in combination with statistical analysis allow adequate description of experimental I–V dependencies for the cold cathodes. But traditional interpretation of FE sites as metal type conductor leads to contradictions between the experimental results and original FN theoretical assumptions. A self-consistent interpretation based on the assumption of non-metallic conductivity and nonclassical FE mechanism was proposed. This specificity of nC emitters, in fact, originates from the possibility of carbon atoms being arranged with sp3 (diamond-like) and sp2 (graphite-like) hybridization. These two different carbon phases may be combined with a very narrow interface providing immediately high electrical conductivity and reduction of the potential barrier to electrons escaping into vacuum. The low macroscopic field threshold providing relatively high intensity electron emission is very attractive for device application of the nC cold cathode. But the proposed explanation of low-field emission from nC materials predicts some limitations in

the current density of emitted electron beam. The required electron beam current for specific applications may be obtained by appropriate choice of the cathode dimensions and geometry. To evaluate applicability of nC cold cathodes obtained by using CVD method CL lighting elements were fabricated and tested. The CL lamps with cylindrical diode configuration lamps show record efficiency of electrical energy conversation into light up to 30%.

Acknowledgements This work has been supported in part by INTAS Grant no. 01-0254.

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