Potential Application of CNTs in Vacuum Microelectronics

Field emission is an attractive source for electrons compared to thermionic emission. It is a quantum effect. When subject to a sufficiently high electric field, electrons near the Fermi level can overcome the energy barrier to escape to the vacuum level. The basic physics of electron emission is well developed. The emission current from a metal surface is determined by the Fowler-Nordheim equation: I = aV2 exp(-b$3/2/@V) where I, V, (, are the current, applied voltage, work function, and field enhancement factor, respectively [26,27].

Electron field emission materials have been investigated extensively for technological applications, such as flat panel displays, electron guns in electron microscopes, microwave amplifiers [28]. For technological applications, electron emissive materials should have low threshold emission fields and should be stable at high current density. A current density of 1-10mA/cm2 is required for displays [29] and > 500mA/cm2 for a microwave amplifier [30]. In order to minimize the electron emission threshold field, it is desirable to have emitters with a low work function and a large field enhancement factor. The work function is an intrinsic materials property. The field enhancement factor depends mostly on the geometry of the emitter and can be approximated as: ¡3 = 1/5r where r is the radius of the emitter tip. Processing techniques have been developed to fabricate emitters such as Spindt-type emitters, with a sub-micron tip radius [28]. However, the process is costly and the emitters have only limited lifetime. Failure is often caused by ion bombardment from the residual gas species that blunt the emission tips. Table 1 lists the threshold electrical field values for a 10 mA/cm2 current density for some typical materials.

Table 1. Threshold electrical field values for different materials for a 10mA/cm2 current density (data taken from [31,32])

Material Threshold electrical field (V/|im)

Table 1. Threshold electrical field values for different materials for a 10mA/cm2 current density (data taken from [31,32])

Material Threshold electrical field (V/|im)

Mo tips

50-

100

Si tips

50-

100

p-type semiconducting diamond

130

Undoped, defective CVD diamond

30-

120

Amorphous diamond

20-

-40

Cs-coated diamond

20-

30

Graphite powder(<1 mm size)

17

Nanostructured diamond"

3-5 (unstable >30mA/cm )

Carbon nanotubes6

1-3 (stable at lA/cm2)

a Heat-treated in H plasma. Random SWNT film a Heat-treated in H plasma. Random SWNT film

Carbon nanotubes have the right combination of properties - nanometer-size diameter, structural integrity, high electrical conductivity, and chemical stability - that make good electron emitters [27]. Electron field emission from carbon nanotubes was first demonstrated in 1995 [33], and has since been studied intensively on various carbon nanotube materials. Compared to conventional emitters, carbon nanotubes exhibit a lower threshold electric field, as illustrated in Table 1. The current-carrying capability and emission stability of the various carbon nanotubes, however, vary considerably depending on the fabrication process and synthesis conditions.

The I-V characteristics of different types of carbon nanotubes have been reported, including individual nanotubes [33,34], MWNTs embedded in epoxy matrices [35,36], MWNT films [37,38], SWNTs [39,40,41,42] and aligned MWNT films [32]. Figure 2 shows typical emission I-V characteristics measured from a random SWNT film at different anode-cathode distances, and the Fowler-Nordheim plot of the same data is shown as the inset. Turn-on and threshold fields are often used to describe the electrical field required for emission. The former is not well-defined and typically refers to the field that is required to yield 1 nA of total emission current, while the latter refers to the field required to yield a given current density, such as 10mA/cm2. For random SWNT films, the threshold field for 10mA/cm2 is in the range of 2-3V/|im. Random and aligned MWNTs [fabricated at the University of North Carolina (UNC) and AT&T Bell Labs] were found to have threshold fields slightly larger than that of the SWNT films and are typically in the range of 3-5 V/|im for a 10mA/cm2 current density [32] (Fig. 3). These values for the threshold field are all significantly better than those from conventional field emitters such as the Mo and Si tips which have a threshold electric field of 50-100 V/|im (Table 1). It is interesting to note that the aligned MWNT films do not perform better than the random films. This is due to the electrical screening effect arising from closely packed nanotubes [43]. The low threshold field for electron emission observed in carbon nanotubes is a direct result of the large field enhancement factor rather than a reduced electron work function. The latter was found to be 4.8eV for SWNTs, 0.1-0.2eV larger than that of graphite [44].

SWNTs generally have a higher degree of structural perfection than either MWNTs or CVD-grown materials and have a capability for achieving higher current densities and have a longer lifetime [32]. Stable emission above 20 mA/cm2 has been demonstrated in SWNT films deposited on Si substrates [40]. A current density above 4 A/cm2 (measured by a 1mm local probe) was obtained from SWNTs produced by the laser ablation method [40]. Figure 4 is a CCD (Charge Coupled Device) image of the set-up for electron emission measurement, showing a Mo anode (1mm diameter) and the edge of the SWNT cathode in a vacuum chamber. The Mo anode is glowing due to bombarding from field emitted electrons, demonstrating the high current capability of the SWNTs. This particular image was taken at a current density of 0.9 A/cm2. The current densities observed from the carbon nanotubes are significantly higher than from conventional emitters, such as nano-diamonds which tend to fail below 30mA/cm2 current density [31]. Carbon nanotube emitters are particularly attractive for a variety of applications including microwave amplifiers.

Although carbon nanotube emitters show clear advantageous properties over conventional emitters in terms of threshold electrical field and current density, their emission site density (number of functioning emitters per unit area) is still too low for high resolution display applications. Films presently

Fig. 2. (Top): Emission I-V characteristics of a random single-walled carbon nano-tube film measured at different anode-cathode distances at 10~8 torr base pressure. The same data are plotted as [ln(I/V2) vs 1/V] in the inset. Deviations from the ideal Fowler-Nordheim behavior are observed at high current. (Bottom): Stability test of a random laser-ablation-grown SWNT film showing stable emission at 20 mA/cm2 (from [40])

Fig. 2. (Top): Emission I-V characteristics of a random single-walled carbon nano-tube film measured at different anode-cathode distances at 10~8 torr base pressure. The same data are plotted as [ln(I/V2) vs 1/V] in the inset. Deviations from the ideal Fowler-Nordheim behavior are observed at high current. (Bottom): Stability test of a random laser-ablation-grown SWNT film showing stable emission at 20 mA/cm2 (from [40])

fabricated [32] have typical emission site densities of 103-104/cm2 at the turn-on field, and < is typically required for high resolution display devices.

2 3 4 5 6 7 8

Fig. 3. Current density versus electric field measured for various forms of carbon nanotubes (data taken from Bower et al. [32])

Fig. 4. A CCD image showing a glowing Mo anode (1 mm diameter) at an emission current density of 0.9 A/cm2 from a SWNT cathode. Heating of the anode is due to field emitted electrons bombarding the Mo probe, thereby demonstrating a high current density (image provided by Dr. Wei Zhu of Bell Labs)

1.1 Prototype Electron Emission Devices Based on Carbon Nanotubes

1.1.1 Cathode-Ray Lighting Elements

Cathode ray lighting elements with carbon nanotube materials as the field emitters have been fabricated by Ise Electronic Co. in Japan [45]. As illustrated in Fig. 5, these nanotube-based lighting elements have a triode-type design. In the early models, cylindrical rods containing MWNTs, formed as a deposit by the arc-discharge method, were cut into thin disks and were glued to stainless steel plates by silver paste. In later models, nanotubes are now screen-printed onto the metal plates. A phosphor screen is printed on the inner surfaces of a glass plate. Different colors are obtained by using different fluorescent materials. The luminance of the phosphor screens measured on the tube axis is 6.4 x 104 cd/cm2 for green light at an anode current of 200 [A,

Fig. 5. Demonstration field emission light source using carbon nanotubes as the cathodes (fabricated by Ise Electronic Co., Japan) [45]

which is two times more intense than that of conventional thermionic Cathode Ray Tube (CRT) lighting elements operated under similar conditions [45].

1.1.2 Flat Panel Display

Prototype matrix-addressable diode flat panel displays have been fabricated using carbon nanotubes as the electron emission source [46]. One demonstration (demo) structure constructed at Northwestern University consists of nanotube-epoxy stripes on the cathode glass plate and phosphor-coated Indium-Tin-Oxide (ITO) stripes on the anode plate [46]. Pixels are formed at the intersection of cathode and anode stripes, as illustrated in Fig. 6. At a cathode-anode gap distance of 30|j.m, 230 V is required to obtain the emission current density necessary to drive the diode display 76 ^mA/ mm2). The device is operated using the half-voltage off-pixel scheme. Pulses of ±150 V are switched among anode and cathode stripes, respectively to produce an image.

Recently, a 4.5 inch diode-type field emission display has been fabricated by Samsung (Fig. 6), with SWNT stripes on the cathode and phosphor-coated ITO stripes on the anode running orthogonally to the cathode stripes [47]. SWNTs synthesized by the arc-discharge method were dispersed in isopropyl alcohol and then mixed with an organic mixture of nitro cellulose. The paste was squeezed into sodalime glasses through a metal mesh, 20 ^m in size, and then heat-treated to remove the organic binder. Y2O2S:Eu, ZnS:Cu,Al, and ZnS:Ag,Cl, phosphor-coated glass is used as the anode.

1.1.3 Gas-Discharge Tubes in Telecom Networks

Gas discharge tube protectors, usually consisting of two electrodes parallel to each other in a sealed ceramic case filled with a mixture of noble gases,

Fig. 6. Left: Schematic of a prototype field emission display using carbon nanotubes (adapted from [46]). Right: A prototype 4.5 field emission display fabricated by Samsung using carbon nanotubes (image provided by Dr. W. Choi of Samsung Advanced Institute of Technologies)

is one of the oldest methods used to protect against transient over-voltages in a circuit [48]. They are widely used in telecom network interface device boxes and central office switching gear to provide protection from lightning and ac power cross faults on the telecom network. They are designed to be insulating under normal voltage and current flow. Under large transient voltages, such as from lightning, a discharge is formed between the metal electrodes, creating a plasma breakdown of the noble gases inside the tube. In the plasma state, the gas tube becomes a conductor, essentially short-circuiting the system and thus protecting the electrical components from overvoltage damage. These devices are robust, moderately inexpensive, and have a relatively small shunt capacitance, so they do not limit the bandwidth of high-frequency circuits as much as other nonlinear shunt components. Compared to solid state protectors, GDTs can carry much higher currents. However, the current Gas Discharge Tube (GDT) protector units are unreliable from the standpoint of mean turn-on voltage and run-to-run variability.

Prototype GDT devices using carbon nanotube coated electrodes have recently been fabricated and tested by a group from UNC and Raychem Co.[49]. Molybdenum electrodes with various interlayer materials were coated with single-walled carbon nanotubes and analyzed for both electron field emission and discharge properties. A mean dc breakdown voltage of 448.5 V and a standard deviation of 4.8 V over 100 surges were observed in nanotube-based GDTs with 1 mm gap spacing between the electrodes. The breakdown reliability is a factor of 4-20 better and the breakdown voltage is ^30% lower than the two commercial products measured (Fig. 7). The enhanced performance shows that nanotube-based GDTs are attractive over-voltage protection units in advanced telecom networks such as an Asymmetric-Digital-Signal-Line

Applications of Carbon Nanotubes 401 SOW -T-1-,-1-,-1-,-1-,-

JS . ° □ □ iiq3in ""in □ na ° a °n u ri

CQ my _ A Commercial GDT (manufacturer 1)

° Commercial GDT (manufacturer 2)

0 20 40 60 80 100

Surge Number

Fig. 7. DC breakdown voltage of a gas discharge tube with SWNT coated electrodes, filled with 15 torr argon with neon added and 1 mm distance between the electrodes. The commercial GDTs are off-the-shelf products with unknown filling gas(es), but with the same electrode-electrode gap distances. The breakdown voltage of the GDT with SWNT coated electrodes is 448.5 V, with a standard deviation of 4.8 V over 100 surges. The commercial GDT from one manufacturer has a mean breakdown voltage of 594 V and a standard deviation of 20 V. The GDT from the second manufacturer has a breakdown voltage of 563 V and a standard deviation of 93 V (from Rosen et al. in [49])

(ADSL), where the tolerance is narrower than what can be provided by the current commercial GDTs.

0 0

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