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JOURNAL OF APPLIED PHYSICS

VOLUME 88, NUMBER 12

15 DECEMBER 2000

Growth and emission characteristics of vertically well-aligned carbon nanotubes grown on glass substrate by hot filament plasma-enhanced chemical vapor deposition Jae-hee Han, Won-Suk Yang, and Ji-Beom Yooa) Department of Materials Engineering, Sungkyunkwan University, 300 Chunchun-dong, Jangan-ku, Suwon 440-746, Korea

Chong-Yun Park Department of Physics, Sungkyunkwan University, 300 Chunchun-dong, Jangan-ku, Suwon 440-746, Korea

共Received 8 March 2000; accepted for publication 8 September 2000兲 Vertically well-aligned multiwall carbon nanotubes 共MWNTs兲 were grown on nickel-coated glass substrates by plasma-enhanced chemical vapor deposition at low temperatures, below 600 °C, with and without hot filament. Acetylene and ammonia gas were used as the carbon source and a catalyst. Effects of growth parameters, such as plasma intensity, filament current, and substrate temperature, on the growth characteristics of MWNTs were investigated. Plasma intensity was found to be the most critical parameter controlling the growth of MWNTs. Field emission from the MWNTs was obtained using a phosphor anode with an onset electric field of 1.5 V/␮m. © 2000 American Institute of Physics. 关S0021-8979共00兲00724-6兴

Since their first discovery in 1991,1 carbon nanotubes have received considerable attention because of the prospect of new fundamental science and many potential applications.2–15 Carbon nanotubes are especially promising candidates for cold-cathode field emitter because of their electrical properties, high aspect ratios, and small radius of curvature at their tips. Various studies on the electron field emission have been reported including a single carbon nanotube16,17 and a carbon nanotube matrix.12 For applications such as flat panel displays, vertical alignment of the carbon nanotubes is important. Recently, aligned carbon nanotubes have been grown above 700 °C on mesoporous silica by thermal decomposition of acetylene gas.6 The growth of well-aligned carbon nanotubes on nickel coated glass at 660 °C has also been reported using hot filament plasma enhanced chemical vapor deposition 共HFPECVD兲.18 However, systematic study on the growth characteristics and emission characteristics of multiwall carbon nanotubes 共MWNTs兲 using HFPECVD has not been reported yet. In this communication, the electron emission from the vertically well-aligned carbon nanotubes grown on glass using HFPECVD is reported. The growth characteristics of vertically well-aligned carbon nanotubes on nickel-coated glass substrates over large areas 共2 cm⫻2 cm兲 were investigated at low temperatures 共⬍600 °C兲 using HFPECVD. Vertically well-aligned carbon nanotubes were grown in the HFPECVD system 共model: MST-CNT 2000兲 shown in Fig. 1. Acetylene (C2H2) and ammonia (NH3) gas were used as a carbon source and catalyst, respectively. A dc plasma was used. A thin nickel layer was coated on glass by dc

magnetron sputtering. For deposition of the nickel film, the base and operating pressures of the chamber were kept below 7⫻10⫺5 and 5⫻10⫺3 Torr, respectively, the operating current was 0.3 A, the flow rate of argon gas was 10 sccm, and the deposition was carried out at room temperature. The thickness of the nickel film was 300–1000 Å. Prior to carbon nanotube growth, the substrate was cleaned in trichloroethylene, acetone, and methanol for 10 min. and rinsed in deionized water. The substrate was transferred to the chamber and pumped down below 2⫻10⫺5 Torr by mechanical and diffusion pumps. Then NH3 was introduced into the chamber. After the working pressure had been stabilized, the tungsten filament coil and the dc power supply were turned on. The distance between filament and substrate was 5–10 mm. The bias voltage for plasma generation varied from 400 to 650 V. The pretreatment for surface etching of the nickel layer was conducted by NH3 plasma for 4 min. C2H2 was introduced into the chamber for the growth of carbon nano-

a兲

FIG. 1. Schematic diagram of plasma enhanced hot filament chemical vapor deposition reactor.

Author to whom correspondence should be addressed; electronic mail: [email protected]

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© 2000 American Institute of Physics

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FIG. 4. SEM image of carbon nanotube grown on Ni coated glass without filament current: 共a兲 650 V 共0.23 A兲, growth time 共25 min兲, 共b兲 640 V 共0.23 A兲, growth time 共40 min兲.

FIG. 2. Effect of plasma intensity on the growth of carbon nanotubes at a constant filament current of 14 A: 共a兲 550 V 共0.15 A兲, 共b兲 504 V 共0.11 A兲, 共c兲 450 V 共0.06 A兲, 共d兲 430 V 共0.02 A兲.

tubes. The substrate temperature was measured using a thermocouple and pyrometer 共IR-GPD, CHINO Co.兲. The morphology of grown carbon nanotubes was characterized by scanning electron microscopy 共SEM兲. Electron emission was measured as a function of applied voltage using a phosphor anode in the vacuum chamber. It is very important to identify the dominant factor to control the carbon nanotube growth in the HFPECVD system. The effect of plasma intensity on the carbon nanotube growth was investigated. The carbon nanotubes were grown under the same condition except for plasma intensity. The flow rates of NH3 and C2H2 were 160 and 80 sccm, respectively. The filament current was 14 A and the growth time was 14 min. The bias voltage for plasma decreased from 550 to 429 V. The substrate temperature changed from 580 to 530 °C with decrease in the bias voltage. As shown in Fig. 2共a兲, the vertically aligned carbon nanotubes were grown at

FIG. 3. Effect of plasma intensity on the growth characteristics of the carbon nanotubes at a constant substrate temperature of 556 °C: 共a兲 500 V 共0.13 A兲共plasma intensity兲, 12.1 A 共filament current兲; 共b兲 551 V 共0.15 A兲, 11.1 A; 共c兲 600 V 共0.21 A兲, 9 A; 共d兲 650 V 共0.25 A兲, 7 A.

the plasma intensity of 550 V and 0.15 A. The diameter of the carbon nanotubes was about 50 nm and the growth temperature of the carbon nanotubes is much lower than the temperature reported by Ren et al.18 As shown in Figs. 2共b兲 and 2共c兲, the growth of carbon nanotubes was not observed at a plasma intensity lower than 550 V 共0.15 A兲 and only initial growth of carbon nanotubes was found at a plasma intensity of 504 V 共0.11 A兲 and 450 V 共0.06 A兲. At a plasma intensity of 429 V 共0.01 A兲, no trace of carbon nanotube growth was found as shown in Fig. 2共d兲. The filament current is expected to play important roles such as heating the substrate and electron generation. The possibility of C2H2 gas dissociation by filament current can be excluded because of the thermal stability of C2H2 gas. The fact that the growth of carbon nanotubes changes under the same filament current implies that plasma intensity plays a more important role than the filament current in carbon nanotube growth. But an increase in plasma intensity induces an increase in the plasma power as well as the substrate temperature. Hence, in order to confirm the dominant factor for the growth of the carbon nanotubes, effects of plasma intensity on the growth of carbon nanotubes were investigated at constant substrate temperature. The plasma intensity and the filament current were simultaneously varied to maintain a constant substrate temperature in the HFPECVD system. The substrate temperature was kept constant at 556 °C, which is quite lower than the reported value of 650 °C by other researchers.6,18 The filament current changed from 12.1 to 7 A, while the plasma intensity changed from 500 V 共0.13 A兲 to 650 V 共0.25 A兲. Figure 3 is a series of SEM micrographs

FIG. 5. Variation of emission current with applied voltage: field-emission current vs electric field plot 共inset plot兲 Fowler–Nordheim plot.


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J. Appl. Phys., Vol. 88, No. 12, 15 December 2000

showing the effect of plasma intensity on the growth of the carbon nanotubes. As shown in Fig. 3共a兲, with a plasma intensity of 500 V 共0.13 A兲, the carbon nanotubes were rarely grown. This result is consistent with that of the experimental results in Fig. 2, which showed only the initial stage of growth if the plasma intensity was lower than 550 V 共0.15 A兲. As shown in Figs. 3共b兲, 3共c兲, and 3共d兲, an increase in the plasma intensity enabled the carbon nanotubes to be grown dramatically even though the filament current decreased. The typical diameter of the carbon nanotubes decreased and the tip of the carbon nanotubes became sharp as the plasma intensity increased. From these results it may be proposed that the plasma intensity is the most crucial factor for determining the growth characteristics of the carbon nanotubes. Moreover, to investigate the critical effect of the plasma on the growth of the carbon nanotubes, the growth of carbon nanotubes without filament current was tested. Without the filament current, the growth of vertically well-aligned carbon nanotubes was observed and shown in Fig. 4共b兲. Electron emission from the carbon nanotube was investigated. Field emission tests were performed in a vacuum changer at a pressure of 1⫻10⫺5 Torr. A positive voltage was applied to the anode using Keithley-248 and current was detected at the anode as a function of applied field. The anode was phosphor-coated indium–tin–oxide glass. An area for field emission measurement was 0.185 cm2 . An onset electric field intensity was defined as the electric field intensity where the emission current density was 10 ␮A/cm2. As shown in Fig. 5, the emission current density on the order of 10 ␮A/cm2 can be detected at the electric field of 1.5 V/␮m. The onset electric field intensity of 1.5 V/␮m is similar to the value reported in the carbon nanotubes grown at a temperature higher than 900 °C.19 An emission current of 0.4 mA was observed at an electric field of 3.75 V/␮m. The experimental data were well fitted into a Fowler–Nordheim plot as shown in the inset of Fig. 5, which implies that electron transport mechanism is governed by the Fowler– Nordheim model.

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Vertically well-aligned MWNTs were grown on nickelcoated glass by HFPECVD at low temperatures below 600 °C. Effects of growth parameters such as plasma intensity, filament current, and substrate temperature on the growth characteristics of MWNTs were investigated. Plasma intensity was found to be the critical parameter to determine the growth of MWNTs. Growth of MWNTs without filament current was also observed. Field emission from the MWNTs was obtained using a phosphor anode with an onset electric field of 1.5 V/␮m.

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