APPLIED PHYSICS LETTERS
VOLUME 78, NUMBER 7
12 FEBRUARY 2001
Patterned selective growth of carbon nanotubes and large field emission from vertically well-aligned carbon nanotube field emitter arrays Jung Inn Sohn and Seonghoon Leea) Department of Materials Science and Engineering, Kwangju Institute of Science and Technology (K-JIST), Kwangju, 500-712, Korea
Yoon-Ho Song, Sung-Yool Choi, Kyoung-Ik Cho, and Kee-Soo Nam Micro-Electronics Tech. Labs., Electronics and Telecommunications Research Institute (ETRI), Taejon, 305350, Korea
共Received 6 September 2000; accepted for publication 31 October 2000兲 We have grown well-aligned carbon nanotube arrays by thermal chemical vapor deposition at 800 °C on Fe nanoparticles deposited by a pulsed laser on a porous Si substrate. We also attain a selective growth of carbon nanotubes on a patterned Fe film on Si substrates in terms of pulsed laser deposition and a liftoff patterning method. Field emission measurement has been made on the carbon nanotube 共CNT兲-cathode diode device at room temperature and in a vacuum chamber below 10⫺6 Torr. The distance between the CNT cathode and the anode is 60 m and is kept through an insulating spacer of polyvinyl film. The measured field emitting area is 4.0⫻10⫺5 cm2. Our vertically well-aligned carbon nanotube field emitter arrays on the Si-wafer substrate emit a large current density as high as 80 mA/cm2 at 3 V/m. The transmission electron microscope image shows that they are multiwalled and bamboolike structures and that the tips of some of the carbon nanotube emitters are open. The open tip structure of our CNTs and their good adhesion via Fe nanoparticles to the Si substrate are part of the reason why we can attain a large field emission current density within a low field. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1335846兴
Carbon nanotubes are known for their superior mechanical strength and low weight,1 good heat conductance,2 varying electronic properties depending on their helicity and diameter,3 large surface area useful for adsorption of hydrogen or other gases,4 and their ability to emit a cold electron at relatively low voltages due to high aspect ratios and nanometer size tips.5 Therefore, carbon nanotubes can be applied to field emitters for flat-panel displays6 and vacuum microelectronic devices like microwave power amplifier tubes, nanofield effect transistors 共FETs兲,7 nano-Schottky diodes, ion storage for batteries, and mechanical structures requiring low weight and high strength. In this study, we will focus on the field emission properties of carbon nanotubes. The electric field E T at the tip of end radius r, and voltage V with respect to a distant anode is given approximately as E T ⬃V/5r. 8 It is known that positive ions formed in the gate region sputter off the emitting diamond or Mo tips. Because of carbon nanotubes nanomter scale and their high aspect ratio more than ⬃300 and high mechanical strength and chemical stability, carbon nanotubes are very attractive as electron field emitters. Several groups have reported electron field emission from carbon nanotubes.9–14 In those reports, emitters in, single-wall nanotube 共SWNT兲 films deposited on Si or polytetrafluoroethylene 共PTFE兲 substrates were not well distributed. The reported current densities were rather low and the thresh field was high. Very recently Zhu et al. reported that emission current density from emitters in SWNT films is 10 mA/cm2 at 6.5 V/m.15 Selective growth of vertically aligned carbon a兲
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nanotubes on a patterned glass or Si substrate will be more useful for carbon nanotube-based field emission displays or other electronic devices. We report a large field emission current density 80 mA/cm2 within a low field 共3 V/m兲 from vertically well-aligned carbon nanotube field emitter arrays directly grown on a Fe-deposited porous Si substrate. We achieved a selective growth of carbon nanotubes on a patterned Fe-catalyst deposited on a porous Si substrate. Porous Si is a good substrate for growing well-aligned carbon nanotubes.16 We prepared porous silicon substrates by electrochemical etching of p-type Si共100兲 wafers with resistivities of 3–6 ⍀ cm. Before etching p-type Si, we deposited thin film Al on the backside of the silicon wafers and annealed the material at 450 °C in nitrogen gas atmosphere for 20 min in order to obtain good electrical Ohmic contact to the electrode of the power supply. Electrochemical etching was carried out for 2 min in a Teflon cell containing HF 共48% aqueous solution兲 and ethanol with a 1:1 volume ratio. Pt was used as a cathode. The anodization current density was kept constant at 10 mA/cm2. The resulting porous Si has a thin nanoporous layer. Fe playing a role as a catalyst for carbon nanotube growth, was deposited on the porous silicon substrates by pulsed laser deposition 共PLD兲. Fe deposited by PLD adhered well to a porous Si substrate. Fe-deposited porous Si substrates were placed on a quartz boat and then were slid into the center of a quartz tube of 7 cm in diameter and 90 cm long located in the middle of a high temperature cylindrical tube furnace. Ar gas flowed into the quartz tube while the furnace was heated. Ammonia (NH3) was then introduced into the quartz reaction tube at a flow rate of 300 sccm for 20 min without introducing acetylene (C2H2) gas.
0003-6951/2001/78(7)/901/3/$18.00 901 © 2001 American Institute of Physics Downloaded 08 Oct 2003 to 128.200.91.174. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
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Appl. Phys. Lett., Vol. 78, No. 7, 12 February 2001
FIG. 1. 共a兲 SEM micrograph of the part of 1 cm by 1 cm carbon nanotubes block grown on a Fe-coated Si substrate showing vertically well-aligned carbon nanotubes over a large area. 共b兲 The enlarged and perpendicular view of the carbon nanotubes to the substrate shows they are well-aligned.
After NH3 treatment, acetylene gas was introduced into the quartz tube at a flow rate of 40 sccm for 20 min, and carbon nanotubes were grown on Fe-coated Si substrates by pyrolysis of acetylene at 800 °C. The growth period was varied from 10 to 20 min. The length of carbon nanotubes was adjusted by the variation of the growth period. After the growth process was done, the tube furnace was cooled slowly to room temperature in Ar gas ambient. To see the orientation and alignment, we took SEM images of the grown samples. Their images are shown in Figs. 1共a兲 and 1共b兲. The alignment of the carbon nanotubes across the whole substrate surface is uniform. The length of the carbon nanotubes is 8 m. Many efforts are being devoted to the fabrication of arrays of field emitters all over the world. The selective growth technique would be very useful for these applications, including electron guns for FEDs and cold cathodes for rf amplifiers. Thus, we made a patterned Fe film on Si substrates to perform a selective growth of carbon nanotubes. Si substrates were patterned with Fe films with thicknesses of 300 Å and side lengths of 30 m by 30 m at a pitch distance of 125 m in terms of PLD and a liftoff patterning method. Briefly describing a liftoff method, we first coat Si substrate with Hexamethyldisilazane 共HMDS兲 to enhance adhesion between the Si wafer and the photoresist 共PR兲. Then we spin coated the PR on Si substrate at 4000 rpm for 40 s and soft-baked it in an oven at 88 °C. The PR-coated Si substrate made contact with an optical mask bearing fine patterns and was exposed to light. The PR pattern obtained after the development was used as a mask for Fe deposition by PLD. Dissolving the PR used as a mask in acetone, we finally obtained a patterned Fe thin film with thicknesses of 300 Å and side lengths of 30 m by 30 m at a pitch distance of 125 m. Using the CVD experimental setup and procedures mentioned above, we grew the carbon nanotubes on a patterned substrate and took SEM images. We indeed accomplished a selective growth of carbon nanotubes on the patterned Fe-coated Si substrate. We can clearly see the selective growth of the well-aligned carbon nanotubes on the square of 30 m by 30 m in Figs. 2共a兲 and 2共b兲. The vertical alignment of the carbon nanotubes is due to van der Waals interactions between neighboring carbon nanotubes
Sohn et al.
FIG. 2. SEM micrographs of self-oriented and well-aligned carbon nanotubes grown on patterned Fe thin film-coated Si substrates. 共a兲 On the lefthand side, SEM image of carbon nanotube blocks grown on 30 m by 30 m Fe catalyst pattern at a pitch distance of 125 m. 共b兲 On the right-hand side the SEM image shows that carbon nanotubes in a block are well aligned to the direction perpendicular to the substrate surface.
along the direction normal to the substrate. The appropriate density of Fe nanoparticles on the Si substrate was crucial in growing vertically aligned carbon nanotubes. Using the grown self-oriented and vertically wellaligned carbon nanotubes without further processing, we have studied their emission properties. We measured electron emission from 1 cm by 1 cm well-aligned carbon nanotubes block shown in Figs. 1共a兲 and 1共b兲 as a cathode at room temperature and in a vacuum chamber below 10⫺6 Torr. The spacing between the CNT-cathode and the anode electrode was 60 m and was maintained by an insulating material of polyvinyl film with a thickness of 60 m. The measured field emitting area was 4.0⫻10⫺5 cm2. The brief schematic diagram of an experimental setup used for the measurement of emission is shown in Fig. 3共a兲. The plot of current density 共j兲 versus field 共V/m兲 characteristics of the well-aligned carbon nanotubes is given in Fig. 3共b兲. The plot of ln(I/V2) vs 1/V yields a straight line in agreement with the Fowler–Nordheim equation, confirming the current results from field emission. Our carbon nanotube field emitters emit 1 mA/cm2 at an electric field of 2 V/m. At 3 V/m, they emit a large current density as high as 80 mA/cm2 which is an order of magnitude higher than any other results reported so far. The end geometries of carbon nanotubes where the electron emission actually occurs will be an important factor. According to the reports,17,18 open tips stabilized with bridged adatoms between layers or slanted-cut open tips with zigzag atoms are more efficient field emitters than closed tips. We took a SEM and HRTEM image of carbon nanotubes we grew in order to see the end geometries and detailed structures. They are shown in Figs. 4共a兲 and 4共b兲. The carbon nanotube field emitters were multiwalled and bamboolike structures and the tips of some of the carbon nanotube emitters were open. Fe catalyst deposited by PLD adhered well to porous Si substrate, remaining at the bottom of the carbon nanotubes. Our growth of carbon nanotubes followed a base-growth mode mechanism. The carbon nanotubes with a metal catalyst at the bottom maintained good electrical contact to the electrode and so it was useful for field emission applications. The open tip structure of our carbon nanotubes and their good adhesion via Fe nanopar-
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Sohn et al.
Appl. Phys. Lett., Vol. 78, No. 7, 12 February 2001
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FIG. 4. 共a兲 SEM image showing the end structures of carbon nanotubes grown on a Fe-dposited on porous Si substrates. It shows an open tip structure. 共b兲 HRTEM image showing a detailed structure of carbon nanotube field emitters. They are multiwalled and bamboolike structures and the end geometries of the carbon nanotubes are open.
This work was supported by the Ministry of Information and Communication and partially by KOSEF. M. M. Treachy, T. W. Ebbesen, and J. M. Gibson, Nature 共London兲 381, 678 共1996兲. 2 R. S. Ruoff and D. C. Lorents, Carbon 33, 925 共1995兲. 3 R. Saito, G. Dresselhaus, and M. S. Dresselhaus, Physical Properties of Carbon Nanotubes 共Imperial College Press, London, 1999兲, Chap. 4. 4 C. Liu, Y. Y. Fan, M. Liu, H. T. Cong, H. M. Cheng, and M. S. Dresselhaus, Science 286, 1127 共1999兲. 5 A. G. Rinzler, J. H. Hafner, P. Nikolaev, L. Lou, S. G. Kim, D. Tomanek, P. Nordlander, D. T. Colbert, and R. E. Smalley, Science 269, 1550 共1995兲. 6 W. B. Choi, D. S. Chung, J. H. Kang, H. Y. Kim, Y. W. Jin, I. T. Han, Y. H. Lee, J. E. Jung, N. S. Lee, G. S. Park, and J. M. Kim, Appl. Phys. Lett. 75, 3129 共1999兲. 7 S. J. Tans, A. R. M. Verschueren, and C. Dekker, Nature 共London兲 393, 49 共1998兲. 8 R. Gomer, Field Emission and Field Ionization 共Harvard University Press, Cambridge, MA, 1961兲. 9 A. G. Rinzler, J. H. Hafner, P. Nikolaev, L. Lou, S. G. Kim, D. Tomanek, P. Nordlander, D. T. Colbert, and R. E. Smalley, Science 269, 1550 共1995兲. 10 W. A. de Heer, A. Chatelain, and D. Ugarte, Science 270, 1179 共1995兲. 11 Y. Saito, S. Uemura, and K. Hamaguchi, Jpn. J. Appl. Phys., Part 2 37, L346 共1998兲. 12 Q. H. Wang, A. A. Setlur, J. M. Lauerhaas, J. Y. Dai, E. W. Seelig, and R. P. H. Chang, Appl. Phys. Lett. 72, 2912 共1998兲. 13 J. M. Bonard, J. P. Salvetat, T. Stockli, W. A. de Heer, L. Forro, and A. Chatelain, Appl. Phys. Lett. 73, 918 共1998兲. 14 Q. H. Wang, T. D. Corrigan, J. Y. Dai, R. P. H. Chang, and A. R. Krauss, Appl. Phys. Lett. 70, 3308 共1997兲. 15 W. Zhu, C. Bower, O. Zhou, G. Kochanski, and S. Jin, Appl. Phys. Lett. 75, 873 共1999兲. 16 S. Fan, M. G. Chapline, N. R. Franklin, T. W. Tombler, A. M. Cassell, and H. Dai, Science 283, 512 共1999兲. 17 D. T. Colbert and R. E. Smalley, Carbon 33, 921 共1995兲. 18 J. Ihm and S. Han, The Fifth Workshop on Developments and Industrial Application of Carbon Nanotubes, 1999, pp. 1–7. 1
FIG. 3. 共a兲 Brief schematic diagram of an experimental setup used for the measurement of emission. The CNTs cathode-to-anode electrode spacing was kept at 60 m with a spacer of polyvinyl film and the measured field emitting area was 4.0⫻10⫺5 cm2. 共b兲 Current density 共j兲 vs field 共V/m兲 characteristics of the vertically aligned carbon nanotubes on Fe-coated porous Si substrate. The data were taken in a vacuum chamber below 10⫺6 Torr and at room temperature. It was highly reproducible over repeated voltage scans.
ticles to the Si substrate are part of the reason why we can attain a large field emission current density within a low field. In summary, we have grown well-aligned carbon nanotube arrays by thermal chemical vapor deposition at 800 °C on Fe nanoparticles deposited by a pulsed laser on a porous Si substrate. Using a PLD and a liftoff method, we can achieve a selective growth of carbon nanotubes on a patterned Fe-coated porous Si substrate. At 3 V/m, they emit a large current density as high as 80 mA/cm2 which is an order of magnitude higher than any other results reported. Our growth of carbon nanotubes followed a base-growth mode mechanism. The open tip structure of our carbon nanotubes and their good adhesion via Fe nanoparticles to the Si substrate are part of the reason why we can attain a large field emission current density within a low field.
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