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Acknowledgements This work is partially supported by Florida High Tech Corridor Research Project under contract [2006-815. The authors would like to thank Dr Jeff Bindell of Agere System Inc. for comments on the manuscript. The authors also thank Zia Ur Rahman of UCF / Cirent Material Characterization facility for technical support.

References [1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56–8. [2] Trans SJ, Verschuren ARM, Dekker C. Room-temperature transistor based on a single carbon nanotube. Nature 1998;393:49–52. [3] Trans SJ, Devoret MH, Dai H, Thess A, Smalley RE, Geerligs LJ, Dekker C. Individual single wall carbon nanotubes as quantum wires. Nature 1997;386:474–7. [4] Bockrath M, Cobden DH, McEuen PL, Chopra NG, Zettl A, Thess A, Smalley RE. Single-electron transport in ropes of carbon nanotubes. Science 1997;275:1922–5. [5] Saito Y, Uemura S, Hamaguchi K. Cathode ray tube lighting elements with carbon nanotube field emitters. Jpn J Appl Phys 1998;37:L346; Collins PG, Zettl A. A simple and robust electron beam source from carbon nanotubes. Appl Phys Lett 1996;69(13):1969–71. [6] Bonard JM, Salvetat JP, Stockli T, de Heer WA, Forro L, Chatelain A. Field emission from single wall carbon nanotube films. Appl Phys Lett 1998;73:918–20; de Heer WA, Chatelain A, Ugarte D. A carbon nanotube field-emission electron source. Science 1995;270:1179–80. [7] Che G, Lakshmi BB, Fisher ER, Martin CR. Carbon nanotube membranes for electrochemical energy storage and production. Nature 1998;393:246–8.

[8] Dai H, Franklin N, Han J. Exploiting the properties of carbon nanotubes for nonolithography. Appl Phys Lett 1998;73:1508. [9] Gadd GE, Blackford M, Moricca S, Webb N, Evans PJ, Smith AM, Jacobsen G, Leung S, Day A, Hua Q. The World’s smallest gas cylinders? Science 1997;277:933. [10] Bethune S, Kiang CH, de Vries MS, Gorman G, Savoy P, Vazquez J, Beyers R. Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls. Nature 1993;363:605; Ebbesen TW, Ajayan PM. Large-scale synthesis of carbon nanotubes. Nature 1992;358:220. [11] Guo T, Nikolaev P, Thess A, Cobert DT, Smalley RE. Catalytic growth of single-walled nanotubes by laser vaporization. Chem Phys Lett 1995;243:49. [12] Amelinckx S, Zhang XB, Berbaerts D, Zhang XF, Ivanov V, Nagy JB. A formation mechanism for catalytically grown helix-shaped graphite nanotubes. Science 1994;265:635. [13] Bindell JB, Houge EC, Plew LE, Griffith JE, Bryson CE. Stylus nanoprofilometry: A new approach to CD metrology. Solid State Technol 1999;42(6):45–53. [14] Dai H, Hafner JH, Rinzler AG, Colbert DT, Smalley RE. Nanotubes as nanoprobes in scanning probe microscopy. Nature 1996;384:147–50; Nagy G, Levy M, Scarmozzino R, Osgood RM, Dai JH, Smalley RE, Michaels CA, Flynn GW, McLane GF. Carbon nanotube tipped atomic force microscopy for measurement of ,100 nm etch morphology on semiconductors. Appl Phys Lett 1998;73:529. [15] Cheung CL, Hafner JH, Odom TW, Kim K, Lieber CM. Growth and fabrication with single-walled carbon nanotubes probe microscopy tips. Appl Phys Lett 2000;76:3136. [16] Halperin WP. Quantum size effects in metal particles. Rev Mod Phys 1986;58:533. [17] Kleckley S. Synthesis of novel carbon materials and their applications. PhD thesis, University of Central Florida, Orlando, FL, 1999

Chemical vapor deposition of carbon films: in-situ plasma diagnostics a, a a a b A.N. Obraztsov *, A.A. Zolotukhin , A.O. Ustinov , A.P. Volkov , Yu.P. Svirko a

Department of Physics, Moscow State University, Leninskie gory, 1 – 2, Moscow 119992, Russia b Department of Physics, University of Joensuu, Joensuu FIN 80101, Finland Received 7 July 2002; accepted 14 November 2002

Keywords: A Diamond, Graphitic carbon, Carbon nanotubes, Carbon / carbon composites; B Plasma deposition

*Corresponding author. Tel.: 17-095-939-4408; fax: 17-095939-2988. E-mail address: [email protected] (A.N. Obraztsov).

Chemical vapor deposition (CVD) is conventionally used to create a variety of carbon mesomaterials including polycrystalline diamond films [1] and carbon nanotubes (CNT) [2]. The remarkable flexibility of the CVD process

0008-6223 / 02 / $ – see front matter  2002 Elsevier Science Ltd. All rights reserved. doi:10.1016 / S0008-6223(02)00402-5

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Table 1 Experimental parameters for carbon films deposition in dc discharge plasma [3,4] Type of CVD film material

Substrate temperature (8C)

Methane concentration (%)

Total gas pressure (Torr)

Diamond Nano-diamond Graphite-like Soot

850–900 900–1000 1050–1100 1100–1250

0.5–2 2–5 5–10 over 15

60–90 60–100 60–100 50–100

allows one to create materials with dramatically different properties (e.g. diamond, graphite and CNT films) by changing the geometry of the reaction chamber and discharge type. Moreover, very recently we have shown that diamond, graphite and CNT thin film materials can be grown in the same dc discharge CVD system [3,4] by varying the pressure and composition of hydrogen– methane mixture along with discharge current and voltage. These results, which have been obtained from the Raman measurements, electron microscopy, and also from the measurements of the electron field emission efficiency, are summarized in Table 1. However, the parameters of CVD process used have been found to be very close to an unstable regime, in which spontaneous arcing may destroy the substrate and electrodes. In this work, we reveal the characteristics of the CVD process that ensure a stable discharge. Also, we observe the presence of CH-, C 2 -, H-, and H 2 -activated species responsible for different carbon species formation in the CVD plasma on substrate and direct condensation of carbon in plasma gas phase at higher methane concentrations. Our CVD system, which is described in detail elsewhere [5], is schematically shown in Fig. 1. The system allows us to create carbon films on the substrate with a diameter of up to 50 mm. The substrate can be made from a standard

Si wafer, metal (Ni, W, Mo and other) sheets, and metal films deposited onto a dielectric (Si, fused silica, quartz etc) plate. The distance between the cathode and tungsten anode is 50 mm. The internal diameter and height of the cylindrical reactor chamber with water-cooled walls is 400 mm. The plasma emission from the deposition chamber is collected by a lens and is transferred to the entrance slit of a monochromator. The optical emission spectra (OES) of different areas of the plasma are measured by adjusting the slit position (Fig. 1). The spatial resolution of the OES measurements along the reactor axes is about 5 mm. To examine the electrical parameters of the plasma we measure the dependence of the interelectrode voltage (V ) as a function of the total discharge current (I), which is given by a dc current source. The current–voltage (I–V ) characteristics and OES are measured at a hydrogen– methane mixture pressure from 10 to 150 Torr and methane concentration from 0 to 25%. We find that the discharge plasma does not depend on the substrate temperature variation over a wide range, 600–1200 8C. The data presented in Figs. 2 and 3 was taken at the substrate temperature of 950 8C. The I–V characteristic of the dc plasma discharge at the total gas pressure of 60 Torr and methane concentration of 2% is shown in Fig. 2 and resembles that of a conventional gas discharge [6]. Four distinct regions are labeled as A, B,

Fig. 1. Schematic presentation of the dc discharge CVD and OES optical system.

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Fig. 2. Typical I–V dependence obtained for dc discharge in a hydrogen–methane gas mixture with 2% CH 4 and total pressure of 60 Torr.

C and D. In the low-current region A, the slope of the I–V characteristic is negative. In this region, the light emitting area of a positive column of the discharge is smaller than the anode (and substrate) surface. Since the glow area becomes wider as the current increases, the discharge current density is nearly the same indicating a decrease in the total plasma conductivity in region A due to enlargement of the discharge area. We arrive at the region B as soon as the glow finally covers the whole anode surface. In region B (see Fig. 2), the slope of the I–V characteristics is positive, i.e. the bigger the discharge current, the higher the cathode-to-anode voltage. The nonlinearity of the I–V characteristics corresponds to an increase in the plasma resistance as the current increases. Since in region B the discharge area covers the whole anode surface, the observed current dependence of the plasma resistance is associated with the change of the carrier density and mean free path rather than with the change of the discharge area geometry. Correspondingly, we can conclude that in this

region, the increase of plasma resistance indicates a reduction in the carrier density and / or mean free path at a higher current [6]. In region C, we observe nearly step-like behavior of the I–V characteristics. Such a behavior indicates that in region C the plasma resistance is much lower than in region B, i.e. the free carrier density and / or mean free path shows a sharp increase. In region D plasma resistance increases in a similar way to region B, however the developing plasma instability leads to spontaneous arcing. The arcing threshold depends on the methane concentration and gas pressure. In particular, for 2% methane concentration and 60 Torr gas pressure (Fig. 2) the arcing takes place at a current of 10 A and a voltage of 620 V. The stability of the discharge in regions B and C makes them most suitable for controllable film deposition. Figs. 3 and 4 show the dc discharge shapes and the plasma emission spectra taken at a gas pressure of 60 Torr and various methane concentrations. Figs. 3a and 4a correspond to discharge in a pure hydrogen atmosphere at V5580 V and I56 A. In this case the recombination lines of the atomic (H b , 486 nm and H a , 656 nm) and molecular (H 2 , 550 to 650 nm) hydrogen dominate the emission spectrum. Figs. 3b and 4b correspond to the discharge at 10% methane concentration and V5650 V and I56 A. In Fig. 4b, one can observe the characteristic emission of CH radicals (390 and 430 nm) and C 2 dimers (515 and 560 nm). By performing spectral measurements in different areas of the plasma we found that the intensities of hydrogen-related OES lines are nearly independent of the area position, while intensities of OES lines associated with CH and C 2 are significantly higher near the substrate surface than in discharge periphery areas. In our experimental conditions, we observed these lines for methane concentration in the range 2–25%. When the methane concentration exceeds 15%, an intense orange-yellow emission is observed at the discharge plasma periphery. The spectral profile of the plasma periphery emission at a methane concentration of 25% is shown in Fig. 4c for V5700 V and I56 A. This spectrum

Fig. 3. Typical images of dc discharges taken for (a) pure hydrogen and (b) hydrogen–methane gas mixtures with 10% and (c) 25% CH 4 . The total gas pressure is 60 Torr. A Si wafer 50 mm in diameter is used as the substrate, which is located on the anode in the CVD reactor chamber. The images are obtained at voltages (a) 580, (b) 650 and (c) 700 V. The current is 6 A.

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Fig. 4. Typical OES for (a) pure hydrogen and (b) for hydrogen–methane gas mixtures with 10% and (c) 25% methane. The total gas pressure is 60 Torr, applied voltage is (a) 580, (b) 650 and (c) 700 V. The discharge current is 6 A.

resembles that of black body radiation, indicating that carbon oversaturation of the gas results in the direct condensation of carbon and formation of soot in the discharge area. Such carbon oversaturation may take place in the relatively cold periphery zone of the plasma at high methane concentration [1]. Our recent Raman measurements [3,4] have shown that in our experimental conditions, polycrystalline diamond film is produced at the methane concentration of 0.1–2% depending on the substrate temperature. Methane concentrations in the range 2–5% correspond to the formation of so-called nanocrystalline diamond films. Graphite-like carbon materials composed of CNT and nanosized graphite crystallites (NGC) can be obtained with 5–10% methane in the gas mixture. These NGC and CNT species present in various carbon materials are responsible for very low-field electron emission from carbon materials, including diamond films [3,4]. When the methane concentration exceeds 15%, only highly disordered soot-like carbon materials are deposited in our CVD system [1–5]. Moreover, an increase of methane concentrations up to 15% and higher, results in the deterioration of the discharge stability and overheating of the substrate. These findings are summarized in Table 1. We believe that the presence of C 2 dimers in the plasma during CVD is crucial for the formation of various nanocarbon materials. Carbon dimers may play a dominant role in the synthesis of the nanocrystalline diamond (e.g. Refs. [6,7]). In particular, insertion of the C 2 into an acetylenelike C=C bond to produce a carbene structure is the most energetically favorable scenario. Empirical calculations have shown [7] that the addition of C 2 may result in the evolution of critical nuclei into two-dimensional graphitelike sheets, which become mechanically unstable and roll up spontaneously to form CNT in a non-catalytic process [4]. In conclusion, the range of electrical parameters of a stable dc discharge in a hydrogen–methane gas mixture is

determined for different total pressures and methane concentrations. Variation of the gas mixture composition allows deposition of different thin film carbon materials in the dc plasma by using a discharge current density in the range 0.2–0.5 A / cm 2 . Optical emission spectra of the dc discharge plasma measured during CVD process show the presence of CH, C 2 , H, H 2 activated species in the CVD plasma determining the carbon film formation. We also observe a direct condensation of carbon in the plasma gas phase for methane concentration exceeding 15%.

Acknowledgements This work has been supported in part by INTAS grant No 01-0254 and a grant from the Academy of Finland No.78013.

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