Structural characteristicsofcarbonnanostructuressynthesized by ECR-CVD

Microelectronics Journal 39 (2008) 1600–1604

Structural characteristics of carbon nanostructures synthesized

by ECR-CVD

Te-Hua Fang



, Tong Hong Wang

, Deng-Maw Lu

, Wen-Chieh Lien

a_{Institute of Mechanical and Electromechanical Engineering, National Formosa University, Yunlin 632, Taiwan} b_{Thermal Laboratory, Advanced Semiconductor Engineering, Inc., Kaohsiung 811, Taiwan}

c

Department of Mechanical Engineering, Southern Taiwan University, Tainan 710, Taiwan Received 21 December 2007; accepted 7 February 2008

Available online 8 April 2008

Abstract

In this study, carbon nanotubes (CNTs) and nanoparticles were synthesized by an electron cyclotron resonance-chemical vapor deposition (ECR-CVD) system. Results show that both high- and low-aspect-ratio CNTs and nanoparticles are found. The CNTs range in length from tens of nanometers to micrometers, and in outer diameter from about 5 to 50 nm. Transmission electron microscope (TEM) images show that the faceted nanoparticles exhibit polyhedral or onion or irregularly profiled fullerene structures, and the CNTs growth is from the interlayers lamination. The surface sheet resistance and average surface roughness of the CNT films are about 360 O per square and 7–17 nm, respectively. When the CNT sample has a higher amount of nanoparticles, the current density will be increased. r2008 Elsevier Ltd. All rights reserved.

Keywords: Carbon nanotubes; Nanoparticles; Carbon onion; Raman; ECR-CVD; Nanoindentation; Field emission

1. Introduction

Carbon nanotubes (CNTs) have attracted great interest and have the potential for several promising applications

[1–9], such as probes for atomic force microscopes (AFM)

[2], electron emitters for field emission displays (FED)[3], nanofillers for composite materials[4,5], and electrodes for fuel cells [6,7]. In addition, CNTs can interface with various materials, and are thus very practical. The other materials can be biomolecules, inorganic materials, and polymer coatings [8]. These CNTs, however, are strictly influenced by the processing parameters, which may have different mechanical, electrical, and optical properties.

Many deposition methods have been developed, and each method has its relative advantages for certain applications. Chemical vapor deposition (CVD) [9] is a popular way to synthesize CNT-based nanohybrids onto a specific substrate with the oxidation coupling with the metallic catalyst. The most commonly used metal catalysts are Fe, Co, Ni, and Mo in the form of particles.

The catalyst should be small particles so that the carbon precursor can diffuse into the catalyst via the surface diffusion paths with the lowest activation energy. Electron cyclotron resonance-chemical vapor deposition (ECR-CVD) [10] is a promising CVD technique because of its superior plasma species production rate and low substrate temperature during deposition.

In this paper the fabrication of CNTs on silicon substrate by ECR-CVD is conducted. The surface mor-phology, Raman spectra, energy-dispersive X-ray spectro-scopy (EDX), transmission electron microspectro-scopy (TEM) and the current density–electric field properties of the samples and their different appearances are discussed. 2. Experimental

Using ion beam sputtering (IBS), nickel (Ni) was used as the catalyst in this work to deposit on an n-type 10  10 mm2 Si substrate at room temperature. The thickness of the nickel catalyst was about 5–10 nm. In turn, an ECR-CVD system[10] was used to grow CNTs. The base pressure and power were set at 10 6Torr and 400 W, respectively. A flow ratio of propane (C3H8) to

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Corresponding author. Tel.: +886 5 6315395; fax: +886 5 6315397. E-mail address:[email protected] (T.-H. Fang).

future the phase transformation and aggregated mechan-ism behavior of hybrid CNTs and fullerenes would be investigated using molecular dynamics simulation[19].

Fig. 7 shows the nanoindentation load-depth curve for MWCNTs grown on Si substrates. The higher indentation load caused a deeper penetration depth. The indentation hardness and the contact stiffness of the samples were about 0.4–4 GPa and 10–70 kN/m, respectively. The con-tact stress was about 0.9–1.6 GPa within a concon-tact strain of 0.03. The calculated Young’s modulus was about 12–25 GPa. The lower hardness and Young’s modulus of the samples were due to their defects and porous effects.

Fig. 8 shows the current density–electric field strength properties of the CNT samples. Sample A had a great amount of nanoparticles than that of sample B. From the experimental data it can be found that no clear turn-on electric field was obtained. The field emission properties are

measured to evaluate the validity of the growth of CNTs. The result shows the CNT sample has a higher amount of nanoparticles, the current density will be increased. 4. Conclusions

The growth of the CNTs is only from the internal tilt wall lamination, while the external layer is gradually etched over time and ultimately a tubular cone is formed. It is shown that the interlayers of a CNT are formed by the lamination growth. The faceted nanoparticles exhibit polyhedral or onion or irregularly profiled fullerene structures. The outer shape of the nanoparticles may depend on the size of their inner cavity. The surface sheet resistance and average surface roughness of the CNT films are about 360 Oper square and 7–17 nm, respectively. The CNT sample has a higher amount of nanoparticles, the current density will be increased.

Acknowledgment

The authors gratefully acknowledge the support by the National Science Council of Taiwan, under Grant no. NSC 95-2221-E150-066.

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Fig. 6. TEM image of the CNTs and nanoparticles.

Fig. 7. Nanoindentation load-depth curve of the MWCNTs sample.

Fig. 8. Field emission characteristics of the MWCNTs samples (sample A with larger nanoparticles and sample B with smaller nanoparticles). T.-H. Fang et al. / Microelectronics Journal 39 (2008) 1600–1604 1603

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