Effects of source gases on the growth of carbon nanotubes

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Abstract

We carry out the in situ analysis of chemical species for the growth of carbon nanotubes (CNTs) in direct current plasma enhanced chemical vapor deposition (DC-PECVD) with C2H2–NH3, C3H4–NH3 and CO–NH3 mixtures by optical emission spectroscopy (OES). From OES analysis, it was shown that the hydrogen related radical was regarded as etching species in CNTs growth and the C2 and CH radical as available carbon sources for the growth of CNTs irrespective of carbon source gas. As the NH3 flow rate changed, different chemical species in plasma made effects on the growth of CNTs depending on the source gas.

Introduction

CNTs have been extensively investigated in the past few years because of their potentials in science and applications in many fields [1], [2], [3], [4], [5]. Many methods to synthesize the CNTs have been reported including arc discharge [6], laser vaporization [7], pyrolysis [8], and plasma enhanced chemical vapor deposition (PECVD) [9]. Especially, PECVD method has been reported as a promising candidate for the synthesis of CNTs owing to its several advantages such as low temperature growth, easy scale up, vertical alignment, and compatibility with a conventional Si process. It was reported that source gases played a critical role in CNTs growth and various gases including C2H2, CH4, C3H4 and CO were used for CNTs growth. Few works on the effect of source gas on the CNTs growth especially in the PECVD system were reported.

In this work, the effects of source gases on the CNTs growth were investigated. We have grown CNTs using C2H2, C3H4 and CO gas as a carbon source and NH3 gas as a catalytic gas, and studied the effect of the NH3 flow rate on chemical species in the plasma of various gas mixtures by optical emission spectroscopy (OES) measurement. We investigated the relationship between plasma characteristics and growth of CNTs in various source gases. The role of radical species during the CNTs growth was studied.

Section snippets

Experiment

The CNTs were grown on different catalyst metals (Ni, Co and Fe) coated glass substrate with Cr buffer layers using DC-PECVD. A Cr layer of 1500 Å thick was coated on a glass substrate. Subsequently, catalyst metals layer of 300 Å in thickness was deposited using electron beam evaporation. The CNT growth was performed at a temperature of approximately 550 °C by the PECVD method. A C2H2, C3H4 and CO gas was used as the carbon source and the NH3 gas was used as the catalyst and a dilution gas. A

Results and discussion

Fig. 1 shows that OES spectra detected from the plasma of C2H2–NH3, C3H4–NH3 and CO–NH3 gas mixtures for CNTs growth. For the C2H2–NH3 and C3H4–NH3 gas mixture, the following emission lines were commonly observed. Chemical species identified in this study include NH at 336 nm, N2+ at 358.5 nm, CN at 386, 387 and 388 nm, CH at 431 nm, C2 at 436 nm, H2 at 541.5, 546, 611, and 611.5 nm, Hα at 656.5 nm, and Hβ at 486 nm. Among those mentioned above, the Hα, N2, N2+ and NH bands resulted from NH3

Conclusions

We carried out in situ diagnosis on chemical species from the plasma of the C2H2–NH3, C3H4–NH3 and CO–NH3 mixtures for the growth of CNTs in DC-PECVD using OES. The change in chemical species in plasma with various carbon source gases affected the growth characteristics of CNTs. In the C2H2–NH3 system, as the NH3 flow rate increased, relative intensities of Hα and CN radical increased. Thus, the CNTs growth was suppressed by the enhanced etching effect. In the case of C3H4–NH3 system, as the NH3

Acknowledgements

The Ministry of industry and Energy of Korea through Samsung Advanced Institute of Technology and CNNC supported this work.

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