Elsevier

Thin Solid Films

Volumes 420–421, 2 December 2002, Pages 219-224
Thin Solid Films

Iron and cobalt silicide catalysts-assisted carbon nanostructures on the patterned Si substrates

https://doi.org/10.1016/S0040-6090(02)00749-6Get rights and content

Abstract

Catalyst-assisted carbon nanotubes (CNTs) and carbon nano-rods were synthesized on the patterned or un-patterned Si wafer by microwave plasma chemical vapor deposition, using iron or cobalt silicide catalysts. Controllable carbon nanostructures were achieved by manipulating carbon and nitrogen concentration in the source gases, catalyst materials, and patterned wafer application. CNTs were synthesized under a high ratio of CH4/H2=0.1, while carbon nano-rods were synthesized under a low ratio of CH4/H2=0.01. Introducing N2 gas into CH4/H2 source gases gives rise to bamboo-like CNTs formation. Selective CNTs depositions were applied on (a) parallel Fe-coated line arrays, (b) CoSix-coated hole arrays. This is a novel method that is compatible with Si microelectronic device manufacturing. The field emission results indicate that the emission current density can be above 1 mA/cm2 at 3.97 V/μm, and hollow like CNTs belong to better emission current than bamboo-like CNTs. Growth models of different carbon nanostructures are proposed.

Introduction

Carbon nanotubes (CNTs) that consist of sheet(s) of graphite (a hexagonal lattice of carbon) rolled into a cylinder, were discovered in 1991 by Iijima [1]. This novel material has inspired much excitement in recent years and a large amount of research has been dedicated to their understanding. Mechanically, the axial Young's modulus of multi-walled CNTs can be as high as 1800 GPa, and the tubes can thus be much stiffer than commercial carbon fibers [2]. Electrically, the tubes are either metallic or semiconducting depending on their chirality [3], [4]. Chemically, the tubes are inert and highly resistant to oxidation [5]. The unique properties of CNTs are also predicted to possess a great potential in applications of scanning probes [6], field emission (FE) display [7], anode for lithium ion batteries [8], nano-electronic devices [9], supercapacitors [10], molecular sensors [11] and hydrogen storages [12].

The goals of CNTs synthesis can be considered to be CNTs with highly ordered orientations, fewer defects and reproducibly controllable properties. Chemical vapor deposition (CVD) method offers advantages of controlling CNT orientations and properties, by varying synthesis parameters and substrate pretreatment. For example, an enhanced CVD approach can obtain directionally suspended SWNTs networks in which the SWNTs are aligned and parallel to the substrate [13]. Also, CVD methods can be used to obtain various kinds of CNTs, such as CNTs with a perfect Y-junction or straight line, bridging CNTs on two parallel patterned structures, the selective, lateral growth of bamboo-like CNTs [14], [15].

This study systematically explores the carbon nanostructures synthesized by microwave plasma CVD (MPCVD) with various compositions and ratios of source gases. The novel catalyst of CoSix, much employed as a contact and as gate electrode films for Si microelectronic devices, is used, and Fe catalyst is used for comparison. The selective growth of CNTs on parallel line arrays, and hole arrays is also explored. Growth models of different carbon nanostructures are proposed.

Section snippets

Experimental

CNTs and carbon nano-rods were synthesized on Si wafers and patterned Si wafers with parallel line arrays and holes, as shown in Fig. 1, by a MPCVD system with source gases of CH4+H2 or CH4+H2+N2. Two kinds of catalytic films were employed on Si and on the patterned wafers before carbon nanostructures synthesis: (1) Fe film (20 Å) and (2) Co film (75 Å) followed by two-step rapid thermal annealing (RTP) at 600 °C for 60 s and 760 °C for 20 s, under N2 ambient. The solid-state interaction

Nanostructures morphologies on line arrays and on flat substrate

The process conditions in this study can be divided into two series. The first uses different source gases, either CH4/H2=10/100 sccm (Sample 1) or CH4/H2/N2=10/100/100 sccm gases (Sample 2). The second uses the same source gases but different catalysts, either Fe catalysts (Samples 3 and 4) or CoSix catalyst (Sample 5). For the first series, Fig. 2a presents the morphologies of CNTs (Sample 1) on the patterned Si wafers with Fe as catalysts. According to this figure, the CNTs of 18 μm in

Conclusions

This work develops the synthesis processes to selectively deposit different carbon nanostructures, including CNTs and carbon nano-rods, on patterned or un-patterned Si substrates using Fe and CoSix as catalysts. Effects of N2 addition, catalyst materials and carbon concentration on the formation of various nanostructures were studied. The results show that adding nitrogen is crucial to the formation of bamboo-like CNTs. A possible mechanism is also proposed. Various carbon nanostructures, such

Acknowledgements

The authors would like to thank the supports of the National Science Council (Contract No.: NSC90-2216-E-009-034, -035 and -040) and the Ministry of Education of Taiwan (Contract No.: 89-E-FA06-1-4).

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