Iron and cobalt silicide catalysts-assisted carbon nanostructures on the patterned Si substrates
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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2007, Applied Surface ScienceGrowth of self-aligned carbon nanotube for use as a field-effect transistor using cobalt silicide as a catalyst
2007, CarbonCitation Excerpt :Nevertheless, the SWCNT growth between two pre-defined CoSi2 patterns and its feasibility for CNTFET application were not reported in above works. To continue the previous efforts [10–14], we explore the self-aligned SWCNT growth using the pre-defined CoSi2 patterns [15], both as the catalyst and source/drain electrodes, and its feasibility for CNTFET fabrication. Though the CoSi2 is not as active as Co, Ni or Fe for the use as a catalyst for CNT growth, there are other advantages of CoSi2 which make it attractive and worth of further investigation.
Carbon nanotubes - Production and industrial applications
2007, Materials and DesignMicrowave activated combustion synthesis of bulk cobalt silicides
2005, Journal of Alloys and CompoundsGrowth of vertically aligned carbon nanotube emitters on patterned silicon trenches for field emission applications
2005, Thin Solid FilmsCitation Excerpt :The trench structure can permit a favorable condition for a vertically aligned growth of CNT emitters. A selective growth of CNTs has been carried out by many research groups on plat substrates [11–16], while there were rare reports on the vertically aligned growth of CNTs in triode type trench structure by chemical vapor deposition (CVD) method. In this work, CNT emitters have been obtained on patterned Si trenches with various shapes and sizes by thermal CVD process.