Abstract
Carbon nanotubes/amorphous carbon fibres were grown using a newly installed Thermal Chemical Vapour Deposition (TCVD) system. A Focused Ion Beam microscope was used to fabricate carbon nanotube devices and to understand the I-V characteristics of the carbon nanotubes. As a first step of the study, optimisation work was carried out to grow single wall carbon nanotubes and multi wall carbon nanotubes with narrow diameters, which can be used for electronic device applications. Carbon fibre growth was carried out using methane, hydrogen and helium. Nickel and molybdenum/iron films were used as a catalyst, and aluminium, silicon oxide and chromium were used as an interlayer. When growth was carried out with the nickel catalyst, the carbon fibres had an amorphous structure. Large catalyst particles appeared at their tips which suggested that the tip growth and bulk diffusion are dominant in the growth mechanism. By changing the growth conditions, vertically standing amorphous carbon fibres were obtained, with high yields. On the other hand, when molybdenum/iron catalyst was used, carbon nanotubes were obtained. The crystallinity and the yield of carbon fibres varied when changing the hydrogen concentration at the initial period of growth regime. We revealed that hydrogen can contribute positively and negatively to the carbon nanotube growth depending on the concentration and the process temperature. The TCVD grown nanotubes were used in prototype electronic devices to understand their I-V characteristics. The FIB technique enables the fabrication and testing of the devices with great flexibility. By depositing tungsten pads on carbon nanotubes, which are touching on an Au electrode, low contact resistances were achieved. The I-V characteristics show diffusive conductance and high resistance. However, by shortening the gate length, the current dramatically increased, with an increase of the exponent of the power law describing the conduction. The maximum conductance obtained in this study was 12 times larger than the quantum conductance. We concluded that localisation resulted in the power law I-V characteristics, and by shortening the gate length, the conduction was shifted from diffusive to weakly localised carrier transports with multi channels.