Characterisation of the growth mechanism during PECVD of multiwalled carbon nanotubes

The growth mechanism of multiwalled carbon nanotubes has been a subject of considerable research interest. Published results show that nanotubes may be formed by a variety of methods using different combinations of carbon-rich gases and etchant gases. We present an overview of different forms of carbon, showing how nanotubes relate to other carbon structures and how they may be produced. The use of plasma-enhanced chemical vapour deposition (PECVD) for growing vertically aligned nanotubes for electronic device applications is reported, including an analysis of the species present in the plasma during PECVD. It is shown that the presence of a reactive species such as ammonia suppresses decomposition of carbon feedstock gas and favours the formation of carbon nanotubes. It is further shown that gas-phase reactions remove excess carbon, allowing the production of nanotubes without unwanted amorphous carbon deposits, which is essential for electronic applications. Plasma analysis is used to determine an optimum carbon-source to etchant gas ratio, which is verified by postdeposition analysis.