Numerical analysis of pressure dependence on carbon nanotube growth in CH4/H2 plasmas

The pressure dependence of a CH4/H2 gas mixture plasma was analyzed for carbon nanotube (CNT) growth by one-dimensional fluid modeling and compared with the experimental results obtained under the same condition as the analysis. When the gas pressure was 10 Torr, vertically aligned CNTs are grown with a high number density. As the pressure increases, densities of the non-radical neutrals, radical neutrals and ions with a lager molecular weight than CH4 tend to increase but those of the smaller species including CH3 and CH2 decrease. This is explained by the hydrogen abstraction reaction of CH4 with H atoms and the production reaction of C2H5 due to incorporation of CH3 radicals. The fluxes of these carbon-bearing species onto the substrate surface were evaluated. As the gas pressure increased, the flux of positive ions decreased. Instead, the contribution of higher-order non-radical species, including C3H8, to CNT formation at high gas pressure is suggested.