| In this study, various plasma diagnostics were used to determine which gas-phase species were present under conditions that were favorable for plasma-enhanced chemical vapor deposition (PECVD) growth of carbon nanotubes on metal catalysts. The nature of the deposited material and the identification and relative concentrations of species were determined as a function of varying pressures, plasma source powers, gas compositions, and bias on the substrate.;PECVD was mostly performed at 100 mTorr with C2H2/H 2. Removal of the oxide on the metal catalysts with a H2 plasma pretreatment improved the density of carbon nanotube growth. Carbon nanotubes were well formed in H2 rich gas mixtures, but shapeless carbon structures were observed with high C2H2 content due to large coverage of carbon atoms on the catalyst particles and formation of larger hydrocarbon products in the plasma. High power caused a cessation of carbon nanotube growth relative to low power because the plasma density increased abruptly at high power and H atoms were generated at considerably high levels, leading to conditions under which carbon nanotube are etched faster than they can grow. The improved vertical alignment of carbon nanotubes at high bias powers is most likely caused by the presence of a large electric field adjacent to the substrate at high bias power.;The plasma chemistry of NH3 decomposition in NH3/Ar/He plasmas at 1 Torr was also investigated. The number densities of NH3 decreased with increasing power and substrate heater temperature. N 2 densities measured by using a new "self-actinometry" method increased with power and substrate heater temperature, and account for all of the nitrogen in the dissociated NH3. The major hydrogen-containing species appeared to be H2, however, a substantial amount of H-atoms (comparable to H2) was present at the highest powers.;The nanotubes grown by PECVD at 900 K, 1 Torr, and no rf bias with C 2H2/NH3 feed gas were not vertically aligned even with rf bias applied to the substrate because an electric field on the substrate was too low to cause alignment by a large sheath width and low bias voltage at 1 Torr pressure and low plasma density. |
