Paper GR-TuP1
Growth Mechanism of Carbon Nanowalls Synthesized by Irradiation of Independently Controlled Ar Ions and CF_X/H Radicals

Tuesday, October 21, 2008, 6:30 pm, Room Hall D

Carbon nanostructures such as carbon nanotubes, fullerenes, etc. are investigated intensively in the world. Among a nanocarbon family, we are focusing on carbon nanowalls (CNWs). The CNWs can be described as two-dimensional carbon nanostructures with edges comprising stacks of plane graphene sheets standing almost vertically on the substrate. The CNWs have several unique characteristics, e.g. high aspect ratio and large surface area. Hence, the CNWs attract great attentions for nanoscale electronic devices, storage materials for hydrogen gas, and fuel cells. Previously, we have fabricated CNWs using parallel-plate capacitively coupled plasma with hydrogen (H) radical injection. Considering the practical applications of CNWs, further investigations are required to clarify the growth mechanism for control of their morphologies and properties. In this study, two radical sources, inductively coupled plasmas (ICPs) using 13.56 MHz, and an ion source, a ICP using 13.56 MHz, were used in order to understand which radicals or ions contributed to the CNWs formation. The heated Si substrate was exposed to fluorocarbon (CF_X) and H radicals as well as Ar ions, of which densities were controlled in each source independently. The correlation between the characteristics of irradiated species, such as Ar ion energy and H radical density, and the CNWs morphologies was investigated employing a spectroscopic ellipsometry for in-situ observation on the substrate surface. It was found that CNWs were not formed with Ar ions of energies below 100 eV with CF_X and H radical injection. On the other hand, CNWs were successfully formed by Ar ions of energies between 100-200 eV, which were confirmed by SEM and Raman spectroscopy. The deposition rate of CNWs was proportional to the acceleration voltage of the Ar ions. As a result, not only fluorocarbon and hydrogen radicals but also high energy ions accelerated at energies of more than 100 eV were required for the nucleation of CNWs because higher energy ions created dangling bonds on the edge of CNWs, which will be a key for their growth. Furthermore, it was found out that the best value of hydrogen gas flow rate for the highest deposition rate and the better morphology of CNWs existed. The growth of high quality of CNWs with a high growth rate is determined by the balance of the ratio of H radicals to CF_X radicals. These results will be crucial to elucidate the mechanism of CNWs.