Evaluation of metal-free carbon nanotubes formed by SiC thermal decomposition
Thermal decomposition on SiC surface at high temperature and at low residual oxygen pressure yields a thin film of metal-free carbon nanotubes (CNTs) aligned on the SiC. In this study, CNT films on carbon-face 4H-SiC are produced under vacuum from 10−3 to 10−7 Torr and at temperatures ranging from 1400 to 1700 °C. Ex situ spectroscopic ellipsometry (SE) technique is applied to investigate the structure of the CNT films. It is found that two thin interfacial layers on the top and bottom of the CNTs are crucial in modeling the film structure from the SE data. We suggest a five layer structural model that consists of a CNT film as the main component, two interfacial layers, an amorphous carbon layer, and a surface roughness layer. This structural model based on the SE measurements is confirmed by cross-section transmission electron microscopy and can be applied for all CNT/SiC structures grown at various pressures and temperatures. At the growth pressure of 10−5 Torr, the CNT layer on SiC consists of 30%–50% CNTs, 35%–60% voids, and 5%–12% amorphous carbon in volume.