Surface and interface reactions of catalysts for carbon nanotube growth on Si substrates studied by soft X-ray photoelectron spectroscopy

The substrate temperature dependence of the chemical state of Si substrate surface and metal catalysts for the growth of carbon nanotubes (CNTs), Co and Fe, and their morphology were investigated using soft X-ray photoelectron spectroscopy. We found that the temperature at which the chemical states of Co changes depends on the initial thickness of the underlying SiO2 layer. In the case of Fe thin film on a thin SiO2 layer, temperatures of transformation to metal and silicides are higher than those for the Co layer. In addition, Co silicides coexist with the SiO2 layer on the Si substrate at the substrate temperature of 590°C, but Fe silicides do not. These results indicate that, in the case of CNT growth by chemical vapor deposition, we have to contrive different ways to control the chemical states of metal catalysts for different substrate surfaces and metal catalysts.