Critical silicon dioxide thickness for CVD growth of single-walled carbon nanotubes

Chemical vapor deposition (CVD) has shown remarkable control over the efficient and directed assembly of single-walled carbon nanotubes, making CVD a primary growth method for device applications. Due to the high temperatures involved in CVD, the chemical compatibility between the substrate, feedstock, and catalyst must be understood. Using x-ray photoelectron spectroscopy (XPS), we have studied the evolution of the chemical state of an iron nitrate catalyst during the initial temperature ramp of a standard CVD process. Heating the catalyst on clean silicon or on silicon with a native oxide leads to the formation of a silicide at 800~$^{o}$C, inhibiting single-walled nanotube growth. By 900~$^{o}$C, a typical growth temperature, all of the iron catalyst has been incorporated into the silicide. Thicker silicon oxide layers, on the order of 10~nm, effectively prevent silicide formation, enabling high yield growth.