  The ability to integrate carbon nanotubes, especially single-walled carbon
nanotubes, seamlessly onto silicon would expand the range of applications
considerably. Though direct integration using chemical vapor deposition is the
simplest method, the growth of single-walled carbon nanotubes on bare silicon
and on ultra-thin oxides is greatly inhibited due to the formation of a
non-catalytic silicide. Using x-ray photoelectron spectroscopy, we show that
silicide formation occurs on ultra-thin oxides due to thermally activated metal
diffusion through the oxide. Silicides affect the growth of single-walled
nanotubes more than multi-walled nanotubes due to the increased kinetics at the
higher single-walled nanotube growth temperature. We demonstrate that nickel
and iron catalysts, when deposited on clean silicon or ultra-thin silicon
dioxide layers, begin to form silicides at relatively low temperatures, and
that by 900C, all of the catalyst has been incorporated into the silicide,
rendering it inactive for subsequent single-walled nanotube growth. We further
show that a 4 nm silicon dioxide layer is the minimum diffusion barrier
thickness which allows for efficient single-walled nanotube growth.
