Using electron energy loss spectroscopy in a 100 kV VG
scanning transmission electron microscope we study nitrogen doped carbon
nanotubes grown via electron cyclotron resonance (ECR) microwave plasma
techniques. The process is controlled by direct current (dc) biasing the
grid separating the ECR source and the substrate. We show that plasma
induced sputtering of the ECR source wall (stainless steel) can lead to
significant iron and chromium contamination of growth samples. We identify
various Fe, Cr, Ni nitride phases, and propose a growth model based on
nitridation-induced metal segregation of steel based FeCrN alloys. Trace Cr
doping of nanotube catalysts appears a promising route for introducing large
nitrogen concentrations into both single and multi-walled nanotubes and may
accelerate nanotube growth rates.