In this paper, a set of amorphous carbon nitride (a-C:N) films with di
fferent nitrogen contents were analysed. The most relevant deposition
parameter is the nitrogen partial pressure, and the role of nitrogen i
s to reduce the disordered sp(2) phase and the sp(3)/sp(2) ratio. The
origin of the electron spin resonance active centres is identified to
be the disordered sp(2) phase.