  Nitrogen is frequently included in chemical vapour deposition feed gases to
accelerate diamond growth. While there is no consensus for an atomistic
mechanism of this effect, existing studies have largely focused on the role of
sub-surface nitrogen and nitrogen-based adsorbates. In this work, we
demonstrate the catalytic effect of surface-embedded nitrogen in nucleating new
layers of (100) diamond. To do so we develop a model of nitrogen overgrowth
using density functional theory. Nucleation of new layers occurs through C
insertion into a C--C surface dimer. However, we find that C insertion into a
C--N dimer has substantially reduced energy requirements. In particular, the
rate of the key dimer ring-opening and closing mechanism is increased 400-fold
in the presence of nitrogen. Full incorporation of the substitutional nitrogen
defect is then facilitated through charge transfer of an electron from the
nitrogen lone pair to charge acceptors on the surface. This work provides a
compelling mechanism for the role of surface-embedded nitrogen in enhancing
(100) diamond growth through the nucleation of new layers. Furthermore, it
demonstrates a pathway for substitutional nitrogen formation during chemical
vapour deposition which can be extended to study the creation of
technologically relevant nitrogen-based defects.
