  Scalable realizations of quantum network technologies utilizing the nitrogen
vacancy center in diamond require creation of optically coherent NV centers in
close proximity to a surface for coupling to optical structures. We create
single NV centers by $^{15}$N ion implantation and high-temperature vacuum
annealing. Origin of the NV centers is established by optically detected
magnetic resonance spectroscopy for nitrogen isotope identification. Near
lifetime-limited optical linewidths ($<$ 60 MHz) are observed for the majority
of the normal-implant (7$^\circ$, $\approx$ 100 nm deep) $^{15}$NV centers.
Long-term stability of the NV$^-$ charge state and emission frequency is
demonstrated. The effect of NV-surface interaction is investigated by varying
the implantation angle for a fixed ion-energy, and thus lattice damage profile.
In contrast to the normal implant condition, NVs from an oblique-implant
(85$^\circ$, $\approx$ 20 nm deep) exhibit substantially reduced optical
coherence. Our results imply that the surface is a larger source of
perturbation than implantation damage for shallow implanted NVs. This work
supports the viability of ion implantation for formation of optically stable NV
centers. However, careful surface preparation will be necessary for scalable
defect engineering.
