  Nitrogen impurities help to stabilize the negatively-charged-state of NV$^-$
in diamond, whereas magnetic fluctuations from nitrogen spins lead to
decoherence of NV$^-$ qubits. It is not known what donor concentration
optimizes these conflicting requirements. Here we used 10-MeV $^{15}$N$^{3+}$
ion implantation to create NV$^-$ in ultrapure diamond. Optically detected
magnetic resonance of single centers revealed a high creation yield of
$40\pm3$% from $^{15}$N$^{3+}$ ions and an additional yield of $56\pm3$% from
$^{14}$N impurities. High-temperature anneal was used to reduce residual
defects, and charge stable NV$^-$, even in a dilute $^{14}$N impurity
concentration of 0.06 ppb were created with long coherence times.
