  We characterize single nitrogen-vacancy (NV) centers created by 10-keV N+ ion
implantation into diamond via thin SiO$_2$ layers working as screening masks.
Despite the relatively high acceleration energy compared with standard ones (<
5 keV) used to create near-surface NV centers, the screening masks modify the
distribution of N$^+$ ions to be peaked at the diamond surface [Ito et al.,
Appl. Phys. Lett. 110, 213105 (2017)]. We examine the relation between
coherence times of the NV electronic spins and their depths, demonstrating that
a large portion of NV centers are located within 10 nm from the surface,
consistent with Monte Carlo simulations. The effect of the surface on the NV
spin coherence time is evaluated through noise spectroscopy, surface
topography, and X-ray photoelectron spectroscopy.
