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The negatively-charged nitrogen-vacancy (NV) center in diamond has been shownrecently as an excellent sensor for external spins. Nevertheless, their optimumengineering in the near-surface region still requires quantitative knowledge inregard to their activation by vacancy capture during thermal annealing. To thisaim, we report on the depth profiles of near-surface helium-induced NV centers(and related helium defects) by step-etching with nanometer resolution. Thisprovides insights into the efficiency of vacancy diffusion and recombinationpaths concurrent to the formation of NV centers. It was found that the range ofefficient formation of NV centers is limited only to approximately $10$ to$15\,$nm (radius) around the initial ion track of irradiating helium atoms.Using this information we demonstrate the fabrication of nanometric-thin($\delta$) profiles of NV centers for sensing external spins at the diamondsurface based on a three-step approach, which comprises (i) nitrogen-dopedepitaxial CVD diamond overgrowth, (ii) activation of NV centers by low-energyhelium irradiation and thermal annealing, and (iii) controlled layer thinningby low-damage plasma etching. Spin coherence times (Hahn echo) ranging up to$50\,$ $\mu$s are demonstrated at depths of less than $5\,$nm in material with$1.1\,\%$ of $^{13}$C (depth estimated by spin relaxation (T$_1$)measurements). At the end, the limits of the helium irradiation technique athigh ion fluences are also experimentally investigated.

Towards optimized surface $\delta$-profiles of nitrogen-vacancy centers activated by helium irradiation in diamond