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

TL;DR

This study investigates the formation and optimization of near-surface nitrogen-vacancy centers in diamond activated by helium irradiation, providing a method to create nanometric NV profiles for enhanced quantum sensing.

Contribution

It introduces a quantitative analysis of NV center depth profiles using step-etching and demonstrates a three-step fabrication process for ultra-thin NV layers with high spin coherence.

Findings

NV centers form within 10-15 nm of helium ion tracks

Achieved spin coherence times up to 50 μs at less than 5 nm depth

Identified limits of helium irradiation at high ion fluences

Abstract

The negatively-charged nitrogen-vacancy (NV) center in diamond has been shown recently as an excellent sensor for external spins. Nevertheless, their optimum engineering in the near-surface region still requires quantitative knowledge in regard to their activation by vacancy capture during thermal annealing. To this aim, we report on the depth profiles of near-surface helium-induced NV centers (and related helium defects) by step-etching with nanometer resolution. This provides insights into the efficiency of vacancy diffusion and recombination paths concurrent to the formation of NV centers. It was found that the range of efficient 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 diamond surface based on a three-step approach, which comprises (i) nitrogen-doped epitaxial CVD diamond overgrowth, (ii) activation of NV centers by low-energy helium irradiation and thermal annealing, and (iii) controlled layer thinning by 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 at high ion fluences are also experimentally investigated.