Toward Optimized Surface δ-Profiles of Nitrogen-Vacancy Centers Activated by Helium Irradiation in Diamond

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 (δ) 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 μs are demonstrated at depths of less than 5 nm in material with 1.1% of (13)C (depth estimated by spin relaxation (T1) measurements). At the end, the limits of the helium irradiation technique at high ion fluences are also experimentally investigated.

Nanoscale Sensing Using Point Defects in Single-Crystal Diamond: Recent Progress on Nitrogen Vacancy Center-Based Sensors

“…NV yields reached values of 15% [52] and 50% [50], however, remaining quite low (1.9%) in [49]. As discussed below, often, δ-doping enhances the coherence times of shallow NVs compared to N implantation.…”

“…To be used in scanning probe sensing, NV centers should have a depth below ≈10 nm. As mentioned before, this can be achieved by tuning the dimension of the cap layer [47] or adding a last etching step [52]. In this depth range, the maximum reached T 2 is 200 µs obtained when irradiating an isotopically-purified sample with electrons [47].…”

“…Very recently, overgrowth of a nitrogen-terminated diamond surface has been employed for δ-doping [51]. -Vacancies are created ex situ by implanting helium ions [52,53], carbon ions [49] or irradiating with electrons [47,50]. A subsequent annealing at high temperature causes vacancy diffusion and creates NV centers (Figure 2c).…”

“…With this method, it is possible to increase the NV density in the δ-doped layer by increasing the 12 C dose without activating NV centers in the substrate. An alternative method, applied to low energy helium implantation, consists of growing a thicker N-doped layer (d = 18 nm) and adding a last etching step to create NV centers near the surface [52].…”

“…In this section, we discuss how N δ-doping [47,51] and ion implantation [52,75,76] or electron irradiation [50] can lead to improved spin coherence. Figure 4 summarizes recently obtained T 2 values for different methods and works.…”