This thesis presents research on improving spin coherence times of bulk and single NV centres in CVD diamond via material engineering, alongside electrical readout of the NV electronic ground state. A chemical-mechanical polishing (CMP) technique is investigated to remove sub-surface damage on (001) surfaces caused by scaife polishing. 13C-limited coherence times of shallow implanted NV centres were compared across samples prepared with ICP-RIE or CMP, with CMP samples showing an improvement of 340µs. The effect of co-doping with phosphorus and sulphur on charge stability, creation yield, spin coherence times, and AC sensitivity of shallow NV centres was studied. 10 CVD diamond samples were characterised using above-bandgap imaging, photoluminescent, and Raman spectroscopy. Coherence times in four samples were measured with a home-built confocal microscope. A sample implanted with 110keV32S ions at 1011cm-2 showed the best shot noise-limit to the AC sensitivity. Preferentially-oriented NV centres were produced on pseudo-(111) surfaces. (001) diamonds were nickel-etched to produce (111) trenches overgrown with nitrogen-containing diamond. Preferential orientation was evidenced via optically detected magnetic resonance under an external ⟨111⟩ magnetic field. High-density nitrogen overgrowth in trenches gave short T2 times (<2µs), while lower-density samples showed mean spin coherence times of 160µs, closer to the unetched surface. Finally, a home-built device for electrical spin-state readout is described, including a confocal microscope, metallic contacts, and a readout PCB. Photoelectrically detected magnetic resonance contrast of 63% (compared to 5% optically) is demonstrated , with initial RF device development.
Scalable realizations of quantum network technologies utilizing the nitrogen vacancy center in diamond require creation of optically coherent NV centers in close proximity to a surface for coupling to optical structures. We create single NV centers by
15N ion implantation and high-temperature vacuum annealing. Origin of the NV centers is established by optically detected magnetic resonance spectroscopy for nitrogen isotope identification. Near lifetime-limited optical linewidths (
< 60 MHz) are observed for the majority of the normal-implant (7
∘,
≈ 100 nm deep)
15NV centers. Long-term stability of the NV
− charge state and emission frequency is demonstrated. The effect of NV-surface interaction is investigated by varying the implantation angle for a fixed ion-energy, and thus lattice damage profile. In contrast to the normal implant condition, NVs from an oblique-implant (85
∘,
≈ 20 nm deep) exhibit substantially reduced optical coherence. Our results imply that the surface is a larger source of perturbation than implantation damage for shallow implanted NVs. This work supports the viability of ion implantation for formation of optically stable NV centers. However, careful surface preparation will be necessary for scalable defect engineering.
We investigate the correlation between surface roughness and corresponding
T2 times of nearsurface nitrogen-vacancy centers (~7 nm/ 5 keV implantation energy) in diamond. For this purpose we compare five different polishing techniques, including both purely mechanical as well as chemical mechanical approaches, two different substrate sources (Diam2tec and Element Six) and two different surface terminations (O- and H-termination) during nitrogen-vacancy forming. All coherence times are measured and compared before and after an oxygen surface treatment at 520 °C. We find that the coherence times of shallow nitrogen-vacancy centers are surprisingly independent of surface roughness.
We demonstrate nanometer-precision depth control of nitrogen-vacancy (NV) center creation near the surface of synthetic diamond using an in situ nitrogen delta-doping technique during plasma-enhanced chemical vapor deposition. Despite their proximity to the surface, doped NV centers with depths (d) ranging from 5 to 100 nm display long spin coherence times, T2 > 100 μs at d = 5 nm and T2 > 600 μs at d ≥ 50 nm. The consistently long spin coherence observed in such shallow NV centers enables applications such as atomic-scale external spin sensing and hybrid quantum architectures.
Scaling of diamond defect center based quantum registers relies on the ability to position nitrogen vacancy (NV) centers with high spatial resolution. Using ion implantation, shallow (<10 nm) NVs can be placed with accuracy below 20 nm, but generally show reduced spin properties compared to bulk NVs. We demonstrate the enhancement of spin properties for shallow implanted NVs using a diamond overgrowth technique. An increase of coherence times up to an order of magnitude (T2 = 250 μs) was achieved, ms decoherence times were realized using dynamical decoupling. This marks a further step towards achieving strong coupling among defects positioned with nm precision.
Negatively charged nitrogen-vacancy (NV
−) centers located a few nanometers below the diamond surface are key quantum defects for nanoscale sensing of external spins. However, the creation of shallow NV
− centers with high yield remains a materials challenge. Here, we demonstrate that high-angle ion implantation enhances the creation efficiency of shallow NV
− centers. By implanting
15N ions at angles exceeding 60
∘, we achieve high NV
− yields approaching 10% with effective NV
− depths below 10 nm. These yields are significantly higher than those typically reported for shallow NV
− creation. The enhanced NV
− yield is consistent with an increased vacancy-to-nitrogen ratio in the near-surface region, which is expected to promote NV formation during annealing. The created NV
− ensembles show coherence properties comparable to those of single shallow NV
− centers at similar depths, and allow detection of nuclear spins in van der Waals materials attached to the diamond surface. Our results establish geometric control of ion implantation as a simple and broadly applicable approach to engineer shallow vacancy-related quantum defects.