Mechanical polishing of diamond is known to be detrimental to the spin coherence time and strain environment of near-surface defects via intrinsic introduction of subsurface damage: this damage is typically removed by inductively coupled plasma reactive ion etching (ICP-RIE). By utilizing a chemical mechanical polishing (CMP) process to prepare ⟨001⟩ diamond surfaces, we demonstrate that we can achieve 13C-limited spin lifetimes of shallow implanted (≤34 nm) nitrogen vacancy (NV) centers in an industrially scalable process. We compare spin lifetimes (T2) of three diamonds processed with CMP with one processed by ICP-RIE and observe an increased median T2 of 340 μs in the CMP-processed samples for 15NV centers implanted and annealed under identical conditions.
Nitrogen vacancy (NV) color centers in diamond have shown great potential for various applications in quantum technology due to their long coherence times, high sensitivity to magnetic fields and atomic scale resolution. However, one major challenge in utilizing near surface NV centers is the decoherence caused by spins and charges fluctuating on the surface, which affects the spin properties of the sensors. To reduce the induced noise, various oxygen surface treatments such as low power oxygen plasma treatment and annealing under oxygen atmosphere have been explored to terminate the diamond surface and reduce its impact on NV coherence. We showed that the NV center’s coherence time can be enhanced up to a factor of 3 over a large spectral range of noise. Double electron–electron resonance measurements revealed an extra source of decoherence, scaling similarly as the P1 spin bath. The improvement in coherence times is accompanied with an increase in measured ketone/ether content and reduction of sp2 signal in x-ray photoelectron spectroscopy measurements. Finally we compared the performance of different NV ensembles and surface treatments for sensing external proton spins. The oxygen annealing is an effective procedure of enhancing the spin coherence times and reducing broad band spin noise experienced by shallow implanted ensemble NV centers in diamond.
Abstract Nitrogen vacancy (NV) color centers in diamond have shown great potential for various applications in quantum technology due to their long coherence times, high sensitivity to magnetic fields and atomic scale resolution. However, one major challenge in utilizing near surface NV centers is the decoherence caused by spins and charges fluctuating on the surface, which affects the spin properties of the sensors. To reduce the induced noise, various oxygen surface treatments such as low power oxygen plasma treatment and annealing under oxygen atmosphere have been explored to terminate the diamond surface and reduce its impact on NV coherence. We showed that the NV center’s coherence time can be enhanced up to a factor of 3 over a large spectral range of noise. Double electron–electron resonance measurements revealed an extra source of decoherence, scaling similarly as the P1 spin bath. The improvement in coherence times is accompanied with an increase in measured ketone/ether content and reduction of sp 2 signal in x-ray photoelectron spectroscopy measurements. Finally we compared the performance of different NV ensembles and surface treatments for sensing external proton spins. The oxygen annealing is an effective procedure of enhancing the spin coherence times and reducing broad band spin noise experienced by shallow implanted ensemble NV centers in diamond.
Noise is a detrimental issue for nitrogen-vacancy (NV) centers in diamond, causing line broadening and decreasing the coherence time
(T2). Following our previous electric and magnetic field noise work, we investigate noise caused by the diamond surface roughness, which is a source for charge density fluctuations and incoherent photon scattering. We find that the varying surface charge density noise source is prevalent throughout the entire NV dynamical decoupling frequency range, while the photon scattering noise is almost negligible. Next, we combine the results from various noise sources to perform comprehensive analyses on
T2 and how it varies with NV depth. At a given NV depth of 5
nm below a hydrogen- or fluorine-terminated surface, we find that these magnetic nuclei reduce the NV coherence time the most, followed by the surface electric field noise sources. The photon scattering and bulk magnetic field noise effects on
T2 are weak compared to the varying charge density, electric dipole, and surface impurity noise. However, with oxygen surface termination, the surface electric field noise sources are comparable to the surface magnetic field noise. Our calculated values of
T2,Hahn (few $\ensuremath{\mu}\mathrm{s}$ to 10 $\ensuremath{\mu}\mathrm{s}$) are in good agreement with the experimental values reported elsewhere. Finally, we calculate an anticipated signal-to-noise ratio (SNR) for NV AC magnetometry of external nuclear spins. In our simplified assessment, where some depth-dependent parameters (e.g., NV conversion efficiency) are held constant, we find that shallower NV layers should yield the best SNR, which is consistent with experimental findings.
The nitrogen-vacancy (NV) center in diamond exhibits spin-dependent fluorescence and long spin coherence times under ambient conditions, enabling applications in quantum information processing and sensing. NV centers near the surface can have strong interactions with external materials and spins, enabling new forms of nanoscale spectroscopy. However, NV spin coherence degrades within 100 nm of the surface, suggesting that diamond surfaces are plagued with ubiquitous defects. Prior work on characterizing near-surface noise has primarily relied on using NV centers themselves as probes; while this has the advantage of exquisite sensitivity, it provides only indirect information about the origin of the noise. Here we demonstrate that surface spectroscopy methods and single-spin measurements can be used as complementary diagnostics to understand sources of noise. We find that surface morphology is crucial for realizing reproducible chemical termination, and use this insight to achieve a highly ordered, oxygen-terminated surface with suppressed noise. We observe NV centers within 10 nm of the surface with coherence times extended by an order of magnitude.
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.