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Register a new userShallow NV centers augmented by exploiting n-type diamond
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Creation of nitrogen-vacancy (NV) centers at the nanoscale surface region in diamond, while retaining their excellent spin and optical properties, is essential for applications in quantum technology. Here, we demonstrate the extension of the spin-coherence time ($it{T}$${_2}$), the stabilization of the charge state, and an improvement of the creation yield of NV centers formed by the ion-implantation technique at a depth of $sim$15 nm in phosphorus-doped n-type diamond. The longest $it{T}$${_2}$ of about 580 $mu$s of a shallow NV center approaches the one in bulk diamond limited by the nuclear spins of natural abundant $^{13}$C. The averaged $it{T}$${_2}$ in n-type diamond is over 1.7 times longer than that in pure non-doped diamond. Moreover, the stabilization of the charge state and the more than twofold improvement of the creation yield are confirmed. The enhancements for the shallow NV centers in an n-type diamond-semiconductor are significant for future integrated quantum devices.
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Near-surface nitrogen-vacancy ({NV}) centers in diamond have been successfully employed as atomic-sized magnetic field sensors for external spins over the last years. A key challenge is still to develop a method to bring NV centers at nanometer proximity to the diamond surface while preserving their optical and spin properties. To that aim we present a method of controlled diamond etching with nanometric precision using an oxygen inductively coupled plasma (ICP) process. Importantly, no traces of plasma-induced damages to the etched surface could be detected by X-ray photoelectron spectroscopy (XPS) and confocal photoluminescence microscopy techniques. In addition, by profiling the depth of NV centers created by 5.0 keV of nitrogen implantation energy, no plasma-induced quenching in their fluorescence could be observed. Moreover, the developed etching process allowed even the channeling tail in their depth distribution to be resolved. Furthermore, treating a 12C isotopically purified diamond revealed a threefold increase in T2 times for NV centers with <4 nm of depth (measured by NMR signal from protons at the diamond surface) in comparison to the initial oxygen-terminated surface.
The study establishes that the degree of optically induced spin polarization that can be achieved for NV$^- $in 1b diamond is limited by the concentration of single substitutional nitrogen, N$^0$ . The polarization of the individual NV centres in the diamond is dependent on the separation of the NV$^-$ and the nitrogen donor. When the NV$^-$ - N$^+$ pair separation is large the properties of the pair will be as for single sites and a high degree of spin polarization attainable. When the separation decreases the emission is reduced, the lifetime shortened and the spin polarization downgraded. The deterioration occurs as a consequence of electron tunneling in the excited state from NV$^-$ to N$^+$ and results in an optical cycle that includes NV$^0$. The tunneling process is linear in optical excitation and more prevalent the closer the N$^+$ is to the NV$^-$ centre. However, the separation between the NV$^-$ and its donor N$^+$ can be effected by light through the excitation of NV$^-$ and/or ionization of N$^0$. The optical excitation that creates the spin polarization can also modify the sample properties and during excitation creates charge dynamics. The consequence is that the magnitude of spin polarization, the spin relaxation and coherence times T$_1$ and T$_2$ have a dependence on the nitrogen concentration and on the excitation wavelength. The adjacent N$^+$ gives an electric field that Stark shifts the NV$^-$ transitions and for an ensemble results in line broadening. It is observation of changes of these Stark induced effects that allow the variation in NV$^-$ - N$^+$ separation to be monitored. Spectroscopic measurements including that of the varying line widths are central to the study. They are made at low temperatures and include extensive measurements of the NV$^-$ optical transition at 637 nm, the infrared transition at 1042 nm and ODMR at 2.87 GHz.
We investigate the influence of plasma treatments, especially a 0V-bias, potentially low damage O$_2$ plasma as well as a biased Ar/SF$_6$/O$_2$ plasma on shallow, negative nitrogen vacancy (NV$^-$) centers. We ignite and sustain using our 0V-bias plasma using purely inductive coupling. To this end, we pre-treat surfaces of high purity chemical vapor deposited single-crystal diamond (SCD). Subsequently, we create $sim$10 nm deep NV$^-$ centers via implantation and annealing. Onto the annealed SCD surface, we fabricate nanopillar structures that efficiently waveguide the photoluminescence (PL) of shallow NV$^-$. Characterizing single NV$^-$ inside these nanopillars, we find that the Ar/SF$_6$/O$_2$ plasma treatment quenches NV$^-$ PL even considering that the annealing and cleaning steps following ion implantation remove any surface termination. In contrast, for our 0V-bias as well as biased O$_2$ plasma, we observe stable NV$^-$ PL and low background fluorescence from the photonic nanostructures.
We present an enhancement of spin properties of the shallow (<5nm) NV centers by using ALD to deposit titanium oxide layer on the diamond surface. With the oxide layer of an appropriate thickness, increases about 2 up to 3.5 times of both relaxation time and evolution time were achieved and the shallow NV center charge states stabilized as well. Moreover, the coherence time kept almost unchanged. This surface coating technique could produce a protective coating layer of controllable thickness without any damages to the solid quantum system surface, making it possible to prolong T1 time and T2* time, which would be a possible approach to the further packaging technique for the applicating solid quantum devices.
We report on an ion implantation technique utilizing a screening mask made of SiO$_2$ to control both the depth profile and the dose. By appropriately selecting the thickness of the screening layer, this method fully suppresses the ion channeling, brings the location of the highest NV density to the surface, and effectively reduces the dose by more than three orders of magnitude. With a standard ion implantation system operating at the energy of 10 keV and the dose of 10$^{11}$ cm$^2$ and without an additional etching process, we create single NV centers close to the surface with coherence times of a few tens of $mu$s.
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