Nitrogen doped diamond single crystals grown using either HPHT synthesis or CVD method were examined. After the diamond growth, electron beam irradiation and post annealing at 1000℃ for 2h were carried out to form NV-centers in the diamond crystals. For creating NV- center whose amount is comparable to that of neutral substitutional nitrogen Ns0, the total fluences of electron beam was set in the range of 1×1017 to 1×1018 e/cm-2. The 1/T2 can be expressed as summation of 1/T2{Ns0}, 1/T2{NV-} and T2{NV0}.2 By removing of the relaxation rate from Ns0 and NV0 center from 1/T2, we obtained the inverse relationship between T2{NV-} and [NV-] as shown in figure. The inverse relationship shows that DDI between NV centers is successfully detected. It is clarified that strength of DDI from NV- center: DNV- is about twice as large as those of Ns0 and NV0 center.2 It is expected that the large strength of DDI leads to shorten T2 with increasing of [NV-].
Nitrogen doped diamond single crystals grown using either HPHT synthesis or CVD method were examined. After the diamond growth, electron beam irradiation and post annealing at 1000℃ for 2h were carried out to form NV-centers in the diamond crystals. For creating NV- center whose amount is comparable to that of neutral substitutional nitrogen Ns0, the total fluences of electron beam was set in the range of 1×1017 to 1×1018 e/cm-2. The 1/T2 can be expressed as summation of 1/T2{Ns0}, 1/T2{NV-} and T2{NV0}.2 By removing of the relaxation rate from Ns0 and NV0 center from 1/T2, we obtained the inverse relationship between T2{NV-} and [NV-] as shown in figure. The inverse relationship shows that DDI between NV centers is successfully detected. It is clarified that strength of DDI from NV- center: DNV- is about twice as large as those of Ns0 and NV0 center.2 It is expected that the large strength of DDI leads to shorten T2 with increasing of [NV-].
We investigated spin-echo coherence times T2 of negatively charged nitrogen vacancy center (NV−) ensembles in single-crystalline diamond synthesized by either the high-pressure and high-temperature and chemical vapor deposition methods. This study specifically examined the magnetic dipole–dipole interaction (DDI) from the various electronic spin baths, which are the source of T2 decoherence. Diamond samples with NV− center concentration [NV−] comparable to those of neutral substitutional nitrogen concentration [Ns0] were used for DDI estimation. Results show that the T2 of the ensemble NV− center decreased in inverse proportion to the concentration of nitrogen-related paramagnetic defects [NPM], being the sum of [Ns0], [NV−], and [NV0], which is a neutrally charged state NV center. This inversely proportional relation between T2 and [NPM] indicates that the nitrogen-related paramagnetic defects of three kinds are the main decoherence source of the ensemble NV− center in the single-crystalline diamond. We found that the DDI coefficient of NVH− center was significantly smaller than that of Ns0, the NV0 center, or the NV− center. We ascertained the DDI coefficient of the NV− center DNV− through experimentation using a linear summation of the decoherence rates of each nitrogen-related paramagnetic defect. The obtained value of 89 μs ppm for DNV− corresponds well to the value estimated from the relation between DDI coefficient and spin multiplicity.
To improve the sensitivity of magnetic sensors using negatively charged nitrogen-vacancy (NV−) centers, it is essential to improve the quality of diamond crystals and precisely control the nitrogen concentration in the crystals. We have utilized high-pressure, high-temperature synthesis [1] and chemical vapor deposition [2] for the growth of nitrogen-doped diamond. The concentration of substitutional nitrogen in the neutral charge state [NS0] were evaluated by electron paramagnetic resonance (EPR) measurements. As shown in Fig. 1, [NS0] were precisely controlled by the weight ratio of nitrogen getter material in the metal solvent for the HPHT method (Fig. 1(a)) and by the carbon to nitrogen gas ratio for the CVD method (Fig. 1(b)). Then, the diamond crystals were irradiated with electron beams followed by vacuum annealing so that 10% of the nitrogen at the substitutional site became NV− centers, and the negatively charged NV− centers were successfully formed preferentially [3]. T2 of these diamond crystals was inversely proportional to the nitrogen concentration. The product of T2 and nitrogen concentration of the obtained HPHT diamond crystal is slightly higher than the maximum value reported so far [4], suggesting that the crystal is expected to have high magnetic sensitivity.
NV- center has been paid attention as a quantum sensor for highly sensitive detection of magnetic field, electric field, temperature and strain, etc.. Electron beam irradiation followed by subsequent thermal annealing is known as the effective procedures to create NV- center in diamond. The electron irradiation generates vacancies in diamond and the annealing enables vacancies to diffuse in the lattice. Then diffused vacancies are captured by nitrogen atoms, and as a result, NV centers are created. In addition, substitutional nitrogen atom at a lattice site (P1 center) also acts as a donor for NV center, the charge state of NV centers changes from neutral (NV0) to NV-. Creation high concentration of ensemble NV- centers is needed to achieve high sensitively sensing. The magnetic sensitivity is proportional to the reciprocal of the number of NV centers and T2 (spin coherence time), i.e., high concentration NV- centers with long T2 increase the sensitivity. P1 center has an important role for being a component of NV- centers and acting as donor to the NV center. But it also plays a role as decoherence source (decrease in magnetic field sensitivity). Therefore, it is important to create high concentration of NV- centers considering the amount of P1 centers. In current study, we investigate irradiation fluence dependence of the creation behavior of NV centers in type-Ib diamonds by using electron spin resonance (ESR) and photo-luminescence (PL) to understand the creation mechanism of NV- center. The values of T2 are also evaluated by Hahn-echo measurement, since it is one of the key parameters for highly sensitive magnetometry. Then, we discuss the creation mechanism of NV centers based on concentrations of defects.
Negatively charged nitrogen-vacancy (NV − ) centres formed in diamond crystals are point defects that have potential applications in various quantum devices such as highly sensitive magnetic sensors. To improve the sensitivity of magnetic sensors using NV − centres, it is essential to precisely control the nitrogen concentration in the crystals. In this paper, we demonstrated that nitrogen concentration in diamond can be controlled with high precision for the following two representative growth methods. One is the high-pressure/high-temperature (HPHT) synthesis method and the other is the chemical vapour deposition (CVD) method. The nitrogen concentration of HPHT-grown diamond decreased semi-logarithmically with increasing contents of titanium or aluminium as nitrogen getter materials. The nitrogen concentration of CVD-grown diamond increased linearly with increasing the flow rate ratio of nitrogen to carbon. NV − centres were formed by controlling the total fluence of electron beams so that approximately 20% of the nitrogen became NV − centres. The coherence time of electron spin of NV − centres obtained by the Hahn-echo pulse sequence T 2 of these diamond crystals was inversely proportional to the nitrogen concentration. A comparison of T 2 of the NV − centres for HPHT-synthesized and CVD-grown diamonds showed no significant difference between them. This article is part of the Theo Murphy meeting issue ‘Diamond for quantum applications’.
Diamond color centers such as the nitrogen-vacancy (NV) center have been extensively studied as a popular platform that enables quantum sensing and computing applications. For highly sensitive sensing NV ensembles have significantly stronger signals than individual NV centers and can create strong light fields, which are also essentially required by novel concepts that need a large sensing volume, such as cavity coupling and readout, laser threshold magnetometry and multi-pass geometries. On top of strong signals, low optical material losses are the other key consideration for these techniques, by means of low optical absorption of signal light and low birefringence of the diamond crystal. In this work, we study systematically the behavior of absorption in the NV emission range around 700nm and birefringence with increasing nitrogen- and NV-doping, as well as their behavior during NV creation via diamond growth, electron beam irradiation and annealing treatments. Absorption correlates with increased nitrogen-doping yet single substitutional nitrogen atoms (Ns0 or P1 centers) do not seem to be the direct absorber. The high nitrogen doping level in the growth leads to high absorption in the whole UV-Vis range, which is the main contributors to the high absorption at 700 nm. When treating the diamond for an enhanced NV concentration, ‘over irradiating’ the diamond with too high fluence is harmful in the sense of creating neutral vacancies that increase absorption at the wavelength range of interest. Opposite to the absorption, birefringence reduces with increasing nitrogen doping and shows better homogeneity. After-growth treatments show a minor and if any a positive (i.e. reducing) influence on diamond birefringence. Multiple crystal defect concentrations and their changes are identified via absorption spectroscopy during the material processing steps and thus we identify potential causes of absorption and birefringence. From this we suggest strategies to fabricate CVD diamonds with high NV density yet low absorption and low birefringence
We systematically investigate the NV center creation and properties in bulk chemical vapor deposition (CVD) diamond, throughout growth, electron-beam irradiation, and annealing steps. We study diamond series grown with a varying nitrogen flow over 4 orders of magnitude, resulting in a broad range of single substitutional nitrogen (P1 center) densities from 0.2 to 20 ppm. We investigate in-situ P1 and NV creations, as well as NV charge states distributions. As the next step, we optimize electron-irradiation fluences for a fixed nitrogen concentration, with two different accelerated electron energies. We bring up a general approach to determine the optimal irradiation conditions, and discuss criteria for P1 to NV conversion ratios in the context of stable NV charge states. Through all these optimizations, we enhance the NV- concentration remarkably in our CVD diamond samples, without sacrificing their Hahn-echo coherence time T2. We achieve NV concentrations from 1 to 168 ppb with T2 from 549 to 45.5 µs respectively. This study shows a pathway to engineer targeted, optimized properties of NV-ensembles in bulk CVD diamonds for improved sensitivity.