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arXiv:1105.0092 · Physical Review B

Optimal conditions for NV− center formation in type-1b diamond studied using photoluminescence and positron annihilation spectroscopies

Jacques Botsoa, Thierry Sauvage, Marie‐Pierre Adam, P. Desgardin, Elisa Leoni, Blandine Courtois, François Treussart, M.F. Barthe

Published Sep 28, 2011• 65 citations
arXivHTMLPDF

Abstract

We studied the parameters to optimize the production of negatively charged nitrogen-vacancy color centers (NV${}^{\ensuremath{-}}$) in type-1b single crystal diamond using proton irradiation followed by thermal annealing under vacuum. Several samples were treated under different irradiation and annealing conditions and characterized by slow positron beam Doppler-broadening and photoluminescence (PL) spectroscopies. At high proton fluences another complex vacancy defect appears limiting the formation of NV${}^{\ensuremath{-}}$. Concentrations as high as $2.3\ifmmode\times\else\texttimes\fi{}{10}^{18}$ cm${}^{\ensuremath{-}3}$ of NV${}^{\ensuremath{-}}$ have been estimated from PL measurements. Furthermore, we inferred the trapping coefficient of positrons by NV${}^{\ensuremath{-}}$. This study brings insight into the production of a high concentration of NV${}^{\ensuremath{-}}$ in diamond, which is of utmost importance in ultrasensitive magnetometry and quantum hybrid systems applications.

DOI
10.1103/physrevb.84.125209

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We studied the parameters to optimize the production of negatively charged nitrogen-vacancy color centers (NV\ensuremath−{}^{\ensuremath{-}}\ensuremath−) in type-1b single crystal diamond using proton irradiation followed by thermal annealing under vacuum. Several samples were treated under different irradiation and annealing conditions and characterized by slow positron beam Doppler-broadening and photoluminescence (PL) spectroscopies. At high proton fluences another complex vacancy defect appears limiting the formation of NV\ensuremath−{}^{\ensuremath{-}}\ensuremath−. Concentrations as high as 2.3\ifmmode×\else\texttimes\fi10182.3\ifmmode\times\else\texttimes\fi{}{10}^{18}2.3\ifmmode×\else\texttimes\fi1018 cm\ensuremath−3{}^{\ensuremath{-}3}\ensuremath−3 of NV\ensuremath−{}^{\ensuremath{-}}\ensuremath− have been estimated from PL measurements. Furthermore, we inferred the trapping coefficient of positrons by NV\ensuremath−{}^{\ensuremath{-}}\ensuremath−. This study brings insight into the production of a high concentration of NV\ensuremath−{}^{\ensuremath{-}}\ensuremath− in diamond, which is of utmost importance in ultrasensitive magnetometry and quantum hybrid systems applications.
Electron irradiation is one of the most powerful methods to create negatively charged nitrogen-vacancy centers (NV-s) in diamonds, which act as qubits and quantum sensors even at room temperature (RT). For quantum sensing, it is important to create ensemble NV-s with long coherence time (T2) since the sensitivity can be affected by both amount of NV- and T2. For high concentration NV-, electron irradiation with higher fluence is needed. However, if electron fluence is too high, neutral NVs (NV0s) are created because of lack of N donors. For T2, since residual defects act as origins of T2 decoherence, it is important to optimize the condition of subsequent annealing as well as irradiation fluence. Thus, we need to consider initial nitrogen (P1) concentration in diamond, electron fluence, and annealing conditions. In this paper, we create NV-s in Ib diamonds by electron irradiation and subsequent annealing with different conditions, and study the amounts of NV-s and their T2 for high-sensitive quantum sensing. The samples used in this study were Ib diamond with P1 concentration around 100 ppm. Those samples were irradiated with 2 MeV-electrons at a fluence of 1x1018/cm2 at RT. After irradiation, the samples were annealed between 600 and 1400oC in Ar. Photoluminescence (PL) measurements were carried out at RT to confirm NV creation. For PL measurements, the laser with a wavelength of 532 nm was used as excitation. PL intensity for samples annealed at 600 oC is about half even after annealing for 600 min, comparing with that for 800 oC-annealed samples although the values increase with increasing annealing time. Thus, the annealing at 650 oC is not enough to create NV. The samples annealed between 800 and 1200 oC show the highest PL intensity even after annealing for 10 min. The 800 and 1000 oC-annealed samples keep the PL intensity after annealing for 1200 min although the PL intensity for 1200 oC-annealed samples decrease with increasing annealing time above 100 min. The PL intensity for the samples annealed at 1400 oC shows smaller value and decreases with increasing annealing time. For T2, the samples annealed at 1400 oC show 2 microsec after annealing for 10 min. However, NV- cannot be observed after annealing at 1400 oC for 30min. Therefore, it can be concluded that annealing at 1400 oC is not suitable for the creation of NV-. For samples annealed between 600 and 1000 oC, the values of T2 are constant (between 0.5 and 0.9 microsec) after annealing up to 1200 min. For samples annealed at 1200 oC, no significant difference in T2 value is observed from those annealed between 600 and 1000 oC up to 30min, and the value increases with increasing annealing time above 30 min. From the point of view of sensing, it is important to consider both the amounts of NV- centers and T2. Therefore, we compare the values of PL intensity times T2. As a result, the samples annealed between 800 and 1200 oC show the highest value although the value for samples annealed at 1200 oC decreases after annealing for 1200 min. This suggests that annealing at 800 oC for 10 min is enough to create NV-s. We also study the creation of NV-s in diamonds by electron irradiation at elevated temperature. The samples with P1 concentration around 60 and 100 ppm were irradiated with 2 MeV-electrons at fluences up to 5x1018/cm2 at 800 or 1000 oC. The samples were annealed at 1000 oC for 120 min after irradiation. The amounts of NV are estimated from electron spin resonance (ESR) signals. As a result, samples irradiated at 800 and 1000 oC show higher NV- concentration than that for RT-irradiated ones, and 800 oC-irradiated ones show higher NV concentration than 1000oC-irradiated ones. Since the T2 values are not affected by irradiation temperature, the elevated temperature irradiation, especially 800 oC irradiation is a useful method to realize quantum sensing with high sensitivity.
Nitrogen-vacancy (NV) centers in single-crystal CVD diamond are promising candidates for quantum sensing due to their remarkable optical and spin properties [1]. These defects exhibit stable photoluminescence (PL), long spin coherence times, and high sensitivity to external fields [2]. In this study, we present a room-temperature PL characterization of NV centers excited with a 532 nm laser. The setup was optimized for high signal-to-noise ratio and spectral resolution. Key parameters analyzed include emission intensity, zero-phonon line (ZPL) at ~637 nm, phonon sidebands, and the charge state distribution between NV⁰ and NV⁻. The NV⁻ state, crucial for spin-based sensing, is evaluated to understand its stability under different nitrogen doping conditions [3]. The relative intensities of NV⁰ and NV⁻ provide insights into nitrogen incorporation and vacancy formation. These results contribute to optimizing synthesis conditions for enhanced NV⁻ concentration, supporting the advancement of diamond-based quantum sensing technologies.
Abstract The nitrogen-vacancy (NV) center in diamond is a promising quantum system for magnetometry applications exhibiting optical readout of minute energy shifts in its spin sub-levels. Key material requirements for NV ensembles are a high NV − concentration, a long spin coherence time and a stable charge state. However, these are interdependent and can be difficult to optimize during diamond growth and subsequent NV creation. In this work, we systematically investigate the NV center formation and properties in bulk chemical vapor deposition (CVD) diamond. The nitrogen flow during growth is varied by over four orders of magnitude, resulting in a broad range of single substitutional nitrogen concentrations of 0.2–20 parts per million. For a fixed nitrogen concentration, we optimize electron-irradiation fluences with two different accelerated electron energies, and we study defect formation via optical characterizations. We discuss a general approach to determine the optimal irradiation conditions, for which an enhanced NV concentration and an optimum of NV charge states can both be satisfied. We achieve spin–spin coherence times T 2 ranging from 45.5 to 549 μ s for CVD diamonds containing 168 to 1 parts per billion NV − centers, respectively. This study shows a pathway to engineer properties of NV-doped CVD diamonds for improved sensitivity.
We report depth-resolved photoluminescence measurements of nitrogen-vacancy (NV−) centers formed along the tracks of swift heavy ions (SHIs) in type Ib synthetic single crystal diamonds that had been doped with 100 ppm nitrogen during crystal growth. Analysis of the spectra shows that NV− centers are formed preferentially within regions where electronic stopping processes dominate and not at the end of the ion range where elastic collisions lead to the formation of vacancies and defects. Thermal annealing further increases NV yields after irradiation with SHIs preferentially in regions with high vacancy densities. NV centers formed along the tracks of single swift heavy ions can be isolated with lift-out techniques for explorations of color center qubits in quasi-1D registers with an average qubit spacing of a few nanometers and of order 100 color centers per micrometer along 10 to 30-μm-long percolation chains.
Two-photon fluorescence spectroscopy of negatively charged nitrogen-vacancy [(N-V)-] centers in type Ib diamond single crystals have been studied with a picosecond (7.5 ps) mode-locked Nd:YVO(4) laser operating at 1064 nm. The (N-V)- centers were produced by radiation damage of diamond using a 3 MeV proton beam, followed by thermal annealing at 800 degrees C. Prior to the irradiation treatment, infrared spectroscopy of the C-N vibrational modes at 1344 cm(-1) suggested a nitrogen content of 109 +/- 10 ppm. Irradiation and annealing of the specimen led to the emergence of a new absorption band peaking at approximately 560 nm. From a measurement of the integrated absorption intensity of the sharp zero-phonon line (637 nm) at liquid nitrogen temperature, we determined a (N-V)- density of (4.5 +/- 1.1) x 10(18) centers/cm3 (or 25 +/- 6 ppm) for the substrate irradiated at a dose of 1 x 1016) H(+)/cm(2). Such a high defect density allowed us to observe two-photon excited fluorescence and measure the corresponding fluorescence decay time. No significant difference in the spectral feature and fluorescence lifetime was observed between one-photon and two-photon excitations. Assuming that the fluorescence quantum yields are the same for both processes, a two-photon absorption cross section of sigma(TPA) = (0.45 +/- 0.23) x 10(-50) cm(4).s/photon at 1064 nm was determined for the (N-V)- center based on its one-photon absorption cross section of sigma(OPA) = (3.1 +/- 0.8) x 10(-17) cm2 at 532 nm. The material is highly photostable and shows no sign of photobleaching even under continuous two-photon excitation at a peak power density of 3 GW/cm(2) for 5 min.
Quantum information applications place stringent demands on the development of platforms that can host them. Color centers in diamond have been identified as important media for quantum information processing. Accordingly, the photoluminescence properties of nitrogen-vacancy (N-V) centers in diamond created by implantation and annealing are studied at cryogenic temperatures (below 10 K). We examine high pressure high temperature and chemical vapor deposition synthetic diamonds with varying nitrogen concentration and present an accurate method to estimate the concentration of the (N-V) centers created by ion implantation. The ion irradiation route produced up to 6 ppm of optically active (N-V) centers, while nitrogen implantation yielded up to 3 ppm of optically active (N-V) with 8% conversion efficiency. However, a broadening of the (N-V)− zero phonon line was observed in all samples.