# Reproducible generation of green-emitting color centers in hBN using oxygen annealing

Helmi Fartas, Saïd Hassani, Jean-Pierre Hermier, Ngoc Diep Lai, Stéphanie

Buil, Aymeric Delteil

► To cite this version:

Helmi Fartas, Saïd Hassani, Jean-Pierre Hermier, Ngoc Diep Lai, Stéphanie Buil, et al. Reproducible generation of green-emitting color centers in hBN using oxygen annealing. Applied Physics Letters, 2025, 127 (1), 10.1063/5.0261073. hal-05152518

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Submitted on 9 Jul 2025

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# Reproducible generation of green-emitting color centers in hBN using oxygen annealing

Helmi Fartas<fnref n="1" />,<fnref n="2" />, Saïd Hassani<fnref n="1" />, Jean-Pierre Hermier<fnref n="1" />, Ngoc Diep Lai<fnref n="2" />, Stéphanie Buil<fnref n="1" />, Aymeric Delteil<fnref n="1" />,*

The ability to generate quantum emitters with reproducible properties in solid-state matrices is crucial for quantum technologies. Here, we show that a high density of close-to-identical single-photon emitters can be created in commercial hexagonal boron nitride using annealing under oxygen atmosphere. This simple procedure yields a uniform in-plane distribution of color centers consistently emitting around 539.4 nm, with a wavelength spread smaller than 1 nm. We present an extensive characterization of their photophysical properties, showing that the emitters are bright and stable, and exhibit narrow lines at low temperatures with minimal spectral diffusion. These characteristics make this family of quantum emitters highly appealing for applications to quantum information science.

Hexagonal boron nitride (hBN) has gained attention as an advantageous material platform for integrated quantum photonics<fnref n="1" />,<fnref n="2" />. As a van der Waals (vdW) material, hBN supports diverse fabrication techniques that enable its integration with other materials, including vdW crystals, in highly compact and complex devices, down to monolayer thicknesses. Moreover, hBN hosts optically active point defects that act as efficient single-photon emitters (SPEs) across various wavelength ranges³⁻⁵. While some of these defects occur naturally, primarily near crystal edges or structural imperfections, several activation techniques, such as high-temperature annealing⁶ or surface etching⁷, have been shown to yield a more uniform spatial distribution of the SPEs, away from flakes boundaries and defects. In particular, annealing in various conditions of temperature and atmosphere is an efficient technique to generate or activate a wide variety of emitters in the ultraviolet⁸, visible³,⁹ and near infrared ranges⁶. The nature and properties of the generated quantum emitters depend on both the crystal fabrication method and the annealing conditions, with argon, oxygen and methane being the most commonly used gas atmospheres.

Nonetheless, the broad wavelength variability of hBN color centers poses a challenge for scalable integration into photonic devices. While most methods, including annealing, yield SPEs with strongly inhomogeneous properties – and in particular a large wavelength spread –, a few methods have been recently identified to generate SPEs with reproducible wavelength using electron irradiation⁸,¹⁰⁻¹² and carbon implantation¹³,¹⁴. Here, we introduce a simple and efficient method to reproducibly create a large number of SPEs through the annealing of commercial hBN under oxygen atmosphere. We generate and optically characterize SPEs emitting in the green wavelength range, with a narrow zero-phonon line (ZPL) and a minimal wavelength spread, as opposed to most prior combinations of hBN crystal and annealing conditions. The generated emitters are bright and stable, which is essential for quantum information applications.

The hBN crystals are exfoliated from commercial hBN (HQ Graphene) on SiO₂/Si substrate, which is subsequently annealed at 1000 °C under 1000 sccm oxygen flow during

<figure><img src="image_2.png" /><figcaption>FIG. 1. (a) Confocal map of a 12 × 12 µm<fnref n="2" /> area of the flake. (b) Room-temperature photoluminescence spectra of 4 representative emitters as a function of the photon energy (top horizontal axis) and wavelength (bottom horizontal axis). The 523 nm peaks originates from Raman scattering of the 485 nm laser.</figcaption></figure>

2 hours. The sample is then characterized in photoluminescence in a confocal microscope at room and cryogenic temperature. We focus on flakes of thickness in the 100-200 nm range, which are well suited for the realization of mono-

<footnote n="1">¹ Université Paris-Saclay, UVSQ, CNRS, GEMaC, 78000, Versailles, France.</footnote>

<footnote n="2">² Université Paris-Saclay, ENS Paris-Saclay, CNRS, LuMIn, 91190, Gif-sur-Yvette, France.</footnote>

<footnote>* aymeric.delteil@uvsq.fr</footnote>
<figure><img src="image_3.png" /><figcaption>FIG. 2. Room temperature photophysics of the green centers. (a) Count rate as a function of the laser power measured in CW regime for three emitters, revealing the saturation of the emitters. (b) Fluorescence decay in pulsed regime. The dashed line is a fit to the data. (c) Second-order photon correlations in pulsed regime, as a function of the time delay between detections. The dashed line is a fit to the data.</figcaption></figure>

lithic photonic structures$^{15,16}$ and allow for high stability of the SPEs$^{3,17}$. The sample is optically excited by a 485 nm diode laser which can perform both in continuous-wave (CW) and pulsed regimes. The laser is focused on the sample by an air objective of numerical aperture 0.95 located on top of the sample. The emitted photons are collected by the same objective, and channeled to a spectrometer or avalanche photodiodes with a resolution of 65 ps. We first perform confocal mapping by scanning the sample position with piezoelectric positioners. Fig. 1a shows a typical confocal map measured at room temperature on a flake of thickness 156 nm, as measured by atomic force microscopy. The confocal map exhibits multiple localized spots, which correspond to emitters activated by the annealing process. Their repartition is quite uniform, and their density is high ($\sim 1 \ \mu\text{m}^{-2}$), yet not so high as to hinder individual addressing. Indeed, the vast majority of localized spots correspond to individual emitters. We then perform spectroscopy of individual emitters by characterizing the photon emission using a grating spectrometer. Fig. 1b shows a few representative examples of emission spectra at room temperature. The emission exhibits a zero-phonon line (ZPL) at 540 nm, with a linewidth of about 9 nm. It displays a pronounced kurtosis, a characteristic already observed in other families of hBN emitters$^{18}$, which takes its origin in emitter-phonon coupling$^{19}$. We therefore attribute the room-temperature broadening mechanism to interaction with acoustic phonons, like most other hBN SPEs$^{3,18-20}$. Additionally, a bimodal optical phonon replica can be observed at 580 and 590 nm. This 165-200 meV replica is visible in most other quantum emitters in hBN$^{3,19,20}$.

We then perform characterizations aiming to assess the potential of the SPEs for room temperature operation. To infer the emitter brightness, we measure the count rate $C$ as a function of laser power $P$ in CW regime. As shown on Fig. 2a, the SPEs exhibit a saturation behavior that can be fitted with the standard two-level saturation function $C = C_\infty/(1+P_{\text{sat}}/P)$, with $P_{\text{sat}}$ the saturation power, found in the 1-1.5 mW range, and $C_\infty$ the asymptotic count rate. In many cases, $C_\infty$ is close to, or higher than, 2 million counts per second. Given that

this count rate is measured out of any photonic structure, and most of the emission is directed toward the high index substrate, this places the emitter among the brightest emitters in this material platform. Then, the emitter lifetime is measured in pulsed regime. Fig. 2b shows an example of photoluminescence decay. It can be fitted with a single-exponential function providing $\tau = 4.2$ ns. This decay time lies in the typical range for hBN color centers. The combined measurement of saturation count rate $C_\infty$ and emitter lifetime leads us to expect a high quantum efficiency for this family of emitters. A confirmation would require the observation of Purcell modification of the decay time$^{17}$. Another important figure of merit is the photon purity, which can be assessed by measuring the second-order correlation function in a Hanbury Brown and Twiss setup. Fig. 2c shows the measured $g^{(2)}(\tau)$ as a function of the time delay $\tau$ between photon detections, measured in pulsed regime during 3 min. It allows us to extract $g^{(2)}(0) = 0.09 \pm 0.01$, which unequivocally demonstrates single-photon emission.

To further investigate the potential of the green emitters for experiments based on Hong-Ou-Mandel interference, we perform additional characterization at cryogenic temperature, where the emission spectrum narrows down drastically. Fig. 3a shows eight SPE spectra measured at 4 K. All exhibit a narrow ZPL, as well as phonon replica with a complex substrate, which could help identifying the defect from ab initio calculations. We note that the lineshape of the phonon replica is similar to that of ref$^{20}$. Although the emission wavelength is different, this suggests a comparable emitter-phonon coupling. Fig. 3b shows a higher resolution spectrum of the ZPL, yet the observed linewidth is still limited by the spectrometer resolution (120 $\mu$eV). Inferring the exact linewidth will require more advanced techniques, such as resonant excitation$^{21-23}$ or interferometry$^{24,25}$. Fig. 3c shows the evolution of the emission spectrum as a function of time during 3 hours. No spectral fluctuations can be observed at our spectral resolution. Additionally, we notice no intensity fluctuations or blinking, demonstrating the remarkable stability of the SPE.

To obtain further insight into the statistical dispersion of
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>FIG. 3. Low-temperature spectroscopy. (a) Low-resolution of the PL spectrum of 6 emitters. (b) High-resolution spectrum of the ZPL of a SPE. (c) Time-dependence of the ZPL emission as measured during 3 h.</figcaption></figure>

the SPE properties, on Fig. 4a we show a histogram of the ZPL position for 30 emitters. The distribution is centered at 539.35 nm, and has a standard deviation of 0.45 nm. This ensemble distribution is remarkably narrow for hBN emitters, only comparable to that of B-centers¹¹. We have also performed a statistical analysis of the SPE lifetime, shown on

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>FIG. 4. (a) Histogram of the ZPL center wavelength. (b) Histogram of the emitter lifetime.</figcaption></figure>

Fig. 4b, which yields $\tau = 4.22 \pm 0.10$ ns, showing the high reproducibility of the emitter photophysical properties.

We characterize the orientation of the emission dipole at room temperature by inserting a polarizer on the detection path and measuring the count rate as a function of the polarizer angle. The excitation laser polarization is set to a circular polarization. Fig. 5a shows the emission polarization of a few representative SPEs. All are linearly polarized, indicating an in-plane dipole – like the vast majority of hBN SPEs. The polarization axes are bunched in three groups, as is the case of SPEs created by carbon implantation¹³ and electron irradiation²⁶. This is consistent with a $C_{2v}$ symmetry of the defect center. The polarization axes are close to crystal axes as identified by the twin boundaries²⁷ visible on optical and SEM images (Fig. 5b). However, further investigations are needed to establish the type of crystal direction (i.e. zigzag or armchair) along which the dipole is oriented, which should provide helpful information for defect identification.

While the nature of the green SPEs is unknown, oxygen is likely a constituent of the color center, since we verified that annealing under the same conditions but without oxygen does not generate any emitter. Moreover, the defect formation also possibly involves a preexisting impurity in the crystal. Indeed, while we verified that the green defects can be reproducibly
<figure><img src="image_9.png" /><figcaption>FIG. 5. (a) Count rate as a function of the polarizer angle measured for 20 emitters. The dashed lines represent the crystal axes visible on Fig. 5b. (b) Optical microscope image of the flake, revealing the crystal axes.</figcaption></figure>

generated in commercial hBN (HQ Graphene), on the other hand we could not generate them in high-purity hBN grown by the high-pressure, high-temperature technique²⁸. Instead, oxygen annealing of the latter material yields quantum emitters in the red to infrared spectral range, with no wavelength control¹⁶. We also note that it is possible to generate color centers emitting in the infrared range using a different procedure²⁹, showing the variety and complexity of quantum emitter generation in hBN. Oxygen is expected to form stable defects in hBN, both in substitutional positions and in complexes with vacancies.³⁰⁻³². Additionally, it can also form complexes with preexisting impurities such as carbon³³ – commonly found natively in hBN crystals depending on the growth process. Finally, we performed ODMR experiments, without detecting any signal in the range 0-8 GHz, both with and with-

out external magnetic field, which discards easily accessible spin-dependent optical transitions, in contrast to other families of hBN defects³⁴,³⁵. Further investigations will be needed to unequivocally identify the defect at the origin of the green emission, based on additional characterization combined with DFT calculations³⁰.

To conclude, we presented a general investigation of the photophysics of a family of quantum emitters in hBN that can be reproducibly generated at a large scale, and that exhibits remarkable photophysical properties, with, in particular, a highly reproducible emission wavelength and lifetime. Its narrow linewidth and high brightness makes it an appealing contender for applications to quantum photonics and quantum information science. Our study also provides valuable insights for future identification of this color center. The SPE fabrication could be improved by investigating the relation between annealing conditions and emitter density, and by gaining spatial control through top-side masking of the crystal prior to annealing. Generation of SPEs in mono- or few-layer crystals is another appealing prospect. In addition, a better knowledge of the SPE properties will entail further studies, such as investigations of the effect of magnetic field and ODMR at low temperature, as well as resonant laser spectroscopy to assess the presence of dephasing or spectral diffusion²²,²³.

## DATA AVAILABILITY

The data that support the findings of this study are available in Zenodo at https://doi.org/10.5281/zenodo.15720988.

## ACKNOWLEDGMENTS

We acknowledge D. Gérard, C. Arnold and T. Bergèse for fruitful discussions and technical assistance. This work is supported by the French Agence Nationale de la Recherche (ANR) under reference ANR-21-CE47-0004-01. This work has also been supported by Region Île-de-France in the framework of DIM QuanTiP.

## CONFLICT OF INTEREST

The authors have no conflicts to disclose.

## REFERENCES

<footnote n="1">¹J. Wang, F. Sciarrino, A. Laing, and M. G. Thompson, Nature Photon. **14**, 273 (2020).</footnote>

<footnote n="2">²E. Pelucci, G. Fazhgas, I. Aharonovich *et al.*, Nature Rev. Phys. **4**, 194 (2022).</footnote>

<footnote n="3">³T. T. Tran, K. Bray, M. J. Ford *et al.*, Nat. Nanotechnol. **11**, 37 (2016).</footnote>
⁴R. Bourrelier, S. Meuret, A. Tararan et al., Nano Lett. 16, 4317 (2016).

⁵L. J. Martínez, T. Pelini, V. Waselowski et al., Phys. Rev. B 94, 121405 (2016).

⁶Y. Chen, C. Li, S. White et al., ACS Appl. Mater. Interfaces 13, 47283 (2021).

⁷N. Chejanovsky, M. Rezai, F. Paolucci et al., Nano Lett. 16, 7037 (2016).

⁸A. Gale, C. Li, Y. Chen et al., ACS Photonics 9, 2170 (2022).

⁹C. Lyu, Y. Zhu, P. Gu, J. Qiao, K. Watanabe, T. Taniguchi, and Y. Ye, Appl. Phys. Lett. 117, 244002 (2020).

¹⁰B. Shevitski, S. M. Gilbert, C. T. Chen et al., Phys. Rev. B 100, 155419 (2019).

¹¹C. Fournier, A. Plaud, S. Roux et al. Nat. Commun. 12, 3779 (2021).

¹²A. Kumar, C. Cholsuk, A. Zand et al., APL Mater. 11, 071108 (2023).

¹³D. Zhong, S. Gao, M. Saccone et al., Nano Lett. 24, 1106 (2024).

¹⁴M. Hua, W.-Y. Chen, H. Hou et al., arXiv:2410.13169 (2024).

¹⁵J. E. Fröch, L. P. Spencer, M. Kianinia et al., Nano Lett. 21, 6549 (2021).

¹⁶D. Gérard, M. Rosticher, K. Watanabe, et al., Appl. Phys. Lett. 122, 264001 (2023).

¹⁷D. Gérard, A. Pierret, H. Fartas et al., ACS Photonics 11, 5188 (2024).

¹⁸T. T. Tran, C. Elbadawi, D. Totonjian et al., ACS Nano 10, 7331 (2016).

¹⁹D. Wigger, R. Schmidt, O. Del Pozo-Zamudio et al., 2D Mater. 6, 035006 (2019).

²⁰P. Khatri, I. J. Luxmoore, and A. J. Ramsay, Phys. Rev. B 100, 125305 (2019).

²¹J. Horder, S.J.U. White, A. Gale, Phys. Rev. Appl. 18, 064021 (2022).

²²C. Fournier, K. Watanabe, T. Taniguchi et al., Phys. Rev. B 107, 195304 (2023).

²³D. Gérard, S. Buil, J.-P. Hermier and A. Delteil, Phys. Rev. B 111, 085304 (2025).

²⁴X. Brokmann, M. Bawendi, L. Coolen, and J.-P. Hermier, Photon-correlation Fourier spectroscopy, Opt. Exp. 14, 6333-6341 (2006).

²⁵B. Spokoyny, H. Utzat, H. Moon et al., J. Phys. Chem. Lett. 2020, 11, 1330 (2020).

²⁶J. Horder, D. Scognamiglio, A. Ganyecz et al., ACS Photonics 11, 3954 (2024).

²⁷A.P. Rooney, Z. Li, W. Zhao, A. Gholinia et al., Nature Commun. 9, 3597 (2018).

²⁸T. Taniguchi, and K. Watanabe, J. Cryst. Growth 303, 525 (2007).

²⁹S. S. Mohajerani, S. Chen, A. Alaei, et al., ACS Photonics 11, 2359 (2024).

³⁰S. A. Tawfik, S. Ali, M. Fronzi et al., Nanoscale 9, 13575 (2017).

³¹S. Li and A. Gali, J. Phys. Chem. Lett. 13, 9544 (2022).

³²Y. S. Na, J. H. Kim, S. Kang et al., 2D Mater. 8, 045041 (2021).

³³C. Cholsuk, A. Zand, A. Çakan, T. Vogl, The Journal of Physical Chemistry C 128, 12716 (2024).

³⁴A. Gottscholl, M. Kianinia, V. Soltamov, S. Orlinskii, G. Mamin et al., Nature Mater. 19, 540 (2020).

³⁵H. L. Stern, C. M. Gilardoni, Q. Gu, S. E. Barker, O. F. J. Powell et al., Nature Mater. 23, 1379 (2024).