Comparison of different methods of nitrogen-vacancy layer formation in diamond for wide-field quantum microscopy

A. J. Healey, A. Stacey, B. C. Johnson, D. A. Broadway, T. Teraji, D. A. Simpson, J.-P. Tetienne, and L. C. L. Hollenberg

Phys. Rev. Materials 4, 104605 – Published 29 October 2020

Abstract

Thin layers of near-surface nitrogen-vacancy (NV) defects in diamond substrates are the workhorse of NV-based wide-field magnetic microscopy, which has applications in physics, geology, and biology. Several methods exist to create such NV layers, which generally involve incorporating nitrogen atoms (N) and vacancies (V) into the diamond through growth and/or irradiation. While there have been detailed studies of individual methods, a direct side-by-side experimental comparison of the resulting magnetic sensitivities is still missing. Here we characterize, at room and cryogenic temperatures, $\approx 100 − nm$ -thick NV layers fabricated via three different methods: (1) low-energy carbon irradiation of N-rich high-pressure high-temperature (HPHT) diamond, (2) carbon irradiation of $δ$ -doped chemical vapor deposition (CVD) diamond, (3) low-energy $N^{+}$ or ${CN}^{-}$ implantation into N-free CVD diamond. Despite significant variability within each method, we find that the best HPHT samples yield similar magnetic sensitivities (within a factor 2 on average) to our $δ$ -doped samples, of $< 2 μ T {Hz}^{- 1 / 2}$ for dc magnetic fields and $< 100 nT {Hz}^{- 1 / 2}$ for ac fields (for a $400 nm$ $\times 400 nm$ pixel), while the $N^{+}$ and ${CN}^{-}$ implanted samples exhibit an inferior sensitivity by a factor 2–5, at both room and low temperatures. We also examine the crystal lattice strain caused by the respective methods and discuss the implications this has for wide-field NV imaging. The pros and cons of each method, and potential future improvements, are discussed. This study highlights that low-energy irradiation of HPHT diamond, despite its relative simplicity and low cost, is a competitive method to create thin NV layers for wide-field magnetic imaging.

Received 9 June 2020
Revised 3 September 2020
Accepted 8 October 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.104605

Physics Subject Headings (PhySH)

Diamond Nitrogen vacancy centers in diamond

Crystal growth Ion implantation Optically detected magnetic resonance

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

A. J. Healey¹, A. Stacey ^2,3, B. C. Johnson^1,2, D. A. Broadway^1,2, T. Teraji⁴, D. A. Simpson¹, J.-P. Tetienne ^1,2,*, and L. C. L. Hollenberg^1,2,†

¹School of Physics, University of Melbourne, Parkville, VIC 3010, Australia
²Centre for Quantum Computation and Communication Technology, School of Physics, University of Melbourne, Parkville, VIC 3010, Australia
³School of Science, RMIT University, Melbourne, VIC 3001, Australia
⁴National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan

^*jtetienne@unimelb.edu.au
^†lloydch@unimelb.edu.au

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Vol. 4, Iss. 10 — October 2020

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Images

Figure 1
Overview of the NV layer formation methods investigated: (a) Schematic illustrating sample production. From left to right: method 1, carbon irradiation of high-N, low-NV HPHT diamond to create a localized NV-rich layer; carbon irradiation of $δ$ -doped CVD layer to improve NV conversion; implantation of a pure CVD substrate with a chosen nitrogen-containing species to incorporate nitrogen into the crystal at the cost of extra damage by vacancy overcreation. The color scale used and relative NV densities are schematic only. (b), (c) srim simulations for the $C^{-}$ and $N^{+}$ implantation cases, respectively, showing depth distributions of vacancies produced (both) and implanted ions (method 3 only) produced by representative implantation procedures.
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Figure 2
Sample characterization at room temperature: measured variation of (a) $R$ with implant dose; and (b) $T_{1}$ , (c) Rabi contrast, (d) free-induction decay time $T_{2}^{*}$ , and (e) Hahn echo $T_{2}$ with $R$ . All measurements made at room temperature and at 60 G except for the Hahn echo, which was conducted at 475 G to reduce the impact of ${}^{13}C$ revivals on the fits. Red lines in (d) and (e) denote lines of constant per 400 nm $\times$ 400 nm dc and ac sensitivity (in $μ T {Hz}^{- 1 / 2}$ and nT ${Hz}^{- 1 / 2}$ , respectively) at a constant Rabi contrast of 3% to guide the eye. The error bars indicate one standard deviation for $R$ and the standard fitting errors for the spin properties. Where not visible, error bars are smaller than the marker size.
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Figure 3
Sensitivity vs NV creation method: values of (a) dc and (b) ac magnetic sensitivities from Eq. (1) (using $T_{2}^{*}$ and $T_{2}$ , respectively) for a 400 nm $\times$ 400 nm pixel. Error bars obtained using standard propagation using measurement errors detailed in Fig. 2. Sensitivities obtained on a setup with $t_{R} = 5 μ s$ , $t_{D} = 1.5 μ s$ , and a laser intensity of $3 kW / {cm}^{2}$ .
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Figure 4
Extended decoupling characterization: (a) $T_{2}$ scaling of selected representative samples of each sample type as a function of the number of pulses $N$ in the CPMG- $N$ sequence. Data displayed collected from samples $δ$ -1 (green), CN-3 (yellow), N-3 (purple), and HPHT-6 (blue). (b) Extension of $T_{2}$ from $N = 1$ (Hahn echo) to $N = 1024$ , with gray lines showing 10-fold and 100-fold improvement. (c) Scaling of CPMG-1024 $T_{2}$ with $R$ , with isosensitivity curves in ${nTHz}^{- 1 / 2}$ . (d) ac sensitivities calculated using Eq. (1) (using the CPMG-1024 $T_{2}$ ), grouped by method. Data points marked by squares in (b)–(d) belong to samples that could not be extended beyond $N \approx 256$ and see only modest improvement over Hahn echo values.
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Figure 5
Representative strain maps from each method. (a) Sample HPHT-4: HPHT sample; (b) sample N-5: $N^{+}$ implant; (c) sample CN-1: ${CN}^{-}$ implant sample; (d) sample $δ$ -2: $δ$ -doped sample. The images show the variation $Δ f$ of the zero-field splitting constant $D$ . The insets show line cuts taken along the red and blue solid lines.
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Figure 6
Spin property scaling with temperature: (a) $T_{1}$ , plotted on a log scale. (b) Rabi oscillations taken at 4 K a field of 60 G (blue) and 1970 G (red) in sample CN-1 showing increased measurement contrast at higher fields. Points marked by triangles show PL measurements, showing reduced fluorescence at the higher field but not enough to explain the difference in contrast. (c), (d) Show $T_{2}^{*}$ and $T_{2}$ variation with temperature, respectively. (e), (f) Show the dc and ac magnetic sensitivity scaling, respectively. Data presented are for samples $δ$ -2 (green), N-2 (purple), CN-1 (yellow), and HPHT-1 (blue) in all comparative plots, with the addition of sample HPHT-3 as the bottom blue set in (a) and (d).
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