Research articlesSiV0 centres in diamond: effect of isotopic substitution in carbon and silicon
Abstract
The neutrally charged silicon-vacancy defect (SiV0) is a colour centre in diamond with spin S = 1, a zero-phonon line (ZPL) at 946 nm and long spin coherence, which makes it a promising candidate for quantum network applications. For the proper performance of such colour centres, all of them must have identical optical characteristics. However, in practice, there are factors that influence each individual centre. One of these factors is non-uniform isotope composition for both carbon atoms in diamond lattice and silicon atoms of dopant. In this work, we studied the isotopic shifts of SiV0 centres for CVD-grown epitaxial layers of isotopically enriched 12C and 13C diamonds, as well as for diamond with natural isotope composition but doped only with one isotope of Si (28Si, 29Si and 30Si). The detected shift was 1.60 meV for 12C/13C couple and 0.33 meV for 28Si/29Si and 29Si/30Si couples, which are close to the previously obtained values of the isotopic shift for the negatively charged silicon vacancy (SiV−), which indicates a similar model of interaction with the environment for these two charge states of the SiV colour centres.
This article is part of the Theo Murphy meeting issue 'Diamond for quantum applications'.
1. Introduction
Colour centres in diamond that emit light across the visible and near-infrared range offer a promising foundation for quantum information technologies, as well as applications in photonics and quantum applications [1–15]. The negatively charged silicon-vacancy (SiV−) centre in diamond has gained attention in the field of nanophotonics due to its exceptional spectral characteristics [1–5,16–20]. These include a potent zero-phonon line (ZPL) at a wavelength of 737 nm, which accounts for around 70% of the fluorescence emitted by the centre, and a relatively weak phonon sideband [19–23]. Furthermore, phonons of diamond lattice with colour centres can be used as the universal quantum transducers to couple point defects and control spin of the system [24,25]. The lack of spectral diffusion in SiV− makes it possible to generate and detect indistinguishable single photon emitters [26–28]. In addition, there is another, neutral charge state of the SiV centre (SiV0) that emits light in the near-infrared range at a wavelength of 946 nm, which is closer to the telecommunication wavelengths [29–31]. The SiV0 centre is an attractive candidate for quantum memory, network and computing due to its unique optical properties for such purposes [32–36]. The SiV0 has a four orders longer spin coherence time than SiV− (T2 ≈ 0.945 ms at 20 K versus T2 ≈ 35 ns at 4.5 K, respectively), more stable optical transitions than NV− and longer-lived nuclear spin (T2n ≈ 0.45 s) [33]. This set of extraordinary characteristics makes SiV0 nicely fit for quantum technologies such as quantum memory and qubits of quantum computers.
The study of isotopic effects is important for a number of problems related both to general physical questions and from the point of view of practical quantum applications. The isotopic disorder originated due to isotopes with different masses of carbon and colour centres isotopes in the diamond lattice. It impacts the electron–phonon interaction and changes the volume of media. These changes affect the optical characteristics of diamond such as optical transitions and the broadening of spectral lines. The magnitude of the line shift caused by isotopic substitution can show the relationship between the optical subsystem and the diamond lattice, and the line broadening indicates the need to use isotopically pure materials for quantum technologies. Nowadays, the most known physical mechanism of isotopic shift is changing in the oscillation of lattice modes and zero-point energy due to different masses of isotopes [37].
In this work, we study the isotopic shifts of the neutrally charged colour centre SiV0 in diamond, both for the substitution of the carbon isotope (12C and 13C) and for the silicon isotope (28Si, 29Si and 30Si).
2. Methods
The spectroscopic study was performed using Si-doped diamond samples that were grown using microwave plasma-assisted chemical vapour deposition in methane–hydrogen–silane gas mixtures. The details of the synthesis of samples can be found in [26] concerning monoisotopic Si-enrichment in diamond of natural carbon isotope composition, and in [38] for monoisotopic carbon enrichment of diamond with Si impurity of natural isotope composition. The optical absorption spectra at low temperatures (5 K) were measured using a high-resolution FTIR spectrometer Bruker IFS 125 HR equipped with Si-detector and a closed-cycle Helium cryostat Sumitomo SRP-092. The spectral resolution was 0.5–2 cm−1, depending on the intensity of the ZPL signal for SiV0 centres. In all cases, it was sufficient to investigate the values of full width at half maximum (FWHM) of the investigated peaks of ≈10 cm−1. For that, the experimental curves near ZPL of SiV0 and SiV− centres were fitted by the Gaussian function. The FWHM values for SiV− centres were measured by using the most intensive component of the four-line fine structure (or ‘C-component’, as defined in [39]). A vacuum sample chamber contains an X-ray tube for studying absorption spectra at X-ray irradiation. The X-ray radiation allowed us to observe for the first time a sufficiently intense absorption of SiV0 centres since it increased the concentration of the SiV0 centres due to a change in the charge state of the SiV− centres [19]. A cooled sample was placed in a vacuum chamber simultaneously illuminated by both optical and X-ray radiation. The rated electrical power of the X-ray tube was 600 W, and the radiation energy was 8.027 keV (Cu Kα). The spectral resolution in these measurements was better than 1 cm−1.
3. Results and discussion
Using the technique of X-ray irradiation, we were able to observe the absorption spectra of SiV0 centres with a good signal-to-noise ratio in samples with different isotopic substitutions for both carbon and silicon. Figure 1 schematically shows the mechanism of conversion of the SiV− centre to the SiV0 centre by X-ray assisted removal of an electron from the former. Figure 2 shows the low-temperature absorption spectra of the SiV0 lines belonging to samples with different isotopic substitutions for both silicon and carbon. Samples with different Si isotopic compositions have a natural carbon isotope. Considering that in the natural isotope of carbon, the proportions of 12C and 13C are 98.89% and 1.11%, respectively, we can assume that the absorption peaks for the 12C and natC isotopes will differ slightly. Similarly, it can be assumed that the contribution of various silicon iso topes to 13-carbon monoisotopic diamond will be insignificant, since the natural isotopic composition of silicon has a 28Si, 29Si and 30Si ratio of 92.23%, 4.67% and 3.10%, respectively. Therefore, the absorption line of natSiV0 in the monoisotopic 13C-diamond can be attributed with good accuracy to 28SiV0. It can be seen that the width of the SiV0 line is larger for the natural carbon isotope, which indicates a greater contribution to the width of the SiV0 line from isotopic disordering with respect to carbon.
Figure 1. Schematic of the charge exchange of the SiV− centre in SiV0 using X-rays with the respective energy schemes for absorption. Figure 2. Absorption spectra of 28,29,30SiV0 in silicon-monoisotopic natC diamonds (three spectra from top to bottom: 30SiV0, 29SiV0 and 28SiV0, respectively) and in carbon-monoisotopic 13C diamond with natSiV0 (bottom spectrum) at temperature T = 5 K and under X-ray irradiation. Note the shift of the peaks with Si mass increase to lower frequencies. The spectra are displaced vertically for clarity.
The analysis of the neutral SiV0 absorption peaks shows that the silicon isotope shift is 0.33 meV per a.m.u, while the carbon isotope shift is 1.61 meV (table 1 for peak positions and widths). To compare these values with the previously known data on isotopic shifts for charged SiV− centres in the same samples [26,38], we show in figure 3 the absorption spectra SiV− centres for the whole series of samples but with additional X-ray irradiation. No additional X-ray-related shifts were registered either for SiV0 or for SiV−. The FWHM values of SiV0 peaks are much larger than the FWHM values of SiV− peaks (table 1). However, the widths of the SiV− lines in the studied samples were among the narrowest ever observed for the SiV− ensembles, which imply small values of stress in the samples. Thus, we conclude that the fact that the registered SiV0 lines are wider than SiV− is due to the nature of these centres, and not to stresses in the crystal. The insufficient signal-to-noise ratio did not allow the reliable registration of any peaks at the ZPL sideband of SiV0, specifically, its local vibrational mode peaks near 918 nm (ΔE = 40 meV) or 902 nm (ΔE = 64 meV) [40], while the observed structure of ZPL sideband for the SiV− peak was in good coordination with previous research (e.g. [26,38,41]).
| E (eV) | ΔE28Si,29Si,30Si (meV) | ΔE12C,13C (meV) | FWHM (meV) | |
|---|---|---|---|---|
| natC 30SiV0 | 1.30968 | 0.695 | ||
| natC 29SiV0 | 1.31001 | 0.33 | 0.812 | |
| natC 28SiV0 | 1.31034 | 1.61 | 0.726 | |
| 13C natSiV0 | 1.31195 | 0.690 |
From the analysis of the SiV− spectra (figure 3 and table 2), it can be seen that: (i) the SiV0 peak width is at least one order of magnitude larger compared with that for SiV−; (ii) the isotopic shifts for SiV− differ slightly upwards in comparison with SiV0, while the relative isotopic shifts for both silicon and carbon are similar to the SiV0 (the shift ratio SiV0/SiV− for the measured isotopic shifts is ≈0.95).
Figure 3. Absorption spectra of 28,29,30SiV− in silicon-monoisotopic natC diamonds and in carbon-monoisotopic 13C diamond with natSiV− at temperature T = 5 K and under X-ray irradiation.
| E (eV) | ΔE28Si,29Si,30Si (meV) | ΔE12C,13C (meV) | FWHM (meV) | |
|---|---|---|---|---|
| natC 30SiV− | 1.68144 | 0.247 | ||
| natC 29SiV− | 1.68179 | 0.35 | 0.078 | |
| natC 28SiV− | 1.68214 | 1.67 | 0.086 | |
| 13C natSiV− | 1.68381 | 0.144 |
4. Conclusion
We have investigated the isotopic shifts of SiV0 centres for CVD-grown 12C and 13C diamonds doped with only one isotope of Si (28Si, 29Si, 30Si) in optical absorption spectra at low temperatures (T = 5 K). We analysed experimental data and found that shift values per a.m.u. for SiV0 and SiV− are close, 0.33 and 0.35 meV, respectively (the shift ratio of SiV0/SiV− ≈ 0.95). These results show that shifts in both neutral and charged states of SiV centres can use the similar physical model of interaction with carbon lattice. We also detected the isotopic shift of 12C and 13C, it was 1.60 meV. One of our findings is that for quantum applications of SiV0 centres, it is more important to use isotopically pure carbon than isotopically pure silicon in the aspect of getting a narrower spectral line.
Data accessibility
This article does not contain any additional data.
Authors' contributions
K.N.B.: conceptualization, formal analysis, investigation, visualization, writing—original draft, writing—review and editing; E.S.S.: data curation, investigation, software, writing—original draft; A.P.B.: investigation, methodology, resources; V.G.R.: supervision, validation, writing—review and editing; V.S.S.: funding acquisition, project administration, resources, writing—original draft, writing—review and editing.
All authors gave final approval for publication and agreed to be held accountable for the work performed therein.
Conflict of interest declaration
We declare we have no competing interests.
Funding
The work was funded by the Russian Science Foundation, grant no. 21-72-10153, https://rscf.ru/project/21-72-10153/.
Acknowledgements
The authors thank Roman Khmelnitskiy and Artem Martyanov for the help with preparation of isotopically enriched diamond samples.
