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Point defects in diamond are being intensively investigated for their applications in processing and communication of quantum information, as well as for metrology. So far, the negatively charged nitrogen-vacancy center (NV−) has been the most studied defect [1]. Thanks to its efficient optical spin polarization and spin-state dependent fluorescence, it is being exploited, for example, in the context of high-sensitivity magnetometers [2]. More recently, owing to their superior optical properties, the group-IV-vacancy centers (SiV− [3], GeV− [4], SnV− [5,6], and PbV− [7]) have emerged as one of the leading types of point defects for quantum computing and communication applications. Whereas it is generally accepted that the N atom in the NV− center occupies a substitutional C site, the group-IV atoms in group-IV-vacancy centers are expected to form a so-called split-vacancy configuration where they are centered in between two vacancies. However, experimentally, these structural configurations have so far been only indirectly determined. A detailed, direct and quantitative characterization of the structure of these defects is especially important for the field, since the superior properties of the group-IV-vacancy centers are to a large extent a consequence of the D3d inversion symmetry of the split-vacancy configuration rather than C3v, as in the case of NV−. The D3d symmetry, however, will only be found if the impurity is exactly centered in the double vacancy, which corresponds to the ideal bond-center (BC) position.
We have used the beta- emission channeling method from 121Sn (t1/2=27.1 h) implanted at the low fluence of 2.3E12 atoms/cm2 into natural diamond in order to identify the lattice sites of Sn and hence also possible configurations of SnV defects. Following room temperature implantation ~60% of the implanted 121Sn occupied substitutional sites, while ~40% was found on a position that corresponds to a bond-center site and which is attributed to the SnV split-vacancy configuration. While the as-implanted lattice location indicated displacements from the ideal S and BC sites of the order of ~0.15 Å, following annealing at 920°C ~70% of the Sn atoms were found on the ideal substitutional and ~30% on ideal bond-center positions. The same diamond sample was subsequently implanted with the stable isotope 120Sn and studied by means of confocal photoluminescence (PL) spectroscopy, which, after 920°C annealing, revealed the characteristic luminescence signal [5,6] of SnV− at a wavelength of 621 nm.
Besides 121Sn, suitable EC probes for further studies of group IV-vacancy centers could be 31Si (157 min), 75Ge (82.8 min), or 209Pb (3.2 h), and ideas for possible future experiments will be outlined.
[1] M.W. Doherty et al., “The nitrogen-vacancy colour centre in diamond”, Physics Reports 528 (2013) 1–45
[2] L. Rondin et al., “Magnetometry with nitrogen-vacancy defects in diamond”, Reports on Progress in Physics 77 (2014) 056503/1–26
[3] A. Sipahigil, et al., “An integrated diamond nanophotonics platform for quantum optical networks”, Science 354 (2016) 847–50
[4] M.K. Bhaskar, et al., “Quantum nonlinear optics with a germanium-vacancy color center in a nanoscale diamond waveguide”, Physical Review Letters 118 (2017) 223603/1–6
[5] S.D. Tchernij et al., “Single-photon-emitting optical centers in diamond fabricated upon Sn implantation”, ACS Photonics 4 (2017) 2580–2586.
[6] T. Iwasaki et al., “Tin-vacancy quantum emitters in diamond”, Physical Review Letters 119 (2017) 253601/1–6
[7] S.D. Tchernij et al., “Single-photon emitters in lead-implanted single-crystal diamond”, ACS Photonics 5 (2018) 4864–4871.
