Calculations show group-IV–vacancy centers in diamond convert to substitutional and XV2 defects
A DFT preprint finds that XV centers capture carbon interstitials or a second vacancy, which may explain their low yield after ion implantation.
A group working on first-principles calculations has posted an arXiv · preprint, not yet peer-reviewed, on how group-IV–vacancy (XV) color centers form in diamond. Here X is Si, Ge, Sn or Pb, and V is a carbon vacancy. The authors examined the roles of vacancies and carbon self-interstitials (C_i), which ion implantation inevitably introduces into the lattice. arxiv.org
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XV centers are considered for quantum computing and quantum information because of their optical and spin properties. In the split-vacancy configuration the impurity sits between two adjacent carbon vacancies, which gives the center inversion symmetry. The negatively charged center has spin 1/2, a large Debye-Waller factor, strong zero-phonon line emission and a short radiative lifetime. arxiv.org
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Formation and competing reactions
According to the calculations, XV formation is energetically favorable, with large binding energies and low diffusion barriers. However, XV readily captures C_i and turns into the substitutional defect X_s, or captures an additional V and forms the complex XV_2. arxiv.org
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The authors say these competing reactions destabilize the split-vacancy configuration. They could account for the low formation yield: XV centers appear at concentrations well below those expected from the implanted dose. Earlier experiments cited in the paper showed that annealing of implanted Ge lowers the split-vacancy fraction and raises the substitutional one. For Sn the conversion is weaker, and a substantial fraction stays in the split-vacancy configuration. arxiv.org
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Charge conditions and method
Charge transition levels show that oppositely charged reactants never meet in these reactions. Coulomb repulsion blocks a reaction only when both reactants carry charges of the same sign. For the capture of C_i this requires strongly n- or p-type conditions. On this basis the authors suggest that Fermi-level control may suppress the defect conversions and stabilize the split-vacancy configuration. arxiv.org
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The calculations used density functional theory in VASP with a 4×4×4 cubic supercell of 512 atoms and a 420 eV plane-wave cutoff. Formation and binding energies, electronic structures and charge transition levels were obtained with the HSE06 functional. The calculated bandgap is 5.4 eV against an experimental 5.5 eV. Migration barriers were found with the climbing-image nudged elastic band method using PBE. As a functional check for one Si reaction, HSE06 gave a barrier of 4.37 eV against 4.23 eV in PBE. arxiv.org
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