Photoluminescence excitation study of split-vacancy centers in diamond

Two known representatives of the split-vacancy complexes in diamond, the negatively charged silicon-vacancy ${\mathrm{SiV}}^{-}$ and recently discovered germanium-vacancy ${\mathrm{GeV}}^{-}$ defects, were comparatively studied for their photoluminescence (PL) and complementary optical absorption spectra. The observed strong difference between luminescence and absorption spectra indicates a strong frequency defect, that is the difference of binding energies of impurity atom in the ground and excited electronic states, in these color centers. The presence of frequency defect is well supported by first-principle calculations. The obtained results accompanied with isotopic effects shed light on the structure of these centers in the ground and excited electronic states that would open the doorway to their theoretical description.

Figure 1

(a) The structure of a split-vacancy complex in diamond. Dark gray balls stand for carbon atoms, translucent for vacant cites, and the large ball in the middle for an impurity atom. (b) Schematic representation of the Huang-Rhys model with the frequency defect. Tick marks throughout the text denote the excited electronic state. The arrows demonstrate the transitions corresponding to ZPL ($0^{\prime }\to 0$) and the first LVM ($0^{\prime }\to 1$) in the luminescence sideband. (c) Splitting of one electron energy levels due to symmetry breaking $\overline{3}m\to 3$. The open arrows represent transitions accompanied by the emission of $X$($Y$)-polarized photons, the solid ones to $Z$-polarized ones. (d) The ZPL splitting of ${\mathrm{SiV}}^{-}$ center in the samples studied at designated temperature. ZPL is decomposed into four Lorentzians with the known separation in frequency. The polarization of individual ZPL components was attributed according to Ref. [10].

Figure 2

ZPL slitting of ${\mathrm{GeV}}^{-}$ center in ${}^{12}\mathrm{C}$ (solid line) and ${}^{13}\mathrm{C}$ (dashed line) diamonds. The spectra were shifted by 2.0622 and 2.0593 eV for ${}^{13}\mathrm{C}$ and ${}^{12}\mathrm{C}$ respectively.

Figure 3

PL/PLE spectra of ${\mathrm{SiV}}^{-}$ (upper panel) and ${\mathrm{GeV}}^{-}$ (lower panel) centers at intermediate ($T=77$ K) and low ($T=5$ K) temperatures. Both panels have the same $x$-axis scales, which were selected for visual coincidence of respective ZPLs (marked by vertical dashed line). PL and PLE spectra correspond to energies lower and higher than the ZPL one respectively. Asterisks mark the positions of the background maximum in both emission and absorption sidebands.

Figure 4

Calculated relative energies of ${\mathrm{SiV}}^{-}$ center depending on the displacement of impurity atom along $Z$ axis ($\circ$) and in $XY$ plane ($\square$). Filled symbols correspond to the excited electronic state and the open ones to the ground electronic state. The thick lines represent the calculated energies of ${\mathrm{GeV}}^{-}$ center in comparison to ${\mathrm{SiV}}^{-}$.

Figure 5

PLE spectra of ${\mathrm{SiV}}^{-}$ (a) and ${\mathrm{GeV}}^{-}$ (b) in the excitation region far from ZPL at several temperatures. The strongest line observed in this region (marked by dotted lines) is possibly due to the excitation from the electronic level laying deep into the valence band and subsequent complicated relaxation of the obtained excited state [24]. Weak lines in the vicinity of it are likely to be its vibronic replicas and strongly attenuates at room temperature.