High-resolution resonant excitation of NV centers in $6H-\mathrm{SiC}$: A matrix for quantum technology applications

Atomlike defect levels in silicon carbide (SiC) polytypes have been proposed and proven to be an excellent platform for various quantum technology applications. Single-photon emitters, coherent control at room temperature, and temperature and magnetic field sensing at the nanoscale have already been demonstrated for the case of vacancy and divacancy defects in SiC and more recently proposed for negatively charged NV centers. NV centers, which allow a better control of their generation, offer in addition a further shift of the spectral range in the near infrared, i.e., in the $O$- and $S$-band telecom range. We demonstrate here that the association of high resolution optical spectroscopy and electron paramagnetic resonance spectroscopy combined with first-principles calculations allow the identification of the microscopic structure of the six distinct NV centers in $6H-\mathrm{SiC}$ and the assignment of their associated zero-phonon photoluminescence lines. Time resolved photo-EPR measurements at $T=4\mathrm{K}$ show that NV centers in $6H-\mathrm{SiC}$ present spin lattice relaxation times of several seconds. These excellent qubit properties should enable their application and implementation in quantum information devices.

Figure 1

Crystal structure of $6H-\mathrm{SiC}$ showing the three nonequivalent Si and C (black circles) lattice sites, hexagonal ($h$) and quasicubic ($k1$ and $k2$), whereby $k1$ is always neighboring one $k2$ and three $h$ sites ($k2$ has quasicubic neighbors exclusively). Thus we obtain the following NV center configurations: basal: $hk1, k2k2, k1h$ and $axial$: $hh, k2k1, k1k2$ (for further details see the text). As an example we show also the magnetization density of the $basal$ ($k2k2$, left) and the $axial$ ($k1k2$, right) NV pair providing $C_{1h}$ and $C_{3v}$ symmetry, respectively.

Figure 2

High-resolution photoluminescence spectrum of $6H-\mathrm{SiC}$ displaying under nonresonant excitation the ZPL of the VV° and $\mathrm{N}{\mathrm{V}}^{-}$ centers; $T$ = 10 K, optical excitation at 1000 nm. On the left (smaller wavelength), the PL corresponding to the divacancy centers are shown in the small (blue) rectangular frame; on the right (for larger wavelength) the NV center PL peaks are framed by the larger (red) rectangle. We observe five ZPL as two ZPL (NV2a/NV2b) are nearly degenerate at 1183 nm and not resolved in this PL spectrum. This is also the case for the VV° ZPL where two ZPL QL5/QL6 are nearly degenerate.

Figure 3

$X$-band EPR spectrum for $B$//$c$ and $T$ = 4 K displaying the six doublet spectra of the three axial and three basal spin $S$ = 1 $\mathrm{N}{\mathrm{V}}^{-}$ centers (see also Fig. 1). Basal spectra can be easily identified: due to their off-axis orientation, their resonance fields are very different for the orientation of $B$//$c$.

Figure 4

EPR spectra of the $\mathrm{N}{\mathrm{V}}^{-}$ centers at $T$ = 4 K under nonresonant excitation displaying the six $\mathrm{N}{\mathrm{V}}^{-}$ centers. (a) Angular variation of the six NV centers for a rotation of the magnetic field from $B$//$c$ to $B$//[11-20], allowing the determination of the spin Hamiltonian parameters compiled in Table 2. (b) For the highest-intensity axial NV center ($k2k1$) the central hyperfine interaction with one ${}^{13}\mathrm{C}$ nucleus (one of the three C neighbors) is resolved (blue); (red line): spectrum multiplied by 200. (c) Higher resolution EPR spectrum of the axial NV center ($k1k2$) displaying directly the SHF interaction with nuclear spin $I$ = 1 of the ${}^{14}\mathrm{N}$ neighbor. (d) EPR spectrum of the $hk1$ basal NV center showing a strongly reduced SHF with ${}^{14}\mathrm{N}$ as compared to the other NV centers.

Figure 5

(a), left: Transient behavior of the EPR signal of the axial $k2k1 \mathrm{N}{\mathrm{V}}^{-}$ center after switch-off of the optical excitation at $t=20s$; $T=4\mathrm{K}$, circles experiment, blue line fit with a single exponential decay. (b), right: Time constants of the relaxation time of the optically induced ground state spin polarization of the six $\mathrm{N}{\mathrm{V}}^{-}$ centers; $T$ = 4 K.

Figure 6

(a)–(e) Resonant excitation EPR spectra measured (at $T$ = 4 K) individually for each $\mathrm{N}{\mathrm{V}}^{-}$ center on its low field EPR line. These spectra correspond to the ZPL absorption lines of the NV centers; the ZPL expected at 1155 nm is out of range of the tunable NIR laser but can be attributed from the knowledge of the other five ZPL lines.

Figure 7

Superposition of the photoluminescence ZPL (black line) and the absorption ZPL lines (color lines) as determined by high-resolution resonant excitation of the EPR spectra. The nearly degenerate ZPL of the axial ($hh$) and basal $\left(k1h\right) \mathrm{N}{\mathrm{V}}^{-}$ centers are not resolved in the PL spectrum due to insufficient resolution.

Figure 8

(a), left: Comparison of experimental and calculated ZFS parameters $D$ for the $\mathrm{N}{\mathrm{V}}^{-}$ centers. (b), right: Comparison of the experimental and DFT-calculated absorption vertical ZPL wavelength of the six $\mathrm{N}{\mathrm{V}}^{-}$ centers. As expected the PBE-calculated excitation energies are systematically underestimated by about 100 meV, but the trend is correctly reproduced.