Electron nuclear interactions in spin-3/2 color centers in silicon carbide: A high-field pulse EPR and ENDOR study

High-frequency pulsed electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) were used to determine electron nuclear interactions on remote ligand shells of silicon and carbon in spin-3/2 color centers with an optically induced high-temperature spin alignment in hexagonal $4H$-, $6H$-, and rhombic $15R$-silicon carbide (SiC) polytypes. The EPR and ENDOR experimental data relate unambiguously to spin-3/2 centers in which the optically induced alignment of the spin-level populations occurs. The identification is based on resolved ligand hyperfine interactions with carbon and silicon nearest, next-nearest, and the more distant neighbors and on the determination of the unpaired electron spin densities. The hyperfine interactions with ${}^{29}\mathrm{Si}$ and ${}^{13}\mathrm{C}$ nuclei are unambiguously separated due to the selective population of the fine-structure levels with certain values of ${\mathrm{M}}_{\mathrm{S}}$. The signs of these interactions and, as a result, the signs of oscillating spin density at ${}^{29}\mathrm{Si}$ and ${}^{13}\mathrm{C}$ nuclei, are determined. On the basis of the EPR and optically induced ENDOR measurements, signs of the fine-structure splitting for all the centers were demonstrated, which made it possible to establish the character of optically induced spin alignment, including the inverse populations of the spin levels for these centers. The values of hyperfine interaction with ${}^{29}\mathrm{Si}$ and ${}^{13}\mathrm{C}$ nuclei, including those remote from the localization of the spin-3/2 center, are tabulated, which can be used by a number of algorithms in quantum information processing as long-term memory.

Figure 1

ESE-detected ENDOR spectra at $W$ band of the V2 and V1/V3 centers in $6H$-SiC, $B\left|\right|c$, recorded with a scan in a wide frequency range, for the hf transitions indicated in optically induced ESE-detected EPR spectra shown at the right. The ENDOR lines with the strongest HF interactions (in absolute value) are marked. There is a mirror reflection of the HF interaction lines with different coordination spheres of silicon relative to the Larmor frequency of ${}^{29}\mathrm{Si}$, which unambiguously indicates opposite signs of fine-structure splitting. The inset shows one EPR line of V2 center in $6H$-SiC crystal with a natural isotope silicon content and with a modified isotopic composition of silicon (0.7% of ${}^{29}\mathrm{Si}$ isotope).

Figure 2

$W$-band angular dependence ESE-detected EPR (a) and corresponding ESE-detected ENDOR (b) spectra of the optically aligned V2 color centers in single $6H$-SiC crystal. (c) Expanded-scale ESE-detected ENDOR spectrum from (b). The EPR transitions for the lf and hf magnetic fields are indicated in optically induced ESE spectra. The dotted lines in (b), (c) correspond to the signals recorded in the additional $6H$-SiC sample. The angular dependencies of the ENDOR lines are highlighted in gray. (b) The signals at $f_{\mathrm{L}}\pm 1/2\left|A_{\mathrm{SiNNN}}\right|$ and $f_{\mathrm{L}}\pm 3/2\left|A_{\mathrm{SiNNN}}\right|$ correspond to HF interactions with the NNN Si atoms (with ${}^{29}\mathrm{Si}$ nuclei) with respect to the negatively charged silicon vacancy ${{\mathrm{V}}_{\mathrm{Si}}}^{-}$. The light-induced inverse population of the spin sublevels of V2 centers is shown in the top inset. Bottom inset displays EPR spectrum demonstrating resolved anisotropic hyperfine structure originates from NN carbon atoms (with ${}^{13}\mathrm{C}$ nuclei) with respect to negatively charged silicon vacancy ${{\mathrm{V}}_{\mathrm{Si}}}^{-}$; and almost isotropic hyperfine structure for the next-nearest neighbors Si atoms (with ${}^{29}\mathrm{Si}$ nuclei) with respect to the negatively charged silicon vacancy VSi ${{\mathrm{V}}_{\mathrm{Si}}}^{-}$.

Figure 3

Angular dependence of $W$-band ESE-detected ENDOR for V2 color centers in single $6H$-SiC crystal. Hyperfine interactions with the surrounding silicon nuclei ${}^{29}\mathrm{Si}$ (left-hand side) and carbon ${}^{13}\mathrm{C}$ (right-hand side) are presented. The angular dependencies of the ENDOR lines are highlighted in gray and by dashed lines.

Figure 4

$W$-band angular dependence ESE-detected EPR (a) and ESE-detected ENDOR (b) spectra of the optically aligned V1/V3 color centers in single $6H$-SiC crystal. (c) Expanded-scale ESE-detected ENDOR spectrum from (b). The EPR transitions for the low-field and high-field magnetic fields are indicated in optically induced ESE spectra. The signals at $f_{\mathrm{L}}\pm 1/2\left|A_{\mathrm{SiNNN}}\right|$ and $f_{\mathrm{L}}\pm 3/2\left|A_{\mathrm{SiNNN}}\right|$ (b) correspond to HF interactions with the next-nearest neighbors Si atoms (with ${}^{29}\mathrm{Si}$ nuclei) with respect to the negatively charged silicon vacancy ${{\mathrm{V}}_{\mathrm{Si}}}^{-}$. (inset) The light-induced inverse population of the spin sublevels of V1/V3 centers ($D<0$). In the $B\left|\right|c$ orientation, the ESE-detected ENDOR spectrum is additionally presented, recorded from the low-field EPR line under conditions of optical pumping in a $6H$-SiC crystal enriched with the ${}^{13}\mathrm{C}$ isotope (12%). As a result, the transitions associated with HF interactions with ${}^{13}\mathrm{C}$ are sharply enhanced. The spectra for the perpendicular orientation $\left(B\perp c\right)$ are given for demonstrating a small orientation dependence for some ENDOR signals.

Figure 5

ESE-detected ENDOR spectra at $W$ band of the V2 centers in $15R$-SiC, $B\sim \left|\right|c$ for the low-field and high-field transitions in optically induced ESE-detected EPR spectra. The same designations were used as shown in Fig. 1 for V2 center.

Figure 6

ESE-detected ENDOR spectra at $W$ band of the V2, V3, and V4 color centers in $15R$-SiC measured at orientation $\theta \approx {10}^{\circ }$ for the low-field and high-field transitions indicated in optically induced ESE spectra shown at the right.

Figure 7

ESE-detected ENDOR spectra at $W$ band of the V2 color centers in $4H$-SiC measured at orientation $\theta \approx 0^{\circ }$ for the low-field and high-field transitions indicated in optically induced ESE spectra shown at the right. The inset shows the level system for the V2 center in $4H$-SiC.

Figure 8

ESE-detected ENDOR spectra at $W$ band of the negatively charged silicon vacancy in regular environment, ${{\mathrm{V}}_{\mathrm{Si}}}^{-}$ center, in $15R$-SiC (for two orientations), which are characterized by zero-field splitting $D=0$. Inset shows the spin-level diagram and ESE-detected EPR for spin-3/2 centers with $D=0$ (optically not active) in $15R$-SiC.

Figure 9

Optically induced level spin alignment of V1, V2, V3, and V4 centers in $4H$-, $6H$-, and $15R$-SiC.