# Defects in Silicon Carbide as Quantum Qubits: Recent Advances in Defect Engineering
## 3 Defects in Silicon Carbide
### Divacancies (V\({}_{Si}\)-V\({}_{c}\))
Using above-bandgap excitation they efficiently converted divacancy into its neutral-charge state, and more importantly drastically increased the PL intensity, without affecting the spin properties. Another well-studied defect in SiC is the negatively charged nitrogen vacancy (NV). This defect consists of a nitrogen-shallow donor at a carbon lattice site paired with a neighboring silicon vacancy (Figure 10). [PERSON] et al. [74] employed nitrogen ion implantation with an incident energy of 30 keV to introduce NV centers into 4H-SiC. To optimize the formation of divacancies, they systematically varied the annealing temperature from 800 \({}^{\circ}\)C to 1050 \({}^{\circ}\)C. Their study successfully demonstrated coherent control of NV center spins and characterized the fluorescence and ODMR properties of single NV centers in 4H-SiC at room temperature. By refining the annealing conditions, they achieved a significant increase in the NV center concentration. [PERSON] et al. [75] utilized high-energy (12 MeV) proton irradiation to generate NV centers in 4H-SiC. The irradiated samples were subsequently annealed at elevated temperatures, leading to the formation of NV centers with an estimated concentration of approximately 10\({}^{16}\) cm\({}^{-3}\). [PERSON] et al. [76] investigated the creation of NV centers across multiple SiC polytypes (3C, 4H, and 6H-SiC) using high-energy electron irradiation followed by post-irradiation annealing. Their study provided valuable insights into the defect formation processes and the impact of different irradiation methods on NV center generation in SiC. The summary of the main experimental methods used for creating divacancies and NV centers in SiC is given in Table 4. \begin{table}
\begin{tabular}{c c c c} \hline
**Defect** & **Method** & **Source** & **Energy** & **Reference** \\ \hline VV & Electron irradiation & e\({}^{-}\) & 2 MeV & [69] \\ \hline VV & Femtosecond laser writing & \(-\) & \(-\) & [70] \\ \hline VV & Focused ion beam & He\({}^{+}\) & 30 keV & [28] \\ \hline VV & Ion implantation & H\({}^{+}\) & 250 keV & [71] \\ \hline NV & Electron irradiation & e\({}^{-}\) & \(>\)MeV & [76] \\ \hline NV & Proton irradiation & H\({}^{+}\) & 12 MeV & [75] \\ \hline NV & Ion implantation & N\({}^{+}\) & 30 keV & [74] \\ \hline \end{tabular}
\end{table} 
Table 4: Experimental methods used for creation of divacancies and NV centers in SiC. Figure 10: Microscopic structure of the NV center in SiC (blue, orange, gray, and black spheres represent Si, C, N, and missing atoms, respectively). Figure taken from Ref. [73]. While the aforementioned defects have demonstrated the greatest potential for quantum applications, new defect candidates are continuously emerging. Among these are the carbon antisite-vacancy pair (CAV) and chromium (Cr)-related defects in SiC. CAV defects are typically introduced via irradiation [77, 78], while Cr-related centers can be incorporated through Cr ion implantation [79]. The main challenges related to CAV and Cr-related defects are well-controlled fabrication [77, 78, 79] and charge state control [64]. Despite these limitations, continued advancements in the field suggest that substantial improvements can be expected in the coming years.