# Defects in Silicon Carbide as Quantum Qubits: Recent Advances in Defect Engineering
## 3 Defects in Silicon Carbide
### Divacancies (V\({}_{Si}\)-V\({}_{c}\))  
Together with negatively charged V\({}_{\text{Si}}\), a neutral divacancy V\({}_{\text{Si}}\)-V\({}_{\text{c}}\) is considered as a very promising candidate for quantum applications. A divacancy is formed when two neighboring silicon and carbon atoms are missing from the SiC lattice, as shown in Figure 5.  
Like V\({}_{\text{Si}}\), divacancies are often introduced through high-energy irradiation or ion implantation. [PERSON] et al. [69] used HPSI 4H and 6H SiC samples and studied the selection rules of divacancies that were introduced via electron irradiation (fluences in the 10\({}^{17}\)-10\({}^{18}\) cm\({}^{-3}\) range) and subsequent annealing (800 \({}^{\circ}\)C for 20-30 min). [PERSON] et al. [70] used room-temperature 250 keV proton irradiation of up to three fluencies 10\({}^{14}\) cm\({}^{-2}\), 10\({}^{15}\) cm\({}^{-2}\), and 10\({}^{16}\) cm\({}^{-2}\) to introduce both silicon vacancies and divacancies. [PERSON] et al. [71] achieved the direct writing of divacancy arrays in 4H-SiC using the femtosecond laser and subsequent thermal annealing. [PERSON] et al. [28] used an n-type 4H-SiC sample with a 10 \(\upmu\)m epitaxy layer and a doping level of 5 \(\times\) 10\({}^{14}\) cm\({}^{-3}\), and fabricated single divacancy arrays using the He FIB with an energy of 30 keV. The samples were annealed after implantation at 500 \({}^{\circ}\)C for 2 h to generate monovacancies, followed by post annealing at 900 \({}^{\circ}\)C for 1 h to facilitate monovacancy migration and divacancy formation. Additionally, the authors used C ions with the same energy, but enhanced optical and spin characteristics were notably \begin{table}
\begin{tabular}{c c c c} \hline
**Method** & **Source** & **Energy** & **Reference** \\ \hline Electron & e\({}^{-}\) & 2 MeV & [16] \\ irradiation & & & \\ \hline Neutron irradiation & \(n^{0}\) & 0.18–2.5 MeV & [50] \\ (reactor) & & & \\ \hline Ion implantation & He\({}^{+}\) & 6 keV & [61] \\ He\({}^{+}\), C\({}^{+}\) & 20 keV & [26] \\ \hline \multirow{3}{*}{Focused ion beam} & Li\({}^{+}\) & 80 keV & [25] \\ Li\({}^{+}\) & 100 keV & [18] \\ He\({}^{+}\) & 30 keV & [62] \\ \hline Focused proton beam & H\({}^{+}\) & 2 MeV & [17; 63] \\ \hline Laser writing & – & – & [14; 64; 65] \\ \hline \end{tabular}
\end{table} 
Table 3: Experimental methods used for single V\({}_{\text{Si}}\) creation. Data adapted from Ref. [25]. observed after He ion compared to C ion implantation. Figure 8 shows a confocal scanning image of the introduced divacancy arrays. As already said, the divacancy has four configurations in 4H-SiC (hh, kk, hk, and kh), which are shown in Figure 5. Correspondingly, there are four PL lines in 4H-SiC, known as P11, PL2, PL3, and PL4 [22; 56; 69]. The PL5 and PL6 lines, which have been previously detected but not convincingly assigned, are suggested to be related to the modified divacancies located at or near Frank-type stacking faults [28]. Figure 9 shows the PL spectrum of the various divacancies in 4H-SiC, with six PL lines being observed. While investigating the individual divacancy arrays in 4H-SiC introduced by [PERSON], [PERSON] et al. [28] studied the so-called modified divacancies, PL5 and PL6. Their results indicated PL6 as a defect with exceptional properties for quantum applications. Figure 8: Confocal scanning image displaying the divacancy arrays generated using a dose of 300 ions/spot. The white bar represents a distance of 3 \(\upmu\)m. The yellow square and white circle enclose the individuals PL4 and PL6, respectively. Figure taken from [28]. Figure 9: PL spectrum with 976 nm excitation of the various divacancies in 4H-SiC. Figure taken from Ref. [72]. Six PL lines are detected. Significant progress in divacancy defect engineering has been achieved by [PERSON] et al. [72]. They achieved the optical charge state control of the divacancy in 4H-SiC.