# Defects in Silicon Carbide as Quantum Qubits: Recent Advances in Defect Engineering
## 3 Defects in Silicon Carbide
### Silicon Vacancies (V\({}_{Si}\))  
The negatively charged V\({}_{\text{Si}}\) is one of the fundamental intrinsic defects in SiC. V\({}_{\text{Si}}\) is an optically active defect and plays a crucial role in quantum applications. Here, an overview of the key experimental methods used for the creation of silicon vacancies in SiC is given. The commonly employed methods include irradiation and ion implantation, both of which typically require post-treatment annealing to remove undesired defects introduced during the process and to reach the preferable state. Across various available studies, 4H-SiC has been selected as the preferred SiC polytype. [PERSON] et al. [16] irradiated an isotopically purified 100 \(\upmu\)m-thick 4H-SiC epitaxial layer (\({}^{28}\)Si-99.85%, \({}^{12}\)C-99.98%) with 2 MeV electrons (fluence: 10\({}^{12}\) cm\({}^{-2}\)) to generate isolated V\({}_{\text{Si}}\) centers. To ensure that the vacancies were in the desired negatively charged state, a slightly n-type doped material was used. The samples were then annealed at 300 \({}^{\circ}\)C for 30 min to eliminate unwanted defects. [PERSON] et al. [60] used 110 \(\upmu\)m-thick 4H-SiC,  
Figure 6: Schematic illustration of experimental methods used to introduce defects (from left to right): high-energy particle irradiation, ion implantation, and direct writing techniques such as laser writing or focused ion beam. On the far right, a top-view image of the sample after direct writing or FIB implantation reveals a well-defined array of defects created with high spatial precision. but instead of electron irradiation, they employed reactor neutrons with energies ranging from 0.18 to 2.5 MeV. The highest neutron irradiation fluence, 5 \(\times\) 10\({}^{17}\) cm\({}^{-2}\), required a multi-step thermal annealing process at temperatures ranging from 125 to 700 \({}^{\circ}\)C for 90 min [60]. [PERSON] et al. [18] used low-doped n-type SiC epitaxial layers (10 \(\upmu\)m-thick). They performed room-temperature 100 keV Li ion beam implantation in the form of 10 \(\times\) 10 square matrix arrays of spots using one of the nine different fixed doses in the range of 10-6000 ions/spot (2.01 \(\times\) 10\({}^{12}\)-1.20 \(\times\) 10\({}^{15}\) ions cm\({}^{-2}\)) on a particular cell. Figure 7 shows the PL spectrum at 4.8 K, acquired from an individual spot of a 10 \(\times\) 10 array implanted at a dose of 1.2 \(\times\) 10\({}^{14}\) ions/cm\({}^{2}\). The single spot (600 ions/spot) is identified (red circle) in the low-temperature confocal fluorescence map (Figure 7, inset). [PERSON] et al. [17] used a high-purity semi-insulating (HPSI) 4H-SiC crystal, which was subsequently uniformly irradiated with 1 and 2 MeV protons to an irradiation fluence of 1 \(\times\) 10\({}^{13}\) cm\({}^{-2}\) for each energy. They successfully demonstrated a method by using focused proton beams for controlled defect generation without the need for pre- or post-annealing treatments. By matching the proton energy to the stopping power of the material, the generation depth and resolution could be precisely controlled, and the number of defects in a specific volume was tunable. Similar results were achieved via laser writing [14]. The focused ion beam and laser writing methods represent the most promising methods for highly localized and well-controlled defect formation in SiC. The key experimental methods used for introducing VSi into SiC are summarized in Table 3.  
Figure 7: PL spectrum at 4.8 K acquired from an individual spot of a 10 \(\times\) 10 array implanted at a dose of 1.2 \(\times\) 10\({}^{14}\) ions/cm\({}^{2}\) (600 ions/spot), which is identified (red circle) in the low-temperature confocal fluorescence map (inset). Figure taken from Ref. [18]. Silicon vacancy in 4H-SiC gives rise to well-known PL lines, commonly referred to as V1 (h site) and V2 (k site) [52]. Moreover, V1 has a high temperature companion labeled as V1\({}^{\prime}\)[52], as shown in Figure 7. This leads us to another noteworthy issue that adds to the complexity of defect engineering.