# Defects in Silicon Carbide as Quantum Qubits: Recent Advances in Defect Engineering
## 3 Defects in Silicon Carbide
Red uppercase letters indicate the stacking of Si–C double layers, while lowercase letters denote whether the double layers and their immediate surroundings follow a cubic (k) or hexagonal (h) stacking order. Green lowercase letter pairs represent variants of a pair defect and blue axis. Figure taken from Ref. [53]. Figure 5 shows the structure of V\({}_{\text{Si}}\) and V\({}_{\text{C}}\)-V\({}_{\text{Si}}\) in 4H-SiC. The divacancy has four, denoted as hh, kk, hk, and kh, while the silicon vacancy has two non-equivalent sites, denoted as k and h.  
Adding to the complexity, each defect can exist in multiple charge states within the SiC bandgap, further influencing their electronic transitions, optical emission properties, and spin characteristics. The ability to precisely control and identify these charge states is critical for optimizing defect-based qubits, as unwanted charge state fluctuations can degrade coherence and readout fidelity [55; 56]. This remains one of the most significant challenges: the selection and stabilization of the desired charge states. As discussed in the following sections, color centers in SiC can exist in multiple charge states, but only specific ones are suitable for quantum applications. The first step in the creation of defect-based qubits is the controlled introduction of defects into SiC. This is typically achieved through ion implantation or high-energy irradiation, both of which introduce lattice vacancies and interstitials that can later form stable defect centers. Figure 6 (left side) schematically illustrates ion implantation and high-energy irradiation, two widely used techniques for defect engineering in SiC. In the case of ion implantation, a high-energy ion beam (such as He, C, N, or other species) is used to displace Si and C atoms in a specific volume of the SiC material. The implantation energy determines how deeply the defects are introduced, enabling precise spatial control over defect placement. However, a significant drawback of this method is that other unwanted defects are created along the ion cascade, which can impact the material's properties. Unlike ion implantation, which introduces defects in a localized manner, high-energy irradiation (e.g., electron or neutron irradiation) generates vacancies and interstitials uniformly throughout the material. This technique ensures a more homogeneous distribution of defects, but it lacks the spatial precision offered by ion implantation. Figure 5: The 4H-SiC lattice structure showing the non-equivalent configurations of the divacancy (V\({}_{\text{C}}\)-V\({}_{\text{Si}}\)) and silicon vacancy (V\({}_{\text{Si}}\)). Black uppercase letters indicate the stacking of Si–C layers (red and blue spheres represent Si and C atoms, respectively). Adapted from Ref. [54]. To determine the implantation or irradiation conditions, such as estimating the depth profile and distribution of vacancy-related defects in SiC, various simulation tools are commonly employed. For ion implantation, SRIM code [57] is typically used, while FLUKA code [58] is often applied for simulating neutron-induced damage. A representative example is provided in the study by [PERSON] et al. [59], where they compared the spatial distribution of vacancy-related defects introduced by ion implantation (He\({}^{+}\), C\({}^{+}\)) and neutron irradiation within the volume of 4H-SiC. However, in both cases (ion implantation and irradiation), a high-temperature annealing step is often required to rearrange displaced atoms, remove unwanted defects, and stabilize the desired charge state of the defect centers. Figure 6 (right) schematically illustrates the most advanced approach to defect engineering, where defects are introduced at precise locations and with controlled concentrations. Achieving such a level of precision is crucial for the development of scalable quantum technologies, but remains a considerable challenge. Among the most promising techniques moving toward this goal are laser writing and focused ion beam (FIB) implantation.