Kohn–Sham density functional theory is widely used for screening color centers in semiconductors. While the Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation functional is efficient, its accuracy in describing defects is often not sufficient. The Heyd–Scuseria–Ernzerhof (HSE) functional is more accurate but computationally expensive, making it impractical for large-scale screening. This study evaluates the strongly constrained and appropriately normed (SCAN) family of meta-GGA functionals as potential alternatives to PBE for characterizing NV-like color centers in 4H-SiC using the Automatic Defect Analysis and Qualification (ADAQ) framework. We examine nitrogen, oxygen, fluorine, sulfur, and chlorine vacancies in 4H-SiC, focusing on applications in quantum technology. Our results show that SCAN and r2SCAN achieve a greater accuracy than PBE, approaching HSE's precision at a lower computational cost. This suggests that the SCAN family offers a practical improvement for screening new color centers, with computational demands similar to PBE.  

Defects in semiconductors have in recent years been revealed to have interesting properties in the venture towards quantum technologies. In this regard, silicon carbide has shown great promise as a host for quantum defects. In particular, the ultrabright AB photoluminescence lines in 4⁢H-Si⁢C are observable at room temperature and have been proposed as a single-photon quantum emitter. These lines have previously been studied and assigned to the carbon–antisite-vacancy (CAV) pair. In this paper, we report on new measurements of the AB lines’ temperature dependence, and carry out an in-depth computational study on the optical properties of the CAV defect. We find that the CAV defect has the potential to exhibit several different zero-phonon luminescences with emissions in the near-infrared telecom band, in its neutral and positive charge states. However, our measurements show that the AB lines only consist of three nonthermally activated lines instead of the previously reported four lines; meanwhile, our calculations on the CAV defect are unable to find optical transitions in full agreement with the AB-line assignment. In light of our results, the identification of AB lines and the associated room-temperature emission require further study.

As humanity strives to see the potential of second-generation quantum technology, finding the most suitable quantum system for each application is vital. Leading components have limiting operational requirements, like cryogenic temperatures, and are highly specialized. In contrast, color centers in wide-bandgap semiconductors show versatility and promise. Historically significant in semiconductor technology, that brought the modern information age, point defects now show potential as qubits for data processing with room-temperature operation, quantum nanoscale sensors, and single-photon emitters useful in quantum networks. These systems are nonetheless complex, and their behavior arises from many-body problems influenced by chemical composition, hosting material, and coupling to the environment. Modern first-principles methods and efficient modeling enable accurate verification and prediction of these systems, helping to design and optimize defect-based quantum applications. In this thesis, I present my application of ab initio methods and spin dynamics modeling to color centers in SiC, which contribute to the verification, discovery and optimization of defect systems in optical, qubit, and sensing applications.

I investigated the carbon-antisite vacancy pair (CAV) as a previously proposed model for the AB-lines, which are among the brightest lines in SiC and observable at room temperature. Its optical transitions and zero-phonon lines were characterized using a combination of constrained-occupation density functional theory and GW calculations, and their brightness were estimated using modern post-processing methods. A discrepancy with experimental data emerged, reinforced by new experimental observations from co-authors, prompting further research into both the AB-lines and the CAV defect. This demonstrated both the difficulty of identifying a point defect system and the importance of theoretical verification. As a contrasting approach, data-driven defect design was attempted using high-throughput methods to calculate defects, specifically in the search for a telecom-emitting qubit system ideal for long-range fiber-optic transmission. A handful of novel qubits were predicted, among which the chlorine-vacancy center was further characterized and expected to emit in the optimal telecom C-band. Considering its stability, optical properties, and spin properties, it was shown to share many qualitative features with well-established systems, such as the diamond nitrogen-vacancy center, which has seen wide applicability in quantum technologies, indicating similar potential.

I applied cluster-based methods, the extended Lindbladian method and cluster-correlation expansion, to identify spin relaxation profiles of divacancy and silicon vacancy systems due to relevant spin sources, evaluating dominant contributions to guide the design of application samples. For the divacancy, we quantified the impact of nuclear and electron-spin sources on coherence-limiting relaxation. The silicon vacancy was studied to determine how to produce an increased, but feature-rich, relaxation profile for its application in relaxation-based magnetometry. We similarly quantified the most relevant spin sources and provided guidelines for optimal relaxation sensitivity at low magnetic fields. Possible improvement of the general qubit performance of the silicon vacancy was also considered, but degeneracy in the defect-bath states in coupling to electron spins was discovered in the model. This would imply a coherence-limiting relaxation rate, which has not been observed, unless the degeneracy is lifted. Nuclear-spin-induced effective splitting was determined as the most probable cause. Removing the nuclear spin contribution was therefore predicted to increase the electron-spin coupling effect, which would eventually become counterproductive to prolonging the coherence time, in contrast to popular belief, further emphasizing the value of defect spin modeling.