The divacancy comprising two neighboring vacant sites in the SiC lattice is a promising defect for applications in quantum technology. So far, most research has focused on the divacancy in 4H-SiC, whereas the divacancies in 6H- and 3C-SiC have received much less attention. Here, we outline arguments showing that the neutral charge state of the divacancies in the latter two polytypes is intrinsically stable, in contrast to that in 4H-SiC, where the photoluminescence quenches in most materials for certain excitation energies (below approximately 1.3 eV). Divacancies in 6H- and 3C-SiC are anticipated to remain stable for all excitation energies above resonant excitation. We provide ab initio calculation results for the charge transfer levels of divacancies in 6H- and 3C-SiC. We also show that the luminescence from the divacancy in 3C-SiC vanishes with increasing temperature toward room temperature because of the proximity of the excited state to the conduction band.

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.

Doping of a two-dimensional (2D) material by impurity atoms occurs via two distinct mechanisms: absorption of the dopants by the 2D crystal or adsorption on its surface. To distinguish the relevant mechanism, we systematically dope 53 experimentally synthesized 2D monolayers by 65 different chemical elements in both absorption and adsorption sites. The resulting 17,598 doped monolayer structures were generated using the newly developed ASE DefectBuilder—a Python tool to set up point defects in 2D and bulk materials—and subsequently relaxed by an automated high-throughput density functional theory (DFT) workflow. We find that interstitial positions are preferred for small dopants with partially filled valence electrons in host materials with large lattice parameters. In contrast, adatoms are favored for dopants with a low number of valence electrons due to lower coordination of adsorption sites compared to interstitials. The relaxed structures, characterization parameters, defect formation energies, and magnetic moments (spins) are available in an open database to help advance our understanding of defects in 2D materials.

The negatively charged silicon vacancy (V-Si(-)) in silicon carbide is a well-studied point defect for quantum applications. At the same time, a closer inspection of ensemble photoluminescence and electron paramagnetic resonance measurements reveals an abundance of related but so far unidentified signals. In this study, we search for defects in 4H-SiC that explain the above magneto-optical signals in a defect database generated by automatic defect analysis and qualification (ADAQ) workflows. This search reveals only one class of atomic structures that exhibit silicon-vacancy-like properties in the data: a carbon anti-site (C-Si) within sub-nanometer distances from the silicon vacancy only slightly alters the latter without affecting the charge or spin state. Such a perturbation is energetically bound. We consider the formation of V-Si(-) + C-Si; up to 2 nm distance and report their zero phonon lines and zero field splitting values. In addition, we perform high-resolution photoluminescence experiments in the silicon vacancy region and find an abundance of lines. Comparing our computational and experimental results, several configurations show great agreement. Our work demonstrates the effectiveness of a database with high-throughput results in the search for defects in quantum applications.

Automatic Defect Analysis and Qualification (ADAQ) is a collection of automatic workflows developed for high-throughput simulations of magneto-optical properties of point defects in semiconductors. These workflows handle the vast number of defects by automating the processes to relax the unit cell of the host material, construct supercells, create point defect clusters, and execute calculations in both the electronic ground and excited states. The main outputs are the magneto-optical properties which include zero-phonon lines, zero-field splitting, and hyperfine coupling parameters. In addition, the formation energies are calculated. We demonstrate the capability of ADAQ by performing a complete characterization of the silicon vacancy in silicon carbide in the polytype 4H (4H-SiC).

Point defects in semiconductors have been and will continue to be relevant for applications. Shallow defects realize transistors, which power the modern age of information, and in the not-too-distant future, deep-level defects could provide the foundation for a revolution in quantum information processing. Deep-level defects (in particular color centers) are also of interest for other applications such as a single photon emitter, especially one that emits at 1550 nm, which is the optimal frequency for long-range communication via fiber optics.

First-principle calculations can predict the energies and optical properties of point defects. I performed extensive convergence tests for magneto-optical properties, such as zero phonon lines, hyperfine coupling parameters, and zero-field splitting for the four different configurations of the divacancy in 4H-SiC. Comparing the converged results with experimental measurements, a clear identification of the different configurations was made. With this approach, I also identified all configurations for the silicon vacancy in 4H-SiC as well as the divacancy and silicon vacancy in 6H-SiC. The same method was further used to identify two additional configurations belonging to the divacancy present in a 3C stacking fault inclusion in 4H-SiC. I extended the calculated properties to include the transition dipole moment which provides the polarization, intensity, and lifetime of the zero phonon lines. When calculating the transition dipole moment, I show that it is crucial to include the self-consistent change of the electronic orbitals in the excited state due to the geometry relaxation. I tested the method on the divacancy in 4H-SiC, further strengthening the previous identification and providing accurate photoluminescence intensities and lifetimes.

Finding stable point defects with the right properties for a given application is a challenging task. Due to the vast number of possible point defects present in bulk semiconductor materials, I designed and implemented a collection of automatic workflows to systematically investigate any point defects. This collection is called ADAQ (Automatic Defect Analysis and Qualification) and automates every step of the theoretical process, from creating defects to predicting their properties. Using ADAQ, I screened about 8000 intrinsic point defect clusters in 4H-SiC. This thesis presents an overview of the formation energy and the most relevant optical properties for these single and double point defects. These results show great promise for finding new color centers suitable for various quantum applications.

In quantum technologies, point defects in semiconductors are becoming more significant. Understanding the frequency, intensity, and polarization of the zero phonon line is important. The last two properties are the subject of this paper. I present a method for calculating these properties and show the importance of using wave functions from both the ground and excited state. The validity of this method is demonstrated on the divacancy in 4H-SiC. Here, the calculated polarization and radiative lifetimes are in excellent agreement with experimental measurements. In general, this method can help to identify point defects and estimate suitable applications.

The identification of a microscopic configuration of point defects acting as quantum bits is a key step in the advance of quantum information processing and sensing. Among the numerous candidates, silicon-vacancy related centers in silicon carbide (SiC) have shown remarkable properties owing to their particular spin-3/2 ground and excited states. Although, these centers were observed decades ago, two competing models, the isolated negatively charged silicon vacancy and the complex of negatively charged silicon vacancy and neutral carbon vacancy [Phys. Rev. Lett. 115, 247602 (2015)], are still argued as an origin. By means of high-precision first-principles calculations and high-resolution electron spin resonance measurements, we here unambiguously identify the Si-vacancy related qubits in hexagonal SiC as isolated negatively charged silicon vacancies. Moreover, we identify the Si-vacancy qubit configurations that provide room-temperature optical readout.