Theoretical polarization of zero phonon lines in point defects

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 lifetimes are in excellent agreement with experimental measurements. In general, this method can help to identify point defects and estimate suitable applications

Ab Initio Predictions of Spin Defects in Simple Oxides

Simple oxides hold promises as hosts of spin qubits with coherence times tens of times longer than those measured in diamond and silicon carbide. However, no suitable spin defect has yet been found in these systems. Using high-throughput first principles calculations, we predict spin defects in calcium oxide with electronic properties remarkably similar to those of the NV-center in diamond. These defects are charged complexes where a dopant atom -- Sb, Bi or I -- occupies the volume vacated by adjacent cation and anion vacancies. We show that the Bi complex emits in the telecommunication range and the computed many-body energy levels indicate that an optical cycle required for qubit initialization can be realized. Our results pave the way to designing quantum states with long coherence times in simple oxides, which are attractive materials for quantum technologies

Identification of Si-vacancy related room temperature qubits in 4H silicon carbide

Identification of 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, still two competing models, the isolated negatively charged silicon vacancy and the complex of negatively charged silicon vacancy and neutral carbon vacancy [Phys. Rev. Lett.\ \textbf{115}, 247602 (2015)] are 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.Comment: 3 figure

ADAQ: Automatic workflows for magneto-optical properties of point defects in semiconductors

Automatic Defect Analysis and Qualification (ADAQ) is a collection of automatic workflows developed for high-throughput simulations of magneto-optical properties of point defect 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).Comment: Typo corrected in eq. 3, references adde

Absorption \textit{versus} Adsorption: High-Throughput Computation of Impurities in 2D Materials

Doping of a two-dimensional (2D) material by impurity atoms occurs \textit{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 \texttt{DefectBuilder} -- a Python tool to set up point defects in 2D and bulk materials -- and subsequently relaxed by an automated high-throughput DFT workflow. Interstitial positions are preferred for small dopants with partially filled valence electrons in host materials with large lattice parameters. On the contrary, 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.Comment: Joel Davidsson and Fabian Bertoldo contributed equall

Orbital-free Density-Functional Theory in a Finite Element Basis

In this work, we have implemented an orbital-free density functional theory (OF-DFT) solver using the finite element method. In OF-DFT, the total ground state energy is minimized directly with respect to the electron density, rather than via orbitals like in the standard Kohn-Sham approach. For this to be possible, one needs an approximation of a universal density functional of the non-interacting kinetic energy. Presently available approximations allow for computation with very low computational expense, but which gives inaccurate energies. A stable OF-DFT code can be used as a testbed for new kinetic energy functionals and provide the necessary tool for investigating the accuracy of OF-DFT calculations for complex systems. We have implemented Thomas-Fermi theory with and without nuclear cusp condition, as well as additional exchange terms of Dirac and Amaldi. The program uses an extended version of the steepest descent in order to find the minimizing density in the variational principle. Our results include convergence tests for the hydrogen atom, weak bonding in the H2 molecule, and accurate results for the lightest noble gases (He, Ne, Ar). For heavier atoms (Kr, Xe, Rn), the results are less accurate. In addition, we consider hydrogen in the simple cubic structure without the cusp condition, which is a first attempt to use the code for periodic systems. Lastly, we discuss some possible improvements for the iterative process towards the minimizing density, as well as other possible directions for future development

Color Centers in Semiconductors for Quantum Applications : A High-Throughput Search of Point Defects in 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