Point defects in silicon carbide for quantum technologies: Identification, tuning and control

Point defects strongly affect the electrical and optical properties of semiconductors, and are therefore of vast importance for device performance. Over recent years, however, point defects have been shown to possess properties that are highly suitable for applications related to quantum computing, sensing and communication. Single-photon emission and coherent spin manipulation at room temperature have been established for several systems, with the nitrogen-vacancy center in diamond counting among the first solid-state and semiconductor-based quantum platforms. Despite the long coherence times and established entanglement protocols of diamond-based qubit systems, diamond is only marginally compatible with advanced device fabrication methodology, and methods for integration of quantum emitters with electrically and optically controlled devices remain immature. For this reason, silicon carbide (SiC) has gained the attention of the quantum community, having a wide band gap, low spin-orbit coupling, mature device fabrication, and playing host to several promising quantum emitters of both extrinsic and intrinsic type. In this work, electrically and optically active point defects in silicon carbide have been studied using a combination of theoretical and experimental methods, with the aim of elucidating the role of different defects in power electronics and quantum technology devices. Hybrid density functional theory (DFT) calculations were employed to establish defect formation energies and chargestate transition levels, explore defect migration, and develop a new framework for studying the effect of electric fields on defect quantum emission. The calculations are correlated to experimental findings, where deep level transient spectroscopy (DLTS) and photo/cathodoluminescence (PL/CL) measurements reveal electrical and optical defect properties, respectively. The thesis places a particular emphasis on the silicon vacancy (VSi) in SiC, a room temperature single-photon source and qubit candidate exhibiting long spin coherence times. By monitoring VSi emission and comparing to DLTS spectra of proton-irradiated 4H-SiC samples, the VSi(-/2-) and VSi(2-/3-) charge-state transitions are assigned to the S-center, enabling electrical control over the VSi charge state. Depositing Schottky barrier diodes (SBDs) on the 4H-SiC sample surface enhances VSi emission by almost an order of magnitude, and sequential biasing of the SBD results in VSi charge-state switching, as detected by monitoring the V1 and V10 zero-phonon lines attributed to the negatively charged VSi at a hexagonal lattice site. The framework of bulk 4H-SiC epitaxial layers is compared to that of a microparticle matrix of predominantly the 6H polytype, with the former ensuring a homogeneous environment for the qubit defect and the latter enabling self-assembly, flexibility and ease of addressability. Importantly, both external and internal perturbations to the solid-state matrix wherein the VSi is embedded are shown to influence the emitted photon energies, as evidenced by an electric field-induced Stark effect and strain tuning in SiC microparticles. Furthermore, a set of emitters observed in the vicinity of the V1/V10 lines and having consistent subset spacings of 1.45 meV and 1.59 meV are tentatively attributed to vibronic replicas of the VSi emission. The VSi is unstable at elevated temperatures, and this thesis addresses the topics of VSi conversion and defect migration in p-type, intrinsic and n-type 4H-SiC material at 400 °C and above. Indeed, we find that hydrogen and VSi are likely to form complexes in the case that both species are present and in close proximity. In the absence of H, the VSi may convert to the carbon antisitevacancy (CAV) pair in p-type material, however, temperatures above 1000 °C are needed in n-type 4H-SiC, where both recombination with interstitials and divacancy formations prove to be more favorable annealing pathways for the VSi. The carbon vacancy (VC) is far more stable than VSi, and by comparing the two defect species using muon spin rotation (μSR) spectroscopy, we establish the μSR technique as a powerful tool for distinguishing different defect relaxation mechanisms and probing near-surface semiconductor defects in a non-destructive and depth-resolved manner. Annealing temperatures above 1200 °C are shown to be needed to induce VC migration, which is further demonstrated to be anisotropic in 4H-SiC, with the VC favoring in-plane atomic hops over the axial migration path. Finally, above temperatures of 2300 °C the lattice atoms themselves become mobile, and secondary ion mass spectrometry (SIMS) is employed to investigate the influence of a carbon cap covering the surface during annealing on self-diffusion of Si and C in 4H-SiC