Electron paramagnetic resonance study of defects in SiC

Sammanfattning: Silicon carbide (SiC) is a wide bandgap semiconductor (energy gap of 3.26 eV and 3.03 eV for 4Hand 6H-SiC, respectively). With outstanding physical and electronic properties, SiC is a promising material for high-power, high-frequency and high-temperature applications. The electronic properties of a semiconductor are to a large extent determined by point defects in the crystal. As known from other semiconductors, defect control is crucially important for the successful device applications. Point defects can be impurities, such as the shallow nitrogen (N) donor or boron acceptor (the residual n- and p-type dopants in SiC), or intrinsic defects, such as vacancies, antisites, interstitials or combinations thereof. One of the key issues in the SiC technology is to develop semi insulating (SI) SiC substrates required for SiC MEtal Semiconductor Field Effect Transistors (MESFETs) and also for III-nitride based High Electron Mobility Transistors (HEMTs), to reduce the parasitic capacitance and to improve the device performance. For achieving the SI behavior the Fermi level should be pinned near the middle of the bandgap. This can be realized using defects with deep acceptor level(s) to compensate the residual shallow N donors which cause the natural ntype doping of as-grown SiC.Vanadium (V) doped SI SiC has been developed since the 1990s. However, SiC MESFETs using V-doped SI SiC substrates are shown to have severe problems with electron trapping to eep levels in the SI substrates which causes reduction of the drain current and instability of the device performance. Since the beginning of this decade, V-free high-purity SI (HPSI) SiC substrates using intrinsic defects to compensate the N donors have been developed. The work in this thesis has been devoted to characterize defects in HPSI SiC using electron paramagnetic resonance (EPR). EPR detects transitions between energy levels split up by the interaction of unpaired electron spins (localized at the defect and neighboring atoms) with an applied magnetic field. Thanks to the sensitivity of the electron spins to their surroundings; especially to nearby nuclear spins that further splits the energy levels by the so-called hyperfine (hf) interaction, one can extract information on the structure and electronic configuration of a defect.The work has been focused on (i) the identification of prominent defects, (ii) the determination of their energy levels and roles in the carrier compensation processes, (iii) the defect interaction and the stability of the SI properties at high temperatures, in order to identify the optimal defect(s) to be used for controlling the SI properties. EPR and ab initio supercell calculations have been the main tools for defect identification and all three common polytypes 3C-, 4H- and 6H-SiC of different conducting types (n-, p-type and SI) have been investigated. For determination of the energy levels in the bandgap, the combined results of EPR and photoexcitation EPR (photo-EPR), Deep Level Transient Spectroscopy (DLTS), the temperature dependence of the resistivity, and ab initio calculations have been evaluated. Annealing studies up to 1600 °C for samples with different defect compositions have been carried out for obtaining knowledge on the defect interaction and thermal stability of the SI properties as well as the change in resistivity, activation energy and defect concentration. Below is a short summary of the papers included in the thesis.In paper 1, the identification of the neutrally charged divacancy (VCVSi 0) in 4H-SiC, by PR and ab initio calculations, is presented. The divacancy is a common defect in SiC and it is thought to play a role in carrier compensation in HPSI SiC. Annealing studies show that it is formed during migration of carbon vacancies (VC) and silicon vacancies (VSi) and in the studied samples it is thermally stable up to at least 1500 °C.Paper 2 presents EPR identification of prominent defects in different types of HPSI 4H-SiC substrates grown by high-temperature chemical vapor deposition (HTCVD) and physical vapor transport (PVT), the determination of some of their deep acceptor levels and their roles in carrier compensation processes. VSi, VC, carbon antisite-vacany pair (CSiVC), and VCVSi were found to be the most common defects in different types of HPSI 4H-SiC. The samples could be grouped into three activation energy ranges Ea~0.8–0.9 eV, ~1.1–1.3 eV, and ~1.5 eV, and the possible defect levels related to these energies were discussed for each group. The samples with Ea~1.5 eV contain high concentrations of VC and VCVSi and low concentrations of VSi and as these samples had the most thermally stable SI properties, due to the increased thermal stability of VC when VSi is absent, we concluded that this defect composition is preferable.A similar study is presented in paper 4 of different types of HPSI 6H-SiC substrates grown by HTCVD. The samples could be grouped into two activation energy ranges Ea~0.6-0.7 eV and ~1.0-1.2 eV. VC, CSiVC and VCVSi were found to be the prominent defects and the relationship between their energy levels and the activation energies was discussed. The  materials were still SI after annealing up to 1600°C although the activation energies were lowered. The (+|0) level of VC was also specifically studied by photo-EPR and determined to be located at ~1.47 eV above the valence band, similar to 4H-SiC.The content of Paper 3 concerns an EPR study of two defects, labeled L5 and L6, in electron irradiated n-type 3C-SiC. The L5 defect could be related to the neutrally charged divacancy as it shows some features similar to the divacancy in 4H-SiC. The L5 defect anneals out at low temperatures (~200°C) and could possibly be carbon interstitial related.Paper 5 presents an attempt to study the energy levels of VC by photo-EPR without the usual interference from other defect levels. By using pure free-standing n-type 4H-SiC epilayers with very low defect concentrations and low-energy electron (200 keV) irradiation we could combine photo-EPR and DLTS to study energy levels related to VC.VC+ and VC- could be detected simultaneously and from the study we concluded that the (+|0) is located at ~EC–1.77 eV and suggested that the (0|?) and (1?|2?) levels are located at ~EC–0.8 eV and ~ EC–1.0 eV, respectively.The investigation in paper 6 concerns the identification of the EI4 EPR center in 4H- and 6HSiC. Based on detailed studies of the hf interactions, the annealing behavior and ab initio supercell calculations we believe the corresponding defect is a complex between a carbon vacancy-carbon antisite and a carbon vacancy at the third neighbor site of the antisite in the neutral charge state, (VC-CSiVC)0. It could be directly involved in carrier compensation in some samples before it anneals out (at ~850 °C in irradiated samples or higher temperatures in as-grown sample) and also seems to be an intermediate state in the formation of the divacancy.In Paper 7, an EPR study of a radiation-induced defect, labeled LE5, in 4H- and 6H-SiC is presented. The observation of the LE5 spectra in samples irradiated at low temperatures (77-100 K) indicates that it is a primary defect. From the low symmetry (C1), the Si hf structures, and the low anneal-out temperature (~600-750 °C) we suggested that the defect may be a complex involving a silicon antisite (SiC) perturbed by a nearby defect.