Using a combinatorial approach, Cr, Al and C have been deposited onto sapphire wafer substrates by High Power Impulse Magnetron Sputtering (HiPIMS) and DC magnetron sputtering. X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and X-ray diffraction were employed to determine the composition and microstructure of the coatings and confirm the presence of the Cr2AlC MAX phase within both coatings. One location in both the DCMS and HiPIMS coatings contained only MAX phase Cr2AlC. The electrical resistivity was also found to be nearly identical at this location and close to that reported from the bulk, indicating that the additional energy in the HiPIMS plasma was not required to form high quality MAX phase Cr2AlC.

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 (V_CV_Si 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 (V_C) and silicon vacancies (V_Si) 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. V_Si, V_C, carbon antisite-vacany pair (CSiVC), and V_CV_Si 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 E_a~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 E_a~1.5 eV contain high concentrations of V_C and V_CV_Si and low concentrations of V_Si and as these samples had the most thermally stable SI properties, due to the increased thermal stability of V_C when V_Si 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 E_a~0.6-0.7 eV and ~1.0-1.2 eV. V_C, C_SiV_C and V_CV_Si 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 V_C 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 V_C 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 V_C.V_C+ and V_C- could be detected simultaneously and from the study we concluded that the (+|0) is located at ~E_C–1.77 eV and suggested that the (0|−) and (1−|2−) levels are located at ~E_C–0.8 eV and ~ E_C–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, (V_C-C_SiV_C)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 (Si_C) perturbed by a nearby defect.

We present new results from electron paramagnetic resonance (EPR) studies of the EI4 EPR center in 4H- and 6H-SiC. The EPR signal of the EI4 center was found to be drastically enhanced in electron-irradiated high-purity semi-insulating materials after annealing at 700-750°C. Strong EPR signals of the EI4 center with minimal interferences from other radiation-induced defects in irradiated high-purity semiinsulating materials allowed our more detailed study of the hyperfine (hf) structures. An additional large-splitting 29Si hf structure and 13C hf lines of the EI4 defect were observed. Comparing the data on the defect formation, the hf interactions and the annealing behavior obtained from EPR experiments and from ab initio supercell calculations of different carbon-vacancy related complexes, we suggest 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, as a new defect model for the EI4 center.

Electron paramagnetic resonance (EPR) was used to study high-purity semi-insulating 4H-SiC irradiated with 2 MeV electrons at room temperature. The EPR signal of the EI4 defect was found to be dominating in samples irradiated and annealed at 750 C. Additional large-splitting Si-29 hyperfine (hf) lines and also other C-13 and Si-29 hf structures were observed. Based on the observed hf structures and annealing behaviour, the complex between a negative carbon vacancy-carbon antisite pair (VCCSi-) and a distance positive carbon vacancy (V-C(+)) is tentatively proposed as a possible model for the EIO4 defect.

We present the results of our recent electron paramagnetic resonance (EPR) studies of the EI4 EPR centre in electron-irradiated high-purity semi-insulating 6H SiC. Higher signal intensities and better resolution compared with previous studies have enabled a more detailed study of the hyperfine (hf) structure. Based on the observed hf structure due to the interaction with Si and C neighbours, the effective spin S = 1, the C-1h-symmetry and the annealing behaviour, we suggest a carbon vacancy-carbon antisite complex in the neutral charge state, VCVCCSi0, with the vacancies and the antisite in the basal plane, as a new defect model for the centre.

The novel technique microwave detected photo induced current transient spectroscopy (MD-PICTS) was applied to semi-insulating 6H-SiC in order to investigate the properties of inherent defect levels. Defect spectra can be obtained in the similar way to conventional PICTS and DLTS. However, there is no need for contacting the samples, which allows for non-destructive and spatially resolved electrical characterization. This work is focused on the investigation of semi-insulating 6H-SiC grown under different C/Si-ratios. In the corresponding MD-PICTS spectra several shallow defect levels appear in the low temperature range. However the peak assignment needs further investigation. Additionally different trap reemission dynamics are obtained for higher temperatures, which are supposed to be due to different compensation effects.

Deep levels introduced by low-energy (200 keV) electron irradiation in n-type 4H-SiC epitaxial layers grown by chemical vapour deposition were studied by deep level transient spectroscopy (DLTS) and photoexcitation electron paramagnetic resonance (photo-EPR). After irradiation, several DLTS levels, EH1, EH3, Z(1/2), EH5 and EH6/7, often reported in irradiated 4H-SiC, were observed. In irradiated freestanding films from the same wafer, the EPR signals of the carbon vacancy in the positive and negative charge states, V-C(+) and V-C(-), respectively, can be observed simultaneously under illumination with light of certain photon energies. Comparing the ionization energies obtained from DLTS and photo-EPR, we suggest that the EH6/7 (at similar to E-C - 1.6 eV) and EH5 (at similar to E-C - 1.0 eV) electron traps may be related to the single donor (+ vertical bar 0) and the double acceptor (1- vertical bar 2-) level of V-C, respectively. Judging from the relative intensity of the DLTS signals, the EH6/7 level may also be contributed to by other unidentified defects.

Semi-insulating (SI) 4H-SiC substrates doped with vanadium (V) in the range 5.5×1015 –1.1×1017 cm–3 were studied by electron paramagnetic resonance. We show that only in heavily V-doped 4H-SiC vanadium is responsible for the SI behavior, whereas in moderate V-doped substrates with the V concentration comparable or slightly higher than that of the shallow N donor or B acceptor, the SI properties are thermally unstable and determined by intrinsic defects. The results show that the commonly observed thermal activation energy Ea~1.1 eV in V-doped 4H-SiC, which was previously assigned to the single acceptor V4+/3+ level, may be related to deep levels of the carbon vacancy. Carrier compensation processes involving deep levels of V and intrinsic defects are discussed and possible thermal activation energies are suggested.

High-purity, semi-insulating 6H-SiC substrates grown by high-temperature chemical vapor deposition were studied by electron paramagnetic resonance (EPR). The carbon vacancy (VC), the carbon vacancy-antisite pair (VCCSi) and the divacancy (VCVSi) were found to be prominent defects. The (+|0) level of VC in 6H-SiC is estimated by photoexcitation EPR (photo-EPR) to be at ~ 1.47 eV above the valence band. The thermal activation energies as determined from the temperature dependence of the resistivity, Ea~0.6-0.7 eV and ~1.0-1.2 eV, were observed for two sets of samples and were suggested to be related to acceptor levels of VC, VCCSi and VCVSi. The annealing behavior of the intrinsic defects and the stability of the SI properties were studied up to 1600°C.

Photoexcitation electron paramagnetic resonance (photo-EPR) was used to determine deep levels related to the carbon vacancy (VC) in 4H-SiC. High-purity free-standing n-type 4H-SiC epilayers with concentration of intrinsic defects (except the photo-insensitive SI1 center) below the detection limit of EPR were irradiated with low-energy (200 keV) electrons to create mainly VC and defects related to the C sublattice. The simultaneous observation of and signals, their relative intensity changes and the absence of other defects in the sample provide a more straight and reliable interpretation of the photo-EPR results. The study suggests that the (+|0) level of VC is located at ~EC–1.77 eV in agreement with previously reported results and its single and double acceptor levels may be at ~ EC–0.8 eV and ~ EC–1.0 eV, respectively.