Silicon (Si) is the most commonly used n-type dopant in AlGaN, but the conductivity of Si-doped Al_xGa_1-xN was often reported to drop abruptly at high Al content (x>0.7) and the reason was often speculated to be due to either compensation by deep levels or self-compensation of the so-called DX (or negative-U) center. Understanding the electronic structure of Si and carrier compensation processes is the essential for improving the n-type doping of high-Al-content Al_xGa_1-xN. In our studies of Si-doped AlGaN layers grown by metal-organic chemical vapor deposition, Electron Paramagnetic Resonance (EPR) was used to study the electronic structure of Si in high-Al-content AlxGa_1-xN.

From the temperature dependence of the concentration of the Si donor on the neutral charge state Ed determined by EPR, we showed that Si already forms a stable DX center in Al_xGa_1-xN with x ~0.77. However, with the Fermi level locating only ~3 meV below E_d, Si still behaves as a shallow donor and high conductivity at room temperature could be achieved in Al_0.77Ga_0.23N:Si layers. In samples with the concentration of the residual oxygen (O) impurity larger than that of Si, we observed no carrier compensation by O in Al_0.77Ga_0.23N:Si layers, suggesting that at such Al content, O does not seem to hinder the n-type doping in the material. The result is presented in paper 1.

In paper 2, we determined the dependence of the EDX level of Si on the Al content in Al_xGa_1-xN:Si layers (0.79≤x≤1) with the Si concentration of ~2×10¹⁸ cm^-3 and the concentrations of residual O and C impurities of about an order of magnitude lower (~1÷2×10¹⁷ cm^-3). We found the coexistence of two DX centers (stable and metastable ones) of Si in Al_xGa_1-xN for x≥0.84. For the stable DX center, abruptly deepening of EDX with increasing of the Al content for x≥0.83 was observed, explaining the drastic decrease of the conductivity as often reported in previous transport studies. For the metastable DX center, the E_DX level remains close to Ed for x=0.84÷1 (~11 meV for AlN).

The Z₁/Z₂ defect is the most common deep level revealed by Deep Level Transient Spectroscopy (DLTS) in 4H-SiC epitaxial layers grown by chemical vapor deposition (CVD). It has previously been shown by DLTS to be a negative-U system which is more stable with capturing two electrons. The center is also known to be the lifetime killer in asgrown CVD material and, therefore, attracts much attention. Despite nearly two decades of intensive studies, including theoretical calculations and different experimental techniques, the origin of the Z₁/Z₂ center remains unclear. EPR is known to be a powerful method for defect identification, but a direct correlation between EPR and DLTS is difficult due to different requirements on samples for each technique. Using high n-type 4H-SiC CVD free-standing layers irradiated with lowenergy (250 keV) electrons, which mainly displace carbon atoms creating C vacancies, C interstitials and their associated defects, it was possible to increase the irradiation dose, allowing the application of EPR and DLTS on the same samples. Combining EPR, DLTS and supercell calculations, we identified the negatively charged carbon vacancy at the quasi-cubic (k) site and observed clear negative-U behaviors of the negative carbon vacancies at both hexagonal (h) and k sites. Our results showed that the Z₁/Z₂ center is related to the (2-|0) level of VC and its higher-lying levels Z₁ and Z₂ are related to the (-|0) levels of V_C at the h and k sites, respectively. The result is presented in paper 3.

Point defects in semiconductor materials are known to have important influence on the performance of electronic devices. For defect control, knowledge on the model of defects and their properties is required. Information on defects, such as the symmetry and the localization of spins, is essential for identification of defects and understanding their electronic structure. Such information can be obtained from Electron Paramagnetic Resonance (EPR). In many cases, the energy levels of defects can be determined from photoexcitation EPR (photo-EPR) or temperature dependence of the EPR signal. The thesis contains six papers, focusing on the identification and electronic structure investigation of defects and impurities in Al_xGa_1-xN (x~0.7-1) and silicon carbide (SiC) using EPR in combination with other electrical characterizations and density functional theory calculations.

The two first papers concern EPR studies of silicon (Si) in AlGaN alloys. Due to its direct and wide band gap which can be tailored from 3.4 eV for GaN to 6.2 eV for AlN, high-Al-content wurtzite Al_xGa_1-xN (x≥0.7) has been considered as a promising material for fabrication of compact, high-efficiency and non-toxic deep ultraviolet light-emitting diodes (LEDs) and laser diodes (LDs) for replacing low-efficiency and toxic mercury lamps in water/air purification and sterilization. Si is commonly used for n-type doping in AlGaN and AlN, but the conductivity of Si-doped Al_xGa_1-xN was often reported to drop abruptly at high Al content (x>0.7) and the reason was often speculated to be due to either carrier compensation by other deep levels or Si itself when it transforms from a shallow donor to a DX (or negative-U) center which acts as an acceptor. In paper 1, we showed that Si already forms a stable DX center in Al_xGa_1-xN with x ~0.77. However, with the Fermi level locating only ~3 meV below the neutral charge state, Ed, Si still behaves as a shallow donor. Negligible carrier compensation by oxygen (O) in Al_0.77Ga_0.23N:Si layers was observed, suggesting that at such Al content, O does not seem to hinder the n-type doping in the material. In paper 2, we found the coexistence of two Si DX centers, the stable DX1 and the metastable DX2, in Al_xGa_1-xN for x≥0.84. For the stable DX1 center, abrupt deepening of the energy level of the negative charge state DX^–, E_DX, which determines the ionization energy E_a of the Si donor, with increasing of the Al content for x≥0.83 was observed. The dependence of E_a on the Al content in Al_xGa_1-xN:Si layers (0.79≤x≤1) was determined. The results explain the drastic decrease of the conductivity as often reported for  Al_xGa_1-xN:Si in previous transport studies. For the metastable DX2 center, we found that the E_DX level remains close to E_d for x=0.84÷1.

SiC is a wide band-gap semiconductor having high-thermal conductivity, high breakdown field, and large saturated electron drift velocity which are essential properties for high-voltage and high-power devices. In paper 3, the identification of niobium (Nb) in 4Hand 6H-SiC grown by high-temperature chemical vapor deposition (CVD) by EPR and theoretical calculations is presented. We showed that the incorporated Nb formed asymmetric split-vacancy defect (Nb_SiV_C) in which Nb locates in a divacancy, closer to the Si vacancy, and prefers only the hexagonal-hexagonal configuration. In papers 4 and 5, we present the identification and the electronic structure of the negative-U Z₁/Z₂ center in 4HSiC. The Z₁/Z₂ defect is known to be the most common deep level revealed by Deep Level Transient Spectroscopy (DLTS) in 4H-SiC epitaxial layers grown by CVD. The center is also known to be the lifetime killer in as-grown CVD material and, therefore, attracts much attention. Using high-doped n-type free-standing 4H-SiC layers irradiated with low-energy (250 keV) electrons, which mainly displace carbon atoms creating C vacancies (V_C), C interstitials and their associated defects, it was possible to increase the irradiation dose and, hence, the defect concentration, allowing the application of EPR and DLTS on the same samples. In paper 4, using EPR, photo-EPR, DLTS and capacitance-voltage measurements, we showed that the Z₁/Z₂ center is related to the (2-|0) level of V_C and its higher-lying levels Z₁ and Z₂ are related to the (-|0) levels of V_C at the hexagonal (h) and quasi-cubic (k) sites, respectively. In paper 5, combining EPR and supercell calculations, the negatively charged V_C at the k-site was identified. We obtained the excellent agreement in the energy levels of Z₁/Z₂ determined by DLTS and energy levels of VC calculated by supercell calculations and observed clear negative-U behaviors of the negatively charged V_C at both k and h-sites by EPR measurements, consolidating our assignment of the Z₁/Z₂ levels to the negatively charged states of V_C. In paper 6, we studied a defect related to displaced C atoms in n-type 4H-SiC irradiated by low-energy electrons. In irradiated layers, we observed an EPR center at room temperature. After annealing at temperatures in the range of 300-500 °C, this center transforms to a second configuration which is observed in darkness and can be  changed back to the first configuration under illumination. Based on the observed ²⁹Si and ¹³C hyperfine structures, two observed configurations of the EPR center were suggested to be related to different configurations of a carbon interstitial cluster. The annealing, bistable behaviors and energy levels of this EPR center are discussed.