Kalabukhova E. Physical properties of the impurity and defect paramagnetic centers in silicon carbide polytypes

Dissertation deals with the investigation of the physical properties of the impurities and defects in 6Н and 4H-SiC by High Frequency/High Filed (HF/HF) electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) methods. Nitrogen donors were investigated with EPR at 140 GHz and with ENDOR at 9 GHz in 6Н, 4H and 15R-SiC. The number of nitrogen EPR spectra turned out to be equal of inequivalent positions of nitrogen in the SiC polytypes. The hexagonal site donors in SiC polytypes show more than 10 times smaller isotropic and also anisotropic hyperfine (HF) interactions compared to the donors on quasicubic sites which show only isotropic HF interactions. As a result the ground state of the nitrogen in quasicubic sites is described by fully symmetric 1S(A1) states while anisotropic HF interactions for hexagonal site donors are explained by admixture of the 1S(E) excited state to the 1S(A1) ground state due to the small value of the valley-orbit splitting for hexagonal sites. This conclusion is supported by the investigation of the temperature behavior of the HF splitting for nitrogen substituting quasicubic and hexagonal site. A reduction of the HF interaction with increasing temperature is explained by thermally activated transition of donor electrons from the 1S(A1) state to the 1S(E) state which has a node at the central N atom. Optical recharging of nitrogen between two paramagnetic 1S(A1) and 1S(E) states was discovered after cooling the 6H SiC samples under interband illumination. The investigation of the ligand HF structure in EPR and ENDOR spectrum due to interaction of the unpaired electron spin with Si29 and С13 nuclear spin makes it possible to identify the type of the site substituting by nitrogen in SiC. It was concluded that nitrogen substitutes carbon site in 6HSiC and silicon site in 4H SiC. The temperature dependences of donor EPR line intensities were found to deviate from the Curie law. The temperature dependencies of the line intensities and widths were used to determine ionization energies and the valley-orbit splitting of nitrogen on the quasi-cubic and hexagonal sites. The low temperature hopping motion of the donor electron between paramagnetic and nonparamagnetic state has been established and investigated in 6Н and 4H-SiC heavily doped with nitrogen. Two groups of phosphorous EPR spectra which have different values and character of HF interaction was studied by HF/HF EPR and ENDOR methods in 6Н SiC. The electronic model and energy characteristics of phosphorous centers are discussed. The shallow boron acceptors in 6Н, 4H and 3C SiC were investigated with EPR at 140 GHz in the temperature interval 4.2-100 K. The microscopic model suggested from the EPR and ENDOR data including principle values of g-tensors, HF interaction constants and frequency dependent thermally activated transition of boron EPR spectra in quasi-cubic site from monocline symmetry to axial is as follows: shallow boron viewed as B-induced C acceptors. The hole is located in the connection line between BSi and adjacent C, which is relaxed towards the B Si. The different mechanism of defect formation in Si-, C-rich n-type 4H, 6H SiC was discovered. The intrinsic defect originated from the carbon antisite CSi- with donor-like behavior was discovered in 4H SiC while structural defects in the form of extended defects (microclusters, associates) are present in 6H SiC having deviations in stoichiometry. The correlation between the concentration of the intrinsic defects and Si/C ratio was established. A measurement procedure, using the HF/HF EPR method was proposed to determine the Si/C ratio in 4H SiC. Two set of semi-insulating (s.-i.) 4H SiC have been studied by EPR at 37 GHz. One set of samples revealed EPR spectra of vanadium V3+ (3d2) with S=1. The signs of fine structure constants D were determined at 4.2 K to be positive. A series of undoped high purity semi-insulating (HPSI) 4H-SiC revealed a number of photosensitive paramagnetic centers including shallow nitrogen, boron and intrinsic defects with S=1/2 which were observed in the dark and under illumination of the sample with the light of different photon energy. The electronic model for trapping and recombination process occurring in HPSI 4H SiC was proposed. It was established that the defect which is responsible for the life time of the minority carriers in HPSI 4H SiC is a silicon vacancy in charge state 3- located approximately at 1.15 0.06eV below conduction band EC. Intercenter charge-transfer processes between nitrogen and deep donor-like intrinsic defects are shown to be very efficient. Keywords: HF/HF EPR, ENDOR, silicon carbide, impurities, native defects, electronic structure, stoichiometry, semi-insulating silicon carbide.