The X‐, Q‐band field‐sweep electron spin echo (FS ESE) study of n‐type 6H‐SiC wafers shows besides the well known hyperfine (hf) triplet lines of 14N on two quasi‐cubic (Nc1, Nc2) and hexagonal (Nh) sites additional triplet lines (Nx) of comparatively low intensity with a g ‐tensor of about the average value of the Nc2 and Nh spectrum at 4.2 K. The Nx triplet has half of the hf splitting with respect to the nitrogen residing at cubic sites Nc2. Pulsed electron nuclear double resonance (ENDOR) measurements performed on the Nx EPR triplet line demonstrate that there is an efficient spin‐coupling between Nh and Nx nitrogen related centers. The Nx triplet lines were attributed to distant donor Nc–Nh pairs between nitrogen atoms residing at quasi‐cubic and hexagonal sites having a total electron spin S = 1, which were found before in n‐type 4H‐SiC. Based on Q‐band FS ESE and X‐band pulsed ENDOR measurements the values of the hf interaction for Nc1, Nc2 were reassigned with respect to the assignment accepted hitherto.The super hyperfine (shf) interaction with neighboring atoms of nitrogen on the quasi‐cubic lattice sites was studied by FS ESE and pulsed ENDOR. The new shf lines due to 29Si and 13C atoms with large shf interaction which were not observed previously were found in nitrogen FS ESE and ENDOR spectra. One of them, with the largest shf splitting (23.12 MHz), were attributed to the 29Si atoms along the c‐axis in nearest‐neighbor positions of nitrogen on the quasi‐cubic sites which unambiguously confirms that nitrogen substitute carbon lattice site in 6H‐SiC. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The nitrogen (N) donors in 6H SiC were investigated by field sweep electron spin echo (FS ESE), pulsed electron nuclear double resonance (ENDOR) and pulsed General TRIPLE ENDOR spectroscopy. The 29Si and 13C superhyperfine (shf) lines observed in the FS ESE and ENDOR spectra of N in n‐type 6H SiC were assigned by pulsed General TRIPLE resonance spectroscopy to the specific carbon (C) and silicon (Si) atoms located in the environment of N donors residing at two quasi‐cubic lattice sites (Nk1, Nk2) in 6H SiC. As a result, the largest value of the shf interaction was found with Si atoms for the N donors at the hexagonal (Nh) and quasi‐cubic site Nk1, while for Nk2 the largest value of the shf interaction was found with C atoms. It gives us the argument to consider that N substitute different lattice sites in 6H SiC lattice. The relative signs of the shf interaction (shfi) constants for Nk1 and Nk2 with 29Si and 13C nuclei located in the nearest‐neighbor, next nearest‐neighbor, and outer shells are found from the TRIPLE ENDOR spectra to be positive for C atoms and negative for Si atoms. From the comparison of the experimentally obtained shfi constants with the theory, the electronic spin‐density distribution over the 29Si and 13C nuclei located in the nearest neighbor shells of N donors has been obtained taking into account the Kohn–Luttinger interference effect. The position of the conduction band minimum along the ML‐line was determined to be at k0z/kmax = 0.2 ± 0.05.

Optical and magnetic properties of SiO2:C nanopowders obtained by chemical and thermal modification of fumed silica were studied by Fourier transform infrared spectroscopy, Raman, continuous wave (CW) electron paramagnetic resonance (EPR), echo-detected EPR and pulsed electron nuclear double resonance (ENDOR) spectroscopy. Two overlapping signals of Lorentzian lineshape were detected in CW EPR spectra of the initial SiO2:C. The EPR signal at g = 2.0055(3) is due to the silicon dangling bonds, which vanishes after thermal annealing, and the second EPR signal at g = 2.0033(3) was attributed to the carbon-related defect (CRD). The annealing of the SiO2:C samples gives rise to the increase of the CRD spin density and shift to the higher g-values due to the appearance of the oxygen in the vicinity of the CRD. Based on the temperature-dependent behavior of the CRD EPR signal intensity, linewidth and resonance field position we have attributed it to the spin system with non-localized electrons hopping between neighboring carbon dangling bonds, which undergo a strong exchange interaction with a localized spin system of carbon nanodots. The observed motional narrowing of the CRD EPR signal in the temperature interval from 4 to 20 K indicates that electrons are mobile at 4 K which can be explained by a quantum character of the conductivity in the vicinity of the carbon layer. The electrons trapped in quantum wells move from one carbon nanodot to another by hopping process through the energy barrier. The fact that echo-detected EPR signal at g = 2.0035(3) was observed in SiO2:C sample annealed at T ann ≥ 700 °C serves as evidence that non-localized electrons coexist with localized electrons that have the superhyperfine interaction with surrounding 13C and 29Si nuclei located at the SiO2:C interface. The presence of the superhyperfine interaction of CRD with 1H nuclei indicates the existence of hydrogenated regions in SiO2:C sample.

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