Electron paramagnetic resonance (EPR) and photo EPR measurements were performed on undoped semi-insulating (SI) 6H SiC material grown by the physical vapor transport (PVT) method. EPR lines from a carbon-related surface defect, two photosensitive high temperature stable intrinsic defects with S=1/2 and shallow nitrogen and boron were observed. The EPR spectrum of the intrinsic defect labeled XX was observed in the dark and consists of three single lines XX1, XX2, XX3 due to three inequivalent sites in the 6H SiC. The second defect labeled PP appeared in EPR spectrum during photo-excitation of the SI 6H SiC and consisted of a single EPR line. The photo EPR data placed the energy level of the defects in the region EV + 1.24 ÷1.29 eV. The EPR parameters and symmetry behavior of XX agree well with those of the carbon vacancy-related Ky center observed in p-type electron irradiated 6H SiC and we tentatively identify the XX defect with VC +. The electronic processes occurring in undoped SI 6H SiC under photo-excitation are also described.

The homogeneity of the distribution of impurities and defects in a 6H-SiC wafer cut out from a nominally undoped 6H-SiC ingot grown by the physical vapor transport (PVT) method at the Bandgap Technologies Inc. (USA) has been studied making use of the electron paramagnetic resonance (EPR) and photo EPR methods at a frequency of 37 GHz and the temperature T = 77 K. The researches of the EPR and photo EPR spectra of eleven samples cut off from the wafer have shown that the type of conductivity in the nominally undoped 6HSiC ingot varies from p- to n-type along the growth direction, so that only the middle section of the ingot is characterized by a semi-insulating (SI) behavior due to the mutual compensation of donors and acceptors. In the samples cut off from the middle section of the wafer, the photoresponse of donor and acceptor EPR spectra demonstrates a persistent relaxation effect after the photoexcitation has been turned off, which is a typical characteristic of SI materials. Two intrinsic defects in SI 6H-SiC samples have been found and studied. On the basis of spectroscopic and energy characteristics of defects, one of them was associated with a carbon vacancy in the positive charge state, V+C, while the other was attributed to a silicon vacancy in the negative charge state, V¡Si.