Possible Role of Hydrogen within the So-Called X Center in Semi-Insulating 4H-SiC

Multi-frequency electron paramagnetic resonance (EPR) measurements between 9 and 140 GHz as well as Hall measurements were performed on a series of nominally undoped high-purity semi-insulating (HPSI) 4H-SiC. The investigations in a temperature range from 4 K to 300 K were focused on the photosensitive intrinsic X-defect residing at two inequivalent lattice sites (X(h) and X(k)). Photo-EPR and Hall effect measurements indicate donor-like energy levels at 1.26 0.06 eV and 1.36 0.06 eV below the conduction band for X(h) and X(k), respectively. The EPR spectra recorded at 77 K and at 9 GHz consist of superimposed EPR lines, which were characterized by axially symmetric g-tensors and resolved hyperfine (hf) lines. The hf interactions determined are identical to those published earlier in two independent works for the carbon vacancy in a single positive charge state. However, it is a surprising fact that for one hf line the intensity ratio is different in all works. With the help of additional theoretical investigations, these variations are tentatively assigned to the contamination of a sample-dependent fraction of carbon vacancies with hydrogen.