Asymmetry of the signal could originate from skin effect due to relatively high electrical conductivity of the sample 47 . In particular, skin penetration depth calculated for static electrical resistivity is about 50 μm, which is smaller than the size of the studied crystals. The high conductivity is also manifested via reduction of the quality factor of the spectrometer resonator from 3300 down to 2900 caused by insertion of the sample at 130 K. The variation of estimate of spins concentration with temperature could originate from thermal ionization of donors and increase of the sample conductivity. EPR signals with g-factor close to 1.96 in ZnO have been attributed to shallow donors in paramagnetic state .