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Electron paramagnetic resonance (EPR) and electron spin echo (ESE) at X-band and at high-frequency W-band (95 GHz) have been used to study defects in natural diamond nanocrystals, detonation nanodiamond (ND) with a size of ∼4.5 nm and detonation ND after high-temperature, high-pressure sintering with a size of ∼8.5 nm. Atomic nitrogen centers N 0 and nitrogen pairs N 2 + have been detected and identified and their structure has been unambiguously determined by means of the high frequency EPR and ESE in natural diamond nanocrystals. In detonation ND and detonation ND after sintering atomic nitrogen centers N 0 have been discovered in nanodiamond core. In addition EPR signal of multi-vacancy centers with spin 3/2 seems to be observed in diamond core of detonation ND.

Five new centers with the electron spins S= 1 2 and 1 and axial symmetry along c-axis were shown by EPR to arise in SiC after heavy neutron irradiation and following high-temperature annealing at 1500°C. The striking feature of all the centers discovered was a strong hyperfine interaction with a great number of equivalent host Si (C) atoms in SiC lattice. It can be assumed that these defects have high-symmetry structure which suggests a high stability and consists in multi-vacancy clusters. In addition, antisite Si (C) atoms could be included. There is a good probability that some of the new centers could be related to famous D1 center.

We report the first EPR observation of rare-earth impurities in erbium doped bulk 6H-SiC crystals, grown by the sublimation sandwich-method. Two lines for axial Er 3+ centre with crystalline c -axis being the axis of the g tensor, which seem to arise from different lattice sites in 6H-SiC and three Er 3+ centres with same nearly orthorhombic symmetries, attributed to different lattice sites have been observed. A direct identification of erbium ions has been established by the presence of hyperfine interaction with 167 Er nuclei. The principal g values of the axial and orthorhombic Er 3+ centres were found. The average g values suggest that the parent cubic ground state may have γ 7 representation for both centres. Presumably, erbium substitute for silicon in a regular environment for axial centre. The orthorhombic Er 3+ seems to include another defect at carbon position along with Er 3+ ion at silicon site. The EPR spectrum of the excited state of Er 3+ in 6H-SiC seems to be observed at higher temperature. The trace amounts of the other rare-earth elements, in particular Dy 3+ , had probably been present in EPR spectra.

In this letter we report the EPR observation of the group-III deep acceptor impurities: deep B, deep Al and deep Ga centres in SiC crystals. A direct identification of the B and Ga atoms involved in the defect centre, considered as deep B and deep Ga, has been established by the presence of hyperfine interaction with 11B and 10B nuclei in isotope enriched B doped crystals and 69Ga and 71Ga nuclei in Ga doped crystals. In contrast to the results for the group-III shallow acceptor impurities in SiC, the observed qualitative behaviour for Al and Ga deep level impurities is the same as that for B. No effective-mass-like behaviour was observed for deep group-III acceptors. Deep centres were shown from EPR spectra to have symmetry nearly axial around the hexagonal axis of the crystal. Evidence of strong thermally activated dynamic effects having common features for the EPR spectra of all deep centres have been observed. The structural model for deep centre as an impurity-vacancy pair are presented. For shallow B acceptor off-centre position of B which is induced by chemical rebonding was suggested and as a result no effective-mass-like behaviour was observed. It was indicated from ODMR of deep Al acceptors that deep Al level is about 0.1 eV shallower as compared to deep B.