Paramagnetic defects in silicon carbide crystals irradiated with gamma-ray quanta

The results of the first observations of paramagnetic defects in SiC crystals irradiated with gamma-ray quanta are reported. Three types of defects, designated as gamma1, gamma2, and gamma3, were detected in irradiated 4H-SiC:Al and 6H-SiC:Al crystals using electron spin resonance (ESR) measurements. All these centers have almost the same parameters of the spin-related Hamiltonian with S = 1/2 and feature an appreciable anisotropy of the g-factors. The gamma1 centers are almost coaxial with the local z-axis oriented approximately along one of the directions of the Si-C bond that does not coincide with the c-axis. The gamma2 and gamma3 centers have a lower symmetry, although the orientation along the above bonds is clearly pronounced. The values of the largest g-factor (g(z)) decreases in the sequence from gamma1 to gamma3. The gamma1 signal can be detected at temperatures of 3.5-15 K; the gamma2 and gamma3 signals are detectable at temperatures of 10-35 and 18-50 K, respectively. The hyperfine interaction of an unpaired electron in the gamma1 center with a nucleus of the Si-29 isotope is detected for certain orientations of the crystal. The gamma1, gamma2, and gamma3 centers cease to exist at a temperature of 160 degreesC; it is concluded that the ESR signals of these centers are related to defects in the C sublattice. It is assumed that the gamma1, gamma2, and gamma3 centers have a common origin and are related to the low-temperature (gamma1) and high-temperature (gamma2 and gamma3) modifications of the same center. The models of a defect in the form of either a carbon vacancy or a complex incorporating an Al impurity atom and a C atom that occupies the silicon site or interstice are discussed. (C) 2001 MAIK "Nauka/Interperiodica".