Optically detected magnetic resonance study of SiC:Ti

Abstract

We describe optically detected magnetic resonance (ODMR) experiments carried out on the green luminescence band in 4H, 6H, and 15R polytypes of SiC:Ti. It is found that the luminescence is due to radiative transitions from excited spin triplet (S=1) to ground singlet (S=0) states. Unusually large isotope shifts in the zero-field splittings demonstrate that the light-emitting centers involve a single Ti and six equivalent neighboring silicon atoms. Spectral dependence studies of the ODMR signals allow us to discuss the microscopic nature of the inequivalent lattice sites for the Ti atom as well as to correctly assign the zero-phonon line and the accompanying characteristic phonons associated with each Ti site. We interpret our results in terms of the model proposed by Patrick and Choyke of Ti substituting on the silicon sublattice, the excitonic triplet state being formed from an electron tightly bound in a normally unoccupied $d_e$ orbital of the Ti atom plus a Coulombically bound hole. The ODMR spectra are extremely sensitive to subtle (second- and third-nearest-neighbor) differences between the inequivalent lattice sites. This, plus a sizable Franck-Condon shift observed in the luminescence and the unusual isotope effects, are interpreted as possible evidence for vibronic properties of the bound hole, implying intermediate to strong localization for it also.