Optical Properties of15RSiC: Luminescence of Nitrogen-Exciton Complexes, and Interband Absorption

Optical experiments on $15R$ SiC are reported, and the results are compared with those given earlier for $6H$ SiC. The absorption edge is due to indirect, exciton-producing transitions, across an exciton energy gap of 2.986 eV at 6°K. Two distinct photoluminescence spectra are found, due to two kinds of nitrogen-exciton complexes. These spectra consist of series of lines, from which 18 phonon energies are obtained. Only four series of lines are observed in each spectrum, although there are five inequivalent nitrogen sites in $15R$ SiC. The four nitrogen donor ionization energies are found to be approximately 0.14, 0.16, 0.16, and 0.20 eV. These, and other binding energies, are somewhat smaller than in $6H$ SiC. As in the $6H$ SiC data, we find evidence of exciton hopping, of localized vibrations, of the presence of six conduction band minima, and of a second valence band, split off 4.8 meV by spin-orbit interaction.