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

Absorption measurements of cubic SiC at 4.2°K show that the absorption edge is due to indirect, exciton-creating transitions, with an exciton energy gap of 2.390 eV. The energies of the phonons participating in these transitions are 46, 79, 94, and 103 meV, and suggest that the conduction-band minima are at $X$, as predicted by recent calculations. The phonon energies are accurately determined from the 6°K luminescence spectrum of four-particle nitrogen-exciton complexes. Additional lines in the luminescence spectrum at higher temperatures are attributed to thermally excited states of of the complex. Comparisons of cubic SiC with other polytypes are given. There is close agreement in some phonon energies, but energy gaps are very different. An empirical correlation of the energy gaps with percent "hexagonal" is given for seven polytypes.