Newly Resolved Phonon-Assisted Transitions and Fine Structure in the Low Temperature Wavelength Modulated Absorption and Photoluminescence Spectra of 6H SiC

We measured the low temperature wavelength modulated absorption and low temperature photoluminescence spectra of relatively high purity (n~1014-1015 cm-3) 6H SiC boule and epi layers at high wavelength resolution (0.1-0.7 Å) to adequately separate and identify phonon-assisted absorption and recombination processes due to free excitons. As a result we have identified newly resolved or weak features in both spectra which we associate with previously unidentified momentum conserving (MC) phonons. We obtain the energies of 21 of the 36 possible MC phonons in 6H SiC and a more accurate estimate of the exciton bandgap, Egx = 3.0225±0.0003 eV. In several of the phonon-assisted absorption onsets we also observe fine structure and variations in the measured spin-orbit splitting, which relate to a fourfold splitting of the free exciton energy bands.