Phonon replicas at the M point in 4H-SiC: A theoretical and experimental study

This paper presents a comprehensive study of the phonons at the M point of the Brillouin zone of 4H-SiC, i.e., at the position of the conduction-band minimum, from both theoretical and experimental points of view. The phonon energies are derived from the low-temperature photoluminescence (PL) spectr um. The theoretical section contains the classification of phonons by symme try, the deduction of the polarization selection rules for the photons emit ted during recombination of free or nitrogen-bound excitons, and presents a simple lattice dynamics model used to obtain some results on the distribut ion of the phonon modes with different symmetries above and below the energ y gap in the phonon dispersion. The classification of the 24 phonons by sym metry is shown to be 8M(1) + 4M(2) + 4M(3)+ 8M(4), where M-i are the irredu cible representations of the group of the wave vector at the M point. The p olarization selection rules imply that among the 24 phonon replicas (lines) in 4H-SiC, which can be observed in the PL spectrum, there are 12 with par allel polarization (M-1 and M-3 symmetry), and 12 with polarization perpend icular (M-2 and M-4 symmetry) to the crystal (hexagonal) c axis. Our consid eration shows that the phonons of each symmetry are equally distributed abo ve and below the "phonon" energy gap. Therefore, there are six replicas of each polarization above and below the gap. In the experimental section an a ssignment of the phonon replicas based on the similarity of the sets of rep licas related to the bound and free excitons is carried out, and the result s are compared with the theoretical ones. We have experimentally found 22 o f the 24 phonons at the M point in 4H-SiC. [S0163-1829(98)02144-4].