Exciton Recombination Radiation and Phonon Spectrum of6HSiC

Abstract

A photoluminescence spectrum consisting of about 50 lines has been observed at low temperatures in $6H$ SiC. The spectrum can be attributed to exciton recombination after capture by un-ionized nitrogen atoms. The mechanism is similar to that first reported by Haynes for impurities in Si, but the large unit cell of $6H$ SiC (with 12 atoms) provides three inequivalent sites for nitrogen, and 36 phonon branches, hence the large number of lines. Seventeen phonon energies have been measured and plotted in a plausible way in an appropriate extended k space, leading to some conclusions about the band structure of $6H$ SiC. Spin-orbit splitting of the valence bands is only 4.8 meV suggesting that the hole is confined to the carbon sublattice. Luminescence, due to the annihilation of free excitons, has also been observed. Earlier absorption measurements have been reinterpreted, making use of the new understanding of the $6H$ SiC phonon spectrum.