Exciton Complexes and Donor Sites in33RSiC

Polytype $33R$, which, in some respects, may be considered as part $6H$, part $15R$, proves to be intermediate between those polytypes in many properties, among which are the exciton energy gap (3.003 eV), absorption strength, exciton binding energies (two kinds), and nitrogen donor ionization energies (0.15 to 0.23 eV). The latter three appear to be consequences of an intermediate value of electron effective mass. Nitrogen-exciton complexes (Lampert complexes) are observed in the low-temperature photoluminescence at ten of the eleven inequivalent donor sites. In this polytype, it is possible to assign spectral lines to particular sites, which we distinguish by a simple code. Both four-particle and three-particle complexes are observed, yielding phonon energies and, in addition, exciton binding energies which may be compared with those at corresponding sites in $6H$ and $15R$. The comparison suggests that the $15R$ electron effective mass is about half that of $6H$. For three-particle complexes, the effective-mass approximation appears to be inadequate. Thermally excited states are observed for both kinds of complexes. $33R$ is a member of a special series of SiC polytypes, each of which may be characterized as part $6H$, part $15R$. Some properties of the luminescence of higher members of this series are predicted, and their large zones and conduction-band minima are discussed.