A possible alternative chemical mechanism would thus be that Si2C(and possibly also Si3C) together with C2H2 are the species responsible for SiC growth and that the addition of halogens only serve to suppress  the formation of Si clusters in the gas phase by formation of SiX2, SiX, and SiHX where X is a halogen, but that these halogenated species are not contributing to the SiC growth.