Carbon in beta-rhombohedral boron

Carbon atoms in the beta-rhombohedral boron structure substitute for boron atoms at the polar sites of the B-12 icosahedra. The donor level of the excess electron of carbon coincides with the upper valence band, thus raising the hopping probability within this band. This assumption is consistent with various experimental results. The ionisation energy of the intrinsic electron trapping levels is slightly different for B-12 and B11C icosahedra. The photoconductivity and quotient spectra of the reflectivities of differently doped samples indicate carbon-induced changes in the density of states within the energy bands.