  The atomic and electronic structure of a set of pristine single wall SiC
nanotubes as well as Si-substituted carbon nanotubes and a SiC sheet was
studied by the LDA plane wave band structure calculations. Consecutive
substitution of carbon atoms by Si leads to a gap opening in the energetic
spectrum of the metallic (8,8) SWCNT with approximately quadratic dependence of
the band gap upon the Si concentration. The same substitution for the
semiconductor (10,0) SWCNT results in a band gap minimum (0.27 eV) at ~25% of
Si concentration. In the Si concentration region of 12-18%, both types of
nanotubes have less than 0.5 eV direct band gaps at the Gamma-Gamma point. The
calculation of the chiral (8,2) SWSi_0.15C_0.85NT system gives a similar (0.6
eV) direct band gap. The regular distribution of Si atoms in the atomic lattice
is by ~0.1 eV/atom energetically preferable in comparison with a random
distribution. Time dependent DFT calculations showed that the silicon
substitution sufficiently increases (roughly by one order of magnitude) the
total probability of optical transitions in the near infrared region, which is
caused by the opening of the direct band gap in metallic SWCNTs, the
unification of the nature and energy of band gaps of all SWCNT species, the
large values of <Si3p|r|Si3s> radial integrals and participation of Si3d states
in chemical bonding in both valence and conductance bands.
