  We have performed first-principles calculations of Si/SiO$_2$ superlattices
in order to examine their electronic states, confinement and optical
transitions, using linearized-augmented-plane-wave techniques and
density-functional theory. Two atomic models having fairly simple interface
structure are considered; they differ in the way dangling bonds at interfaces
are satisfied. We compare our first-principles band structures with those from
tight-binding calculations. The real and imaginary parts of the dielectric
function are calculated at the Fermi-Golden-rule level and used to estimate the
absorption coefficients. Confinement is confirmed by the dispersionless
character of the electronic band structures in the growth direction. Optical
enhancement is shown to exist by comparing the direct and indirect transitions
in the band structures with the related transitions in bulk-Si. The role played
by the interface on the optical properties is assessed by comparing the
absorption coefficients from the two models.
