  We present a first-principle study of geometrical and electronic structure of
hexagonal single-walled silicon nanotubes with a monovacancy or a
substitutional defect. The C, Al or P atoms are chosen as substitutional
impurities. It is found that the defect such as a monovacancy or a
substitutional impurity results in deformation of the hexagonal single-walled
silicon nanotube. In both cases, a relatively localized unoccupied state near
the Fermi level occurs due to this local deformation. The difference in
geometrical and electronic properties of different substitutional impurities is
discussed.
