  Based on first-principles calculations we showed that superlattices of
periodically repeated junctions of hydrogen saturated silicon nanowire segments
having different lengths and diameters form multiple quantum well structures.
The band gap of the superlattice is modulated in real space as its diameter
does and results in a band gap in momentum space which is different from
constituent nanowires. Specific electronic states can be confined in either
narrow or wide regions of superlattice. The type of the band lineup and hence
the offsets of valence and conduction bands depend on the orientation of the
superlattice as well as on the diameters of the constituent segments. Effects
of the SiH vacancy and substitutional impurities on the electronic and magnetic
properties have been investigated by carrying out spin-polarized calculations.
Substitutional impurities with localized states near band edges can make
modulation doping possible. Stability of the superlattice structure was
examined by ab initio molecular dynamics calculations at high temperatures.
