  Mechanical properties, atomic and energy band structures of bare and hydrogen
passivated Si$_{n}$Ge$_{n}$ nanowire superlattices have been investigated by
using first-principles pseudopotential plane wave method. Undoped, tetrahedral
Si and Ge nanowire segments join pseudomorphically and can form superlattice
with atomically sharp interface. We found that Si$_{n}$ nanowires are stiffer
than Ge$_{n}$ nanowires. Hydrogen passivation makes these nanowires and
Si$_{n}$Ge$_{n}$ nanowire superlattice even more stiff. Upon heterostructure
formation, superlattice electronic states form subbands in momentum space. Band
lineups of Si and Ge zones result in multiple quantum wells, where specific
states at the band edges and in band continua are confined. The electronic
structure of the nanowire superlattice depends on the length and cross section
geometry of constituent Si and Ge segments. Since bare Si and Ge nanowires are
metallic and the band gaps of hydrogenated ones varies with the diameter,
Si$_{n}$Ge$_{n}$ superlattices offer numerous alternatives for multiple quantum
well devices with their leads made from the constituent metallic nanowires.
