Confined states in multiple quantum well structures of Si_nGe_n nanowire superlattices

Mechanical properties, atomic and energy band structures of bare and hydrogen-passivated Si_nGe_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_nGe_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 vary with the diameter, Si_nGe_n superlattices offer numerous alternatives for multiple quantum well devices with their leads made from the constituent metallic nanowires.