Size and composition modulated superlattices of silicon based nanowires

Ankara : The Program of Material Science and Nanotechnology and the Institute of Engineering and Science of Bilkent University, 2008.Thesis (Master's) -- Bilkent University, 2008.Includes bibliographical references leaves 45-47.Mechanical properties, atomic and energy band structures of bare and hydrogen passivated SinGen nanowire superlattices have been investigated by using firstprinciples pseudopotential plane wave method. Undoped, tetrahedral Si and Ge nanowire segments join pseudomorphically and can form superlattice with atomically sharp interface. 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. Also we showed that hydrogen saturated silicon nanowires of different diameters having different band gaps can form stable junctions. Superlattices formed by the periodically repeated junctions of silicon nanowire segments having different lengths and diameters exhibit electronic states which can be confined in regions having either narrow or wide parts of superlattice. A point defect, such as a missing atom or substitutional impurities with localized states near band edges can make modulation doping possible. Since bare Si and Ge nanowires are metallic and the band gaps of hydrogenated ones varies with the diameter, these superlattices offer numerous options for multiple quantum well devices with their leads made from the constituent metallic nanowires. Finally, we have considered the junction between bare and hydrogenated nanowires to realise metalsemiconductor heterostructure. We have treated this heterostructure within the supercell geometry and deduced the formation of Schottky barrier. We have shown that Si and Ge nanowires can bring about a novel concept in nanocircuit, where interconnects, devices etc are produced on a single rode.Cahangirov, SeymurM.S