Structural and electronic properties of epitaxial core-shell nanowire heterostructures

First principles density functional theory is used to study the composition dependence of the structural and electronic properties of the epitaxial silicon-germanium core-shell [111]-oriented nanowires with diameter ranging from 1.3 to 1.7 nm. An analysis of the results obtained for these nanowires reveals that variations of geometrical and electronic characteristics with composition are strongly type (core/shell) specific. There exist positive and negative deviations from the Vegard's law for the structural parameters (lattice constants, diameters, and mean bond lengths) of the nanowires with compressive strained Ge core and tensile strained Si core, respectively. Direct-to-indirect transition for the fundamental band gap is found in Ge-core/Si-shell nanowires, while Si-core/Ge-shell nanowires preserve a direct energy gap almost over the whole compositional range. An analysis of dependence of the energy gap on the diameter and shell thickness of the nanowires reveals a strong size dependent redshift, consistent with the well-known spectroscopic data. These results provide an understanding of the interplay between variation of structural characteristics and changes in electronic properties.