  Recent advances in nanowire synthesis have enabled the realization of crystal
phases that in bulk are attainable only under extreme conditions, i.e. high
temperature and/or high pressure. For group IV semiconductors this means access
to hexagonal-phase SixGe1-x nanostructures (with a 2H type of symmetry), which
are predicted to have a direct band gap for x up to 0.5 - 0.6 and would allow
the realization of easily processable optoelectronic devices. Exploiting the
quasi-perfect lattice matching between GaAs and Ge, we synthesized hexagonal
phase GaAs-Ge and GaAs-SixGe1-x core-shell nanowires with x up to 0.59. By
combining position-, polarization- and excitation wavelength-dependent u-Raman
spectroscopy studies with first-principles calculations, we explore the full
lattice dynamics of these materials. In particular, by obtaining
frequency-composition calibration curves for the phonon modes, investigating
the dependence of the phononic modes on the position along the nanowire, and
exploiting resonant Raman conditions to unveil the coupling between lattice
vibrations and electronic transitions, we lay the grounds for a deep
understanding of the phononic properties of 2H-SixGe1-x nanostructured alloys
and of their relationship with crystal quality, chemical composition, and
electronic band structure.
