Probing Lattice Dynamics and Electronic Resonances in Hexagonal Ge and Si_<i>x</i>Ge_1–<i>x</i> Alloys in Nanowires by Raman Spectroscopy

Abstract

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 μ-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.