  Hexagonal group IV materials like silicon and germanium are expected to
display remarkable optoelectronic properties for future development of photonic
technologies. However, the fabrication of hexagonal group IV semiconductors
within the vapour-liquid-solid method has been obtained using gold as a
catalyst thus far. In this letter, we show the synthesis of hexagonal Ge on
self-assisted GaAs nanowires using molecular beam epitaxy. With an accurate
tuning of the Ga and As molecular beam flux we selected the crystal phase,
cubic or hexagonal, of the GaAs NWs during the growth. A 500 nm-long hexagonal
segment of Ge with high structural quality and without any visible defects is
obtained, and we show that germanium keeps the crystal phase of the core using
scanning transmission electron microscopy. Finally X-ray Photoelectron
Spectroscopy reveals a strong incorporation of As in the Ge. This study
demonstrates the first growth of hexagonal Ge in the Au-free approach,
integrated on silicon substrate.
