Hexagonal Ge has been attracting significant attention in the field of optoelectronics due to its predicted novel electronic and optoelectronic properties [1]. Recently, direct bandgap and wavelength tunability of hexagonal (Si)Ge nanowires with gold as a catalyst have been investigated [2]. However, it is not compatible with current CMOS technology [3].
In this work, hexagonal Ge was grown on the facets of self-assisted GaAs nanowires by MBE using VLS mechanism [4]. A pure wurtzite segment was maintained by adjusting the V/III ratio [5]. After growing the GaAs core, a Ge shell was grown with a long hexagonal segment. The crystal structure was characterized using high-resolution scanning transmission electron microscopy. X-ray photoelectron spectroscopy was used to study the Ge surface chemistry, and photoluminescence was performed to examine the optical properties of hexagonal Ge. Results showed that growth induced strong As doping in hexagonal Ge, which can be controlled to a certain extent. Additionally, quantum confinement was observed in hexagonal Ge, leading to a blueshift of the emission.

References
[1] S. Barth et al., Chemistry of Materials, 2021, 32, 2703–2741.
[2] E. M. T. Fadaly et al., Nature, 2020, 580, 205.
[3] J. B. Hannon et al., Nature, 2006, 440, 7080.
[4] I. Dudko et al., Crystal Growth & Design, 2022, 22, 32-36.
[5] T. Dursap et al., Nanotechnology, 2021, 32, 155602.

Acknowledgment
The ECLAUSion project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 801512.