Light Emission from Direct Bandgap Hexagonal Silicon Germanium

Efficient light emission from Si and Ge has been a holy grail due to their indirect bandgap nature. Recently, Ge- rich alloys, with a hexagonal structure have been theoretically predicted to exhibit a direct band gap nature. Density functional theory (DFT) calculations predict a 0.3 eV bandgap for hex- Ge, which can be tuned up to 0.9 eV by alloying with Si¹. Yet, the fundamental bottleneck is that Ge and its alloys crystallize naturally in the cubic structure which is optically inactive due to its indirect bandgap nature.

We have realized hex- SiGe by utilizing wurtzite GaAs nanowire cores as a template to transfer the crystal structure to the SiGe shells in a core-shell geometry². We demonstrate photoluminescence of hex-Ge at 3.5 µm at low temperatures and up to room temperature. In addition, we validate the tunability of the wavelength between 1.8 µm and 3.5 µm via alloying Ge with to up 30% Si. These results reveal the potential of this new material system for SiGe based light emitting devices.

References
¹C. Rödl et al. Phys.Rev.B, 2015, 92, 045207
²I. Hauge et al., Nano Lett., 2017, 17 (1), pp 85–90