EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

The Hex-Si$_{1-x}$Ge$_x$ nanowire material system provides a new direct bandgap semiconductor which is compatible with silicon opto-electronics[1]. Due to the change in crystal structure from cubic to hexagonal, a direct bandgap emerges as the L-point is folded towards the Γ-point[2]. For hex-Ge however, theoretical calculations predict a long 20µs radiative lifetime [1]. For efficient light emission a short radiative lifetime is a requirement, and while Hex-Si$_{0.2}$Ge$_{0.8}$ has experimentally been shown[1] to feature a short radiative lifetime, measurements for the lifetime of Hex-Ge remain challenging.

By analyzing the experimentally observed Burstein-Moss bandfilling by using the Lasher-Stern-Würfel (LSW) model[3-4] as a function of excitation density from the low temperature (4K) photoluminescence spectra of Hex-Ge, the quasi fermi-level splitting as function of the excitation density can be determined. By overlaying this data with an analytical model we were able to estimate a lifetime of $\tau=(6\pm3)$ns. This lifetime suggests that, contrary to theoretical predictions, Hex-Ge is also an efficient light emitter and promises to be a building block in silicon integrated opto-electronics.

This project has received funding from the Horizon 2020 program under grant agreement No 735008 (SiLAS) and the Dutch Organization for Scientific Research (NWO).

References:
[1] E.M.T. Fadaly et al., “Direct-bandgap emission from hexagonal Ge and SiGe alloys,” Nature, vol.580, no.7802, pp.205–209, Apr.2020.
[2] C. Rödl, J. Furthmüller, J.R. Suckert, V. Armuzza, F. Bechstedt, and S. Botti, “Accurate electronic and optical properties of hexagonal germanium for optoelectronic applications,” Phys. Rev. Mater., vol.3, no.3, 2019.
[3] G. Lasher and F. Stern, “Spontaneous and stimulated recombination radiation in semiconductors,” Phys. Rev., vol.133, no.2A, p.A553, Jan.1964.
[4] P. Wurfel, “The chemical potential of radiation,” J. Phys. C Solid State Phys., vol.15, no.18, pp.3967–3985, Jun.1982.