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

The search of Si-compatible light emitting material, has been a big challenge to science for several decades. Recently we have shown light emission from silicon [1]. In the hexagonal phase hex-Ge is a direct semiconductor, as the L-point is folded onto the Γ-point [2]. For the theoretical band energies of Hex-Si$_{1-x}$Ge$_{x}$ alloy, it is predicted that the Hex-SiGe compositions have a direct bandgap, above 65% Ge. However there is an indication that lower Ge-contents still exhibit a direct bandgap transition.

We measured the photoluminescence spectra of Hex-Si$_{1-x}$Ge$_{x}$ as function of the alloy composition, with strong emission between 3.5µm at x=1 and 1.7µm at x=0.55, all measured at 4K. The emission at 1.7µm is a previously unpublished record in high energy emission for hex-SiGe. This emission is very close to the U-band of the telecom wavelengths, opening up possibilities of the integration of Hex-SiGe into Si-photonics.

High radiative efficiency at room temperature is very important for applications, as such we show the time-resolved photoluminescence signal of Hex-Si$_{0.2}$Ge$_{0.8}$ nanowires as function of temperature. The lifetime at 4K and at room temperature is equal, and is measured to be τ$\approx$0.9ns. Careful study shows that the lifetime is determined by the radiative lifetime at the doping density of $n≈10^{-19}/cm^3$. The resulting radiative recombination coefficient is $B_{rad}=\frac{1}{τ_{rad}n_0}\approx1.1⋅10^{-10} cm^3 /s$, which approaches the $B_{rad}$ of InP.

As Hex-Si$_{1-x}$Ge$_{x}$ is a group-IV material and with a broad bandgap tunability reaching telecom wavelengths, it is a very promising candidate for a Si-photonics compatible light emitter. This project has received funding from the Horizon 2020 program under grant agreement No 735008 (SiLAS) and the Dutch Organization for Scientific Research (NWO).

[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,S.Botti,“Accurate electronic and optical properties of hexagonal germanium for optoelectronic applications,”Phys.Rev.Mater.,vol.3,2019.