Silicon and germanium cannot emit light efficiently due to their indirect bandgap, hampering the development of Si-based photonics. However, alloys of SiGe in the hexagonal phase are predicted to have a direct band gap. In this work, we exploit the unique feature of the nanowire growth mechanism to control the crystal structure by tuning the contact angle of the catalyst particle. A wurtzite wire is used as an epitaxial template to realise hexagonal Ge, SiGe, or pure Si (see accompanying Figure).^1,2 We show efficient light emission up to room temperature accompanied by a short radiative life time, the hallmarks of a direct band gap material.³ The band gap energy is tunable in the range of 0.35 till 0.7eV by the chemical composition, opening a plethora of new applications. In addition, first signatures of lasing from hexagonal SiGe are obtained using an external cavity. We finally discuss possible routes to integrate this material in Si- technology.

¹H.I.T Hauge et al., Nano Letters 15, 5855 (2015).

²H.I.T Hauge et al., Nano Letters 17, 85 (2017).

³E.M.T. Fadaly et al., Nature 580, 205 (2020).