Unfolding the band structure of hexagonal Si_xGe_1-x alloys

CMOS-compatible group-IV active photonic devices are a key requirement for Si photonics. The indirect band gaps of Si and Ge present a significant challenge, driving efforts to realise direct-gap group-IV semiconductors.

When grown in the lonsdaleite (hexagonal diamond) phase Ge possesses a "pseudo-direct" band gap of ≈0.35 eV [1]. The narrow band gap of hexagonal Ge can be increased via Si incorporation in hexagonal Si_xGe_1-x alloys, which retain a pseudo-direct band gap up to x≈30%. Recent fabrication of high-quality hexagonal Si_xGe_1-x nanowires has led to experimental demonstration of room temperature direct-gap-like light emission [2].

We theoretically investigate the electronic properties of hexagonal Si_xGe_1-x alloys. Using density functional theory we compute the alloy electronic structure across the full composition range, and quantify the evolution of the alloy band gap via zone unfolding. Our results provide fundamental insight into this emerging material, and quantify its potential for photonics applications.

[1] C. Rödl et al, Phys. Rev. Materials 3, 034602 (2019)
[2] E. M. T. Falady et al, Nature 580, 205 (2020)