Novel tunable band gap BC8/ST12 Si_xGe_x−1 alloys: insights from first-principles calculations

The cubic Ia-3 (BC8) and tetragonal P4₃2₁2 (ST12) modifications of Si and Ge are promising candidates for applications in optoelectronic, thermoelectric or plasmonic devices. However the indirect and narrow band gaps are a limiting factor of the pure phases. Si-Ge alloys in these modifications could overcome this drawback and enable tailoring for specific use-cases. To that end, high pressure BC/ST12 S_ixGe_x−1 solid solutions for 0 ≤ x ≤ 1 have been synthesized and reported to be stable at ambient conditions^[1]. Here we employ ab initio calculations to further investigate the electronic properties of these alloys as a function of x. We show how atomic site occupancy affects the band gap and which atomic arrangements stabilize intermediate compositions. We find that the ST12 phase is energetically favorable up to x≈0.75 and that the indirect band gap of the ST12 Ge end-member can be tuned to become direct for 0.05 ≤ x ≤ 0.2. Furthermore, we obtain the effective band structure of intermediate random alloys by calculating special quasi random structures (SQS) for selected compositions.
[1] G. Serghiou et al., Inorg Chem 53, 5656 (2014) doi:10.1021/ic500416s