Ab initio study of band gap properties in metastable BC8/ST12 SixGe1-x alloys

The cubic I a 3 ¯ (BC8) and tetragonal P _{4 3 2 1} 2 (ST12) high pressure modifications of Si and Ge are attractive candidates for application in optoelectronic, thermoelectric, or plasmonic devices. Si_xGe_1-x alloys in BC8/ST12 modifications could help overcome the indirect and narrow bandgaps of the pure phases and enable tailoring for specific use-cases. Such alloys have experimentally been found to be stable at ambient conditions after release from high pressure synthesis; however, their fundamental properties are not known. In this work, we employ ab initio calculations based on density functional theory (DFT) to investigate the electronic properties of these compounds as a function of composition x. We obtain the effective band structures of intermediate alloys by constructing special quasi-random structures (SQSs) and unfolding their band structure to the corresponding primitive cell. Furthermore, we show that the indirect bandgap of the ST12 Ge end-member can be tuned to become direct at _{x Si} ≈ 0.16 . Finally, our investigations also demonstrate that the BC8 modification, on the other hand, is insensitive to compositional changes and is a narrow direct bandgap semiconductor only for the case of pure Si.