Two novel Ge phases and their Si[sbnd]Ge alloys with excellent electronic and optical properties

Abstract

The physical properties of Si12, Ge12 and their alloys (Si8Ge4 and Si4Ge8) in the oC12 and hP12 phases are reported in this work, including their structural properties, stability, mechanical properties (elastic constant, elastic modulus, B/G ratio), mechanical anisotropy properties (the anisotropy in Poisson's ratio, shear modulus, and Young's modulus; shear anisotropic factor; universal elastic anisotropy index), thermal properties (Debye temperature and minimum thermal conductivity), electronic properties (electronic band structure) and optical properties (absorption). Owing to the band structures of Si12, Ge12, Si8Ge4, and Si4Ge8 in the oC12 and hP12 phases are direct band gap or quasi-direct band gap, and the optical property of these alloys and two novel Ge allotropes in oC12 and hP12 are all better than that of diamond Si, oC12-Si12 and hP12-Si12, so they can be used as candidate materials for photovoltaic devices and thin-film solar cell applications. In addition, we proposed the two metastable germanium allotropes in the hP12 and oC12 phases first time, and our calculations demonstrate that they should be mechanically, thermodynamically and dynamically stable at ambient pressure.