Mechanical and electronic properties of Si, Ge and their alloys in P42/mnm structure

Structural, mechanical, and electronic properties of Si-Ge alloys in P4(2)/mnm structure were studied using first-principles calculations by Cambridge Serial Total Energy Package (CASTEP) plane-wave code. The calculations were performed with the local density approximation and generalized gradient approximation in the form of Perdew-Burke-Ernzerhof, PBEsol. The calculated excess mixing enthalpy is positive over the entire germanium composition range. The calculated formation enthalpy shows that the Si-Ge alloys are unstable at 0 K; however, the alloys might exist at specified high temperature scale. The anisotropic calculations show that Sin in P4(2)/mnm structure exhibits the greatest anisotropy in Poisson's ratio, shear modulus, Young's modulus and the universal elastic anisotropy index A(U), but Si8Ge4 has the smallest anisotropy. The electronic structure calculations reveal that Sin and Si-Ge alloys in P4(2)/mnm structure are indirect band gap semiconductors, but Ge-12 in P4(2)/mnm structure is a direct semiconductor. (C) 2016 Elsevier Ltd. All rights reserved.