Theoretical Investigations of Si-Ge Alloys in P42/ncm Phase: First-Principles Calculations

The structural, mechanical, anisotropic, electronic and thermal properties of Si, Si0.667Ge0.333, Si0.333Ge0.667 and Ge in P4₂/ncm phase are investigated in this work. The calculations have been performed with an ultra-soft pseudopotential by using the generalized gradient approximation and local density approximation in the framework of density functional theory. The achieved results for the lattice constants and band gaps of P4₂/ncm-Si and P4₂/ncm-Ge in this research have good accordance with other results. The calculated elastic constants and elastic moduli of the Si, Si0.667Ge0.333, Si0.333Ge0.667 and Ge in P4₂/ncm phase are better than that of the Si, Si0.667Ge0.333, Si0.333Ge0.667 and Ge in P4₂/mnm phase. The Si, Si0.667Ge0.333, Si0.333Ge0.667 and Ge in P4₂/ncm phase exhibit varying degrees of mechanical anisotropic properties in Poisson's ratio, shear modulus, Young's modulus, and universal anisotropic index. The band structures of the Si, Si0.667Ge0.333, Si0.333Ge0.667 and Ge in P4₂/ncm phase show that they are all indirect band gap semiconductors with band gap of 1.46 eV, 1.25 eV, 1.36 eV and 1.00 eV, respectively. In addition, we also found that the minimum thermal conductivity κmin of the Si, Si0.667Ge0.333, Si0.333Ge0.667 and Ge in P4₂/ncm phase exhibit different degrees of anisotropic properties in (001), (010), (100) and (01¯0) planes.