P63/mmc-Ge and their Si–Ge alloys with a mouldable direct band gap

Abstract

Physical properties, such as stability, electronic properties, hole effective mass, electron effective mass, mechanical properties, structural properties, and optical properties of P63/mmc-Ge6 and their Si6-xGex (x = 1, 2, 3, 4, 5) alloys are investigated using ab initio calculations based on density functional theory. The anisotropic mechanical properties of the Young’s modulus of Si6-xGex alloys are all slightly superior than those of diamond-Si, while the Young’s modulus of Si6-xGex alloys in the (001) plane are isotropic (Emax = Emin). With the increase in the number of germanium atoms, Si5Ge and Si4Ge2 exhibit direct band gaps of 0.63 eV and 0.18 eV, respectively. All other Si6-xGex (x = 3, 4, 5) alloys and P63/mmc-Ge6 exhibit metallicity properties. In addition, the effective masses of electrons and heavy holes of Si4Ge2 along the x and y directions become substantially smaller than those of Si6 and Si5Ge.