Comparative studies of clustering effect, electronic and optical properties for GePb and GeSn alloys with low Pb and Sn concentration

The first principle calculations are performed to study the impurity clustering effect, electronic and optical properties of GePb and GeSn alloys. The calculated results show that for a given concentration, the maximum impurity (Sn or Pb) clustered configuration is the most stable equilibrium structure (corresponding to the lowest total energy) which has the highest band gap. The calculated lattice constants and bulk modulus agree well with experimental and others' theoretical values. The calculated band structures of virtual crystal structure and super-cell structure both indicate that GePb alloys undergo a transition from indirect to direct band gap as Pb concentration increases, and the transitional concentration is much lower than that of GeSn alloy. This conclusion indicates that GePb alloy can be a very prospective material for fabricating group-IV laser. The density of states and charge density maps of GeSn and GePb alloys are analyzed comparatively. For optical properties, the dielectric function, absorption spectrum, reflectivity, refractive index and loss function of Ge22Sn2 and Ge22Pb2 are investigated in detail.