Density Functional Study of Structural, Mechanic, Thermodynamic and Dynamic Properties of SiGe Alloys

The first-principles calculations based on the density-functional perturbation theory have been performed using the local-density approximation to investigate many physical properties of Si_1–xGe_x alloys. Specifically, the structural (lattice constant, bulk modulus), mechanical (elastic constant, Zener anisotropy factor, Young's modulus, isotropic shear modulus, and Poisson's ratio, sound velocities), dynamical (Debye temperature, internal energy, free energy, entropy and specific heat), and the vibrational properties (phonon dispersion curves) are calculated and compared with the available theoretical and experimental data. The effect of composition of Ge on these properties are studied using the virtual crystal (VC) and the supercell approximations. A good agreement between the calculated and experimental values of the lattice constant, the bulk modulus and elastic constants is obtained. The composition dependence of the optical and acoustic phonon frequencies at at the high-symmetry points Γ, X and L are found to be non-linear.