Structural, electronic and energetic properties of silicon carbon alloys

We studied the influence of alloying on the structural and electronic properties of the unrelaxed and relaxed Si1−yCy random alloys by means of ab initio theoretical calculations using two methods: (i) a supercell approach in connection with the plane-wave pseudopotential method; (ii) the full-potential augmented plane-wave plus local orbitals (APW+lo) method. The first method is used to obtain the relaxed atomic structure. The relaxed atomic positions obtained by pseudopotential calculations were used to calculate the band structure via the second method. The local density approximation was used for the exchange and correlation energy density functional. We investigated the lattice parameters and band gap energies. We found that a quite smaller gap appears in the neighborhood of y=0.03125 concentration of C atoms. The band gap shows a large anomalous bowing and is strongly composition dependent. The electron densities of states for the unrelaxed and relaxed Si1−yCy are also presented. A model structure of 16- and 32-atom supercells is used. The calculated formation enthalpy and individual energy contributions for y=0.5 show that the instability of Si1−yCy alloys is dominated by the largest term of the elastic energy.