Theoretical investigations of thermodynamic stability for Si_1-x-yGe_xC_y

Thermodynamic stability of Si_1-x-yGe_xC_y solid solutions is systematically investigated by excess energy calculations based on empirical interatomic potentials. The calculated excess energies for disordered Si_1-x-yGe_xC_y have positive values over the entire concentration range. This implies that Si_1-x-yGe_xC_y with a random distribution of Si, Ge and C is thermodynamically unstable at 0 K. The excess energies of Si_1-x-yGe_xC_y with sublattice ordering are also calculated. The calculated results imply that the ordered atomic arrangements reduce the excess energies and promote C incorporation in Si_1-x-yGe_xC_y because of increase of Si-C interatomic bonds. Furthermore, the excess energies of Si_1-x-yGe_xC_y increase with Ge content x when C content y remains constant; this is because an increase of Ge content introduces a large strain energy in Si_1-x-yGe_xC_y. This gives one possible explanation for experimental findings where by the existence of Ge atoms prevents C incorporation in Si_1-x-yGe_xC_y growth.