Abstract
Thermodynamic stability of group IV alloy semiconductors such as Si_<1-x-y>Ge_xC_y solid solutions in bulk and thin film states is systematically investigated by excess energy calculations based on empirical interatomic potentials and Monte Carlo (MC) simulations. In bulk state, the calculated excess energies for Si_<1-x-y>Ge_xC_y have positive values over the entire concentration range. This implies that Si_<1-x-y>Ge_xC_y with a random distribution of Si, Ge and C is thermodynamically unstable at 0K. Furthermore, the excess energies of Si_<1-x-y>Ge_xC_y increase with Ge content x when C content y remains constant. This is because an increase of Ge content introduces large strain energy in Si_<1-x-y>Ge_xC_y. In thin film state, although lattice constraint at the interface reduces the excess energies by 20-30% of those in bulk state, we obtain similar results to those in bulk state. Further MC simulation reveals that Ge atoms segregate in the topmost layer and C atoms accumulate in the second layer. These calculated results suggest that the lattice constraint at the interface enhance the miscibility of C in Si_<1-x-y>Ge_xC_y in thin films, whereas the miscibility tends to reduce near the surface because of the segregation of Ge and C atoms.
