Atomic structure and phase stability of In_xGa_1-xN random alloys calculated using a valence-force-field method

We have calculated the atomic structure and strain energy of the In_xGa_1-xN random alloy (0<~x<~1) based on 592∼13240-atom models. A valence-force-field method with the Keating potential is used for the strain energy calculation. We analyzed the bond-length and bond-angle distribution in the alloy due to the random fluctuation of the atom positions. The change in the average Ga–N and In–N bond lengths is calculated as a function of the composition x. The calculated result is in good agreement with the recent experimental data of the extended x-ray-absorption fine-structure method. The calculated enthalpy of mixing ΔH_m, i.e., the strain energy, versus the composition x is expressed in the regular-solution model; ΔH_m=Ωx(1-x) using the x-dependent interaction parameter Ω=-2.11x+7.41(kcal/mole). This Ω value is the most reliable among those so far calculated. The calculated phase diagram shows a broad and asymmetric miscibility gap, e.g., 0.04<~x<~0.88 at 800°C. The critical temperature for phase separation is 1417°C.