Thermodynamic stability and redistribution of charges in ternary AlGaN,InGaN, and InAlN alloys

A model of the delta lattice parameter is used to study the thermodynamics of AlGaN, InGaN, and InAlN alloys. The phase diagrams obtained indicate that AlxGa_1?xN is stable in the entire range of x, whereas the miscibility gap corresponds to 0.2 < x < 0.69 for InxGa_1?xN and to 0.16 < x < 0.7 for In_xAl_1?xN at 1000 K. Biaxial stresses lower the critical temperature and narrow the miscibility gap. The charge-density distribution is analyzed using the pseudopotential method to obtain an approximation of 32-atom supercells. The results of the analysis show that the stability of these alloys is controlled by the competition between the destabilizing contribution of strains related to the mismatch between the lattice constants and a stabilizing charge exchange between various chemical bonds. Biaxial stress reduces the charge redistribution caused by strains and thus increases the stability of an alloy.