First-principles calculation of the thermodynamics of InxGa1-xN alloys:: Effect of lattice vibrations

The thermodynamics properties of the wurtzite and zinc-blende InxGa1-xN alloys are calculated using first-principles density-functional calculations. Special quasirandom structures are used to describe the disordered alloys, for x=1/4, 1/2, and 3/4. The effect of lattice vibrations on the phase diagram, commonly omitted from semiconductor alloy phase diagram calculations, are included through first-principles calculations of phonon spectra. Inclusion of lattice vibrations leads to a large reduction in the order-disorder critical temperature (similar to 29% and similar to 26% for the wurtzite and zinc-blende structures, respectively) and changes the shape of the solubility and spinodal curve through changes in the entropies of the competing phases. Neglect of such effect produces significant errors in the phase diagrams of complex ordered semiconductor compounds. The critical temperature for phase separation is 1654 K (1771 K) for the wurtzite (zinc-blende) structures. The predicted phase diagrams are in agreement with experimental measurements on metal-organic chemical-vapor deposition InxGa1-xN films.