First principles phase diagram calculations for the wurtzite-structure quasibinary systems SiC-AlN, SiC-GaN and SiC-InN

The cluster-expansion method was used to perform first principles phase diagram calculations for the wurtzite-structure quasibinary systems (SiC) 1 - X (AlN) X , (SiC) 1 - X (GaN) X and (SiC) 1 - X (InN) X ; and to model variations of band gaps as functions of bulk compositions and temperature. In SiC-AlN, plane wave pseudopotential formation-energy calculations predict low-energy metastable states with formation energies, Δ E f < ~ 0.004 eV/mole (mol=one cation+one anion). The crystal structures of these states are all of the form (SiC) m (AlN) n (SiC) o (AlN) p ...( m , n , o , p integers), where (SiC) m indicates m SiC-diatomic-layers ⊥ to the hexagonal c -axis ( c Hex ) and similarly for (AlN) n , (SiC) o and (AlN) p . The presence of low-energy layer-structure metastable states helps to explain why one can synthesize (SiC) 1 - X (AlN) X films, or single crystals with any value of X , in spite of the apparently strong tendency toward immiscibility. In SiC-GaN, ordered structures are predicted at X =1/4, 1/2, and 3/ 4 (Pm, Pmn 2 1 and Pm, respectively). In SiC-InN, one Cmc 2 1 ordered phase is predicted at X =1/2.