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

摘要

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, ΔE_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, Pmn2_1 and Pm, respectively). In SiC-InN, one Cmc2_1 ordered phase is predicted at X = 1/2.