First principles phase diagram calculations for the wurtzite-structure quasibinary systems SiC-AIN,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-AIN,plane wave pseudopotential formation-energy calculations predict low-energy metastable states with formation energies,△E_f ＜ 0.004 eV/mole (mol = one cation 4+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 (A1N)_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.