Phase behavior in the CdTe–CdS pseudobinary system

Polycrystalline thin films deposited by coevaporation of CdTe and CdS form metastable single-phase CdTe–CdS alloys. Subsequent heating in a kinetic-enhancing ambient segregates CdTe1−xSx and CdS1−yTey phases from the original alloy phase. The equilibrium miscibility gap between the resulting CdTe1−xSx and CdS1−yTey alloy phases is determined for CdTe1−xSx films with initial x ∼ 0.4 treated from 360 to 700 °C. At 625 °C, the equilibrium compositions correspond to published results for mixed crystals. Below 625 °C the miscibility gap widens asymmetrically due to different mixing free energies for S in CdTe and Te in CdS. The solubility thermodynamics are modeled with an excess mixing free energy to account for nonideal mixing behavior.