Thermodynamic Modeling of the Ge-X (X = As, Se, S, P) Systems

Storage density is a key target of phase change memory (PCM) in applications, and the development of high-performance ovonic threshold switching (OTS) is a great way to increase the storage density of PCM. Establishing a thermodynamic database for OTS materials is critical for their use. This study carries out thermodynamic modelling of the Ge-X (X = As, Se, S, P) systems by using the calculation of phase diagrams (CALPHAD). The thermodynamic parameters were evaluated in view of the available experimental and first-principles data in the literature. In addition, the enthalpies of formation calculated by first-principles calculations in the present work were used to evaluate the model parameters. Thermodynamic models were established for all the phases in the four systems. The substitutional solution model was employed to describe the solution phases including liquid in the Ge-As and Ge-P systems, (Ge), (As), (Se), (S), and (P). The associate model was adopted to describe the liquid phase in the Ge-Se and Ge-S systems on account of associate species GeSe2 and GeS2 presented in the Ge-Se and Ge-S melts, respectively. The intermetallic compounds GeAs, GeAs2, αGeSe, βGeSe, αGeSe2, βGeSe2, Ge3Se7, GeS, αGeS2, βGeS2, and GeP were treated as stoichiometric compounds. A set of reliable and self-consistent thermodynamic parameters of Ge-X (X=As, P, S, Se) was obtained. Comparisons between the calculated results and experimental data available in the literature show that almost all the reliable experimental information can be satisfactorily accounted for by the present modeling.