Atomistic study of the alloying behavior of crystalline SnSe_1-xS_x

Recently, layered chalcogenide alloys (LCAs) have been extensively investigated for use in various practical applications by selectively controlling the amount of foreign components. However, the alloying behavior of layered chalcogenides has been rarely explored at the atomistic level. Here, we study the microstructural evolution of SnSe_1-xS_x alloys on the atomic scale by combining scanning tunneling microscopy (STM) measurements with first-principles density functional theory (DFT) calculations. STM topographic images suggest that S atoms substituted in SnSe_1-xS_x are not randomly distributed, but tend to form local SnS clusters. The degree of S atom alloying was quantitatively estimated to be about 60% from STM images, indicating that homo-atoms (S-S) are a preferred arrangement over hetero-atoms (S-Se). Our DFT calculations further confirmed that the mixing energy of random SnSe_1-xS_x alloys showed positive behavior over the whole S composition range considered. This result suggests that SnSe_1-xS_x has a tendency toward local phase segregation into SnSe and SnS rather than random alloys. We expect our atomistic study on the alloying behavior to provide important insight for fabricating optimal SnSe_1-xS_x alloys with high thermoelectric properties.