Short-range Order in Group-IV Binary Alloys and Non-universal Deviations from Vegard's Law

Group-IV binary alloys are promising materials for tunable Si-integrated infrared photonics. However, the determination of their structural properties remains elusive. Specifically, for GeSn alloys, previous ab initio calculations based on random alloy model predicted a range of positive deviations of lattice parameters from Vegard’s law, while experiments showed a contradicting range of deviations, from positive to negative to vanishing. The discrepancies between ab initio theory and experiments remain unresolved. Here, we show the short-range order (SRO) that we recently discovered in GeSn¹ and SiSn² alloys plays an important role in their structural properties. For GeSn alloys with SRO, our calculations predict small negative deviations of lattice parameters from Vegard’s law at low Sn content and small positive deviations at higher Sn content, which are in good agreement with the latest experiments. For SiSn alloys, significant negative deviations are observed in the entire composition range. We conjecture that the underlying mechanisms for the non-universal deviations from Vegard’s law are related to local anisotropy and the competition between tetrahedrality breaking and solute-solute repulsion.

1. ACS Appl Mater Interfaces 12, 57245 (2020)

2. Phys Rev Mater 5, 104606 (2021)