Short-range order in binary Group IV alloys*

Group IV alloys based on Si, Ge and Sn have attracted research interest as a potential candidate for mid-infrared photonics due to advantages of Si-compatibility, low-cost, and tunable band gaps. Group IV alloys have been commonly conceived as a random solid solution, but it is unclear how random these alloys truly are. By carrying out extensive DFT-based Monte Carlo sampling, we showed the structures of both GeSn¹ and SiSn² alloys display substantial deviations from random solid solution through a short-range order (SRO). Remarkably, despite their structural resemblance, GeSn and SiSn alloys were found to exhibit very different natures of SRO: In GeSn alloy, the SRO is mainly reflected by solute-solute first-nearest-neighbor repulsion, whereas in SiSn alloy, the SRO carries a more complex form which originates from the competition between solute-solute first- and second-nearest-neighbor repulsion. In both alloys, SRO was further demonstrated to strongly affect electronic structures. Particularly, the predicted band gaps in GeSn alloys with SRO were found to yield an excellent agreement with experiments.  Our findings not only demonstrate the significant role of SRO in material properties of binary group IV alloys, but also contribute to new understandings of natures of SRO.