Short-Range Order in GeSn Alloy.

Group IV alloys have been long viewed as homogeneous random solid solutions since perceiving them as Si-compatible, direct-band gap semiconductors 30 years ago. Such a perception underlies the understanding, interpretation, and prediction of alloys' properties. However, as the race to create scalable and tunable device materials enters a composition domain far beyond the alloys' equilibrium solubility, a fundamental question emerges as to how random these alloys truly are. Here, we show, by combining statistical sampling and large-scale ab initio calculations, that GeSn alloy, a promising group IV alloy for mid-infrared technology, exhibits a clear short-range order for solute atoms within its entire composition range. Such a short-range order is further found to substantially affect the electronic properties of GeSn. We demonstrate that the proper inclusion of this short-range order through canonical sampling can lead to a significant improvement over previous predictions on alloy's band gaps by showing an excellent agreement with experiments within the entire studied composition range. Our finding thus not only calls for an important revision of the current structural model for group IV alloy but also suggests that short-range order may generically exist in different types of alloys.