  Group IV alloys have been long viewed as homogeneous random solid solutions
since they were first perceived 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 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 short-range order is
further found to substantially affect the electronic properties of GeSn. We
demonstrate 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 current structural model for group IV alloy, but also
suggests short-range order may generically exist in different types of alloys.
