Short-range order in GeSn alloy

TL;DR

This study reveals that GeSn alloys exhibit significant short-range order affecting their electronic properties, challenging the traditional view of these alloys as perfectly random solutions and improving band gap predictions.

Contribution

It demonstrates the presence of short-range order in GeSn alloys using statistical sampling and ab initio calculations, and shows its impact on electronic properties and band gap predictions.

Findings

GeSn alloys have short-range order across all compositions.

Including short-range order improves band gap predictions.

Short-range order significantly influences alloy electronic properties.

Abstract

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.