Effects of Sn clustering on GeSn alloy stability and electronic properties

Germanium and tin both belong to group IV of the periodic table, but exhibit different electronic properties. Ge is a semiconductor while 𝛼-Sn is a semi-metal. When combined as an alloy, the bandgap of GeSn can be tuned from 0.67 eV to 0 eV by varying the Sn content; this has promising applications in optoelectronics within the near infrared (IR) to mid-IR ranges. Realization of these alloys is difficult, however, because of the low solubility (<1%) of Sn in Ge owing to the large lattice mismatch (~15%) between the two. 

In this study, we employed the cluster expansion methodology implemented in the Alloy Theoretic Automated Toolkit to generate ~200 structures of Ge_1-xSn_x across the complete x =0-1 range. Their formation energies were calculated, and 17 ground states were predicted. The electronic properties of the alloys were analyzed using the many-body perturbation theory method, G₀W₀ method. We utilized the Warren-Cowley short-range order parameter to quantify the degree of Sn clustering in the alloys and investigated its impact on their electronic properties. A comprehensive validation against experimental data in the literature reveals that the methods employed in this study accurately reproduce the experimental measurements; and gives rigorous insights into the structure, electronic properties, and the role of Sn clustering on the thermodynamic stability and band structures of the alloys.