First-principles investigation of amorphous Ge-Sb-Se-Te optical phase-change materials

Chalcogenide phase-change materials (PCMs) are promising candidates for nonvolatile memory and neuromorphic computing devices. The recently developed Ge₂Sb₂Se₄Te₁ alloy shows superior properties in terms of low optical loss and higher thermal stability with respect to the flagship Ge₂Sb₂Te₅ alloy, making this new quaternary alloy a suitable candidate for high-performance optical switches and modulators. In this work, we carry out ab initio calculations to understand how selenium substitution modifies the local structure and the optical response of the amorphous quaternary alloys. We consider four amorphous Ge₂Sb₂Se_xTe₅₋x (GSST) alloys with x = 1 to 4 and show that the substitution of selenium content induces a gradual reduction in the calculated refractive indices, which is in agreement with experimental observation. This improvement on optical loss stems from the increased band gap size, which is attributed to the larger Peierls-like distortion and the stronger charge transfer in the Se-richer amorphous GSST alloys.