Role of carbon-rings in polycrystalline GeSb₂Te₄ phase-change material

Carbon (C) is used to increase the overall performance of Ge-Sb-Te (GST) phase-change memory material. Yet the C configuration in polycrystalline GST and its microscopic role is unclear. Using the well-studied GeSb₂Te₄ as an example, this work unravels the microscopic C doping role based on ab initio calculations. Our results reveal that carbon prefers occupying the interstitials at very low C concentrations, while with a roughly critical C concentration of over 2%, carbon atoms will cluster at the grain boundaries, with very few carbon atoms might exist at the interstitials. With further increasing the C concentrations, C atoms tend to form ring-like configurations in the grain boundaries rather than chain-like configurations. This is due to that the covalent bonds in ring-like configurations are stronger than that in chain-like configurations. Further analysis on the lone-pair electrons and electronic densities of states show that C dopants significantly change the number of lone-pair electrons of surrounding atoms, thus affecting the electronic structure. Finally, the diffusion coefficient of C is estimated to be of the order of 10⁻¹³ m²/s at 400 K, indicating the good stability of C dopants in GeSb₂Te₄. Our work provides fundamental understanding on the microscopic role of C doping in GST phase-change materials and benefits for improving the properties of the recording materials by controlling the doping concentration.