Anharmonicity of the vibrational modes of phase-change materials: A far-infrared, terahertz, and Raman study

Phase-change materials are employed as active elements in optical and electronic memory devices. Upon crystallization, their visible and near-infrared dielectric properties change dramatically due to the formation of resonant bonds, a unique type of bond related to both metallic and covalent bonds. In this work we study the change of the anharmonicity upon crystallization as well as the impact of vacancy concentration and vacancy ordering on the anharmonicity. Our temperature-dependent study of vibrational modes directly reveals a correlation between anharmonicity, vacancy concentration, and ordering. Previously, such a correlation was derived only indirectly from measurements of the specific heat. The pronounced far-infrared contrast, even larger than the optical one, can enable the application of phase-change materials in photonic devices.