The Origin of Optical Contrast in Sb₂Te₃-Based Phase-Change Materials

The optical contrast between the crystalline and amorphous forms of phase-change materials (PCMs) is so marked as to make it ideal for data storage. Various explanations have been given, including smearing of bands, changes in the local structural order and bonding, and resonant bonding. A consensus has not been reached. Herein, the dielectric function of crystalline and amorphous Sb₂Te₃, a topological insulator and prototype PCM, is studied using density functional theory and ellipsometry measurements; an expression for interplay between the crystalline electric field and the spin–orbit interaction on the valence band orbitals is derived. It is found that the crystalline electric field and spin–orbit interaction are responsible for this difference and the unusually large refractive index in the crystalline phase. Upon amorphization, a second-order electronic phase transition is induced by the spin–orbit interaction's effect on the p_z orbitals. This mechanism explains how a substantial optical gap can open in systems that have a small electronic band gap.