First-Principles Study of Electromigration in the Metallic Liquid State of GeTe and Sb2Te3 Phase-Change Compounds

In the reset process of phase-change memories, the active material is brought rapidly above its melting temperature by Joule heating. Atomic migration in the liquid state due to the high electric field can lead to alloy demixing and eventually to device failure. The electromigration force F responsible for ionic migration is proportional to the electric field E, via the effective charge Z∗ (F = eZ*E, where e is electron charge). The determination of Z∗ is thus of great relevance for the electrothermal modeling of devices. We show that a direct first-principles calculation of the effective charges in metallic liquids is possible by computing the atomic forces in the presence of both an electric field and an electronic current within a nonequilibrium Green's function method based on density functional theory. We present results on the effective charges for GeTe and Sb2Te3 in their liquid state obtained with a calculation including wind forces.