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.