Programming current reduction via enhanced asymmetry-induced thermoelectric effects in vertical nanopillar phase change memory cells

Thermoelectric effects are envisioned to reduce programming currents in nanopillar phase change memory cells. However, due to the inherent symmetry in such a structure, the contribution due to thermoelectric effects on programming currents is minimal. In this work, we propose a hybrid phase change memory structure which incorporates a two-fold asymmetry specifically aimed to favorably enhance thermoelectric effects. The first asymmetry is introduced via an interface layer of low thermal conductivity and high negative Seebeck coefficient, such as, polycrystalline SiGe, between the bottom electrode contact and the active region comprising the phase change material. This results in an enhanced Peltier heating of the active material. The second one is introduced structurally via a taper that results in an angle dependent Thomson heating within the active region. Various device geometries are analyzed using 2D-axis-symmetric simulations to predict the effect on programming currents as well as for different thicknesses of the interface layer. A programming current reduction of up to $60\%$ is predicted for specific cell geometries. Remarkably, we find that due to an interplay of Thomson cooling in the electrode and the asymmetric heating profile inside the active region, the predicted programming current reduction is resilient to fabrication variability.