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

摘要

Thermoelectric effects are envisioned to reduce programming currents innanopillar phase change memory cells. However, due to the inherent symmetry insuch a structure, the contribution due to thermoelectric effects on programmingcurrents is minimal. In this work, we propose a hybrid phase change memorystructure which incorporates a two-fold asymmetry specifically aimed tofavorably enhance thermoelectric effects. The first asymmetry is introduced viaan interface layer of low thermal conductivity and high negative Seebeckcoefficient, such as, polycrystalline SiGe, between the bottom electrodecontact and the active region comprising the phase change material. Thisresults in an enhanced Peltier heating of the active material. The second oneis introduced structurally via a taper that results in an angle dependentThomson heating within the active region. Various device geometries areanalyzed using 2D-axis-symmetric simulations to predict the effect onprogramming currents as well as for different thicknesses of the interfacelayer. A programming current reduction of up to $60\%$ is predicted forspecific cell geometries. Remarkably, we find that due to an interplay ofThomson cooling in the electrode and the asymmetric heating profile inside theactive region, the predicted programming current reduction is resilient tofabrication variability.