  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.
