  Phase change memory devices are typically reset by melt-quenching a material
to radically lower its electrical conductance. The high power and concomitantly
high current density required to reset phase change materials is the major
issue that limits the access times of 3D phase change memory architectures.
Phase change superlattices were developed to lower the reset energy by
confining the phase transition to the interface between two different phase
change materials. However, the high thermal conductivity of the superlattices
means that heat is poorly confined within the phase change material, and most
of the thermal energy is wasted to the surrounding materials. Here, we
identified Ti as a useful dopant for substantially lowering the thermal
conductivity of Sb2Te3-GeTe superlattices whilst also stabilising the layered
structure from unwanted disordering. We demonstrate via laser heating that
lowering the thermal conductivity by doping the Sb2Te3 layers with Ti halves
the switching energy compared to superlattices that only use interfacial phase
change transitions and strain engineering. The thermally optimized superlattice
has (0 0 l) crystallographic orientation yet a thermal conductivity of just
0.25 W/m.K in the "on" (set) state. Prototype phase change memory devices that
incorporate this Ti-doped superlattice switch faster and and at a substantially
lower voltage than the undoped superlattice. During switching the Ti-doped
Sb2Te3 layers remain stable within the superlattice and only the Ge atoms are
active and undergo interfacial phase transitions. In conclusion, we show the
potential of thermally optimised Sb2Te3-GeTe superlattices for a new generation
of energy-efficient electrical and optical phase change memory.
