Electronic and Thermal Properties of GeTe/Sb₂Te₃ Superlattices by ab-initio Approach: Impact of Van der Waals Gaps on Vertical Lattice Thermal Conductivity

In the last decade, several works have focused on exploring the material and electrical properties of $\text{GeTe/Sb}_{2}\text{Te}_{3}$ superlattices (SLs) in particular because of some first device implementations demonstrating interesting performances such as fast switching speed, low energy consumption, and non-volatility. However, the switching mechanism in such SL-based devices remains under debate. In this work, we investigate the prototype $\text{GeTe/Sb}_{2}\text{Te}_{3}$ SLs, to analyze fundamentally their electronic and thermal properties by ab initio methods. We find that the resistive contrast is small among the different phases of $\text{GeTe/Sb}_{2}\text{Te}_{3}$ because of a small electronic gap (about 0.1 eV) and a consequent semi-metallic-like behavior. At the same time the out-of-plane lattice thermal conductivity is rather small, while varying up to four times among the different phases, from 0.11 to 0.45 W/m$^{-1}$K$^{-1}$, intimately related to the number of Van der Waals (VdW) gaps in a unit block. Such findings confirm the importance of the thermal improvement achievable in $\text{GeTe/Sb}_{2}\text{Te}_{3}$ super-lattices devices, highlighting the impact of the material stacking and the role of VdW gaps on the thermal engineering of the Phase-Change Memory cell.