Optimising Thermal Conductivity in Phase Change Materials and Devices

Chalcogenide phase change materials have been commercialised for optical and electrical data storage applications and they are now being developed for displays, optical neural networks, and optical routing. Programming these devices is achieved by switching the phase change material between two structurally distinct states—typically an amorphous and a crystalline state are used. The resulting optical or electrical property contrast is then used to represent the data. Although this process is conceptually simple and commercial data storage devices based on this effect are widely available, the programming process requires a substantial amount of energy to heat the phase change material to its melting temperature. Moreover, the device or media must be designed to quench at a rate typically close to 10⁹ K/s. Thus, there is a design conflict because on one hand the material must heat efficiently to reach its melting temperature yet on the other hand it must also diffuse heat into the surroundings to enable quenching at high rates.<br/>This presentation will demonstrate how we can use transition metal dichalcogenide 2D films, Van der Waals superlattices, and doping to form efficiently switching phase change material based devices. We show that adding WS2 between the PCM and the substrate produces a thermal boundary resistance effect that can halve the switching energy and also protect the substrate from thermal damage. We also show that doping 3.5% Ti into the Sb2Te3 layers of an Sb2Te3-GeTe phase change superlattice also halves the superlattice thermal conductivity by creating phonon scattering centres in the Sb2Te3 layered structure. Again, the result is a radical decrease in the switching energy.<br/>In summary, by controlling heat diffusion in the phase change material and the interfaces, we can enhance the switching energy efficiency and longevity of phase change materials in data storage and programmable photonics devices.