Thermal transport in nanostructured thin films: the challenge of a microscopic understanding

The reduction of energy consumption is one of the major global objectives due to the global warming and increasing cost related to the conventional fossil fuel sources. To solve these problems, several studies have been done on materials that allow the reduction of heat dissipation by increasing thermal insulation and exploiting the heat for other forms of energy. An interesting approach for this context is nanostructuring. Nanostructured materials are important because they allow the combination of low thermal conductivity with good electrical properties, unlike traditional single materials. Despite a multitude of studies on nanocomposites, a whole understanding of their thermal transport is still missing, therefore the opti??mization for heat management is still very limited. In this thesis I investigate nanostructured thin films, in particular the nanocrystalline GeTe (Germa??nium telluride), going to study the reduction of thermal conductivity. Ger??manium telluride is a semiconductor with a rhombohedral-to-cubic structural phase transition which has recently attracted much interest for many appli??cations. One of the most important is thermoelectricity: the combination of low thermal conductivity and good electrical properties of the nanocompos??ites allows the exploitation of thermoelectric effects to transform, through the Seebeck effect, the wasted heat into energy. The improvement of the ther??moelectric properties through nanostructuration would allow a reduction of wasted heat. Another very important application is that of phase change random access memories (PCRAM): the fact that GeTe is a phase-change material allows the material to be used for efficient coding of the information in the phase state. In this case, the study of GeTe and the improvement of its properties would allow increasing the coding efficiency by decreasing the energy required for data storage. All these properties of nanocomposites are due to the presence of interfaces that act in the inhibition of phonons, which are responsible for heat transport in semiconductors. In this thesis, I will study the properties of phonons (energy and lifetime) using a technique that allows us to measure phonons with wavelengths of tens of nanometers in thin films: ultrafast acoustic pump-probe technique.