DFG Research Grant: Ballistic Heat Transport in Thin Oxide Layers

In this project we will continue and finish of our investigations of the thermal conductivity in the transition region from diffusive to ballistic heat transport. Our goal is to identify unambiguously the regime of ballistic phonon transport. As a model system with a large potential for applications, e.g. in power electronics, we will investigate beta-Gallium Oxide as epitaxially grown layers and heterolayers. In these materials the phonon contribution dominates in the heat transport. Due to the temperature dependence of scattering lengths the change to ballistic transport can be adjusted for these layers. The changeover to ballistic transport, known as Casimir limit, is expected if the scattering length exceeds the sample thickness. Here, the thermal conductivity is no longer determined by the Fourier equation but by geometry and boundary conditions. In this regime experimental data is rare. Based on our results from the first funding period of the project on the electrical and thermal transport in single crystals and epitaxial layers we will employ the implemented measurement techniques to layers of ultra-low doping. The phonon scattering responsible for heat transport will be investigated as a function of temperature, layer thickness and doping level. For these thin layers, the methods of growth technology and thermal transport measurements must be further developed to capture the limiting case. Our experimental and methodical results will contribute to the fundamental understanding of thermal transport in ultra-thin electronic layers and add to solving the important issue of heat dissipation from an active electronic layer to a substrate in the case of application. Here, inefficient heat dissipation limits the performance of microelectronic devices, in particular for high power operation.