Time-domain thermoreflectance (TDTR) data analysis using phonon hydrodynamic model

Time-domain thermoreflectance (TDTR) is a powerful pump–probe technique for measuring thermal properties of materials and interface thermal conductance. However, a diffusive thermal transport model is often used for data analysis, leading to underestimated thermal conductivities for high thermal conductivity materials, for example, single-crystalline semiconductors like Si at low temperatures. In this work, we have developed a theoretical model based on phonon hydrodynamics, an approximation of the phonon Boltzmann transport equation, for TDTR data analysis. We apply this model to process the TDTR signals of Si measured between 80 and 300 K. The extracted thermal conductivities using the phonon hydrodynamic model agree remarkably well with the bulk values measured by the steady-state technique, providing a more appropriate way of TDTR data analysis. The effectiveness of the phonon hydrodynamic model is further verified by analyzing TDTR signals of Ge at room temperature.