Numerical Study on the Phonon Thermal Conductivity of Nano-Structured Single-Crystal Silicon Thermoelectric Thin Film

To reduce the thermal conductivity of porous thermoelectric thin film and improve the thermoelectric conversion efficiency, a solver of the phonon Boltzmann transport equation with the relaxation time approximation and discrete ordinates method is developed to numerically study the phonon thermal conductivity of the nanoporous silicon thin film. The effects of film thickness, film porosity, boundary specularity and phonon scattering boundary area on the thermal conductivity are researched. The effects of the porosity and film thickness on the anisotropic thermal conduction are also discussed. The results show that the thermal conductivity gets lower as the porosity increases and the film thickness decreases, and can be reduced at least by two orders of magnitude compared with the bulk silicon when the film porosity rises up to 64% and the film thickness is less than 0.1 times of the phonon mean free path of the bulk silicon. According to the heat flux distribution in the nanoporous thin film, a method is proposed to optimize the porous structure of the nanoporous thin film, which may provide a theoretical guidance for the design of low-thermal conductivity and high-efficient thermoelectric thin films.