Systematic Studies of Periodically Nanoporous Si Films for Thermoelectric Applications

Abstract

As the major heat carriers in dielectrics and semiconductors, phonons are strongly scattered by boundaries and interfaces at the nanoscale, which can lead to a significantly reduced lattice thermal conductivity k_L. In recent years, such phonon size effects have been used to enhance the thermoelectric performance of various nanostructured materials. With dramatically reduced k_L and bulk-like electrical properties, high thermoelectric performance has been demonstrated for nanoporous Si films at room temperature. Despite these encouraging results, however, challenges still exist in the theoretical explanation of the observed low k_L values. Existing studies mainly attribute the observed low k_L to phononic effects and/or amorphous pore edges. These two effects can be separated when the specific heat of the film can be measured along with k_L to provide more insight into the phonon dispersion modification. In this work, both the specific heat and k of a suspended nanoporous Si film is extracted from the 3ω measurements. The result is compared to the reported k_L values of various porous Si films. The influence of employed phonon mean free path spectrum on the data analysis is discussed.