Thermal Investigations of Periodically Nanoporous Si Films --- The Impact of Structure Sizes and Pore-Edge Amorphization

In recent years, nanoporous Si films have been intensively studied as promising thermoelectric materials, which mainly benefits from their dramatically reduced lattice thermal conductivity $k_{L}$ and bulk-like electrical properties.$^{1,2}$ Despite many encouraging results, challenges still exist in the theoretical explanation of the observed low $k_{L}$.$^{3}$ Existing studies mainly attribute the low $k_{L}$ to 1) phonon bandstructure modification by coherent phonon processes in a periodic structure (phononic effects), and/or 2) pore-edge defects. In this work, temperature-dependent $k_{L}$ is measured for nanoporous Si films with different pore sizes and spacing to compare with model predictions. For systematic studies, two fabrication techniques are used to drill the nanopores: 1) reactive ion etching, and 2) a focus ion beam to introduce more pore-edge defects. The results from this work will provide guidance for phonon engineering in general materials with periodic interfaces or boundaries. References: 1. Tang et al., \textit{Nano Letters} \textbf{10}, 4279-4283 (2010). 2. Yu et al., \textit{Nature Nanotechnology} \textbf{5}, 718-721 (2010). 3. Cahill et al., \textit{Applied Physics Reviews} \textbf{1}, 011305/1-45 (2014) Nanoscale thermal transport. II. 2003--2012.