Abstract
In recent years, nanoporous Si films have been widely studied for thermoelectric applications due to the low cost and earth abundance of Si. Despite many encouraging results, inconsistency still exists among experimental and theoretical studies of reduced lattice thermal conductivity for varied nanoporous patterns. In addition, divergence can also be found among reported data, due to the difference in sample preparation and measurement setups. In this work, systematic measurements are carried out on nanoporous Si thin films with pore pitches on the order of 100 nm, where pores are drilled either by dry etching or a focused ion beam. In addition to thermal conductivity measurements, the specific heat of the nanoporous films is simultaneously measured and agrees with the estimation using bulk values, indicating a negligible change in the phonon dispersion. Without considering coherent phonon transport, the measured thermal conductivity values agree with predictions by frequency-dependent phonon Monte Carlo simulations assuming diffusive pore-edge phonon scattering. In Monte Carlo simulations, an expanded effective pore diameter is used to account for the amorphization and oxidation on real pore edges.
Highlights
In the literature, special attention has been paid to the large discrepancy in the reported kL values of periodic porous Si films
Measurements on Si nanoporous films with 200–300 nm pitches, where incoherent phonon transport was confirmed at 300–1000 K29
Systematic thermal studies have been carried out on nanoporous Si thin films drilled with deep reactive ion etching (DRIE) and a focused ion beam (FIB)
Summary
Special attention has been paid to the large discrepancy in the reported kL values of periodic porous Si films. Besides some studies[5,13,14,15,16,17,18,19,20], the in-plane kL at room temperature was much lower than theoretical predictions that assumed bulk-like phonon transport and diffusive phonon scattering by pore edges[22,23]. More recent comparison studies between periodic and aperiodic nanoporous films further suggested negligible phononic effects above 10 K for films with 300 nm pitches[16], and above 14 K for Si nanomeshes with>100 nm pitches[19]. Measurements on Si nanoporous films with 200–300 nm pitches, where incoherent phonon transport was confirmed at 300–1000 K29
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