Thermal Studies of Nanoporous Si Films with Pitches on the Order of 100 nm —Comparison between Different Pore-Drilling Techniques

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.

Determining phonon mean free path spectrum by ballistic phonon resistance within a nanoslot-patterned thin film

“…Within periodic nanoporous films, the shape variation of individual nanofabricated pores across the whole film can add large uncertainties to the data analysis, along with varied pore-edge defects or roughness to affect the effective pore diameter and thus the neck width. 43,[46][47][48] This makes such periodic nanoporous Si films unsuitable for the study of the phonon MFP distribution.…”

“…The effective 𝑤 value for each neck can be slightly expanded due to the amorphous pore edges, as suggested for nanoporous Si films in electron microscopy studies. 34,46,47…”

“…9,44,45 In addition, varied pore-edge defects and roughness across a real film can add more complexity to the data analysis. 43,[46][47][48] In contrast, an accurate characteristic length 𝐿 𝐶 can be derived for nanoslot-patterned thin films and be directly used for phonon MFP reconstruction in this work. In nanofabrication, patterning a few nanoslots can also have better structure control than patterning numerous periodic nanopores, particularly when a ultra-small 𝐿 𝐶 is required to probe the 𝑘 𝐿 contribution of short-MFP phonons.…”

Heat Transport Control and Thermal Characterization of Low-Dimensional Materials: A Review

“…Other approaches have been suggested to reduce R c , m and improve the membrane temperature uniformity, such as adding high k materials to the membranes [ 120 ]. Furthermore, recent studies showed that the use of an integrated device fabricated from the same device layer as the membrane minimizes the thermal contact resistance between sample and membrane [ 36 , 121 ].…”

“…Using this approach Lu et al measured the thermal properties of platinum wires and multiwalled carbon nanotubes. Later several researchers used the same approach to measure the thermal properties of Si nanowires, multi- and single-walled carbon nanotubes [ 137 , 138 ], and nanoporous Si films [ 121 ] among others.…”