Abstract
Phonons can display both wave-like and particle-like behaviour during thermal transport. While thermal transport in silicon nanomeshes has been previously interpreted by phonon wave effects due to interference with periodic structures, as well as phonon particle effects including backscattering, the dominant mechanism responsible for thermal conductivity reductions below classical predictions still remains unclear. Here we isolate the wave-related coherence effects by comparing periodic and aperiodic nanomeshes, and quantify the backscattering effect by comparing variable-pitch nanomeshes. We measure identical (within 6% uncertainty) thermal conductivities for periodic and aperiodic nanomeshes of the same average pitch, and reduced thermal conductivities for nanomeshes with smaller pitches. Ray tracing simulations support the measurement results. We conclude phonon coherence is unimportant for thermal transport in silicon nanomeshes with periodicities of 100 nm and higher and temperatures above 14 K, and phonon backscattering, as manifested in the classical size effect, is responsible for the thermal conductivity reduction.
Highlights
Phonons can display both wave-like and particle-like behaviour during thermal transport
While thermal transport in silicon nanomeshes has been previously interpreted by phonon wave effects due to interference with periodic structures, as well as phonon particle effects including backscattering, the dominant mechanism responsible for thermal conductivity reductions below classical predictions still remains unclear
By comparing experimental results between samples and against particle model predictions, we show that coherence effects are not necessary to describe thermal transport in the regime where the nanomesh pitch is greater than l but smaller than LU, and that the backscattering effect leads to the k reductions
Summary
Phonons can display both wave-like and particle-like behaviour during thermal transport. Computational works by Jain et al.[20] and Ravichandran and Minnich[21] concluded that some of these experimental results could be explained by particle based models without considering the coherence effects Underlying these various interpretations are differing views of the important coherence length scales in periodic structures. The backscattering concept predicts that adding lateral bridging necks linking the nanowires into a nanomesh (Fig. 1b (left)) would reduce k because ballistic phonons are more likely to be scattered backwards when colliding with the nanomesh necks than with the nanowire walls, providing additional resistance to heat flow This backscattering effect has recently been used to explain why k of a nanomesh could be lower than k of an equivalent nanowire array[21] even in the absence of coherence effects
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