Session: 17-01-01: Research Posters

Paper Number: 150851

150851 - Coherent and Incoherent Phonon Heat Conduction in Silicon Porous Thin Film 

Phononic crystals, particularly nanomeshes (NMs)—porous thin films of 2D or 3D materials—have garnered significant interest due to their distinctive thermal transport properties including ultralow thermal conductivity (κ) and phono coherence. The pursuit of ultralow κ in NMs has been longstanding, driven by their potential applications in thermoelectrics. However, current experimental demonstrations of phonon coherence are rare and indirect, often relying on comparison with numerical modeling.

 

This study investigates the spectral thermal transport properties of silicon nanomeshes (SNMs) with large and small holes arranged periodically, aperiodically in the x direction (ap-x), and fully aperiodically (ap-xy). Our simulations reveal a significant reduction in κ of small-hole SNMs due to ap-x aperiodicity, in contrast to large-hole SNMs. Spectral phonon analysis indicates that low-frequency phonons in small-hole SNMs are notably suppressed by ap-x aperiodicity, similar to 1D superlattices. Spectral energy density analysis shows the existence of well-defined coherent phonon modes in small-hole SNMs in the 0-3 THz range; spectral thermal conductivity analysis shows that these modes transport ballistically in the SNM. In contrast, ap-x aperiodicity has negligible impact on large-hole SNMs, attributed to hindered coherent phonon formation.

 

Furthermore, our study shows that regardless of their size, holes can significantly reduce the κ of phonons across all frequencies, with higher-frequency phonons typically experiencing stronger scattering by holes. Moreover, for SNMs with equivalent overall porosity, smaller holes exhibit a more pronounced scattering effect on phonons, resulting in lower κ values for small-hole SNMs compared to large-hole counterparts. Remarkably, our findings reveal that in small-hole SNMs, low-frequency phonons (in the 0-3 THz range) can even exhibit higher heat transfer capabilities than those in large-hole SNMs within the same frequency range. This observation underscores the unique characteristics of coherent modes, which can traverse the SNM structure without encountering scattering by the holes.

 

Finally, we examined the effectiveness of heat flux splitting factor and shape factor, two parameters derived from macroscopic descriptions of heat conduction, in categorizing the relative κ values of SNMs with different hole distributions. Our analysis concluded that while both factors perform adequately for large-hole SNMs (where the feature size exceeds the spatial coherence length of phonons), their utility for small-hole SNMs may be unreliable, whether for qualitative or quantitative analyses.

 

In summary, the analysis and prediction of thermal transport in small-hole SNMs should consider coherent phonon behaviors, particularly when the hole feature size is comparable to or smaller than the spatial coherence length of phonons, highlighting the pivotal role of wave nature in such scenarios.

 

Presenting Author: Haoran Cui University of Nevada, Reno

Presenting Author Biography: Haoran Cui is a 4th year Ph.D student in the University of Nevada, Reno.

Authors:

Haoran Cui University of Nevada, Reno
Theodore Maranets University of Nevada, Reno
Tengfei Ma University of Nevada, Reno
Yan Wang University of Nevada, Reno