声子平均自由程两步修正法预测薄膜基纳米结构的热导率
Two-Step Modification of Phonon Mean Free Paths for Thermal Conductivity Predictions of Thin-Film-Based Nanostructures
摘要：
作为一种简单的超材料，纳米材料通常在薄膜上制造，从而可以控制热传输。这些纳米结构的蚀刻侧壁通常是粗糙的，这是由于在纳米加工过程中引入的表面缺陷，而顶部和底部膜表面更光滑。在现有的分析模型中，这些表面之间的对比度还没有得到解决，所有的边界被假定为声子反射扩散。在本文中，一个新的两步方法来解决这个问题，提出了一般的薄膜为基础的结构的声子输运建模。在该方法中，首先从体声子MFP修改有效面内声子平均自由程（Λ_Film）以考虑顶部/底部膜表面的影响，在低温温度下可能增强镜面声子反射的概率。该Λ_膜被进一步修改以包括通过蚀刻侧壁的散射，其具有几乎完全扩散的声子散射。这样的两步声子平均自由程修改产生几乎相同的结果作为频率依赖的声子蒙特卡罗模拟蚀刻纳米线和代表性的纳米多孔薄膜。这个简单而准确的分析模型可以应用到一般的薄膜为基础的纳米结构，沿正交方向的声子尺寸效应相结合。
英文摘要：
As one simple metamaterial, nanopatterns are often fabricated across a thin film so that the thermal transport can be manipulated. The etched sidewalls for these nanostructures are usually rough due to surface defects introduced during the nanofabrication, whereas the top and bottom film surfaces are smoother. In existing analytical models, the contrast between these surfaces has not been addressed and all boundaries are assumed to be diffusive for phonon reflection. In this paper, a new two-step approach to address this issue is proposed for phonon transport modeling of general thin-film-based structures. In this approach, the effective in-plane phonon mean free paths (Λ_Film) are first modified from the bulk phonon MFPs to account for the influence of the top/bottom film surfaces, with possibly enhanced probability of specular phonon reflection at cryogenic temperatures. This Λ_Film is further modified to include the scattering by etched sidewalls with almost completely diffusive phonon scattering. Such a two-step phonon mean free path modification yields almost identical results as frequency-dependent phonon Monte Carlo simulations for etched nanowires and representative nanoporous thin films. This simple yet accurate analytical model can be applied to general thin-film-based nanostructures to combine the phonon size effects along orthogonal directions.
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