Phonon Coherence and Scattering Effects in Laterally Periodic Silicon Nanostructures

CBET-1336734PI：K.E.Goodson Stanford University微型和纳米声子/光子晶体在许多新兴技术中找到了应用和/或前景，如纳米光电子器件、高ZT热电材料、用于光学冷却的声子-光子晶体、声绝缘体、微声元件(如波导、腔和过滤器)以及NEMS/MEMS热传感器和执行器。在这些二维周期多孔(2D-PP)和一维纳米级联(1D-NL)结构中的声子相干和散射效应可能导致它们的有效热导率降低两个数量级以上。通过仔细测量硅膜的输运性质，探索这些新结构中声子输运的有趣物理是这个项目的主题，由于绝缘体上硅制造技术的改进，硅膜提供了几乎无与伦比的材料质量。我们探索了一系列的临界尺寸，如薄膜厚度、孔直径和间距从20到1000 nm，以及表面粗糙度从2到10 nm和从10到800K的温度范围。我们采用了新颖的孔设计来打破或控制起始周期并诱导多个声子带隙。虽然这个项目的主要目标是向科学界提供最大的一组准确和具有良好特征的实验数据，但我们也试图开发一个模拟工具/模型来帮助指导实验设计，并对现有模型进行改进，以扩大其有限的适用和精度范围。本项目解决了关于纳米结构中声子传输基本原理的几个悬而未决的问题。例如，我们想知道二维和一维纳米结构导热系数极端降低的原因，以及这种降低是如何受到孔几何形状和温度的影响的。本项目试图确定气孔和边界上的经典声子散射和声子相干效应(色散)对热传导的相对贡献。我们应该学习是否有可能使用2D结构中的多个周期性图案来创建多个声子带隙，并能够理解打破周期性图案的影响。其他可以直接受益于拟议研究的科学和技术领域包括热电发电和制冷。潜在地，一种更有效的热电二维材料可能会导致这些器件的显著改进。了解纳米结构中的输运过程将极大地提高纳米结构热传感器和执行器的性能和可靠性。

CBET-1336734PI: K.E. GoodsonStanford UniversityMicro- and nano-phononic/photonic crystals have found application and/or hold promise in many emerging technologies such as nanophotonic devices, high ZT thermoelectric materials, phononic-photonic crystals for optical cooling, acoustic insulators, micro-acoustic components such as waveguides, cavities and filters, as well as NEMS/MEMS thermal sensors and actuators. Phonon coherence and scattering effects in these two-dimensional periodically porous (2D-PP) and one-dimensional nanoladder (1D-NL) structures may result in more than two orders of magnitude reduction in their effective thermal conductivities. Exploring the intriguing physics of phonon transport in these novel structures is the subject of this project by means of careful transport property measurements on silicon films, which offer nearly unparalleled material quality owing to the refinement of silicon on insulator fabrication technology. We explore a wide range of critical dimensions such as film thickness, hole diameter and separation ranging from 20 to 1000 nm as well as surface roughnesses ranging from 2 to 10 nm and temperatures ranging from 10 to 800 K. We implement novel pore designs to break or control the onset periodicity and induce multiple phononic bandgaps. While the primary focus of this project is to make available the largest set of accurate and well-characterized experimental data to the scientific community, we also attempt to develop a simulation tool/model that can help to guide the experimental design as well as improve upon the existing models to extend their limited range of applicability and accuracy.This project addresses several outstanding questions on the fundamentals of phonon transport in nanostructures. For example, we would like to know the origin of extreme reduction in thermal conductivity two-dimensional and one-dimensional nanostructures and the manner in which this reduction is affected by pore geometry and temperature. This project attempts to identify the relative contributions of classical phonon scattering on pores and boundaries and phonon coherent effect (dispersion) to heat conduction. We should learn if it possible to create multiple phononic bandgaps using multiple periodicity patterns in the 2D structures and be able to understand the impact of breaking the periodicity patterns. Other areas of science and technology that can directly benefit from the proposed research include thermoelectric energy generation and cooling. Potentially, a more efficient thermoelectric two-dimensional material may result in significant improvement of these devices. Understanding of the transport in nanostructures structures will greatly benefit the performance and reliability of nanofabricated thermal sensors and actuators.