纳米晶硅纳米梁的热特性
Thermal properties of nanocrystalline silicon nanobeams
摘要：
在纳米尺度下控制热能传递在许多应用和热性质中已经变得至关重要，因为它常常限制器件性能。在这项工作中，我们研究的影响所产生的纳米级结构的独立的纳米晶硅薄膜和增加的表面体积比时，制造成悬浮的光机械纳米梁的热导率。我们表征了不同晶粒尺寸的结构中的热传输，并阐明了晶粒尺寸和几何尺寸对热导率的相对影响。我们使用微时域热反射法研究的晶粒尺寸分布的影响，从10到400 nm，在独立的纳米晶硅薄膜的热导率考虑表面声子和晶界散射。我们发现热导率急剧下降，下降到10 W.m^{-1}.K^{-1}或更低，这只是单晶硅电导率的一小部分。减小晶粒尺寸进一步降低导热率。我们还观察到，这种效果是较小的OM纳米结构比在膜由于在降低热导率的表面散射的竞争。最后，我们介绍了一种新的多功能非接触式表征技术，可以适应于任何结构支持的热位移光共振，并使用它来评估的热导率。这种方法可以用于与光学共振表现出不同的模式配置文件和数据示出同意定量与热反射测量。这项工作开辟了一条道路，以一个更普遍的热表征的光机械腔，并创建热点与工程形状在所需的位置在结构中作为一种手段，研究耦合光子-声子结构中的热输运。
英文摘要：
Controlling thermal energy transfer at the nanoscale has become critically important in many applications and thermal properties since it often limits device performance. In this work, we study the effects on thermal conductivity arising from the nanoscale structure of free-standing nanocrystalline silicon films and the increasing surface-to-volume ratio when fabricated into suspended optomechanical nanobeams. We characterize thermal transport in structures with different grain sizes and elucidate the relative impact of grain size and geometrical dimensions on thermal conductivity. We use a micro-time-domain thermoreflectance method to study the impact of the grain size distribution, from 10 to 400 nm, on the thermal conductivity in free-standing nanocrystalline silicon films considering surface phonon and grain boundary scattering. We find a drastic reduction in the thermal conductivity, down to values of 10 W.m^{-1}.K^{-1} and below, which is just a fraction of the conductivity of single crystalline silicon. Decreasing the grain size further decreases the thermal conductivity. We also observe that this effect is smaller in OM nanostructures than in membranes due to the competition of surface scattering in decreasing thermal conductivity. Finally, we introduce a novel versatile contactless characterization technique that can be adapted to any structure supporting a thermally shifted optical resonance and use it to evaluate the thermal conductivity. This method can be used with optical resonances exhibiting different mode profiles and the data is shown to agrees quantitatively with the thermoreflectance measurements. This work opens the way to a more generalized thermal characterization of optomechanical cavities and to create hot-spots with engineered shapes at desired position in the structures as a means to study thermal transport in coupled photon-phonon structures.
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