Thermal Properties of Nanocrystalline Silicon Nanobeams
Résumé
Controlling thermal energy transfer at the nanoscale and thermal properties has become critically important in many applications since it often limits device performance. In this work, 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 are studied. Thermal transport in structures with different grain sizes is characterized and elucidated the relative impact of grain size distribution, from 10 to 400 nm, and geometrical dimensions on thermal conductivity. A micro-time-domain thermoreflectance method is used to study free-standing nanocrystalline silicon films and find a drastic reduction in the thermal conductivity, down to values below 10 W m −1 K −1. Decreasing the grain size further decreases the thermal conductivity. In optomechanical nanostructures, this effect is smaller than in membranes due to the competition of surface scattering in decreasing thermal conductivity. Finally, a novel versatile contactless characterization technique is introduced that can be adapted to any structure supporting a thermally shifted optical resonance and used to evaluate the thermal conductivity. The data 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 positions in the structures as a means to study thermal transport in coupled photonphonon structures.
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