Thermal Properties of Nanocrystalline Silicon Nanobeams

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 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 are studied. Thermal transport and elucidate the relative impact of different grain size distributions and geometrical dimensions on thermal conductivity are characterized. A micro time‐domain thermoreflectance method 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 is used, with a stronger decrease for smaller grains. 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 that can be adapted to any structure supporting a thermally shifted optical resonance is introduced. The thermal conductivity data agrees quantitatively with the thermoreflectance measurements. This study opens the way to a more generalized thermal characterization of optomechanical cavities and to create hot‐spots with engineered shapes at the desired position in the structures as a means to study thermal transport in coupled photon‐phonon structures. The performance of optomechanical nanostructures, like many other devices, is impacted by nanoscale thermal energy transfer. Competing contributions of the nanoscale grains and the fabricated nanoscale geometry to the decrease in thermal conductivity of silicon‐based nanostructures lead to values well below 10 W m–1 K–1. A novel measurement technique is introduced to measure the thermal properties of optomechanical nanostructures in working conditions.