Thermal conductivity reduction in silicon fishbone nanowires

International audienceSemiconductor nanowires are potential building blocks for future thermoelectrics because of their low thermal conductivity. Recent theoretical works suggest that thermal conductivity of nanowires can be further reduced by additional constrictions, pillars or wings. Here, we experimentally study heat conduction in silicon nanowires with periodic wings, called fishbone nanowires. We find that like in pristine nanowires, the nanowire cross-section controls thermal conductivity of fishbone nanowires. However, the periodic wings further reduce the thermal conductivity. Whereas an increase in the wing width only slightly affects the thermal conductivity, an increase in the wing depth clearly reduces thermal conductivity, and this reduction is stronger in the structures with narrower nanowires. Our experimental data is supported by the Callaway-Holland model, finite element modelling and phonon transport simulations. Thermal transport in low dimensional and nanostructured materials has attracted high attention over the past decades, in particular with regards to promising prospects in thermoelectric energy generation 1 , including the possibility of using the wave properties of phonons, which can be relevant at cryogenic temperatures 2,3. Nonetheless, the main impact of semiconductor nanostructures on thermal transport comes from scattering of the heat carriers-phonons. In that regard, semiconductor nanowires (NWs) are the focus of much attention 4-6 and remain to date one of the most promising building blocks for thermoelectric 6-9 and other microelectronic devices. Generally, the thermal conductivity of NWs depends on the diameter 4,10-14 and surface properties 4,7,14-18 , because heat conduction in nanostructures is suppressed by diffuse scattering of phonons on the surfaces 19,20. For example, a few experimental works 21,22 have demonstrated a reduction of thermal conductivity in corrugated silicon NWs due to the limited phonon mean free path 21,22. To further enhance this surface scattering, theoretical works 23-26 proposed various diameter-modulated NWs and found that heat conduction is strongly suppressed in these structures. Not only is it possible to reduce thermal conductivity proportionally to the ratio between the corrugation and the central constriction, but this reduction can be larger than an order of magnitude at room temperature for structures of a couple of nanometers in width 25. Despite the difference in scales, lattice dynamics 25 , Monte-Carlo simulations 23,27 , and mixed calculations 24 agree that reducing the width of the central constriction or increasing the depth of the corrugation reduces thermal conductivity. Thus, modification of the sidewall shape of NWs is a promising approach to further thermal conductivity reduction. In this work, we systematically study heat conduction in NWs with periodic wings, called hereafter fishbone NWs, which have features of both NWs and phononic crystals. First, we find that thermal conductivity is reduced as the central part-the neck-becomes smaller. Next, we demonstrate that wing size in the direction parallel to heat flux does not strongly affect heat conduction, whereas wing size in the direction perpendicular to the heat flux can significantly reduce thermal conductivity. We explain this reduction by the trapping and backscattering of phonons in the wings. Overall, we experimentally demonstrate that the transient behaviour of the fishbone NWs follow the mass contrast, and that thermal conductivity and thermal relaxation rates can be reduced at room temperature by more than 20% and 35%, respectively