(Invited) Phonon Dispersion Engineering and Thermal Transport in Si Membranes
In this talk we will report on the modification of the phonon properties in Si membranes, such as the dispersion relation, and its dependence on the thickness. The thermal conductivity in Si results from the cumulative contribution of the transport of phonons with a broad range of wavevectors and mainly from long-wavevector phonons. Thus, heat transport at room temperature can be influenced by the reduction of the membrane thickness as the dispersion relation of the short-wave-phonons starts to be affected by emerging new vibrational modes arising from the boundary conditions at the membrane surface. Our measurements of the thermal conductivity in ultra-thin sub-20 nm Si membranes have demonstrated this extreme and, in fact, we have been able to disentangle the effects of phonon confinement from those related to surface morphology and chemical composition. Finally, we will discuss the prospects of employing the increased modification of phonon dispersion relation achieved in phononic crystals in controlling heat transport. While some applications require the existence of absolute band gaps, the approach to reduce the thermal conductivity is more linked to the decrease of phonon’s group velocity and phonon-phonon interactions in an extended range of frequencies.