Thermal transport in phononic crystals and the observation of coherent phonon scattering at room temperature

Large reductions in the thermal conductivity of thin silicon membranes have been demonstrated in various porous structures. However, the role of coherent boundary scattering in such structures has become a matter of some debate. Here we report on the first experimental observation of coherent phonon boundary scattering at room temperature in 2D phononic crystals formed by the introduction of air holes in a silicon matrix with minimum feature sizes >100 nm. To delaminate incoherent from coherent boundary scattering, phononic crystals with a fixed minimum feature size, differing only in unit cell geometry, were fabricated. A suspended island technique was used to measure the thermal conductivity. We introduce a hybrid thermal conductivity model that accounts for partially coherent and partially incoherent phonon boundary scattering. We observe excellent agreement between this model and experimental data, and the results suggest that significant room temperature coherent phonon boundary scattering occurs. Silicon-based semiconductors are attractive for thermoelectric devices, but their phonon-dominated thermal conductivity is preventing their use. Here, Alaie et al. observe coherent phonon boundary scattering in phononic crystals at room temperature by introducing large (=100 nm) air holes in the Si matrix.