Abstract
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.
Highlights
Large reductions in the thermal conductivity of thin silicon membranes have been demonstrated in various porous structures
Hao et al.[12], used Monte Carlo simulations combined with frequency-dependent phonon MFPs16 to study phonon transport in a 2D square-lattice phononic crystals (PnCs), and concluded that phonon size effects caused by the periodic arrangement of the holes can be remarkable even when the hole size and spacing are much greater than the average phonon meanfree paths (MFPs)
We present evidence that coherent scattering of phonons can take place in 2D PnCs with submicron features including the first, to our knowledge, room temperature experimental observation of this phenomenon in such lattices
Summary
Large reductions in the thermal conductivity of thin silicon membranes have been demonstrated in various porous structures. It has been proposed that coherent boundary scattering in phononic crystals (PnCs) with relatively large feature sizes (Z100 nm) may hold the key to solving this problem by scattering phonons with minimal influence on electrons[7,8,9] As phonons traverse such a lattice, they can undergo two types of scattering processes: simple particle-like incoherent scattering as a result of encountering a boundary, and wave-like coherent Bragg[10] scattering due to the periodic geometry of the artificial lattice of air holes. The thermal conductivity (k) of PnC samples has consistently been measured to be significantly lower than that of an unpatterned film[6,7,9,14,15], even after taking into account the combination of material removal and simple incoherent boundary scattering[7,9,14] This suggests that another k reduction mechanism, possibly coherent boundary scattering, must be taking place. The matter of coherent versus incoherent phonon boundary scattering at room temperature remains an open question, and begs for a control experiment isolating the two scattering mechanisms to enables their independent quantification
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