To reduce thermal conductivity is a key goal in the design of high-performance thermoelectric materials. It is well known that lattice thermal conductivity of thin films and nanowires can be significantly reduced by surface roughness. Usually, this phenomenon is explained and modelled by the diffuse scattering of phonons at the surface. However, experiments have shown that the thermal conductivities of some electrolessly etched  and intentionally roughened VLS-grown  silicon nanowires are lower than their Casimir limit (i.e., fully diffuse case). Several different models have been proposed to explain the mechanism of the ultra-low thermal conductivity, for example, the additional reduction caused by the amorphous or oxide layer on the surface of nanowires revealed in the Molecular Dynamics (MD) simulations , multiple incoherent scattering events caused by the rough surface modeled explicitly in Monte Carlo simulation , and multiple coherent backscattering from correlated surfaces . While all these mechanisms have been shown to contribute the low thermal conductivity, the key mechanism and to what extend the thermal conductivity can be reduced using surface roughness have not been fully studied.
In this work, the reduction mechanism of surface roughness on the thermal conductivity is investigated by studying silicon thin films with different types of structured surface using MD simulations. Based on the phonon dispersion calculated by Lattice Dynamic method, it has been found that in addition to diffuse scattering, the resonance hybridization between the disordered rough surface and the bulk thin film plays an important role in reducing thermal conductivity. The phonon group velocities in the thin film are significantly reduced by resonance hybridizations, and the phonon energy is trapped near the rough surface in the resonance modes. This mechanism has been observed in thin films with regularly patterned nano-pillars on the surfaces, which have been proposed recently as promising thermoelectric materials, due to their reduced thermal conductivity and uncompromised electric conductivity . We have found, via MD simulations of thin films regular pillared surface and the disordered rough surface, that the reduction in thermal conductivity is comparable for the two types of structures. This implies that it is possible to design practically feasible surface “roughness” to achieve ultra-low thermal conductivity.
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