In this paper we have shown that a finite acoustic mismatch between structure and barrier materials in low-dimensional structures leads to the acoustic phonon confinement, which in its turn brings about a corresponding decrease of the phonon groups velocity and modification of the phonon density of states. These factors contribute to the reduction of the in-plane lattice thermal conductivity, thus allowing one to increase the thermoelectric figure of merit. Results of experimental study of confined acoustic phonons in single Si thin films and Si/Ge superlattices are also reported High-resolution Raman spectroscopy of ultra-thin silicon-on-insulator structures reveals multiple peaks in the spectral range from 50 cm~(-1) to 160 cm~(-1). The peak position are consistent with the theoretical predictions and indicate the confined nature of phonon transport in thin films and superlattices with a finite acoustic mismatch between layers. This opens up a novel tuning capability for optimization of the thermoelectric properties of low-dimensional structures.
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