Phonon relaxation rates in silicon thin films determined by molecular dynamics

Silicon thin films with nanometer dimensions are increasingly being used in the electronic and nanotechnology industries. At such small scales, the continuum assumption is no longer valid and the interactions of the energy carriers (phonons) with the boundaries affect the thermal conductivity of the films. For semiconductors and dielectric thin films, understanding phonon properties in the nanometer scale is important not only to predict their thermal transport behavior, but also to propose solutions to a broad range of thermally induced problems, such as self-heating, sub-continuum localized heating effects and thermally induced reliability. In this work, we estimate, by means of molecular dynamics, the phonon relaxation times in silicon thin films, in the out-of-plane direction, at different temperatures and thin film thicknesses. The relaxation times are determined from the temporal decay of the autocorrelation function of the energy components of the phonons allowed in the crystal. The results are compared with the relaxation times obtained from perturbation theory and Mathiessen's rule. Two major trends were observed, the relaxation rates for transversal acoustic modes are lower than those for the longitudinal acoustic mode for all thickness and temperatures studied, and the longitudinal acoustic modes do not follow the theoretical predictions