On the interplay between phonon-boundary scattering and phonon-point-defect scattering in SiGe thin films

International audienceThis paper provides theoretical understanding of the interplay between the scattering of phononsby the boundaries and point-defects in SiGe thin films. It also provides a tool for the design ofSiGe-based high-efficiency thermoelectric devices. The contributions of the alloy composition,grain size, and film thickness to the phonon scattering rate are described by a model for the thermalconductivity based on the single-mode relaxation time approximation. The exact Boltzmannequation including spatial dependence of phonon distribution function is solved to yield anexpression for the rate at which phonons scatter by the thin film boundaries in the presence of theother phonon scattering mechanisms. The rates at which phonons scatter via normal and resistivethree-phonon processes are calculated by using perturbation theories with taking into accountdispersion of confined acoustic phonons in a two dimensional structure. The vibrational parametersof the model are deduced from the dispersion of confined acoustic phonons as functions of tempera-ture and crystallographic direction. The accuracy of the model is demonstrated with reference torecent experimental investigations regarding the thermal conductivity of single-crystal andpolycrystalline SiGe films. The paper describes the strength of each of the phonon scatteringmechanisms in the full temperature range. Furthermore, it predicts the alloy composition and filmthickness that lead to minimum thermal conductivity in a single-crystal SiGe film, and the alloycomposition and grain size that lead to minimum thermal conductivity in a polycrystalline SiGefilm