Direct first-principle-based study of mode-wise in-plane phonon transport in ultrathin silicon films

Abstract

Fundamental understanding of thermal transport properties in ultrathin Si-based films is essential for the thermal management of nanoelectronic and nanophotonic devices. Using Density-Functional-based tight-binding method and direct iterative solution of linearized Boltzmann transport equation without empirical assumptions, we systematically investigated mode-wise in-plane phonon transport in ultrathin Si films of a few nanometer thickness (0.77–1.90 nm) and the effects of surface morphology. The dimensionality reduction of these films leads to quantum confinement and softening of silicon bonds and in turn changes the dispersion and phonon-phonon interactions. The ultrathin Si films with naturally reconstructed surfaces show a counterintuitively high in-plane thermal conductivity (∼30 W/m-K at 300 K) with relatively weak size dependence and large acoustic phonon contribution, demonstrating that dimensionality reduction alone cannot suppress phonon transport in ultrathin films efficiently. The in-plane thermal conductivities of ultrathin films are very sensitive to surface defects and even atomic-level surface defects can induce up to 10-fold reduction in thermal conductivity due to much enhanced phonon scatterings in low frequency regime. Our direct first-principle-based calculations also show that the conventional modeling of thin films with bulk properties and suppression function may lead to large uncertainty when applied to ultrathin films.