Surface phonon mode remarkably limits heat conduction in a silicon ultra-thin film*

Decrease of silicon channel thickness is efficient to suppress short channel effects, which is beneficial for further miniaturizing of MOS transistor. However, this causes larger reduction thermal conductivity, resulting in temperature rise of the channel region. Therefore, accurate analysis of heat conduction in thin film is needed for heat dissipation control. The Sondheimer model, which has been widely used, is known to be valid for reproducing heat conduction in the range of 50-100 nm thickness. However, since the Sondheimer model uses the bulk phonon property, the applicability of the model to ultra-thin film with several nanometers is doubtful. Here, in order to validate the model, we performed the anharmonic lattice dynamics explicitly considering atomic structure of the film to rigorously calculate thermal conductivity and phonon transport properties of the ultra-thin silicon film. Consequently we found that the Sondheimer model cannot reproduce spectral phonon transport properties of ultra-thin film because it ignores the change of phonon dispersion and presence of surface phonon. We will discuss this discrepancy in terms of surface phonon scattering to suggest the way to resolve this.