Investigation of Mechanism for Heat Conduction Suppression in Si Thin-Films Due to Surface Roughness

Improvements in the heat dissipation capability are required for enhancing the performance of integrated circuits. However, in nanoscale silicon transistors, the thermal conductivity significantly decreases compared to bulk silicon due to size effects. This decrease is widely attributed to boundary scattering caused by surface roughness, but the understanding of this mechanism is still incomplete. In this study, we investigated atomic-scale mechanisms underlying the reduction in thermal conductivity due to roughness by applying anharmonic lattice dynamics analysis to silicon thin-film structures with surface roughness. The results showed that surface roughness reduces thermal conductivity through two mechanisms: a decrease in the group velocity due to the hybridization with surface modes and a reduction in relaxation time due to an increase of anharmonic interatomic force constants of surface atoms. Furthermore, these effects depend differently on surface roughness and film thickness. The reduction in group velocity affects thermal conductivity across a wide range of film thicknesses and roughness, whereas the effect of reduced relaxation time strongly suppresses thermal conductivity in rough thin films less than 5 nm thick.