Effect of Surface Modification on Heat Conduction Suppression in Silicon Thin Films

Recently, silicon-based nanostructured thermoelectric generation devices, such as thin films and nanowires, have attracted great attention. However, their conversion efficiency remains insufficient. Some efforts have focused on improving conversion efficiency by suppressing thermal conductivity in nanostructures through surface modification. In such works, localized phonon modes significantly suppressed the group velocity of heat-conducting phonons. Additionally, surface disorder induced by the modification can enhance phonon scattering, potentially further reducing thermal conductivity.

In this study, we investigated the impact of H, O, and Ge surface modifications on the thermal conductivity of ultrathin silicon films. Surface-localized phonon modes were observed in all modified systems, equally reducing the group velocity. However, the effects of modifications on phonon scattering differed significantly. In Si-H/O bonds, the interatomic force constants (IFCs) were greatly modulated, increasing phonon scattering rate. In contrast, no modulation in IFCs was observed in Si-Ge bonds, and the scattering rate was not changed. Therefore, to effectively reduce thermal conductivity through surface modification, the modulation of IFCs rather than group velocity is more important.