Phonon Localization in Ultrathin Silicon Membranes with Surface Nanostructures

Surface nanostructures have been shown to introduce new vibrational modes that locally couple with phonon modes of the base and impact in-plane phonon propagation. Nanoscopic surface imperfections have been experimentally and theoretically demonstrated to decrease the thermal conductivity (TC) of silicon (Si) membranes. However, the tunability of the coupling or hybridization mechanisms due to unique surface geometries, and the extent of their impact on phonon properties are not fully understood. Using direct non-equilibrium molecular dynamics (MD), we investigate the effect of periodic “nanofins” on the thermal properties of ultrathin Si membranes. Our study exhibits that these structures engender a distinct in-plane anisotropy in the TC of the membrane, and create unique localized temperature and phonon-induced strain profiles, in accordance with the surface structure periodicity. Further investigation, using lattice dynamics with the quasi-harmonic approximation, reveals a significant reduction in mode diffusivity, indicating phonon localization, approaching behavior similar to highly disordered solids, e.g., amorphous Si. These results further establish surface nanoengineering as a viable approach to meter and direct heat flow in future microelectronic/quantum technologies.