Shallow periodic patterns on silicon nanostructures for engineered thermal conductivity reduction

Silicon nanostructures have been intensively studied to prove the correlation between thermal conductivity reduction and surface roughness, with the aim to develop sustainable and high efficient thermoelectric devices. In this work, we shed light on the mechanism responsible for the pronounced reduction of the thermal conductivity. We report on the thermal characterization of suspended silicon nanostructures whose surfaces were modified with a shallow periodic grating etched on their top surfaces. A clear correlation between the grating periodicity and the resulting thermal conductivity is observed. Although the etched pattern is only 20 nm deep compared to the total nanostructure thickness of 190 nm, and thus leaving the overall cross-section almost unchanged, a remarkable reduction in thermal conductivity is measured, reaching values up to five times lower than that of unetched nanowires. This reduction lies well below the limit expected from purely diffusive phonon transport: the shallow pattern only marginally affects the boundary mean free path through surface scattering, even when accounting for fully thermalized boundary scattering (Casimir limit) at the modified surface.