Thermal conductivity reduction in Si free-standing membranes investigated using Raman thermometry

We report on the reduction of the thermal conductivity in ultra-thin suspended Si membranes with high crystalline quality at room temperature. A series of membranes with thicknesses ranging from 9 nm to 1.5 μm was investigated using Raman thermometry, a novel contactless optical technique for thermal conductivity determination. The temperature rise of a laser spot focused on the membranes was monitored as a function of the absorbed power. For this purpose, the absorption coefficient of the membranes was experimentally determined and also theoretically modelled. A systematic decrease in the thermal conductivity was observed as reducing the thickness of the membranes which is explained using the Fuchs-Sondheimer model through the influence of phonon boundary scattering at the surfaces of the membranes. The thermal conductivity of the thinnest membrane with d= 9 nm resulted in (9±2)W/mK, thus approaching the amorphous limit but still maintaining a high crystalline quality.