Phonon scattering enhancement in silicon nanolayers

Dimensional confinement in silicon nanowires (NWs) is well-known for enhancing phonon scattering, thus leading to a pronounced reduction of thermal conductivity j with respect to bulk material. The effect of confinement on phonon scattering in nanolayers (NLs), however, has not been fully understood. In this work, thermal conductivity on polycrystalline silicon NLs with roughened surfaces and thicknesses ranging from 30 to 100 nm has been experimentally investigated. For measurement purposes, the nanostructures were fabricated with a dedicated surface nano-machining process, thus producing vertical silicon nanostructures suspended on Al/Si electrodes on a silicon substrate, using SiO2 as a sacrificial layer. By designing such structures in a four-terminal configuration, their j could be determined by the current-voltage method. Boron doped silicon NLs were examined, at resistivity ranging between 2 and 10 mX cm. We found an increase of phonon scattering from the confinement, since j decreased steadily with the thickness from values typical of thick films (around 30 W m-1 K-1) down to\15 W m-1 K-1. Compared to NWs, NLs had displayed figures of merit smaller by one order of magnitude. However, due to the larger filling factor, they were able of generating more than five times the electric power per area unit that could be obtained with high-density stacks of top-efficiency NWs.