Ultralow thermal conductivity in Electrolessly Etched (EE) Silicon Nanowires*

EE process produces single-crystalline Silicon nanowires with rough walls. We use suspended structures to directly compute the heat transfer through single nanowires. Nanowires with diameters less than the mean free path of phonons impede transport by boundary scattering. The roughness acts as a secondary scattering mechanism to further reduce phonon transport. By controlling the amount of roughness it is possible to push limits to the extent that nanowire conductance close to quanta of thermal conductance,${\pi k_B^2 T} \mathord{\left/ {\vphantom {{\pi k_B^2 T} {6\hbar }}} \right. \kern-\nulldelimiterspace} {6\hbar }$ is observed. Traditionally, the lower limit of conductivity is amorphous Silicon at 1 W/mK at room temperature. The measured conductivity of our nanostructures challenges even this amorphous limit pointing towards previously unstudied mechanisms of thermal resistance. We measure thermal conductivity of $\sim $150nm diameter EE wires to be $\sim $1 W/mK.