Thermal conductivity reduction by phonon backscattering in a silicon nanowire with wavy surfaces

Abstract

The silicon (Si) element originally exhibits a low thermoelectric figure of merit due to its inherently high thermal conductivity, which has rendered it unsuitable for use in thermoelectric devices (TEDs). However, with the development of semiconductor process technology, it has become known that silicon nanowires exhibit different physical properties from bulk Si. Among material properties, the thermal conductivity shows the tendency decreasing linearly with decreasing diameter, which opens up the possibility of silicon as a thermoelectric device [1]. Although it still has a lower figure of merit compared to other thermoelectric materials (BiTe, SnSe, PbTe etc.), efforts have been made to improve performance by lowering the thermal conductivity [2]. In this study, We implemented the Bosch process using a deep reactive ion etcher to carve a three-dimensional silicon nanowire with wavy surface (W-SiNW). We modulated the surface of the nanowire to induce backscattering of phonons to reduce thermal conductivity. Phonons are scattered backwards by the wavy surface, ultimately lowering thermal conductivity. Thermal conductivity was measured between 300 and 400 K and show 18.5 W/m·K at 300 K and 16.7 W/m·K at 400 K. In addition to the results, we analyze the amount of backscattering and complement the results with Monte Carlo simulations in nanowires.