Thermoelectric modules based on silicon nanowires (Si-NWs) have recently attracted significant attention as they show an improved thermoelectric efficiency due to a decrease in thermal conductivity. Here, we adopt a top-down fabrication method to dramatically reduce the thermal conductivity of vertical Si-NWs. The thermal conductivity of a vertical Si-NW is significantly suppressed with an increasing surface roughness, decreasing diameter, and increasing doping concentration. This large suppression is caused by enhanced phonon scattering, which depends on the phonon wavelength. The boron- and phosphorus-doped rough Si-NWs with a diameter of 200 nm and surface roughness of 6.88 nm show the lowest thermal conductivity of 10.1 and 14.8 W·m-1·K-1, respectively, which are 5.1- and 3.6-fold lower than that of a smooth intrinsic nanowire and 14.8- and 10.1-fold lower than that of bulk silicon. A thermoelectric module was fabricated using this doped rough Si-NW array, and its thermoelectric performance is compared with previously reported Si-NW modules. The fabricated module exhibits an excellent performance with an open circuit voltage of 216.8 mV·cm-2 and a maximum power of 3.74 μW·cm-2 under a temperature difference of 180 K, the highest reported for Si-NW thermoelectric modules.
Keywords: Vertical silicon nanowire; phonon scattering; surface roughness; thermal conductivity; thermoelectric module; top-down technique.