Porous Nanostructures and Thermoelectric Power Measurement of Electro-Less Etched Black Silicon

We report the morphology evolution of porous silicon nanostructures and thermoelectric characterization of silicon nanowires (SiNWs) of Electro-Less Etched (ELE) black silicon. Along the axial direction of NWs, the nanopore density (porosity) increases gradually for both highly doped <i>n</i>-Si (<i>n</i>^<i>+</i>-Si) and highly doped <i>p</i>-Si (<i>p</i>^<i>+</i>-Si). The porosity of silicon nanostructures has been demonstrated to be determined mostly by the wafer doping level and etching time. The formation of porous <i>n</i>^<i>+</i>-SiNWs and porous <i>n</i>^<i>++</i>-Si film can be understood by an enhanced electron tunnelling from the silicon to the electrolyte through a narrowed space charge layer, while the porous <i>p</i>^<i>+</i>-SiNW formation could be the result of an increased thermionic emission current over a lower barrier due to a lower band bending. With microfabricated heaters and thermometers, we measured simultaneously electrical resistivity and thermoelectric power (TEP) of boron-doped SiNWs prepared from a Si wafer with resistivity of 0.2–0.4 Ω cm. The electrical conductivity of NWs indicates an increased role of conducting surface states in the ELE SiNWs with a negligible role of surface scattering. The TEP values of NWs at room temperature are around 1.2 ± 0.1 mV/K, which is comparable to the values of bulk silicon for a similar range of dopant concentration