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 n-Si (n(+)-Si) and highly doped p-Si (p(+)-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 n(+)-SiNWs and porous n(++)-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 p(+)-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 Omega 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.