Si1-xGex Nanowire Arrays for Thermoelectric Power Generation

TLDR: In this article, the authors show the fabrication and thermoelectric characterisation of SiGe nanowire arrays (NWAs), which are arrays of millions of parallel upstanding upstanding nanowires attached to Si bulk, rather than single NWs as studied before.

Abstract: Thermoelectricity offers an excellent clean energy generation opportunity and has attracted renewed attention in the last few decades. The low conversion efficiency and high costs currently limit its practical application. Much effort is still needed to enhance its efficiency and reduce its cost. Nanostructures have been proven to greatly enhance the thermoelectric figure of merit (ZT) because of increased phonon scattering at the interfaces. It has been demonstrated that single Si nanowires (NWs) exhibit a 60 times higher ZT than Si bulk. Meanwhile, SiGe alloys can also reduce the thermal conductivity via alloy scattering without deteriorating the other performance parameters such as Seebeck coefficient, S and electrical conductivity, σ. SiGe NWs thus promise to offer even better thermoelectric performance than Si. In this work, we will show our recent research results on the fabrication and thermoelectric characterisation of SiGe nanowire arrays (NWAs). The NWAs are arrays of millions of parallel upstanding NWs attached to Si bulk, rather than single NWs as studied before. A Seebeck coefficient of S ≈ 1.1 mV/K is measured for the SiGe NWAs/Si bulk composite and is independent of Ge fraction, consistent with the theoretically expected value. The temperature drop across the SiGe NWA is consistently larger than across a similar Si NWA, indicating reduced thermal conductivity of the SiGe NWs.The use of SiGe improves the output power with a factor of 8 in the bulk TEG configuration. The use of SiGe NWAs in the p-leg only, increases the output power by a factor of 5 in comparison with the Si NWA TEG. These improvements are due to the reduction of the thermal conductance of the SiGe NWs and the reduction of the electrical contact resistance of the SiGe-based wires while the Seebeck coefficient remains unaffected