High efficiency thermoelectric devices made by silicon nanostructures

The focus of the thesis is the fabrication of high efficiency devices for thermoelectric generation (TEGs), based on silicon nanostructures. The high efficiency of a TEG relies on the figure of merit ZT: enhancing it means to improve the electrical conductivity and the Seebeck coefficient S, and to reduce the thermal conductivity kt. Silicon nanostructures satisfy these requirements thanks to the reduced kt compared to bulk silicon, as evidenced by several theoretical and experimental works on single nanowires. However, to deliver high currents and high voltages to an electrical load it is necessary to fabricate TEGs made by several nanostructures connected in parallel. This has been investigated, and here presented, following two possible strategies: building arrays of nanowires perpendicular to the silicon substrate (vertical strategy) or networks of nanostructures parallel to the silicon substrate (planar strategy). Such devices are characterized by good mechanical strength and, at the same time, they can handle large currents and deliver high power. In the context of the vertical strategy, a low-cost process for the fabrication of forests of long silicon nanowires with a top copper contact was developed. Different approaches to reduce the influence of the substrate in the electrical performances of these nanowire forests were examined, such as applying the process on highly doped substrates or reducing the thickness of the residual substrate using double polished wafers. Then, an experimental set-up based on the guarded hot plate concept for the measurement of their thermal conductivity was assembled. In the framework of the planar strategy, we built prototypes of suspended nanomembranes through techniques based on e-beam lithography and conventional CMOS processing. These prototypes were characterized by means of the 3 omega technique, which is an electrical method that allows to estimate the thermal conductivity of the structures under test. To this end, the nanomembranes of the prototypes have been made with smooth or randomly rough surfaces, with the purpose of exploring the influence of surface roughness in the reduction of kt, which is directly linked to the phonon scattering on the nanostructure walls. All the characterization procedures performed on these devices, both in the vertical and in the planar strategy, were essential steps to justify the belief that a TEG made of them can be definitely considered a realistic outcome.