Thermoelectric Properties of Nanostructured Silicon Films.

Abstract

Based on the Seebeck effect, thermoelectric materials can convert temperature heat into electrical energy. Alternatively, based on the Peltier effect, thermoelectric cooling can be achieved by supplying current through thermoelectric materials. Since the best thermoelectric materials are heavily doped semiconductors, and silicon is the most abundant semiconductor on earth, investigation of the thermoelectric properties of strained silicon and ~100 nm thick silicon thin films, and the thermoelectric cooling application of the ~100 nm thick silicon thin film become the focus of this dissertation. In the Seebeck effect, charge carriers thermally diffuse from a high temperature to low temperature, creating an open circuit voltage potential across the thermoelectric material. However, the application of such technology is mainly limited due to its poor efficiency when competing with the traditional engines; the efficiency depends on the temperature difference and a dimensionless figure of merit, ZT. It has been determined that ZT=S2σT/κ, in which S2σ is the power factor, S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the average temperature. The interdependence of S, σ and κ makes it difficult to improve ZT because optimizing one parameter adversely affects another. ZT is the key parameter to determine the thermoelectric performance not only in thermoelectric power generation application based on Seebeck effect, but also in thermoelectric cooling application based on Peltier effect. A higher ZT results in better thermoelectric performance. It has been proved that silicon nanomesh materials result in a reduced thermal conductivity due to the increased phonon-boundary scattering. This work focuses on using novel nanostructured silicon thin films fabricated by block copolymer lithography to study thermoelectric properties of silicon, and to use these results to estimate the thermoelectric performance of two types of silicon thin films with nanomesh structures.