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Thermoelectric and electrical characterization of Si nanowires and GaNAs


Thermo-electricity offers an elegant solution to the problem of heat-to-electricity conversion. As a completely solid-state heat engine, thermoelectric materials confer many advantages in electricity generation and heat pumping. However, the efficiency of thermoelectric material, represented by the figure of merit, Z, is generally low. In order to increase Z, it is desirable to increase a property of material known as the density of states (DOS). In this work, two approaches to increase DOS were explored, one based on the physical effect of quantum confinement in Si nanowires, and another based on the chemical approach where by GaAs property are drastically changed by doping of N. While the Si nanowires fabrication have proved to be extremely challenging, and results were hard to obtain, some xvii interesting insight into dopant redistribution in nanowires were observed. For the GaNAs, the research goal was completed, and the hypothesized enhancement in effective mass was observed. Although the power factor enhancement was not obtained, the work shows that resonant doping could be potentially useful, provided that problem with significant alloy scattering can be overcome. In Chapter 1, a thorough explanation of the theory behind thermoelectric property of materials is given. The discussion will go through the mathematical rigor behind the derivation of n, [mu] S, and Ke. In Chapter 2, the theoretical basis of this work is established. The optimal reduced Fermi level is identified, and its utility is discussed in relation to the identification of the maximum power factor. The two approaches to increasing density of states are discussed. In Chapter 3, the transport coefficient measurement system that was built for this work is described. Device fabrication and measurement procedure are demonstrated. In Chapter 4, preparation of the substrates, electron beam lithography work, oxidation of Si nanowires is discussed as method to prepare nanowires. Result of FET and Seebeck measurement on nanowires are discussed. In Chapter 5, the utilization of resonant states in GaNAs to increase the power factor is explored. Predicted increase in effective mass was observed, but N scattering prevents the realization of a power factor enhancement

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