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Title:Electrical and thermal transport in 2- and 3-dimensional periodic holey silicon
Author(s):Ma, Jun
Director of Research:Sinha, Sanjiv
Doctoral Committee Chair(s):Sinha, Sanjiv
Doctoral Committee Member(s):Braun, Paul V.; Kim, Seok; Nam, SungWoo
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
silicon nanostructure
thermal transport
electrical transport
Abstract:Silicon, one of the most abundant elements on earth, is a promising candidate for thermoelectric applications in the form of different nanostructures. It has been conclusively demonstrated that boundary scattering in nanostructured silicon e ectively reduces thermal transport, resulting in an enhanced thermoelectric Figure of merit ZT. However, claims of high ZT are quantitatively misleading since electrical and thermal properties are often characterized on separate samples due to measurement complexity. In the first part of this dissertation, we design, fabricate and employ a novel integrated microdevice to measure all three thermoelectric properties of 2D holey-silicon thin films. We systematically vary doping across samples using diffusion doping over a barrier layer. While the size of the sample has minimal impact on electrical conductivity, we find the Seebeck coefficient (and hence, the power factor) to be substantially suppressed. By examining the temperature trend and comparing with available bulk data, we find the reduction to be explained through quenched phonon drag resulting from phonon boundary scattering. The thermal conductivity of these samples remain relatively in agreement with the Casimir limit. The total increase of ZT is 4 times when compared against bulk silicon at 300 K. The second part of this dissertation measures the temperature dependent thermal conductivity of 3D periodic silicon inverse opals. Beside the anticipated low thermal conductivity due to high porosity, we observed an anomalous T1.8 dependence at low temperatures, distinct from the typical T3 behavior of bulk polycrystalline silicon. Using phonon scattering theory, we show such dependence arising from coherent phonon scattering in the intergrain region. This unique observation of coherence effect at grain boundary may be attributed to a thinner intergrain region formed when intragrain growth is limited by shell thickness during prolonged annealing. This work provides insight into coupled charge and heat transport in silicon nanostructure with periodic holes in 2- and 3-dimensions.
Issue Date:2016-10-20
Rights Information:Copyright 2016 Jun Ma
Date Available in IDEALS:2017-03-01
Date Deposited:2016-12

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