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Title:Experimental determination of the thermoelectric properties of porous silicon nanowires
Author(s):Tian, Hongxiang
Director of Research:Sinha, Sanjiv
Doctoral Committee Chair(s):Sinha, Sanjiv
Doctoral Committee Member(s):Li, Xiuling; Ertekin, Elif; Toussaint, Kimani
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
porous silicon nanowire
phonon transport in porous silicon nanowire
electron transport
Abstract:The thermoelectric properties of nanostructured silicon have attracted significant attention in recent work. The objective is to reduce thermal conductivity through the introduction of phonon scattering mechanisms while preserving charge transport. Initial reports on electrolessly etched silicon nanowires and periodic \holey" silicon membranes contained several puzzling aspects that remain unresolved. Here, we present measurements on mesoporous silicon nanowires fabricated through electroless etching of degenerately doped silicon. While thermal conductivity in this material at room temperature is attractive for thermoelectric applications at ~2 W/(m K), charge transport is severely degraded due to the disordered yet nanocrystalline structure. We investigate post doping conditions to improve the electrical conductivity by ~3 orders of magnitude. TEM characterization confirmed that porosity and crystalline structure are retained after doping. We characterized the boron concentration before and after doping by secondary ion mass spectroscopy (SIMS). Raman spectroscopy was also employed to extract the free carrier concentrations as well as nanocrystalline size. We measured the Seebeck coefficient and thermal conductivity from 30 K to 400 K by a frequency domain technique. Analysis of Seebeck coefficient reveals the electron scattering mechanism in porous silicon nanowire before and after post-doping. The carrier concentrations extracted from Seebeck coefficient are in good agreement with the Raman spectrum analysis. In order to interpret the thermal conductivity data, we propose a frequency dependent multiple scattering of phonons across the porous wire. This work provides detailed insight into charge and heat transport in disordered yet nanocrystalline materials and advances their engineering for thermoelectric waste heat harvesting amongst other applications.
Issue Date:2015-01-21
Rights Information:Copyright 2014 Hongxiang Tian
Date Available in IDEALS:2015-01-21
Date Deposited:2014-12

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