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Title: Thermoelectric effects in silicon nanowires
Author: Krali, Emiljana
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2013
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The increasing demand for fossil fuels, and the need to reduce greenhouse gases, requires ‘clean’ energy sources and more efficient utilisation of energy. Thermoelectric (TE) materials provide a means towards achieving these objectives, as they convert a temperature difference [Delta]T directly into an electric potential difference [Delta]V. For practical applications, a TE material is chosen only if the dimensionless figure-of-merit ZT=S^2σT/κ ≥ 1. Where S = [Delta]V/[Delta]T, σ and κ are the Seebeck coefficient, electrical and thermal conductivity, respectively, at temperature T. Conventional bulk TE materials, such as Bi2[subscript]Te3[subscript], require a compromise between S, σ and κ. In nano-structured materials, these parameters may be varied quasi-independently, suggesting a new approach to obtain high values of ZT. In this thesis, silicon nanowire (SiNW) arrays were fabricated using a metal-assisted chemical etching process, creating SiNWs from 30 to 400 nm in diameter, a maximum length of 350 μm and aspect ratio up to 3000. A novel transient measurement method was used to characterise the temperature dependence of S in two different n-type doped SiNW arrays, ≈10^{15} cm^{-3} and ≈10^{18} cm^{-3}. In the lightly doped 35 μm long SiNWs, S=1850 μV/K at 300 K, an increase by a factor of 2.5 compared to its parent bulk Si (S_{Bulk}). Furthermore, the phonon drag component, a manifestation of electron-phonon scattering in the sample, is heavily suppressed due to surface scattering. In the moderately doped 30 μm long SiNW array, S=1480 μV/K = 2.5S_{Bulk} at 300 K. An increase in S was also observed in the n- and p-SiNWs measurements in ambient conditions. A transient method was used to characterise the temperature dependence of κ in the range 300 – 30 K. At 300 K, κ=23 W/mK = 0.19\κ_{Bulk}. Finally, the I-V characteristics of the SiNW arrays were measured and only a limited change was observed from bulk Si. Assuming σ is unchanged, in the 30 μm long SiNWs, 0.0255 ≤ ZT_{NW} ≤ 0.34. This corresponds to an increase in ZT from 32 to 55 times than the bulk Si value.
Supervisor: Durrani, Z. A. K. Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available