Enhancement of the figure of merit of silicon germanium thin films for thermoelectric applications

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Copyright: D'abbadie, Lois
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Abstract
Silicon Germanium thin films are the most stable thermoelectric materials at high temperatures. Nonetheless, low efficiency and limited knowledge of such structures are still a challenge to research. Understanding the various mechanisms taking place in the matter and their relationships is the next step to boost research and to enhance the TE efficiencies, widening the range of applications. In this thesis, we develop a mathematical approach based on solid state theory to calculate the figure of merit from an electronic band structure. Added to DFT, this near ab initio method rapidly assesses virtual structures as possible TE materials. In this report, the method is applied to bulk silicon germanium with good agreement with experimental results. Throughout the development of this method, we also conclude that TE semiconductors band-gaps are related to the range of temperature where the material show higher values of ZT. We also show that doping with donors and acceptors, which is a common enhancement, need optimization for high temperatures applications due to its contribution to the thermal conductivity. Moreover the carriers' mobility is a prevalent parameter but its calculation remains complex. So, we implement deformation potential theory with DFT to calculate the electron-phonon interactions in silicon germanium alloys. We find no change of interactions with the alloy composition. With enough computing power, these methods are applicable to low dimensions structures. In addition to our theoretical study, we report a sputter deposition method of silicon germanium alloy thin films with controlled composition and thickness, grown on a sputtered layer of silicon dioxide. XRD study shows the appearance of a crystal phase beginning at deposition temperatures of 650 C . Reflectivity and TEM provide a consistent measurement of deposition thickness in the range of 20 to 100 nm with average interfaces roughness around 2 nm.
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Author(s)
D'abbadie, Lois
Supervisor(s)
Li, Sean
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Publication Year
2013
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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