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Title: Growth of germanium nanowires for thermoelectric applications
Keywords: growth, germanium, nanowires, thermoelectricity
Issue Date: 2-Mar-2010
Citation: YOUCEF BANOUNI (2010-03-02). Growth of germanium nanowires for thermoelectric applications. ScholarBank@NUS Repository.
Abstract: This research focuses on the growth and fabrication of germanium nanowires for the measurement of their resistance and Seebeck coefficient, an important parameter in the thermoelectric performance of a device. Although silicon is known as a poor thermoelectric material, it was shown that silicon in the form of nanowires exhibits thermoelectric performance comparable to the best thermoelectric devices currently in commercial use. This is extremely significant as silicon is widely available and immense knowledge and know-how are available regarding its processing. Naturally comes the question of whether materials somewhat similar to silicon, such as germanium, would exhibit similar interesting thermoelectric behavior when engineered in the form of nanowires. Amongst several ways to grow germanium nanowires, the vapour transport method was chosen, by using a conventional furnace tube. This setup is certainly the simplest to grow nanowires by evaporation, as it uses a solid nanopowder source instead of a gas source. The counterpart is the difficulty in the control of the growth parameters. The literature on growth control of semiconductor nanowires was extensively reviewed. Very little data is available on the growth and control of the characteristics of germanium nanowires using a furnace. Most research focuses on growth using the more traditional chemical vapour deposition with a gas source. These systems allow a high level of control and are by design very different from furnace tubes. Therefore, an important work of translation of the available data was necessary to be put to use in our system. Following attempts on stainless steel substrates, most of the growth effort was spent on more traditional silicon substrates. Next, the challenge of growing wires that can be integrated into a spun-on oxide matrix was overcome. Each step of incremental adaptation of the growth parameters is presented. It was found that despite the high melting point of germanium, evaporation of the germanium powder source occurs even at temperatures 100&176;C below the melting point of solid germanium. Lowering the source temperature to this extent allows one to control significantly the growth rate. With no surprise whatsoever, reducing the growth duration was also critical, though it was beneficial only after the reduction of the source temperature to allow for fine tuning of the growth rate. Reducing the source temperature was also necessary to control the growth rate, it is to come in order as the third fine tuning option. Next, a study of the integration of the resulting nanowires is presented, with a particular emphasis on the surface irregularity due to the presence of obstacles (nanowires) in the oxide matrix layer. Finally, the qualitative usability of the alternating current-polarity Seebeck measurement technique is studied. The inherent challenges and limitations of this technique are presented, and an alternative to the alternating current-polarity method is introduced.
Appears in Collections:Master's Theses (Open)

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