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Title: Synthesis of oriented silicon nanowires by applying bias voltage
Authors: Chau, Penny Chin Yiu (周展耀)
Department: Department of Physics and Materials Science
Issue Date: 2005
Course: AP4116
Programme: BEng (Hons) in Materials Engineering 2004-05
Supervisor: Prof. S T Lee
Subjects: Nanowires
Nanostructure materials
Abstract: An electric field is supposed to play a major role in the formation of silicon nanowires by Oxide-Assisted Growth mechanism. The strong electric field is assumed at the tip of silicon nanowires. Most of the Silicon-oxide molecules are attracted by this E-field and then land on the tip of the wire. Therefore, the nanowires growth only occurs on the tip. Based on this E-field mechanism premise, we hypothesize that, if the formation of silicon nanowires is related to its emitted E-field feature, growth orientation and speed of the nanowires can be altered by applying bias voltage (i.e. external E-field). The basic concept is that a bias voltage is applied to attract the more charges to accumulate on the particular area of the tip, so that it will accelerate the growth process. If we change the direction of the external E-field, the growth direction of the nanowires can be altered to go where we want. Meanwhile, we can synthesize the aligned nanowires. In our course of experiment, hot filament CVD (chemical vapor deposition) was mainly used. Additionally, a plate of stainless steel was installed to act as an electrode which applies bias voltage. A silicon substrate coated with gold was the place for the growth of silicon nanowires. By vaporizing the starting source - silicon monoxide, the nanowires can be formed on the heated substrate. During the formation of the nanowires, the external E-field was applied. As a result, it can be found that the growth orientation of nanowires can really be altered by the external E-field. However, because of the silicon nanowires carrying charges feature, the nanowires were easily tangled. Therefore, it is so difficult to align a large population of the nanowires. In order to prevent from the VIII nanowires from being tangled, the metal catalyst should be fine enough. This is because, if the distance between each of the nanowires is large enough, they would not cross and attach to each other. At the same time, it can maximize the effect of the external electric field. An oriented growth direction can be significantly shown. Different kinds of gold deposition method were used. It has been found that gold pattern made by electron beam lithography can really show that the growth orientation of silicon nanowires is affected by the electric field. Since positive and negative bias voltages give the same effect on the growth orientation, it is reasonable to believe that the silicon nanowire is an induced dipole. As a consequence, under the electric field, an electrostatic force continuously pulls the head of the nanowire, so that it becomes oriented and relatively straight.
Appears in Collections:OAPS - Dept. of Physics & Materials Science

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