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Title: Microstructure of group IV semiconductor nanomaterials
Other Titles: Di 4 zu ban dao ti na mi cai liao de wei jie gou
Di si zu ban dao ti na mi cai liao de wei jie gou
第 4 族半導體納米材料的微結構
Authors: Yao, Yuan (姚湲)
Department: Dept. of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2004
Publisher: City University of Hong Kong
Subjects: Microstructure
Nanostructured materials
Notes: CityU Call Number: TA418.9.N35 Y36 2004
Includes bibliographical references
Thesis (Ph.D.)--City University of Hong Kong, 2004
xvii, 172 leaves : ill. (some col.) ; 30 cm.
Type: Thesis
Abstract: Semiconductor nanomaterials are an area of research of intense interest because of their wide potential applications arising from their different properties than the bulk material. It is very important to develop the fabrication method and understand the formation mechanism of such nanomaterials since they are responsible for obtaining the high-quality materials available for research and applications. As the growth process can influence the crystal structure of the nanomaterials, the study of the microstructures may reveal the mechanism of synthesis and information to improve the fabrication procedures. Due to the small size of nanomaterials, transmission electron microscopy (TEM) and related techniques, such as high resolution TEM (HRTEM), selected area electron diffraction (SAED) and electron energy loss spectroscopy (EELS) were employed to characterize the nanomaterials because these methods can provide a high enough spatial resolution and corresponding chemical information about these tiny objects. Other methods, such as scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), have been used to characterize the morphology and composition of nanomaterials. In this thesis, quasi one-dimensional silicon nanowires have been achieved at relatively low temperatures (between 600 - 900 ºC) with the Au-catalyzing oxide-assisted growth (OAG) method on both TEM copper grids evaporated with Au particles in a tube furnace and on silicon substrates covered with Au film in the hot-filament chemical vapor deposition (HFCVD) system. TEM study shows the Si nanowires have Au caps on the crystalline silicon cores surrounded with silica sheath. The diameters of Si nanowires are limited by the sizes of the Au caps, whose diameter decreases with decreasing synthesis temperature. Statistics indicates that most Si nanowires grow along the <112> direction, same as those grown from the catalyst-free OAG method but different from those grown from the common metal-catalyzing vapor-liquid-solid (VLS) method. Additionally, epitaxially aligned silicon nanowires have been fabricated on the Au-covered silicon substrate in HFCVD. HRTEM reveals the <111> facets should be the interface between the Au catalysts and the silicon cores though the <112> direction is the axis of the nanowires. It is believed that the surface energy and the silica sheath of the nanowires should introduce the favorite <112> growth direction and these oxide layers also retard the growth of silicon nanowires below 600 ºC. TEM and SAED show that 2H SiC nanowhiskers – a Si-related semiconductor nanostructure – can be fabricated directly with SiO and methane. This is the extension of the OAG method besides the Au-catalyst assistance. The microstructure study implies the phase transformation may induce the cubic tip of the 2H SiC nanowhisker during the cooling down of the system. Using EELS, nanodiamonds, a wide-band-gap semiconductor, have been found in diamond-like-carbon (DLC) films deposited by mass-selected pure carbon ions. Graphite layers vertical to the substrate, multiwalled carbon nanotubes, and nano-onions have also been identified in the DLC films deposited under different conditions. This is the first time showing that the diamond can be synthesized under relatively low temperatures and pressures by pure carbon without hydrogen etching. The results provide further insight and understanding of the formation mechanism of diamond.
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