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Title: Microstructural properties of semiconductor nanostructures
Author: Li, Fang
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2011
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Semiconductor nanostructures have attracted great interest owing to their unique physical properties and potential applications in nanoscale functional devices. The enhancement of the physical properties of semiconductor nanostructures and their performance in devices requires a deeper understanding of their fundamental microstructural properties. Thus this thesis is focused on the experimental and theoretical studies of the microstructural properties of two important semiconductor nanostructures: axial heterostructured silicon nanowires with varying doping and indium nitride colloidal nanoparticles. In this thesis, axial heterostructured silicon nanowires with varying doping were synthesized on an oxide-removed Si{111} substrate using a vapour-liquid-solid approach. Their fundamental microstructural properties, including the crystalline structure, wire growth direction and morphologies, were studied using various characterization techniques. It is found that a very small fraction of the silicon nanowires crystallize in a hexagonal (wurtzite) phase, which is thermodynamically unstable in bulk silicon under ambient conditions, while a large majority of the synthesized silicon nanowires exhibit the expected diamond cubic crystalline structure. About 75% of the diamond cubic silicon nanowires synthesized grow in a single <111> direction, while the rest contain growth-related kinks, where the nanowire switches to another direction during the growth. The ~109° silicon nanowire kinks are the most commonly observed, and the growth direction before and after such ~109° kink are both <111>. The sidewalls of silicon nanowires do not change abruptly at the ~109° kink, but exhibit an elbow-shaped structure. It is also found that the nanowire sidewalls exhibit periodic nanofaceting, which is strongly doping-dependent. The nanofaceting is found to occur during the enhanced sidewall growth that arises when the diborane dopant gas is introduced. A thermodynamic model predicting the dependence of nanofacet period on the wire diameter is developed. Another semiconductor nanostructure studied in this thesis is indium nitride colloidal nanoparticles, which were grown using a solution-phase chemical method. The formation of such indium nitride colloidal nanoparticles is confirmed by studying their compositions, crystalline structures and shape using various electron microscopy techniques. The size of the indium nitride colloidal nanoparticles was controlled by varying the time of solution-phase reactions. The most probable size of the colloidal nanoparticles increases and the size distribution broadens with the increase of reaction time. The crystalline structures of the indium nitride colloidal nanoparticles are found to be particle size dependent. The observed dependence of the band gap blueshift of the indium nitride colloidal nanoparticles on the reaction time (hence the particle size) is explained by the quantum-size effect.
Supervisor: Cockayne, David ; Nellist, Peter ; Lang, Christian Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Single crystal semiconductors ; Semiconductors ; Nanostructures ; Microscopy and microanalysis ; High resolution microscopy ; semiconductors ; nanostructures ; transmission electron microscopy