Nanoengineering of ZnO Electrical and Optical Characteristics by Controlling its Morphology and Defects via Reaction Kinetics, Fluid Dynamics, and Stoichiometry in Low-Pressure CVD Synthesis
- Author(s): Lim, Taehoon
- Advisor(s): Martinez-Morales, Alfredo A
- Mangolini, Lorenzo
- et al.
LPCVD synthesis and characterization of ZnO with diverse morphology for semiconducting applications are studied and discussed in this dissertation. Because nanoscale ZnO shows variant characteristics depending on its physical dimensions, morphology, and impurity concentration, the focus of this work is to control the structure and composition of ZnO by adjusting synthesis parameters and methods. In Chapter 2, the synthesis of ZnO with optimized concentration of oxygen vacancies and physical dimensions are discussed. Opaqueness of ZnO is derived from scattering due to its physical dimensions, but visible transparency for the photocatalytic application of ZnO for solar cell devices is required. By controlling the concentration of oxygen vacancies and physical dimensions, fluorescent and transparent ZnO is successfully synthesized. In Chapter 3, a method to stabilize precursor supply and to lower the reaction temperature is discussed. In CVD synthesis, the stable and continuous growth of ZnO is inhibited by the premature oxidation of Zn, and the formation of a zinc oxide layer that blocks the evaporation of Zn precursor. By controlling the fluid dynamics of precursor vapors, a low temperature synthesis method with higher product uniformity is developed. Using this method, a stable synthesis technique for producing high quality ZnO under low temperature is successfully achieved. In Chapter 4, the synthesis of ZnO under relatively low temperature is discussed. During LPCVD synthesis it is observed that as the distance from the source increases, the partial vapor pressure of Zn near the surface of the deposition substrate is drastically decreased (along the axis of gas flow), forming a dimensional gradient on a single substrate. The changing reactant ratio near the surface drives the non-uniformity of product in terms of morphology, physical dimensions, and impurity concentration. In Chapter 5, a novel template-free self-catalyzed synthesis technique for achieving ultra-thin ZnO nanowires is discussed. From the tip of synthesized hexagonal cone ZnO, ultra-thin ZnO nanowires are grown by self-catalysis.