|M.Sc Student||Guy Burshtein|
|Subject||ZnO Nanostructure Growth by Thermal CVD|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Emeritus Lifshitz Yeshayahu|
|Full Thesis text|
ZnO nanostructures are investigated as potential building blocks of future devices. Their commercialization requires controlled processes to meet specific demands. This study attempts at a universal framework providing guidelines for industrial growth processes. It explores three topics: (1) the distribution of growth species in the thermal chemical vapor deposition (TCVD) reactor, (2) the role of reacting gases in TCVD of ZnO and (3) the patterning effect of the catalyst.
The work has three channels of research: (1) Growth of nanostructures utilizing a specially designed TCVD system, (2) Characterization of the nanostructures, (3) Computer simulations of the distribution of the growth species in the reactor.
The TCVD consists of a three zone cylindrical furnace. The heated source material in the central zone generates species that are transported to heated substrates in the two other zones by combined effect of diffusion and convection. Zn species are generated by: (1) conventional carbothermal reaction (a powder of ZnO:C) and (2) reduction of an annealed ZnO powder by H2 a containing carrier gas. The parameters varied were: source and substrate temperatures, carrier gas flow, pressure and composition. The gas composition during the growth process was monitored by residual gas analysis (RGA).
The morphology, structure and chemical composition of the nanostructures were characterized by: High Resolution SEM (HRSEM) with Energy Dispersive Spectroscopy (EDS), Transmission Electron Microscopy (TEM) and X-Ray (XRD).
The distribution of the growth species was simulated by the COMSOL program.
1. The distribution of species in the TCVD system is governed by both convection and diffusion. Convection distributes the growth species downstream while diffusion transports them both downstream and upstream. The experimental data of ZnO growth and the simulations offer insight into the processes governing the evolution of the nanostructures in TCVD systems and point out the optimal conditions for controlled growth.
2. The carbothermal TCVD growth depends on the chemical composition of the carrier gas. Zn evaporation is driven by ZnO reduction in the source. Oxidation of Zn on the substrates forming ZnO nanostructures can be manipulated by O2 or CO2. Reduction of pure ZnO by H2 offers better control than that obtained by the carbothermal reduction.
3. Growth of individual ZnO NWs requires small gold catalyst droplets distributed far from each other. This is achieved by liquid dewetting of a gold film or by e-beam lithography but not via conventional solid state dewetting of neither by the nanosphere lithography (NSL).