Preparation methodologies and microstructural characteristics of selected semiconductor thin films and metal oxides

Abstract

This thesis deals with the preparation methodologies and the microstructural characteristics concerning semiconductor thin films (including SnO2 thin films, Au/Ge bilayer films, and Pd-Ge alloy thin films) and the metal oxides (including SnO, SnO2, Mn2O3 and Mn3O4 nanocrystals: nanoparticles, nanowires, nanorods, and nanofractals). Firstly, the preparation methodologies and the microstructural characteristics of tin oxides have been investigated in detail and described in chapter 2. This covers the following: (i) the application of x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and high resolution transmission electron microscopy to study tin oxide thin films deposited on Si (100) substrates at room temperature using pulsed laser deposition (PLD) techniques with a sintered cassiterite and subsequently heat-treated tin oxide thin films; (ii) measurement of surface topographies of SnO2 thin films prepared by PLD for various substrate temperatures by scanning electron microscopy, where the concept of fractal geometry was proven useful in describing structures and processes in experimental systems; (iii) preparation of low-dimensional nanostructures of SnO2 thin films with some interesting features of tetragonal rutile structure by PLD, where the as-prepared SnO2 thin films were found to be in the polycrystalline state; (iv) growth of nanocrystalline SnO2 thin films onto glass substrates by PLD, where the thin films were determined to be a polycrystalline SnO2 and an amorphous SnO phase. The nucleation and growth processes of SnO2 nanocrystallites were analysed in detail in order to examine how the PLD technique and operating conditions affect the evolution of grain size, shape, crystallographic characteristics and morphology; (v) experimental and theoretical exploration of quantum dot formation and dynamic scaling behavior of SnO2 nanocrystals in coalescence regime for growth by PLD; (vi) investigation of the mystery of porous SnO2 thin film formation by pulsed delivery based on sintered SnO2 target at room temperature; (vii) preparation of a pure orthorhombic SnO2 thin film by PLD at much lower pressures and temperatures than those of traditional methods; (viii) demonstration that SnO2 nanowires can be synthesized by a PLD process deposited on Si (100) substrates at room temperature; and (ix) synthesis of SnO2 nanorods in bulk quantity by a calcining process based on annealing precursor powders. Secondly, the extended version of metal/semiconductor thin films for the crystallization of amorphous Ge, and the formation of nanocrystals and compounds developed with improved micro- and nanostructured features are described in chapter 3. This chapter includes: (i) investigation of microstructural changes and fractal Ge nanocrystallites in polycrystalline Au/amorphous Ge bilayer films upon annealing by scanning electron microscopy, transmission electron microscopy and x-ray energy-dispersive spectroscopy; (ii) interdiffusion assessment of nanoparticles in fat fractal patterns, where the nanoparticles of polycrystalline Ge have been grown in a freshly cleaved single crystal NaCl (100) substrate, starting from Au/Ge bilayer films prepared by evaporation method during annealing; (iii) investigation of nanocrystal formation and fractal microstructural assessment in Au/Ge bilayer films upon annealing by high-resolution transmission electron microscopy, where the crystallization process was suggested to be a diffusion controlled and random successive nucleation and growth mechanism; (iv) investigation of solid-state reactions and amorphous Ge crystallization for various ratios of thickness (or composition) in Pd-Ge alloy thin films after annealing by transmission electron microscopy; and (v) analysis of grain nucleation, growth and aggregation in Pd-Ge alloy thin films during annealing by fundamental kinetic processes. Thirdly, a novel selective synthesis route for various morphologies of manganese oxides nanocrystals (including nanoparticles, nanorods and nanofractals) and their unique microstructural characteristics are presented in chapter 4. Intricate fundamental properties of manganese oxides nanocrystals are studied. This includes: (i) investigation of the influence of grain size on the vibrational properties of Mn2O3 nanocrystals by Raman and Infrared spectroscopy; (ii) investigation of isothermal grain growth of Mn2O3 nanocrystals at various temperatures between 200 and 500 °C for different annealing times and analyzing the grain growth data using two different models; and (iii) development of a widely applicable chemical reaction route to prepare single-crystal Mn3O4 nanocrystals including nanoparticles, nanorods and nanofractals. The Mn3O4 nanocrystals with tetragonal structure were prepared synchronously by a chemical liquid homogeneous precipitation method, which has been employed to synthesize these nanostructured materials using reactants of MnCl2·4H2O, H2O2, and NaOH under the environment of a suitable surfactant and alkaline solution. To sum up, it is expected that the fabrication methodologies developed and the knowledge of microstructural evolution gained in semiconductor thin films (including SnO2 thin films, Au/Ge bilayer films, and Pd-Ge alloy thin films) and metal oxides (including SnO, SnO2, Mn2O3 and Mn3O4 nanocrystals: nanoparticles, nanowires, nanorods, and nanofractals) will provide an important fundamental basis underpinning further interdisciplinary (physics, chemistry and materials science) research in this field leading to promising exciting opportunities for future technological applications involving these thin film materials.

Date of Award	15 Feb 2007
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Ki Leuk Joseph LAI (Supervisor)