Metalorganic Chemical Vapor Deposition Of Titanium Oxide Thin Films At Atmospheric Pressure With Analysis Via X Ray Photoelectron Spectroscopy

Thin titanium dioxide films were produced by metalorganic chemical vapor deposition on sapphire(0001) in an ultrahigh vacuum (UHV) chamber. A method was developed for producing controlled submonolayer depositions from titanium isopropoxide precursor. Film thickness ranged from 0.1 to 2.7 nm. In situ X-ray photoelectron spectroscopy (XPS) was used to determine film stoichiometry with increasing thickness. The effect of isothermal annealing on desorption was evaluated. Photoelectron peak shapes and positions from the initial monolayers were analyzed for evidence of interface reaction. Deposition from titanium isopropoxide is divided into two regimes: depositions below and above the pyrolysis temperature. This temperature was determined to be 300 deg C. Controlled submonolayers of titanium oxide were produced by cycles of dosing with titanium isopropoxide vapor below and annealing above 300 deg C. Precursor adsorption below the pyrolysis temperature was observed to saturate after 15 minutes of dosing. The quantity absorbed was shown to have an upper limit of one monolayer. The stoichiometry of thin films grown by the cycling method were determined to be TiO2. Titanium dioxide film stoichiometry was unaffected by isothermal annealing at 700 deg C. Annealing produced a decrease in film thickness. This was explained as due to desorption. Desorption ceased at approximately 2.5 to 3 monolayers, suggesting bonding of the initial monolayers of film to sapphire is stronger than to itself. Evidence of sapphire reduction at the interface by the depositions was not observed. The XPS O is peak shifted with increased film thickness. The shifts were consistent with oxygen in sapphire and titanium dioxide having different O is photoelectron peak positions. Simulations showed the total shifts for thin films ranging in thickness of 0.1 to 2.7 nm to be -0.99 to -1.23 eV. Thick films were produced for comparison.

The motivation for this work was to study environmentally friendly thin films that have a potential to act as a corrosion protection scheme for Aluminum alloys. To do this, a vacuum system capable of producing physical vapor deposition and plasma enhanced chemical vapor deposition was designed, built and mated to an XPS system. Pure Aluminum was deposited onto a sample substrate of Aluminum 6061 alloy at a rate of one Angstrom per second in a base pressure of 5x10^-7 Torr. This was followed by deposition of an amorphous Silicon Carbide film through PECVD of Trimethylsilane. The sample was then transferred under vacuum into the XPS chamber for a depth profiled spectroscopic analysis. Data collected during multiple cycles of Argon ion sputtering show C-C, C-Si, Si-C, Si-Al, Al-O, Al-Al and Al-Si interactions. The interactions between Aluminum and Silicon give promise that a strong bond exists at the Aluminum interface.

Coatings are applicable in many industries as transparent conductive thin films, thin film solar cells and batteries, nano-laminates, environmental barriers, biocompatible thin films, molecular electronics, and solid-state lubricants. 56 These various coatings exhibit customizable microstructure, surface morphology, tribological, and electronic properties based on specific deposition parameters.57 Deposition technology can also have a significant role in the resulting coating.56 For example, chemical vapor deposition (CVD) at high temperatures involves the decomposition or reduction of precursors on the surface of the substrate at thermodynamic equilibrium. CVD coatings need to be extensively optimized in order to reduce interfacial reactions between the coating and the substrate as well as the coating and the gaseous byproducts.58 In comparison, sputtered coatings are formed by the quenching of high energy sputtered species on the surface of the substrate, so after the nucleation and growth stages there is little effect from the deposition process on the already established coating.59 Surface oxides formed on powder feedstocks used for cold spray deposition can play an important role in the bonding of the particles and in the development of defects in the deposit. A combination of scanning transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) was used to investigate the oxides formed on gas-atomized Al 6061 alloy feedstock powders. The powders were studied in the as-atomized condition and after two different thermal exposures that correspond to typical feedstock pre-treatment conditions. The surface features and internal microstructures are consistent with those reported previously for these powders. The as-atomized powders have 5.2 nm thick amorphous oxide layers, with an outer Mg-rich sublayer and an inner Mg-lean sub-layer. Powders heat-treated at 230 ̊C in air exhibit slightly thicker oxide layers with a crystalline MgAl2O4 spinel outer sub-layer and an amorphous aluminum oxide inner sub-layer. Powders homogenized under Ar at 400 and 530 ̊C have significantly thicker (8.9 nm) oxide layers with evidence for a defect inverse spinel Al (Mg, Al)2O4 inner sub-layer between the MgAl2O4 spinel outer sub-layer and the alloy. Differences between these observations and those reported previously for oxidation of bulk alloys are explained on the basis of Mg surface segregation during the gas atomization process. In depth XPS analysis was necessary to distinguish between the different chemical states on the surface of these powder feedstocks. The high-resolution spectra for five different standards were used to compare the novel surface oxide formation to known XPS spectra. Confident conclusions were reached on the formation process occurring during the heat treatment of these surface oxides and the different deconvoluted chemical states observed in the high-resolution O 1s and Al 2p regions. Inkjet printing is a one-step patterning material deposition technique. Unlike other thin film deposition techniques such as chemical vapor deposition or lithography, patterning is necessary to fabricate complex shapes or circuitry60. In this work, a suspension system is formulated and optimized for the inkjet printing of sodium tungsten bronze (STB). This is a highly conductive material with excellent chemical and heat resistance. A solid-state synthesis of sodium tungsten bronze powder was accomplished using a 3:2:2 ration of Na2WO4 (s), WO3 (s) and W (s), respectively in a high temperature furnace. The powder was purified using deionized water and centrifugation. For ink jet printing, the rheology of the fluid needs to be quite specific for high resolution printing. A solvent exchange was performed to remove the water from the STB without drying the powder out and contaminating the material. Using a rotational evaporator, the water in the system (which promoted aggregation) was exchanged with ethylene glycol. A variety of different solvents and surfactants were utilized to increase the suspension time and optimize the rheology of the ink. The final printed product was characterized using X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) to ensure an unchanging chemical composition after thin film deposition. Carbon-carbon composites are critical in many high temperature aerospace applications. These composites offer low density, low coefficient of thermal expansion, and high thermal resistance among other advantageous properties. These properties as well as other physical properties are retained at temperatures exceeding 2000°C.61 The primary drawback of these composites is the inefficiency of their fabrication process. It can take upward of 600 hours with machining steps to fully densify these composites. This research is focused around using vacuum physics and incorporating gas recirculation into the isobaric isothermal reactor design to improve the efficiency of this chemical vapor infiltration (CVI) process. A design of experiments was formulated and test to investigate how gas recirculation will affect the carbon infiltration process. Once the pressure, temperature and precursor gas flow rates were optimized the incorporation of gas recirculation improved the efficiency by over 10%. Scanning electron microscopy (SEM) and Raman spectrometry were utilized to study the carbon microstructure and ensure rough laminar carbon was the deposited product.

Epitaxial oxide films like strontium titanate (SrTiO3) grown on silicon have a wide range of potential applications, including high k-dielectric devices, ferroelectrics, optoelectronics, and buffer layers for the heteroepitaxy of III-V semiconductor as well other pervoskites and high-Tc superconductors. The crystalline structure of SrTiO3 consists of alternating sublayers of SrO and TiO2. The epitaxy of SrTiO3 on Si(100) must be initiated with the nucleation of the SrO sublayer first. This thesis presents the methodology used for growing SrO on Si(100) surfaces via metal organic chemical vapor deposition (MOCVD) and atomic layer deposition (ALD). Sr(2,2,6,6-tetramethyl-3,5-heptanedionate) 2 [Sr(thd)2] is the beta-diketonate precursor used to conduct these film growth studies, but the use of this class of metal organic sources comes with several challenges. First, their thermal properties change with atmospheric exposure. Second, successful control of vapor delivery is challenging because beta-diketonates have low vapor pressures and their decomposition temperature is close to their vaporization temperature. Additionally, film growth results are difficult to reproduce because these compounds degrade with time. To overcome these challenges, we developed a Sr(thd)2 delivery scheme using a novel source vaporizer that successfully controls the vaporization and vapor transport to the growth surface under steady vapor pressure while preventing the decomposition of the solid source. This vaporization scheme has been able to grow SrO films on Si(100) with high uniformity and low carbon contamination, as shown with ex-situ Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectroscopy (TOF-SIMS). The MOCVD experiments provided enough evidence to encourage ALD investigations which incorporated the integration of the controlled vaporization with a ultra high vacuum (UHV) reaction chamber that provided the ability to conduct growth experiments on functionalized Si(100) surfaces. The ability to tune the chemistry on the Si(100)-2x1 surface can aid in guiding surface reactions of the metal organic precursor with the growth surface. Our goal has been to hydroxyl terminate the Si(100)-2x1 surface in order to nucleate SrO monolayers. Following the desorption of a protective chemical oxide layer, dissociative chemisorption of H2O is carried out in UHV to hydroxyl terminated Si(100)-2x1. Metal oxide growth can be correlated to the concentration of hydroxyl groups on the silicon surface due to the facilitation of ligand exchange from the surface. Furthermore, hydroxyl-terminated surfaces initiate two-dimensional nucleation of the metal oxide while avoiding incubation periods common to the ALD of metal oxide. In-situ AES and low energy electron diffraction LEED were used to investigate the crystalline quality of the nucleated monolayers and the epitaxial orientation of SrO films on Si(100)-2x1 surfaces. The results of the ALD experiments were, unfortunately, inconsistent. Nonetheless, the focus of this thesis is to show the methodology for developing growth protocols that can be used in ALD reactions on functionalized Si(100)-2x1 surfaces for the epitaxy of metal oxides.