Thermal And Photochemical Vapor Deposition Of Aluminum Nitride And Gallium Nitride

Author : Ian Campbell
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (134 download)

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Download or read book Experimental and Computational Investigations of Platinum and Gallium Nitride Vapor Deposition Processes written by Ian Campbell and published by . This book was released on 2022 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Chemical vapor deposition (CVD) and atomic layer deposition (ALD) are widely used and still rapidly growing material synthesis technologies. CVD and ALD are used to prepare thin films and nanoparticles from a chemically diverse library of precursors and, under the right conditions, have excellent process control that enables atomic scale precision. This dissertation primarily focuses on the deposition chemistry associated with the metal organic platinum precursor trimethyl (methylcyclopentadienyl) platinum (MeCpPtMe3) used in ALD and CVD of Pt, and with a special focus on substrates functionalized via ALD pretreatments and doping. The first focus of this dissertation was a platinum deposition process that utilized repeated doses of MeCpPtMe3 to deposit platinum nuclei on SiO2, monolayer graphene, and in mesoporous nitrogen-doped carbon powder (MPNC). The main purpose of this study was to enhance the nucleation density of platinum, further enabling the deposition of ultrathin films and catalytic materials, and to synthesize platinum nuclei with enhanced resistance to agglomeration under thermal stress. The effects of AlN and Al2O3 ALD-based pretreatment processes on the nucleation density and thermal stability of Pt nuclei deposited on planar SiO2 and graphene substrates were assessed. AlN was chosen as a promising pretreatment layer due to its relatively high surface energy as compared to Al2O3, which has been used previously to enhance the density of atomic layer deposited platinum nuclei. It was found that both Al2O3 and AlN pretreatments enhanced Pt nucleation density on SiO2 and graphene dramatically compared to bare SiO2 at 250 °C. The nucleation density for depositions performed at 300 °C was not so greatly enhanced by either pretreatment. However, four-point probe measurements indicated that depositing at 300 °C on pretreated SiO2 substrates, especially those pretreated with AlN, resulted in conductive films with significantly lower sheet resistance than films on untreated SiO2. After annealing these samples at 400 °C for one hour, it was found that Pt nuclei on pretreated samples generally resisted agglomeration more so than on untreated samples. Interestingly, a fully coalesced Pt film was deposited on AlN pretreated graphene at 300 °C. Conductive films with thicknesses in the range of 10-17 nm were deposited using just MeCpPtMe3. However, a combination of Al2O3 pretreatment, extended MeCpPtMe3 exposure, and MeCpPtMe3/H2 ALD was used to prepare films with the lowest thickness and resistivity. Another substrate functionalization strategy employed to influence the properties of deposited nuclei involved doping. Specifically, N doping in graphene-based materials, like MPNC, is proven to enhance catalytic activity and limit the agglomeration of nuclei under working conditions. The MPNC used in this dissertation was prepared elsewhere by carbonizing a composite of polyaniline and SiO2 nanoparticles then etching away the SiO2 to make a disordered, inverse structure composed largely of nitrogen-doped sp2 hybridized carbon. MPNC carbonized at 1000 °C (MPNC-1000) was observed via X-ray photoelectron spectroscopy to contain higher amounts of O and N dopants than MPNC carbonized at 1500 °C (MPNC-1500). It was found that exposure of both MPNC types to static pulses of MeCpPtMe3 resulted in the deposition of Pt nuclei up to 10 nm in diameter. Depositions performed at 250 °C resulted in nearly the same Pt loading (at. %) for both MPNC types, but increasing the deposition temperature to 300 °C resulted in a large increase in Pt loading for MPNC-1000 only, indicating a thermally driven change in reactivity for MPNC-1000 that is attributable to higher levels of O and N. The purpose of the second study was to use density functional theory to assess the effects of N doping on the reactivity of oxidized monovacancies in graphene towards MeCpPtMe3 adsorption and dissociation, and to identify a possible reason for the temperature-dependent reactivity of MPNC-1000. The substrates considered in this investigation consisted of monovacancies in monolayer graphene oxidized by two and three oxygen atoms. Each of these substrates was doped with a single N atom at various locations around the monovacancy. The chosen substrates are believed to be representative of defects found in N-doped, graphene-based substrates like MPNC. Oxidation of monovacancies in graphene with and without N dopants at various locations relative to the vacancy is thermodynamically favorable. N doping at and around monovacancies increased the length of critical oxygen-substrate bonds, indicating increased reactivity of the involved oxygen atoms, and resulted in a more robust interaction between adsorbed MeCpPtMe3 and oxidized defects. Nudged elastic band calculations were used to characterize the effects of N doping on methyl transfer from an adsorbed MeCpPtMe3 molecule to oxygen atoms bound to vacancies in doped and undoped substrates. All methyl transfer reactions were found to have positive activation energies and enthalpies of reaction. In each case, N dopants reduced the enthalpy and activation energy and made the reaction less reversible, with pyridinic N yielding the most pronounced changes. After losing one methyl group during the methyl transfer reaction, MeCpPtMe2 was generally found to bind much more strongly to the substrate than MeCpPtMe3, especially in the case of N-doped substrates. Thus, N doping at or near oxidized monovacancies in graphene significantly enhances the likelihood of precursor dissociation and adsorption. The deposition of GaN via plasma enhanced ALD (PEALD) and high pressure confined CVD (HPcCVD) was the final focus of this dissertation. A PEALD process was used to investigate the synthesis of amorphous GaN (a-GaN) and the subsequent crystallization of a-GaN by thermal annealing while in contact with a highly oriented GaN crystal template. By depositing a-GaN on amorphous Al2O3, it was thought that highly oriented crystalline GaN might be prepared on an amorphous substrate, thus creating a process that would enable crystalline GaN films to be deposited on any substrate capable of withstanding the process conditions regardless of epitaxial mismatch. Whether deposited GaN films were amorphous or crystalline depended largely on oxygen incorporation and its complex relationship with the background level of oxygen in the reactor and plasma chemistry, duration, and power. Shorter plasma duration, higher hydrogen concentration in the plasma, and lower plasma power were found to increase oxygen content in the deposited films. An oxygen concentration of ~17 at. % was required for the deposition of a-GaN. Lastly, HPcCVD was chosen as a potentially new method of preparing high surface area GaN structures for photocatalytic and sensing applications. A homemade reactor was used to contain mixtures of nitrogen gas, trimethyl gallium, and ammonia pressurized up to ~5000 psi. By venting this mixture through hollow core silica optical fiber heated to 650 °C, deposition of centimeter-scale micron-thick gallium nitride films could be performed on the interior surface of the fiber. The findings herein contribute to a deeper understanding of vapor deposition processes at the nanoscale and highlight the role of computational materials science in providing insight into precursor-substrate interactions at the early stages of deposition.