A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item An experimental and graph theoretic study of atomic layer deposition processes for spacecraft applications(2019) Salami, Hossein; Adomaitis, Raymond A; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Accurate understanding of the atomic layer deposition (ALD) process kinetics is necessary for developing new ALD chemistries to produce novel nanomaterials, and also optimization of typical ALD processes used in industrial applications. Proposing a potential reaction sequence alongside with accurate kinetic data is among the very first steps in studying the ALD process kinetics and forms the backbone of further engineering analysis. A valid and proper ALD reaction net work (RN) must be able to reflect the self-limiting and cycle to cycle reproducibility behavior experimentally observed for practical ALD processes. Otherwise, the mathematical model built based on it fails to precisely capture and reproduce ALD behavior no matter how accurate the available kinetic data are. In this work, a RN analysis method based on species-reaction graphs and the principles of convex analysis is developed to study the mathematical structure and dynamical behavior of thin-film deposition RN models. The key factor in ALD RN analysis is the presence of consistent surface-originated invariant states for each ALD half-cycle. Therefore, the primary focus of the proposed approach is on identifying and formulating physically-relevant RN invariant states, and to study the chemical significance of these conserved modes for ALD reaction mechanisms. The proposed method provides a well-defined framework, applicable to all ALD systems, to examine the above criteria of a proper ALD RN without requiring any information on the reaction rates. This method fills a gap in the procedure of ALD process modeling before the time-consuming step of calculating individual reaction rates which is usually done through ALD experiments in reactors equipped with in-situ measurement instruments or computationally expensive computational chemistry-based calculations such as density functional theory. The presented approach is also extended to study the variant states of a RN. The generalized method provides information on different variant states dynamically depending on each individual reaction in the network which facilitates the study and ultimately the formulation of different reaction rates in the system. In the second part of this dissertation, an experimental study of ALD of indium oxide and indium tin oxide films using the trimethylindium, tetrakis (dimethylamino) tin(IV), and ozone precursor system is conducted to first, investigate the potential application of this ALD process for producing high-quality transparent conducting layers; and second, to understand the relationship between the thickness of the deposited films and their electrical and optical properties. The optimized recipe was then used to process commercial Z93 heat radiator pigments used in manufacturing spacecraft thermal radiator panels to enhance their electrical conductivity to avoid the differential charging that may occur due to the interaction with charged particles in Van Allen radiation belts. To this aim a specialized ALD reactor was designed and constructed capable of processing standard flat substrates as well as coating micron-sized particles. The results confirm that the proposed process can be used to coat the heat radiator pigment particles and that the indium oxide film can nucleate and grow on their surface. This provides an example from a variety of potential space-related applications that can benefit from the ALD process.Item PHOTONIC ENGINEERING OF ABSORPTION AND EMISSION IN PHOTOVOLTAICS(2016) Xu, Yunlu; Munday, Jeremy N; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)As modern society advances, the demand for clean and renewable energy resources becomes more and more important. The sun is by far the most abundant source of renewable energy and is indirectly responsible for many other energy resources on earth (e.g. sunlight enables photosynthesis, biofuels, wind, and even carbon-based fuels). A solar cell directly converts the energy of solar illumination into electricity through the photovoltaic effect and is expected to play a crucial role in the future total power generation globally. Our work has focused on photonic approaches to improving the conversion efficiency of solar cells. Toward this goal, we present results describing the use of quantum dot emission to redirect light within a solar cell, as well as the modification of absorption and emission of light from a solar cell using nanostructures and thin films to increase the efficiency to approach (or possibly surpass) the currently understood efficiency limits for traditional devices. The Shockley-Queisser (SQ) limit describes the maximum solar power conversion efficiency achievable for a p-n junction composed of a particular material and is the standard by which new photovoltaic technologies are compared. This limit is based on the principle of detailed balance, which equates the photon flux into a device to the particle flux (photons or electrons) out of that device. Based on this theory, we describe how the efficiency of a photovoltaic cell is altered in the presence of new anti-reflection coatings, nanotexturing (e.g. plasmonic nanoparticle, nanowire), and more advanced photonic structures (e.g. photonic crystals) that are capable of modifying the absorption and emission of photons. Nanostructured solar cells represent a novel class of photovoltaic devices. By careful selection of materials, as well as particle shapes and positions, the device performance can be improved by increasing the optical path length for scattered light, improving the modal distribution of the light within the absorber, and increasing light concentration (or angle restriction). For example, nanowires can yield microscale concentration effects to improve device performance; however, it has been unclear whether or not they can exceed the Shockley-Queisser limit. We show that single-junction nanostructured solar cells have a theoretical maximum efficiency of ∼ 42% under AM 1.5 solar illumination. While this exceeds the efficiency of a non-concentrating planar device, it does not exceed the Shockley-Queisser limit for a planar device with optical concentration. For practical devices, we include the effect of diffuse illumination and find that with the modest optical concentration available from nanostructures (× 1,000), an efficiency of 35.5% is achievable even with 25% diffusive solar radiation. Finally, we discuss how photon emission modification offers an approach for low bandgap materials to achieve higher efficiencies. By incorporating specifically designed photonic structures that restrict the absorption and emission of above bandgap photons, the bandgap of materials can be effectively tuned. Similarly, restriction of the emission angle leads to increased optical concentration. For realistic devices, we consider how both of these effects are affected by non-ideal materials and photonic structures. We find that the photonic crystal bandgap required to achieve maximum efficiency depends critically on the reflectivity of the photonic crystal. We experimentally demonstrated that the semiconductor bandgap of a material need not be an intrinsic property of that material but can be changed through photonic structuring of the surrounding layers. GaAs has a natural bandgap of 1.43 eV; however, we show that optical reflectors can be used to induce photon-recycling effects, which result in a bandgap shift of 0.13 eV. When a p-n junction is created within the GaAs, we find that its electrical properties are also shifted resulting in a 1.71 mV improvement in the open-circuit voltage of the device under 0.6 suns equivalent illumination. These results show that both the optical and electrical properties of a semiconductor can be modified purely by photonic manipulation, which enables a fundamentally new method for designing semiconductor structures and devices. We anticipate that our result will enable a range of optoelectronic devices.Item High Precision Plasma Etch for Pattern Transfer: Towards Fluorocarbon Based Atomic Layer Etching(2016) Metzler, Dominik; Oehrlein, Gottlieb S; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A basic requirement of a plasma etching process is fidelity of the patterned organic materials. In photolithography, a He plasma pretreatment (PPT) based on high ultraviolet and vacuum ultraviolet (UV/VUV) exposure was shown to be successful for roughness reduction of 193nm photoresist (PR). Typical multilayer masks consist of many other organic masking materials in addition to 193nm PR. These materials vary significantly in UV/VUV sensitivity and show, therefore, a different response to the He PPT. A delamination of the nanometer-thin, ion-induced dense amorphous carbon (DAC) layer was observed. Extensive He PPT exposure produces volatile species through UV/VUV induced scissioning. These species are trapped underneath the DAC layer in a subsequent plasma etch (PE), causing a loss of adhesion. Next to stabilizing organic materials, the major goals of this work included to establish and evaluate a cyclic fluorocarbon (FC) based approach for atomic layer etching (ALE) of SiO2 and Si; to characterize the mechanisms involved; and to evaluate the impact of processing parameters. Periodic, short precursor injections allow precise deposition of thin FC films. These films limit the amount of available chemical etchant during subsequent low energy, plasma-based Ar+ ion bombardment, resulting in strongly time-dependent etch rates. In situ ellipsometry showcased the self-limited etching. X-ray photoelectron spectroscopy (XPS) confirms FC film deposition and mixing with the substrate. The cyclic ALE approach is also able to precisely etch Si substrates. A reduced time-dependent etching is seen for Si, likely based on a lower physical sputtering energy threshold. A fluorinated, oxidized surface layer is present during ALE of Si and greatly influences the etch behavior. A reaction of the precursor with the fluorinated substrate upon precursor injection was observed and characterized. The cyclic ALE approach is transferred to a manufacturing scale reactor at IBM Research. Ensuring the transferability to industrial device patterning is crucial for the application of ALE. In addition to device patterning, the cyclic ALE process is employed for oxide removal from Si and SiGe surfaces with the goal of minimal substrate damage and surface residues. The ALE process developed for SiO2 and Si etching did not remove native oxide at the level required. Optimizing the process enabled strong O removal from the surface. Subsequent 90% H2/Ar plasma allow for removal of C and F residues.Item Development of a Spatially Controllable Chemical Vapor Deposition System(2005-01-28) Choo, Jae-Ouk; Adomaitis, Raymond A; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Most conventional chemical vapor deposition (CVD) systems do not have the spatial actuation and sensing capabilities necessary to control deposition uniformity, or to intentionally induce nonuniform deposition patterns for single-wafer combinatorial CVD experiments. In an effort to address these limitations, a novel CVD reactor system has been developed that can explicitly control the spatial profile of gas-phase chemical composition across the wafer surface. In this thesis, the simulation-based design of a prototype reactor system and the results of preliminary experiments performed to evaluate the performance of the prototype in depositing tungsten films are presented. Initial experimental results demonstrate that it is possible to produce spatially patterned wafers using a CVD process by controlling gas phase reactant composition. Based on the evaluation of the first prototype, a second prototype system was designed and constructed, enabling for greater control and programmability. The capability of this prototype for performing combinatorial CVD experiments is discussed. Finally, improvement of intra-segment uniformity and film thickness together with micro structure or composition is discussed.