ASU Electronic Theses and Dissertations
This collection includes most of the ASU Theses and Dissertations from 2011 to present. ASU Theses and Dissertations are available in downloadable PDF format; however, a small percentage of items are under embargo. Information about the dissertations/theses includes degree information, committee members, an abstract, supporting data or media.
In addition to the electronic theses found in the ASU Digital Repository, ASU Theses and Dissertations can be found in the ASU Library Catalog.
Dissertations and Theses granted by Arizona State University are archived and made available through a joint effort of the ASU Graduate College and the ASU Libraries. For more information or questions about this collection contact or visit the Digital Repository ETD Library Guide or contact the ASU Graduate College at gradformat@asu.edu.
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- Creators: Yu, Hongbin
Towards high-efficiency thin-film solar cells: from theoretical analysis to experimental exploration
Thin-film CdTe solar cells have also attracted lots of attention due to the continuous improvements in their device performance. To address the issue of the lower efficiency record compared to detailed-balance limit, the single-crystalline Cd(Zn)Te/MgCdTe double heterostructures (DH) grown on InSb (100) substrates by molecular beam epitaxy (MBE) are carefully studied. The Cd0.9946Zn0.0054Te alloy lattice-matched to InSb has been demonstrated with a carrier lifetime of 0.34 µs observed in a 3 µm thick Cd0.9946Zn0.0054Te/MgCdTe DH sample. The substantial improvement of lifetime is due to the reduction in misfit dislocation density. The recombination lifetime and interface recombination velocity (IRV) of CdTe/MgxCd1-xTe DHs are investigated. The IRV is found to be dependent on both the MgCdTe barrier height and width due to the thermionic emission and tunneling processes. A record-long carrier lifetime of 2.7 µs and a record-low IRV of close to zero have been confirmed experimentally.
The MgCdTe/Si tandem solar cell is proposed to address the issue of high manufacturing costs and poor performance of thin-film solar cells. The MBE grown MgxCd1-xTe/MgyCd1-yTe DHs have demonstrated the required bandgap energy of 1.7 eV, a carrier lifetime of 11 ns, and an effective IRV of (1.869 ± 0.007) × 103 cm/s. The large IRV is attributed to thermionic-emission induced interface recombination. These understandings can be applied to fabricating the high-efficiency low-cost MgCdTe/Si tandem solar cell.
In this doctoral research, a voltage bias was applied on the device silicon layer of Silicon-on-Insulator (SOI) cylindrical single nanopore to analyze how the perpendicular gate electrical field affected the ionic current through the pore. The nanopore was fabricated using electron beam lithography (EBL) and reactive ion etching (RIE) which are standard CMOS processes and can be integrated into any electronic circuit with massive production. The long cylindrical pore shape provides a larger surface area inside the aperture compared to other nanopores whose surface charge is of vital importance to ion transport.
Ionic transport through the nanopore was characterized by measuring the ionic conductance of the nanopore in aqueous hydrochloric acid and potassium chloride solutions under field effect modulation. The nanopores were separately coated with negatively charged thermal silicon oxide and positively charged aluminum oxide using Atomic Layer Deposition. Both layers worked as electrical insulation layers preventing leakage current once the substrate bias was applied. Different surface charges also provided different counterion-coion configurations. The transverse conductance of the nanopore at low electrolyte concentrations (<10-4 M) changed with voltage bias when the Debye length was comparable to the dimensions of the nanopore.
Ionic transport through nanopores coated with polyelectrolyte (PE) brushes were also investigated in ionic solutions with various pH values using Electrochemical Impedance spectroscopy (EIS). The pH sensitive poly[2–(dimethylamino) ethyl methacrylate] (PDMAEMA) PE brushes were integrated on the inner walls as well as the surface of the thermal oxidized SOI cylindrical nanopore using surface-initiated atom transfer radical polymerization (SI-ATRP). An equivalent circuit model was developed to extract conductive and resistive values of the nanopore in ionic solutions. The ionic conductance of PE coated nanopore was effectively rectified by varying the pH and gate bias.
However, many sensing systems are relatively bulky, complicated, and expensive and main advantages of new sensors do not play an important role in practical applications. Many challenges arise due to intricacies for sensor packaging, especially operation in a solution environment. Additional problems emerge when interfacing sensors with external off-chip components. A large amount of research in the field of sensors has been focused on how to improve the system integration.
This work presents new methods for the design, fabrication, and integration of sensor systems. This thesis addresses these challenges, for example, interfacing microelectronic system to a liquid environment and developing a new technique for impedimetric measurement. This work also shows a new design for on-chip optical sensor without any other extra components or post-processing.
With the 1D geometry, nanowires are natural optical waveguides for light guiding and propagation. By introducing feedback mechanisms to nanowire waveguides, such as the cleaved end facets, the nanowires can work as ultra-small size lasers. Since the first demonstration of the room-temperature ultraviolet nanowire lasers in 2001, the nanowire lasers covering from ultraviolet to mid infrared wavelength ranges have been intensively studied. This dissertation focuses on the optical characterization and laser fabrication of two nanowire materials: erbium chloride silicate nanowires and composition-graded CdSSe semiconductor alloy nanowires.
Chapter 1 – 5 of this dissertation presents a comprehensive characterization of a newly developed erbium compound material, erbium chloride silicate (ECS) in a nanowire form. Extensive experiments demonstrated the high crystal quality and excellent optical properties of ECS nanowires. Optical gain higher than 30 dB/cm at 1.53 μm wavelength is demonstrated on single ECS nanowires, which is higher than the gain of any reported erbium materials. An ultra-high Q photonic crystal micro-cavity is designed on a single ECS nanowire towards the ultra-compact lasers at communication wavelengths. Such ECS nanowire lasers show the potential applications of on-chip photonics integration.
Chapter 6 – 7 presents the design and demonstration of dynamical color-controllable lasers on a single CdSSe alloy nanowire. Through the defect-free VLS growth, engineering of the alloy composition in a single nanowire is achieved. The alloy composition of CdSxSe1-x uniformly varies along the nanowire axis from x=1 to x=0, giving the opportunity of multi-color lasing in a monolithic structure. By looping the wide-bandgap end of the alloy nanowire through nanoscale manipulation, the simultaneous two-color lasing at green and red colors are demonstrated. The 107 nm wavelength separation of the two lasing colors is much larger than the gain bandwidth of typical semiconductors. Since the two-color lasing shares the output port, the color of the total lasing output can be controlled dynamically between the two fundamental colors by changing the relative output power of two lasing colors. Such multi-color lasing and continuous color tuning in a wide spectral range would eventually enable color-by-design lasers to be used for lighting, display and many other applications.
This research project focuses on analyzing different growth techniques of thin films of Cobalt, Zirconium Tantalum Boron (CZTB) and the quality of the films resulted. The measurements include magnetic moment measurements using a Vibrating Sample Magnetometer, electrical measurements using 4 point resistivity methods and structural characterization using Scanning Electron Microscopy. Subtle changes in the growth mechanism result in different properties of these films and they are most suited for certain applications.
The growth methods presented in this research are oblique angled sputtering with localized magnetic field and oblique sputtering without presence of magnetic field. The uniaxial anisotropy can be controlled by changing the angle during sputtering. The resulting film of CZTB is tested for magnetic anisotropy and soft magnetism at room temperature by using Lakeshore 7500 Vibrating Sample Magnetometer. The results are presented, analyzed and explained using characterization techniques. Future work includes magnetic field presence during deposition, magnetic devices of this film with giga hertz range operating frequencies.
materials for lithium-based batteries: silicon (Si) and metal lithium (Li). It will focus on
studying the mechanical behaviors of the two materials during charge and discharge and
understanding how these mechanical behaviors may affect their electrochemical
performance.
In the first part, amorphous Si anode will be studied. Despite many existing studies
on silicon (Si) anodes for lithium ion batteries (LIBs), many essential questions still exist
on compound formation, composition, and properties. Here it is shown that some
previously accepted findings do not truthfully reflect the actual lithiation mechanisms in
realistic battery configurations. Furthermore the correlation between structure and
mechanical properties in these materials has not been properly established. Here, a rigorous
and thorough study is performed to comprehensively understand the electrochemical
reaction mechanisms of amorphous-Si (a-Si) in a realistic LIB configuration. In-depth
microstructural characterization was performed and correlations were established between
Li-Si composition, volumetric expansion, and modulus/hardness. It is found that the
lithiation process of a-Si in a real battery setup is a single-phase reaction rather than the
accepted two-phase reaction obtained from in-situ TEM experiments. The findings in this
dissertation establish a reference to quantitatively explain many key metrics for lithiated a
Si as anodes in real LIBs, and can be used to rationally design a-Si based high-performance
LIBs guided by high-fidelity modeling and simulations.
In the second part, Li metal anode will be investigated. Problems related to dendrite
growth on lithium metal anodes such as capacity loss and short circuit present major
barriers to the next-generation high-energy-density batteries. The development of
successful mitigation strategies is impeded by the incomplete understanding of the Li
dendrite growth mechanisms. Here the enabling role of plating residual stress in dendrite
initiation through novel experiments of Li electrodeposition on soft substrates is confirmed,
and the observations is explained with a stress-driven dendrite growth model. Dendrite
growth is mitigated on such soft substrates through surface-wrinkling-induced stress
relaxation in deposited Li film. It is demonstrated that this new dendrite mitigation
mechanism can be utilized synergistically with other existing approaches in the form of
three-dimensional (3D) soft scaffolds for Li plating, which achieves superior coulombic
efficiency over conventional hard copper current collectors under large current density.
On the other hand, high pressure, temperatures that can reach boiling, and the pH of different hot springs ranging from <2 to >9 make hot spring ecosystem a unique environment that is difficult to study. WSN allows many scientific studies in harsh environments that are not feasible with traditional instrumentation. However, wireless pH sensing for long time in situ data collection is still challenging for two reasons. First, the existing commercial-off-the-shelf pH meters are frequent calibration dependent; second, biofouling causes significant measurement error and drift. In this work, 2-dimentional graphene pH sensors were studied and calibration free graphene pH sensor prototypes were fabricated. Test result shows the resistance of the fabricated device changes linearly with the pH values (in the range of 3-11) in the surrounding liquid environment. Field tests show graphene layer greatly prevented the microbial fouling. Therefore, graphene pH sensors are promising candidates that can be effectively used for wireless pH sensing in exploration of hot spring ecosystems.
The main aspect of this report studies the viability of Li-ion battery application in Battery Energy Storage System (BESS) in Modular multilevel converter (MMC). MMC-based BESS is a promising solution for grid-level battery energy storage to accelerate utilization and integration of intermittent renewable energy resources, i.e., solar and wind energy. When the battery units are directly integrated in submodules (SMs) without dc-dc interfaced converters, this configuration provides highest system efficiency and lowest cost. However, the lifetime of battery will be affected by the low-frequency components contained in arm currents, which has not been thoroughly investigated. This paper investigates impact of various low-frequency arm-current ripples on lifetime of Li-ion battery cells and evaluate performance of battery charging and discharging in an MMC-BESS without dc-dc interfaced converters.
Autonomous smart windows may be integrated with a stack of active components, such as electrochromic devices, to modulate the opacity/transparency by an applied voltage. Here, we describe the processing and performance of two classes of visibly-transparent photovoltaic materials, namely inorganic (ZnO thin film) and fully organic (PCDTBT:PC70BM), for integration with electrochromic stacks.
Sputtered ZnO (2% Mn) films on ITO, with transparency in the visible range, were used to fabricate metal-semiconductor (MS), metal-insulator-semiconductor (MIS), and p-i-n heterojunction devices, and their photovoltaic conversion under ultraviolet (UV) illumination was evaluated with and without oxygen plasma-treated surface electrodes (Au, Ag, Al, and Ti/Ag). The MS Schottky parameters were fitted against the generalized Bardeen model to obtain the density of interface states (Dit ≈ 8.0×1011 eV−1cm−2) and neutral level (Eo ≈ -5.2 eV). These devices exhibited photoconductive behavior at λ = 365 nm, and low-noise Ag-ZnO detectors exhibited responsivity (R) and photoconductive gain (G) of 1.93×10−4 A/W and 6.57×10−4, respectively. Confirmed via matched-pair analysis, post-metallization, oxygen plasma treatment of Ag and Ti/Ag electrodes resulted in increased Schottky barrier heights, which maximized with a 2 nm SiO2 electron blocking layer (EBL), coupled with the suppression of recombination at the metal/semiconductor interface and blocking of majority carriers. For interdigitated devices under monochromatic UV-C illumination, the open-circuit voltage (Voc) was 1.2 V and short circuit current density (Jsc), due to minority carrier tunneling, was 0.68 mA/cm2.
A fully organic bulk heterojunction photovoltaic device, composed of poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyli2’,1’,3’-benzothiadiazole)]:phenyl-C71-butyric-acidmethyl (PCDTBT:PC70BM), with corresponding electron and hole transport layers, i.e., LiF with Al contact and conducting
on-conducting (nc) PEDOT:PSS (with ITO/PET or Ag nanowire/PDMS contacts; the illuminating side), respectively, was developed. The PCDTBT/PC70BM/PEDOT:PSS(nc)/ITO/PET stack exhibited the highest performance: power conversion efficiency (PCE) ≈ 3%, Voc = 0.9V, and Jsc ≈ 10-15 mA/cm2. These stacks exhibited high visible range transparency, and provided the requisite power for a switchable electrochromic stack having an inkjet-printed, optically-active layer of tungsten trioxide (WO3), peroxo-tungstic acid dihydrate, and titania (TiO2) nano-particle-based blend. The electrochromic stacks (i.e., PET/ITO/LiClO4/WO3 on ITO/PET and Ag nanowire/PDMS substrates) exhibited optical switching under external bias from the PV stack (or an electrical outlet), with 7 s coloration time, 8 s bleaching time, and 0.36-0.75 optical modulation at λ = 525 nm. The devices were paired using an Internet of Things controller that enabled wireless switching.