No description available.

The invention of the lithium-ion battery has revolutionized the passenger transportation field in recent years, and it has emerged as one of the state-of-the-art solutions to address greenhouse gases emission and air pollution issues. Layered oxide lithium-ion battery cathode materials have become commercially successful in the past few decades due to their high energy density, high power density, long cycle life, and low cost. Yet, with the increasing demand for battery performance, it is crucial to understand the material fading mechanisms further to improve layered oxide materials' performance. A heterogeneous redox reaction is a dominant fading mechanism, which limits the utilization percentage of a battery materials' redox capability and leads to adverse effects such as detrimental interfacial reactions, lattice oxygen release, and chemomechanical breakdown. Crystallographic defects, such as dislocations and grain boundaries, are rich in battery materials. These crystallographic defects change the local lithium-ion diffusivity and have a dramatic effect on the redox reactions. To date, there is still a knowledge gap on how various crystallographic defects affect electrochemistry at the microscopic scale. Herein, we adopted synchrotron-based diffraction, imaging, and spectroscopic techniques to systematically study the correlation between crystallographic defects and redox chemistries in the nanodomain. Our studies shed light on design principles of next-generation battery materials. In Chapter 1, we first provide a comprehensive background introduction on the battery chemistry at various length scales. We then introduce the heterogeneous redox reactions in layered oxide cathode materials, including a discussion on the impacts of heterogeneous redox reactions. Finally, we present the different categories of crystallographic defects in layered oxide materials and how these crystallographic defects affect electrochemical performance. In Chapter 2, we use LiCoO2, a representative layered oxide cathode material, as the material platform to quantify the categories and densities of various crystallographic defects. We then focus on geometrically necessary dislocations as they represent a major class of crystallographic defects in LiCoO2. Combining synchrotron-based X-ray fluorescence mapping, micro-diffraction, and spectroscopic techniques, we reveal that geometrically necessary dislocations can facilitate the charging reactions in LiCoO2 grains. Our study illustrates that the heterogeneous redox chemistries can be potentially mitigated by precisely controlling the defects. In Chapter 3, we systematically investigated how grain boundaries affect redox reactions. We reveal that grain boundaries can guide redox reactions in LiNixMnyCo1-x-yO2 (NMC) materials. Specifically, NMC materials with radially aligned grains have a more uniform charge distribution, less stress mismatch, and better cycling performance. NMC materials with randomly orientated grains have a more heterogeneous redox reaction. These heterogeneous redox reactions are related to the lattice strain mismatch and worse cycling performance. Our study emphasizes the importance of tuning grain orientations to achieve improved performance. Chapter 4 systematically investigated how the grain boundaries and crystallographic orientations affect the thermal stability of layered oxide cathode materials. Combining diffraction, spectroscopic, and imaging techniques, we reveal that a cathode materials' microstructure plays a significant role in determining the lattice oxygen release behavior and, therefore, determines cathode materials' thermal stability. Our study provides a fundamental understanding of how the grain boundaries and crystallographic orientations can be tuned to develop better cathode materials for the next-generation Li-ion batteries. Chapter 5 summarizes the contributions of our work and provides our perspective on future research directions. / Doctor of Philosophy / Lithium-ion battery technology has revolutionized the portable electronic device and electric vehicle markets in recent years. Yet, the performance of current lithium-ion batteries still cannot satisfy customer demands. To further improve battery performance, we need a deeper understanding of why battery materials degrade over long-term cycling. One of the fading mechanisms in lithium-ion batteries is heterogeneous redox reactions, i.e., charge or discharge reactions do not proceed at the same pace at different locations in the electrode materials. Herein, we utilize layered oxide cathode materials as an example to systematically investigate how crystallographic defects in the cathode materials lead to heterogeneous redox reactions. Our study indicates that crystallographic defects, such as geometrically necessary dislocations, contribute positively to the charging reaction of the cathode materials. We also unveil that the grain crystallographic orientations of the primary particles affect the redox reactions directly. By aligning the single grains in the radial direction, the volumetric-change-induced stress can be effectively mitigated to ensure prolonged cycling performance. Our study also points out that the single grain orientations are related to the thermal stability of the battery materials. To summarize, our studies provide new insights into the heterogeneous redox reactions in battery materials and offer critical material design criteria to improve battery performance further.

abstract: The origins of carrier mobility (μe) were thoroughly investigated in hydrogenated indium oxide (IO:H) and zinc-tin oxide (ZTO) transparent conducting oxide (TCO) thin films. A carrier transport model was developed for IO:H which studied the effects of ionized impurity scattering, polar optical phonon scattering, and grain boundary scattering. Ionized impurity scattering dominated at temperatures below ~240 K. A reduction in scattering charge Z from +2 to +1 as atomic %H increased from ~3 atomic %H to ~5 atomic %H allowed μe to attain >100 cm^2/Vs at ~5 atomic %H. In highly hydrogenated IO:H, ne significantly decreased as temperature increased from 5 K to 140 K. To probe this unusual behavior, samples were illuminated, then ne, surface work function (WF), and spatially resolved microscopic current mapping were measured and tracked. Large increases in ne and corresponding decreases in WF were observed---these both exhibited slow reversions toward pre-illumination values over 6-12 days. A hydrogen-related defect was proposed as source of the photoexcitation, while a lattice defect diffusion mechanism causes the extended decay. Both arise from an under-coordination of the In. An enhancement of μe was observed with increasing amorphous fraction in IO:H. An increase in population of corner- and edge-sharing polyhedra consisting of metal cations and oxygen anions is thought to be the origin. This indicates some measure of medium-range order in the amorphous structure, and gives rise to a general principle dictating μe in TCOs---even amorphous TCOs. Testing this principle resulted in observing an enhancement of μe up to 35 cm^2/Vs in amorphous ZTO (a-ZTO), one of the highest reported a-ZTO μe values (at ne > 10^19 cm^-3) to date. These results highlight the role of local distortions and cation coordination in determining the microscopic origins of carrier generation and transport. In addition, the strong likelihood of under-coordination of one cation species leading to high carrier concentrations is proposed. This diverges from the historical indictment of oxygen vacancies controlling carrier population in crystalline oxides, which by definition cannot occur in amorphous systems, and provides a framework to discuss key structural descriptors in these disordered phase materials. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020

Hafnium oxide is widely used for resistive switching devices, and recently it has been discovered that ferroelectricity can be established in (un-)doped hafnium oxide as well. Previous studies showed that both switching mechanisms are influenced by oxygen vacancies. For resistive switching, typically amorphous oxide layers with an asymmetric electrode configuration are used to create a gradient of oxygen vacancies. On the other hand, ferroelectric switching is performed by having symmetric electrodes and requires crystalline structures. The coexistence of both effects has recently been demonstrated. In this work, a detailed analysis of the reversible interplay of both switching mechanisms within a single capacitor cell is investigated. First, ferroelectric switching cycles were applied in order to drive the sample into the fatigued stage characterized by increased concentration of oxygen vacancies in the oxide layer. Afterwards, a forming step that is typical for the resistive switching devices was utilized to achieve a soft breakdown. In the next step, twofold alternation between the high and low resistance state is applied to demonstrate the resistive switching behavior of the device. Having the sample in the high resistance state with a ruptured filament, ferroelectric switching behavior is again shown within the same stack. Interestingly, the same endurance as before was observed without a hard breakdown of the device. Therefore, an effective sequence of ferroelectric—resistive—ferroelectric switching is realized. Additionally, the dependence of the forming, set, and reset voltage on the ferroelectric cycling stage (pristine, woken-up and fatigued) is analyzed giving insight into the physical device operation.

Manifold research has been done to understand the detailed mechanisms behind the performance instabilities of ferroelectric capacitors based on hafnia. The wake-up together with the imprint might be the most controversially discussed phenomena so far. Among crystallographic phase change contributions and oxygen vacancy diffusion, electron trapping as the origin has been discussed recently. In this publication, we provide evidence that the imprint is indeed caused by electron trapping into deep states at oxygen vacancies. This impedes the ferroelectric switching and causes a shift of the hysteresis. Moreover, we show that the wake-up mechanism can be caused by a local imprint of the domains in the pristine state by the very same root cause. The various domain orientations together with an electron trapping can cause a constriction of the hysteresis and an internal bias field in the pristine state. Additionally, we show that this local imprint can even cause almost anti-ferroelectric like behavior in ferroelectric films.

In this report, the operation of a normally-off vertical gallium nitride (GaN) metal-oxide field effect transistor with a threshold voltage of 5 V is demonstrated. A crucial step during device fabrication is the formation of the highly n-doped source layer. The authors infer that the use of molecular beam epitaxy (MBE) is highly beneficial for suppressing diffusion of the magnesium (Mg) p-type dopants from the body layer grown by metal-organic vapor phase epitaxy into the source cap. Repassivation of the previously activated Mg acceptors by a hydrogen out-diffusion treatment is suppressed in the ultrahigh vacuum growth environment. Structural and electrical data indicate that the defect density of the GaN substrate is currently limiting device performance much more compared to other effects like varying surface morphology resulting from fluctuations in III/N stoichiometry during the MBE growth.

The thesis deals with the study and analysis of crystallographic defects on the surface of silicon wafers produced by Czochralski method. It focuses primarily on growth defects and oxygen precipitates, which play an important role in the development of appropriate nucleation centers for growth of stacking faults. The growth of stacking faults near the surface of silicon wafers is supported by their oxidation and selective etching. Such a highlighted stacking faults are known as the OISF (Oxidation Induced Stacking Fault). Spatial distribution of OISF on the wafer gives feedback to the process of pulling silicon single crystal and wafers surface quality. Moreover the work describes the device for automatic detection and analysis of OISF, which was developed for ON Semiconductor company in Rožnov Radhoštěm.

No description available.

III-valent dopants have shown to be most effective in stabilizing the ferroelectric, crystalline phase in atomic layer deposited, polycrystalline HfO₂ thin films. On the other hand, such dopants are commonly used for tetragonal and cubic phase stabilization in ceramic HfO₂. This difference in the impact has not been elucidated so far. The prospect is a suitable doping to produce ferroelectric HfO₂ ceramics with a technological impact. In this paper, we investigate the impact of Al, Y, and La doping, which have experimentally proven to stabilize the ferroelectric Pca21 phase in HfO₂, in a comprehensive first-principles study. Density functional theory calculations reveal the structure, formation energy, and total energy of various defects in HfO₂. Most relevant are substitutional electronically compensated defects without oxygen vacancy, substitutional mixed compensated defects paired with a vacancy, and ionically compensated defect complexes containing two substitutional dopants paired with a vacancy. The ferroelectric phase is strongly favored with La and Y in the substitutional defect. The mixed compensated defect favors the ferroelectric phase as well, but the strongly favored cubic phase limits the concentration range for ferroelectricity. We conclude that a reduction of oxygen vacancies should significantly enhance this range in Y doped HfO₂ thin films. With Al, the substitutional defect hardly favors the ferroelectric phase before the tetragonal phase becomes strongly favored with the increasing concentration. This could explain the observed field induced ferroelectricity in Al-doped HfO₂. Further Al defects are investigated, but do not favor the f-phase such that the current explanation remains incomplete for Al doping. According to the simulation, doping alone shows clear trends, but is insufficient to replace the monoclinic phase as the ground state. To explain this fact, some other mechanism is needed.