We report on the experimental determination of the spontaneous polarization of wurtzite- (Mg,Zn)O by examination of the recombination dynamics of polar ZnO/(Mg,Zn)O quantum wells (QWs). The thickness-dependent decay time of the unscreened single-exciton states inside the QWs was modeled by a self-consistent solution of Schrödinger- and Poisson-equation to deduce the total polarization across the QW for different Mg-contents inside the barriers. By the separation of the piezoelectric components of the polarization, a linear increase in spontaneous polarization with increasing Mg-content x of P/x = (0.151 ± 0.015) C/m2 was determined for Mgx Zn1−x O.

We have investigated the impact of strain on the incorporation and the properties of extended and point defects in (Mg,Zn)O thin films by means of photoluminescence, X-ray diffraction, deep-level transient spectroscopy (DLTS), and deep-level optical spectroscopy. The recombination line Y2, previously detected in ZnO thin films grown on an Al-doped ZnO buffer layer and attributed to tensile strain, was exclusively found in (Mg,Zn)O samples being under tensile strain and is absent in relaxed or compressively strained thin films. Furthermore a structural defect E3′ can be detected via DLTS measurements and is only incorporated in tensile strained samples. Finally it is shown that the omnipresent deep-level E3 in ZnO can only be optically recharged in relaxed ZnO samples.

The low frequency noise of metal-semiconductor field-effect transistors (MESFETs) based on ZnO:Mg thin films grown by pulsed laser deposition on a-plane sapphire was investigated. In order to distinguish between noise generation in the bulk channel material, at the semiconductor surface, and at the gate/channel interface, ohmic ZnO channels without gate were investigated in detail, especially concerning the dependency of the noise on geometrical variations. The experiments suggest that the dominating 1/f noise in the frequency range below 1 kHz is generated within the bulk channel material, both for bare ZnO channels and MESFETs.

The ultrafast carrier dynamics of epitaxial ZnO and BaTiO3 thin films after intense excitation at 3.10 eV and 4.66 eV photon energy has been studied by femtosecond absorption spectroscopy. Modelling the transient transmission changes on the basis of spectroscopic ellipsometry data and pertinent equilibrium model dielectric functions extended by additional terms for the effects at high carrier density (P-band luminescence and stimulated emission from electron-hole-plasma), a self-consistent parameterized description was obtained for both materials. Excited carrier lifetimes in the range of ≈2 to ≈60 ps and long-lived thermal effects after several hundred ps have been identified in both materials. These findings form a reliable basis to quantitatively describe future femtosecond studies on ZnO/BaTiO3 heterolayer systems.

<p>Flash sintering was first demonstrated in 2010, where a ceramic green body was rapidly densified within seconds by applying an electric field during the heating process. The ultra-fast densification can occur as current abruptly flows through the material and self-heats by Joule heating. This process has potentials for large energy savings due to the reduction in furnace temperatures and shortened sintering time compared to conventional sintering. In addition, the ultra-high heating and cooling rates, along with the impact of electric field and current leads to the formation of unique non-equilibrium features in ceramics, which could greatly enhance their properties. Despite the potential of flash sintering, there are many challenges in moving this technique towards practical applications, such as the microstructure inhomogeneity and lack of understanding of the defects characteristics.</p> <p>In this dissertation, flash sintering was performed on ZnO to investigate the influence of various electrical conditions on the microstructure and defects. Detailed characterization was performed on flash sintered ZnO with and without a controlled current ramp, and contrasting types of current (DC and AC). These parameters show significant impact on the gradient microstructure and defects, and provide a way to tailor the desired characteristics for a wide range of applications. On the other hand, flash sintering of ZnO performed with a high electric field and low current density resulted in the growth of nanostructures. These nanostructures are unique compared to other growth techniques as they contain high density basal-plane stacking faults, and exhibit ultraviolet excitonic emission and red emission at room temperature. The nanostructure growth mechanism was investigated by varying the current density limit and revealed the formation of liquid phases which allowed growth by the vapor-liquid-solid mechanism. These findings present a new exciting route for flash sintering to produce highly defective nanostructures for device applications with new functionalities.</p>

Im Rahmen der vorliegenden Arbeit wurden Zinkoxid (ZnO)-basierte Metal-Isolator-Halbleiter Feldeffekttransistoren (MISFETs) mit Wolframtrioxid als transparentes Dielektrikum untersucht. Im ersten Teil werden die morphologischen, optischen, elektrischen und chemischen Eigenschaften der mittels gepulster Laserabscheidung (PLD) gewachsenen Wolframoxiddünnfilme, in Abhängigkeit vom Züchtungsdruck, diskutiert. Mit Hilfe dieser Ergebnisse konnte schließlich das hochisolierende Wolframtrioxid erfolgreich mit einer transparenten Gateelektrode, bestehend aus dem entarteten Halbleiter Zink-Galliumoxid (ZGO) bzw. Zink-Aluminiumoxid (AZO), kombiniert und somit MISFETs auf kristallinen und amorphen Substraten realisiert werden. Zur Optimierung der Transistoreigenschaften wurde die Dicke des Dielektrikums variiert und der Einfluss auf die Transfereigenschaften diskutiert. Des Weiteren wurde zur Verschiebung der Einschaltspannung eine Variation der Kanaldicke und des Elektrodenmaterials des Gates untersucht, wodurch die Möglichkeit der Herstellung von Verarmungs- bzw. Anreicherungstyptransistoren gegeben wurde. Um einen Vergleich der Transfereigenschaften des MISFETs gegenüber einem Metall-Halbleiter Feldeffekttransistor mit einem Schottky-Gatekontakt, bestehend aus oxidiertem Platin bzw. einem Sperrschicht-Feldeffekttransistor mit Zinkkobaltit als p-dotierten Bereich zu ermöglichen, wurden alle drei Transistorarten auf einem Substrat hergestellt und umfassend verglichen. Schließlich wird die Stabilität der Transistoren untersucht. Dabei wird der Einfluss einer permanenten Spannungsbelastung auf die Transfereigenschaften unter verschiedenen einflussnehmenden Bedingungen diskutiert. Abschließend werden aufgrund einer sich ausbildenden Hysterese der Transistoreigenschaften mögliche Ursachen derselben und Wege zur Passivierung der Bauelemente untersucht.

Since our society became industrialised, the levels of carbon dioxide in our atmosphere have been steadily rising, to the point where it in early 2020 at is 413 ppm. The high concentration is causing several troubling effects worldwide because of the increase in mean temperature that it creates, which causes longer draughts, more severe floods, and rising seawater levels to name a few. There are a few measures that can be taken to reduce carbon dioxide in the atmosphere, among which there are a number of methods that currently are being researched and/or used. The prospect of capturing carbon dioxide and using it as a carbon building block to make methanol is one solution that is particularly interesting, since it in theory could provide a fuel for combustion engines that is net neutral regarding carbon emission. Methanol can be synthesised from carbon dioxide using a heterogeneous catalyst consisting of copper, Cu, and zinc oxide, ZnO. This research is focused on one of the components of the catalyst, the metal oxide ZnO in the form of crystallites or nanoparticles (ZnO)n. Quantum chemistry is a branch of computational chemistry which is centered on solving the Schrödinger equation for molecular systems. Density functional theory, DFT, is an approach to quantum theory which in this study was used to calculate the geometry and energy of the particles. The supercomputer Tetralith in the National Supercomputer Centre, NSC, was used to carry out the calculations. The DFT calculations utilized the functional B3LYP and the basis set 6-31G (d,p). One of the largest particle sizes studied, (ZnO)20, with a structure that has a large, flat surface, was found to be the most energetically favourable. According to studies, the presence of an oxygen vacancy on the surface of ZnO reduces the amount of activation energy required for CO2 to bond to the particle, which increases the chance of forming CO and thus continuing the process of forming methanol. Two structures of (ZnO)20 were investigated in this regard, where oxygen atoms were removed at different locations, creating four versions of Zn20O19 in total. This proved yet again that the version with a large, flat surface yields the lesser amount of energy when an O atom is removed from the centre of its surface. The adsorption of CO2 to the ZnO clusters was studied by calculating the energy of adsorption, and this showed that it was the second version of (ZnO)20, without an O vacancy, that yielded the least amount of energy, thus being the most favourable species to engage in physisorption with CO2. Lastly, the activation energy was investigated, and a diagram of the reaction process of CO2 adsorbing to Zn20O19 forming (ZnO)20 and CO is presented in this paper, which shows that the required activation energy is 127 kJ/mol.

Solar cells have extrinsic losses from a variety of sources which can be minimized by optimization of the design and fabrication processes. Reflection from the front surface is one such loss mechanism and has been managed in the past with the usage of planar antireflection coatings. While effective, these coatings are each limited to a single wavelength of light and do not account for varying incident angles of the incoming light source. Three-dimensional nanostructures have shown the ability to inhibit reflection for differing wavelengths and angles of incidence. Nanocones were modeled and show a broadband, multi-angled reflectance decrease due to an effective grading of the index. Finite element models were created to simulate incident light on a zinc oxide nanocone textured silicon substrate. Zinc oxide is advantageous for its ease of production, benign nature, and refractive index matching to the air source region and silicon substrate. Reflectance plots were computed as functions of incident angle and wavelength of light and compared with planar and quintic refractive index profile models. The quintic profile model exhibits nearly optimum reflection minimization and is thus used as a benchmark. Physical quantities, including height, width, density, and orientation were varied in order to minimize the reflectance. A quasi-random nanocone unit cell was modeled to better mimic laboratory results. The model was comprised of 10 nanocones with differing structure and simulated a larger substrate by usage of periodic boundary conditions. The simulated reflectance shows approximately a 50 percent decrease when compared with a planar model. When a seed layer is added, simulating a layer of non-textured zinc oxide, on which the nanocones are grown, the reflectance shows a fourfold decrease when compared with planar models. At angles of incidence higher than 75 degrees, the nanocone model outperformed the quintic model.

The optical properties of solar cells greatly affect their efficiencies. Decreasing the broadband and directional reflectance of solar cells increases the solar irradiance transmitted and absorbed by the cell, thereby increasing the production of electron-hole pairs. Traditional optical enhancements such as light trapping and anti-reflection coatings reduce the reflectance of silicon at an optimized wavelength and angle of incidence. They do not perform as well at high angles of incidence or over the broadband solar spectrum. Theoretical studies suggest that layers with a suitable gradient-index of refraction can create both a broadband and directional anti-reflective coating. Through their variations in height and tapered growth, Zinc oxide (ZnO) nanowires can create a gradient index anti-reflection coating. ZnO is a wide-band gap semiconductor that is non-absorbing over most of the solar spectrum. With low cost, low temperature techniques, ZnO nanowires can be grown with a variety of morphologies. ZnO nanowires were grown by aqueous chemical growth and by electrodeposition on silicon to create a gradient-index anti-reflective coating for solar cell applications. The nanowire arrays were characterized using SEM images, goniometer scattering measurements, and integrating sphere total reflectance measurements. ZnO nanowires grown by aqueous chemical growth on silicon had average diameters between 60 nm and 100 nm and average lengths between 800 nm and 1100 nm. The nanowires had vertical alignment. They exhibited relatively small diffuse reflectivities and relatively large specular reflectivities. ZnO nanowires grown by electrodeposition had greater variances in length and diameter, with average diameters between 85 nm and 180 nm and average lengths between 500 nm and 1200 nm. Electrodeposited ZnO nanowires were randomly arrayed and exhibited relatively large diffuse reflectivities and relatively small specular reflectivities. Total reflectance measurements showed that all nanowire arrays reduced the broadband reflectance of silicon. Smaller nanowire arrays outperformed the larger crystal growths. A five-fold decrease in the broadband reflectance of silicon was obtained from both vertical and randomly oriented nanowire arrays. The reflectances were constant for angles of incident below 35°. Measurements at angles of incidence greater than 35° are required to determine whether ZnO nanowires can perform as directional anti-reflective coatings and whether the morphology of the nanowires affects the directional reflectances.

No description available.