CHARACTERIZATION OF SEED DEFECTS IN HIGHLY SPECULAR SMOOTH COATED SURFACES

Many smooth, highly specular coatings such as automotive paints are subjected to considerable performance demands as the customer expectations for appearance of coatings are continually increasing. Therefore it is vital to develop robust methods to monitor surface quality online. An automated visual assessment of specular coated surface that would not only provide a cost effective and reliable solution to the industries but also facilitate the implementation of a real-time feedback loop. The scope of this thesis is a subset of the inspection technology that facilitates real-time close loop control of the surface quality and concentrates on one common surface defect the seed defect. This machine vision system design utilizes surface reflectance models as a rational basis. Using a single high-contrast image the height of the seed defect is computed; the result is obtained rapidly and is reasonably accurate approximation of the actual height

QUANTIFICATION OF SURFACE DEFECTS USING PRIMARY HIGHLIGHT IN DIFFUSE ANGLE GRAY SCALE IMAGES

The thesis presented is an effort to gather all possible information of one particular type of common paint defect the seed defect, from gray scale images of highly specular painted surface. The proposed approach in the thesis utilizes a white light source to illuminate the surface and utilizes a camera to capture its gray scale image at different diffused angles. While attempting to explain the physics of highlight formation in terms of location on the surface of a seed defect, the thesis also extends to utilize this information from gray scale images to accurately predict the parameters of seed defects including the height, size and position in real time. Since the primary highlight in a gray scale image is more defined, contrary to the past researches on diffuse angle images that use both primary / seed highlight and mirror highlight to estimate height of the seed, this thesis formulates a theory of highlight translation and estimates the height of seed based on primary / seed highlight. The other common type of surface defect - crater defect, is also addressed in the thesis

All in situ ultra-high vacuum study of Bi2Te3 topological insulator thin films

The term "topological insulator" (TI) represents a novel class of compounds which are insulating in the bulk, but simultaneously and unavoidably have a metallic surface. The reason for this is the non-trivial band topology, arising from particular band inversions and the spin-orbit interaction, of the bulk. These topologically protected metallic surface states are characterized by massless Dirac dispersion and locked helical spin polarization, leading to forbidden back-scattering with robustness against disorder. Based on the extraordinary features of the topological insulators an abundance of new phenomena and many exciting experiments have been proposed by theoreticians, but still await their experimental verification, not to mention their implementation into applications, e.g. the creation of Majorana fermions, advanced spintronics, or the realization of quantum computers. In this perspective, the 3D TIs Bi2Te3 and Bi2Se3 gained a lot of interest due to their relatively simple electronic band structure, having only a single Dirac cone at the surface. Furthermore, they exhibit an appreciable bulk band gap of up to ~ 0.3 eV, making room temperature applications feasible. Yet, the execution of these proposals remains an enormous experimental challenge. The main obstacle, which thus far hampered the electrical characterization of topological surface states via transport experiments, is the residual extrinsic conductivity arising from the presence of defects and impurities in their bulk, as well as the contamination of the surface due to exposure to air. This thesis is part of the actual effort in improving sample quality to achieve bulk-insulating Bi2Te3 films and study of their electrical properties under controlled conditions. Furthermore, appropriate capping materials preserving the electronic features under ambient atmosphere shall be identified to facilitate more sophisticated ex-situ experiments. Bi2Te3 thin films were fabricated by molecular beam epitaxy (MBE). It could be shown that, by optimizing the growth conditions, it is indeed possible to obtain consistently bulk-insulating and single-domain TI films. Hereby, the key factor is to supply the elements with a Te/Bi ratio of ~8, while achieving a full distillation of the Te, and the usage of substrates with negligible lattice mismatch. The optimal MBE conditions for Bi2Te3 were found in a two-step growth procedure at substrate temperatures of 220°C and 250°C, respectively, and a Bi flux rate of 1 Å/min. Subsequently, the structural characterization by high- and low-energy electron diffraction, photoelectron spectroscopy, and, in particular, the temperature-dependent conductivity measurements were entirely done inside the same ultra-high vacuum (UHV) system, ensuring a reliable record of the intrinsic properties of the topological surface states. Bi2Te3 films with thicknesses ranging from 10 to 50 quintuple layers (QL; 1QL~1 nm) were fabricated to examine, whether the conductivity is solely arising from the surface states. Angle resolved photoemission spectroscopy (ARPES) demonstrates that the chemical potential for all these samples is located well within the bulk band gap, and is only intersected by the topological surface states, displaying the characteristic linear dispersion. A metallic-like temperature dependency of the sheet resistance is observed from the in-situ transport experiments. Upon going from 10 to 50QL the sheet resistance displays a variation by a factor 1.3 at 14K and of 1.5 at room temperature, evidencing that the conductivity is indeed dominated by the surface. Low charge carrier concentrations in the range of 2–4*10^12 cm^−2 with high mobility values up to 4600 cm2/Vs could be achieved. Furthermore, the degradation effect of air exposure on the conductance of the Bi2Te3 films was quantified, emphasizing the necessity to protect the surface from ambient conditions. Since the films behave inert to pure oxygen, water/moisture is the most probable source of degeneration. Moreover, epitaxially grown elemental tellurium was identified as a suitable capping material preserving the properties of the intrinsically insulating Bi2Te3 films and protecting from alterations during air exposure, facilitating well-defined and reliable ex-situ experiments. These findings serve as an ideal platform for further investigations and open the way to prepare devices that can exploit the intrinsic features of the topological surface states.:Abstract Kurzfassung Acronyms List of Symbols Introduction 1 Topological insulators 1.1 Basic theory of topological insulators 1.2 3D topological insulator materials: bismuth chalcogenides 2 Experimental techniques 2.1 General layout of the UHV-system 2.2 Molecular beam epitaxy 2.3 Structural and spectroscopic characterization 2.3.1 RHEED and LEED 2.3.2 Photoelectron spectroscopy 2.3.3 Ex situ x-ray diffraction 2.4 In situ electrical resistance measurements 2.4.1 In situ transport setup 2.4.2 Measurement equipment and operation modes 2.5 Substrates and sample holders 3 MBE growth and structural characterization of Bi2Te3 thin films 3.1 Bi2Te3 growth optimization and in situ structural characterization 3.1.1 1-step growth on Al2O3 (0001) 3.1.2 2-step growth on Al2O3 (0001) 3.1.3 2-step growth on BaF2 (111) 3.2 Ex situ structural characterization 4 In situ spectroscopy and transport properties of Bi2Te3 thin films 4.1 In situ spectroscopy of Bi2Te3 thin films 4.1.1 XPS 4.1.2 ARPES 4.2 Combined ARPES and in situ electrical resistance measurements of bulk-insulating Bi2Te3 thin films 4.2.1 Quality of the in situ electrical sample contacts 4.2.2 Verification of the intrinsic conduction through topological surface states of bulk-insulating Bi2Te3 thin films 5 Effect of surface contaminants on the TI properties 5.1 Effect of air exposure on the electrical conductivity of Bi2Te3 surfaces 5.2 Determination of the contaminants causing degradation of the TI properties 5.3 Long-time resistance behavior of a Bi2Te3 film exposed to minimal traces of contaminants 6 Protective capping of bulk-insulating Bi2Te3 thin films 6.1 Capping with BaF2 6.1.1 MBE growth and structure of BaF2 on Bi2Te3 thin films 6.1.2 Electron spectroscopy and electrical transport properties of BaF2 capped Bi2Te3 6.2 Capping with tellurium 6.2.1 MBE growth and structure of Te on Bi2Te3 thin films 6.2.2 Photoelectron spectroscopy and electrical transport properties of Te capped Bi2Te3 6.2.3 De-capping of Te 6.2.4 Efficiency of Te capping against air exposure 7 Conclusion and outlook Bibliography Versicherung Curriculum vitae VeröffentlichungenDer Begriff "Topologischer Isolator" (TI) beschreibt eine neuartige Klasse von Verbindungen deren Inneres (engl. Bulk) isolierend ist, dieses Innere aber gleichzeitig und zwangsläufig eine metallisch leitende Oberfläche aufweist. Dies ist begründet in der nicht-trivialen Topologie dieser Materialien, welche durch eine spezielle Invertierung einzelner Bänder in der Bandstruktur und der Spin-Bahn-Kopplung im Materialinneren hervorgerufen ist. Diese topologisch geschützten, metallischen Oberflächenzustände sind gekennzeichnet durch eine masselose Dirac Dispersionsrelation und gekoppelte Helizität der Spinpolarisation, welche die Rückstreuung der Ladungsträger verbietet und somit zur Stabilisierung der Zustände gegenüber Störungen beiträgt. Auf Grundlage dieser außergewöhnlichen Merkmale haben Theoretiker eine Fülle neuer Phänomene und spannender Experimente vorhergesagt. Deren experimentelle Überprüfung steht jedoch noch aus, geschweige denn deren Umsetzung in Anwendungen, wie zum Beispiel die Erzeugung von Majorana Teilchen, fortgeschrittene Spintronik, oder die Realisierung von Quantencomputern. Aufgrund ihrer relativ einfachen Bandstruktur, welche nur einen Dirac-Kegel an der Oberfläche aufweist, haben die 3D TI Bi2Te3 und Bi2Se3 in den letzten Jahren großes Interesse erlangt. Weiterhin besitzen diese Materialien eine merkliche Bandlücke von bis zu ~0,3 eV, welche sogar Anwendungen bei Raumtemperatur ermöglichen könnten. Dennoch ist deren experimentelle Umsetzung nachwievor eine enorme Herausforderung. Das Haupthindernis, welches bis jetzt insbesondere die elektrische Charakterisierung the topologischen Oberflächenzustände behindert hat, ist die zusätzliche Leitfähigkeit des Materialinneren, welche durch Kristalldefekte und Beimischungen, sowie die Verunreinigung der Probenoberfläche durch Luftexposition bedingt wird. Die vorliegende Arbeit liefert einen Beitrag zu aktuellen den Anstrengungen in der Verbesserung der Probenqualität der TI um die Leitfähigkeit des Materialinneren zu unterdrücken, sowie die anschließende Untersuchung der elektrischen Eigenschaften unter kontrollierten Bedingungen durchzuführen. Weiterhin sollen geeignete Deckschichten identifiziert werden, welche die besonderen elektronischen Merkmale der TI nicht beeinflussen sowie diese gegen äußere Einflüsse schützen, und somit die Durchführung anspruchsvoller ex situ Experimente ermöglichen können. Die untersuchten Bi2Te3 Schichten wurden mittels Molekularstrahlepitaxie (MBE) hergestellt. Es konnte gezeigt werden, dass es allein durch Optimierung der Wachstumsbedingungen möglich ist Proben herzustellen, die gleichbleibend isolierende Eigenschaften des TI Inneren aufweisen und Eindomänen-Ausrichtung besitzen. Die zentralen Faktoren sind hierbei die Aufrechterhaltung eines Flussratenverhältnisses von Te/Bi ~8 der einzelnen Elemente, sowie die Wahl einer ausreichend hohen Substrattemperatur, um ein vollständiges Abdampfen (Destillation) des überschüssigen Tellur zu erreichen. Weiterhin müssen Substrate mit gut angepassten Gitterparametern verwendet werden, welches bei BaF2 (111) gegeben ist. Optimales MBE Wachstum konnte durch ein Zwei-Stufen Prozess bei Substrattemperaturen von 220°C und 250°C und einer Bi-Verdampfungsrate von 1 Å/min erreicht werden. Die nachfolgende Charakterisierung der strukturellen Eigenschaften, Photoelektronenspektroskopie, sowie temperaturabhängige Leitfähigkeitsmessungen wurden alle in einem zusammenhängenden Ultrahochvakuum-System durchgeführt. Auf diese Weise wird eine zuverlässige Erfassung der intrinsischen Eigenschaften der TI sichergestellt. Zur Überprüfung, ob die Leitfähigkeit der Proben tatsächlich nur durch die Oberflächenzustände hervorgerufen wird, wurden Filme mit Schichtdicken im Bereich von 10 bis 50 Quintupel-Lagen (QL; 1QL~ 1 nm) hergestellt und charakterisiert. Winkelaufgelöste Photoelektronenspektroskopie (ARPES) belegt, dass das chemische Potential (Fermi-Niveau) in allen Proben innerhalb der Bandlücke der Bandstruktur des Materialinneren liegt und nur von den topologisch geschützten Oberflächenzuständen gekreuzt wird, welche die charakteristische lineare Dirac Dispersionsrelation aufweisen. Die temperaturabhängigen Widerstandsmessungen zeigen ein metallisches Verhalten aller Proben. Bei der Variation der Schichtdicke von 10 zu 50QL wird eine Streuung des Flächenwiderstandes vom Faktor 1,3 bei 14K und 1,5 bei Raumtemperatur beobachtet. Dies beweist, dass die gemessene Leitfähigkeit vorrangig durch die topologisch geschützten Oberflächenzustände hervorgerufen wird. Eine geringe Oberflächenladungsträgerkonzentration im Bereich von 2–4*10^12 cm^−2 und hohe Mobilitätswerte von bis zu 4600 cm2/Vs wurden erreicht. Weiterhin wurden die negativen Auswirkungen auf die Eigenschaften der TI durch Luftexposition quantifiziert, welches die Notwendigkeit belegt, die Oberfläche der TI vor Umgebungseinflüssen zu schützen. Die Proben verhalten sich inert gegenüber reinem Sauerstoff, daher ist Wasser aus der Luftfeuchte höchstwahrscheinlich der Hauptgrund für die beobachtbare Verschlechterung. Darüber hinaus konnte epitaktisch gewachsenes Tellur als geeignete Deckschicht ausfindig gemacht werden, welches die Eigenschaften der Bi2Te3 Filme nicht beeinflusst, sowie gegen Veränderungen durch Luftexposition schützt. Die gewonnenen Erkenntnisse stellen eine ideale Grundlage für weiterführende Untersuchungen dar und ebnen den Weg zur Entwicklung von Bauelementen welche die spezifischen Besonderheiten der topologischen Oberflächenzustände.:Abstract Kurzfassung Acronyms List of Symbols Introduction 1 Topological insulators 1.1 Basic theory of topological insulators 1.2 3D topological insulator materials: bismuth chalcogenides 2 Experimental techniques 2.1 General layout of the UHV-system 2.2 Molecular beam epitaxy 2.3 Structural and spectroscopic characterization 2.3.1 RHEED and LEED 2.3.2 Photoelectron spectroscopy 2.3.3 Ex situ x-ray diffraction 2.4 In situ electrical resistance measurements 2.4.1 In situ transport setup 2.4.2 Measurement equipment and operation modes 2.5 Substrates and sample holders 3 MBE growth and structural characterization of Bi2Te3 thin films 3.1 Bi2Te3 growth optimization and in situ structural characterization 3.1.1 1-step growth on Al2O3 (0001) 3.1.2 2-step growth on Al2O3 (0001) 3.1.3 2-step growth on BaF2 (111) 3.2 Ex situ structural characterization 4 In situ spectroscopy and transport properties of Bi2Te3 thin films 4.1 In situ spectroscopy of Bi2Te3 thin films 4.1.1 XPS 4.1.2 ARPES 4.2 Combined ARPES and in situ electrical resistance measurements of bulk-insulating Bi2Te3 thin films 4.2.1 Quality of the in situ electrical sample contacts 4.2.2 Verification of the intrinsic conduction through topological surface states of bulk-insulating Bi2Te3 thin films 5 Effect of surface contaminants on the TI properties 5.1 Effect of air exposure on the electrical conductivity of Bi2Te3 surfaces 5.2 Determination of the contaminants causing degradation of the TI properties 5.3 Long-time resistance behavior of a Bi2Te3 film exposed to minimal traces of contaminants 6 Protective capping of bulk-insulating Bi2Te3 thin films 6.1 Capping with BaF2 6.1.1 MBE growth and structure of BaF2 on Bi2Te3 thin films 6.1.2 Electron spectroscopy and electrical transport properties of BaF2 capped Bi2Te3 6.2 Capping with tellurium 6.2.1 MBE growth and structure of Te on Bi2Te3 thin films 6.2.2 Photoelectron spectroscopy and electrical transport properties of Te capped Bi2Te3 6.2.3 De-capping of Te 6.2.4 Efficiency of Te capping against air exposure 7 Conclusion and outlook Bibliography Versicherung Curriculum vitae Veröffentlichunge

Eurodisplay 2019

The collection includes abstracts of reports selected by the program by the conference committee

The modification of thin film surface structure via low temperature atmospheric pressure CVD post process treatment

In photovoltaic thin film cells, a transparent conducting oxide (TCO) layer is required to transport current. The most common TCOs used are F:SnO2 (fluorine doped tin oxide), ZnO (zinc oxide) and ITO (indium doped tin oxide). ZnO is normally deposited in a vacuum based process, sputtering or low pressure chemical vapour deposition (LPCVD). Atmospheric pressure chemical vapour deposition (APCVD) is an attractive alternative for ZnO deposition. A critical parameter for TCOs in photovoltaic thin films is the surface morphology which defines the optical scattering properties. The ability to control the spectral sensitivity and degree of scattering are both important process parameters for high performance cells. This thesis investigates APCVD for film growth of ZnO plus dopants (fluorine and aluminium), and the effects of atmospheric pressure plasma etching of ZnO and F:SnO2. ZnO was deposited in multiple system geometries all based on thermal activated CVD. The oxidant source purity is shown to be critical for stable growth at higher temperatures required for dopant incorporation. A fundamental problem was encountered with fluorine doping, whereby the films would crack beyond a critical thickness. A solution was found with the development of a F:SnO2 and F:ZnO composite stack. Photovoltaic testing of this hybrid TCO was encouraging, showing the potential benefit of the composite structure. Modification of the surface morphology was achieved by atmospheric pressure plasma, based on a dielectric barrier discharge configuration. This novel system enables the etching of TCO films without the introduction of hazardous wet chemistry. In this thesis the effects of etching regime and feedgas composition are studied and an etching mechanism is proposed. Isolation of the etching environment enabled investigation into the feed gas mixture, demonstrating which were critical for etching. Both materials showed a dependence on the feed gas mixture for etching, with F:SnO2 requiring HCl and O2 and H2O for ZnO

Exergy-based Planning and Thermography-based Monitoring for energy efficient buildings - Progress Report (KIT Scientific Reports ; 7632)

Designing and monitoring energy efficiency of buildings is vital since they account for up to 40% of end-use energy. In this study, exergy analysis is investigated as a life cycle design tool to strike a balance between thermodynamic efficiency of energy conversion and economic and environmental costs of construction. Quantitative geo-referenced thermography is proposed for monitoring and quantitative assessment via continued simulation and parameter estimation during the operating phase

Characterization of a turbulent separating/ reattaching flow using optical pressure and velocity measurements

The turbulent wake flow behind a generic spacecraft was investigated experimentally in the trisonic wind tunnel Munich at subsonic Mach numbers M = [0.3; 0.7]. The flow/ structure interaction which raised critical safety aspects on the real spacecraft in the past was studied. The characterization of the coherent flow structures was performed by means of transient optical and classical measurement techniques. The topology and dynamics of the wake flow and the pressure field were investigated with the 2C2D-PIV and the instationary PSP. The reattachment position as well as the local dynamic behavior of strong flow structures were successfully characterized and the presence of dominant vortex shedding at expected frequencies around f ≈ [400; 900] Hz was confirmed. It was the first time that the fluid/ structure interaction and the position of strongest stresses could be characterized experimentally with very high spatial and temporal resolution. A PSP system had to be established in order to perform the desired experiments. Therefore, basic components (e.g. calibration chamber, excitation, evaluation tool) had to be developed and the performance of the entire system had to be validated

Metallic Magnetic Hetrostructures

This work studied sputter deposited conventional spin valves (SV) and related structures. In SV layered structures, two ferromagnetic layers are separated by a non-magnetic spacer. Under an external magnetic field, the relative orientation of the magnetization changes in the ferromagnets, exhibiting the giant magnetoresistive effect. The controlled switching of ferromagnets in convention SV is facilitated by the exchange bias (EB) effect, which is achieved by depositing an antiferromagnetic layer next to one of the ferromagnetic layers in a magnetic field. Two highly related investigations were performed in this work. In the first part the exchange bias effect in the Ni80Fe20/Fe50Mn50/Co trilayer structure was studied. Samples were deposited in a low field condition that permitted EB to be established in NiFe/FeMn but not in FeMn/Co bilayer structures. Temperature-dependent magnetic measurements were performed on the trilayer sample, as well as the corresponding NiFe/FeMn and FeMn/Co bilayer samples. Recent literature on similar system showed that an AF spiral could be formed in the trilayer, which was probed by relative EB directions of the NiFe and Co layers. In this work, no exchange bias was found to propagate from the NiFe/FeMn system into the FeMn/Co system, showing that the AF spiral was induced by the specific magnetic treatment and was not the cause of EB effect. Besides, exchange bias field and coercivity of the samples indicated the influence of the EB system in the presence of an adjacent EB system. Explanations of the effect were made with some existing EB models. In the second part of the work, conventional SV of target structure Nb/NiFe/Cu/Co/FeMn/Nb was studied in a ‘built-up samples’ strategy. A batch of these built-up samples, which corresponded to the different stages of the deposition of the target top conventional SV structure, were prepared by terminating the sputtering process after a certain number of layers were deposited. These samples were thoroughly characterized by structural, magnetic and electrical measurements. In terms of structural characterization by x-ray techniques, more reliable information concerning the morphology and microstructure of the layers was obtained by probing the built-up samples, instead of relying solely on the information of the full SV structure. For the electric and magnetic measurements, a number of unexpected observations were made in the built-up samples, although the final performance of the full SV structure was of comparable quality to the literature. These results showed the ability of the ‘built-up samples’ strategy in critical characterization and optimization of magnetic multilayered structures