53 Physik
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The aim of this thesis was to improve a kinetic Monte Carlo simulation of electron movement in a dielectric by including the movement of oxygen vacancies. Starting from the code implemented by G. Jegert during his PhD study, new features has been added and a few adaptions to the new point of interest were made. New defect types, tunneling assisted Poole-Frenkel effect, two interface layers and the movement of oxygen vacancies were added among various smaller changes. The final time depending simulation with a constant bias electric field dealt with the question on the influence of the conduction band offset on the spatial distribution and the types of the defects. It was shown that for a low offset the defects are mainly neutral, while a higher offset led to a higher positive charge, where the majority of the defects were charged double positive. These positive charged vacancies moved to the cathode where there was an accumulation of defects, while the bulk of the dielectric remains almost defect free. Since the charged defects inhibit the changing of the polarization of the dielectric the offset can be adjusted rather low, so that the majority of the defects is neutral. At the same time a lower offset leads to a higher leakage current, since the energetic barrier for Schottky emission is lowered.
CIS (Cu(In,Ga)(S,Se)2) thin film solar cells show a high potential to achieve the efficiencies of Si wafer-based solar cells. The commonly applied patterning processes for the integrated interconnects are based on nanosecond laser ablation and mechanical scribing. Both methods introduce damages on the thin films by thermal effects and mechanical forces. By picosecond laser processing we realized all three patterning steps to the monolithic thin films CIS modules, namely the separation of the molybdenum back electrode, the absorber and the ZnO font electrode (P1, P2 and P3 respectively). We achieved an efficiency of 14.7% for 300 x 300 mm² modules.
Diese Vorlesung entstand aus dem Wunsch der Studenten heraus, einzelne Gebiete der Physik nach dem Absolvieren des Grundkurses Quantenphysik genauer zu durchleuchten. Dabei konnten die Studenten aus einem Gebietskanon auswählen. So ist zu erklären, dass nicht nur Themen der Quantenmechanik enthalten sind, sondern auch elementare Abschnitte über Schwingungen und Wellen sowie Dipole und die elektromagnetische Masse.
Thermodynamik
(2012)
Wir wollen uns mit thermodynamischen Gleichgewichten beschäftigen. Dazu arbeiten wir zunächst mit dem "naiven“ Begriff der Temperatur und untersuchen die thermischen Gesetzmäßigkeiten im Gas. Nach der Definition der Begriffe thermisches Gleichgewicht, thermodynamisches Gleichgewicht, abgeschlossenes, geschlossenes und offenes System erarbeiten wir uns die Grundlagen der kinetischen Gastheorie; dabei werden wir uns einen Einblick in qualitative statistische Methoden verschaffen, die zur Behandlung von Vielteilchensystemen erforderlich sind, bei denen sich zunächst deren Anzahl nicht ändern soll (kanonisches Ensemble). Es schließen sich der 1. und der 2. Hauptsatz der Thermodynamik an. Die Entropie wird über chemische Reaktionen und statistisch über die Mischungsentropie eingeführt, und damit die Richtung des Geschehens bei Prozessen definiert. Mit den Maxwellschen Relationen werden dann die Werkzeuge geschaffen, um bis dahin erwähnte unüberwindliche Schwierigkeiten elegant zu lösen. Mit der Zulassung der Änderung der Teilchenzahl (großkanonisches Ensemble) und der Definition des chemischen Potentials wenden wir uns dann physikalischen und chemischen Gleichgewichten zu. Abschließend widmen wir uns dem Problem, wie Ungleichgewichte durch Fluss von Materie und Wärme ausgeglichen werden.
Physik I : Mechanik
(2012)
Physik beschäftigt sich mit den Erscheinungen der unbelebten Natur. Ihre Beschreibung gelingt mit Hilfe eindeutig definierter Begriffe, die zu Gesetzen verdichtet werden. Dahinter steht die naturwissenschaftliche Denkmethodik, die Annahme der Gültigkeit universeller, reproduzierbarer Naturgesetze, die Beziehungen zwischen den Objekten oder Dingen in der Natur vermitteln. (...)
Dieses Skriptum ist das Ergebnis eigener iterativer Prozesse, grundlegende Denkstrukturen mitzuteilen, die sich in der Physik als erfolgreich erwiesen haben, um mechanische Sachverhalte zu verstehen. Bei den Übungsaufgaben wurde auf eigene Vorräte, aber auch auf Aufgabenstellungen verehrter Kollegen zurückgegriffen, die hier in extenso diskutiert werden.
A femtosecond laser is used for selective structuring of biocompatible sensorchips consisting of a Ta2O5/Pt layer system on glass substrate. It was observed, that for low fluences the Ta2O5 can be selectively lifted-off from the Pt, while high fluences enable a removal of both layers. The underlying physical effects are investigated by pump-probe microscopy allowing the observation of the whole ablation process ranging temporally from femtoseconds to microseconds. Results show the formation of a gas-liquid mixture at 3 ps, causing the Ta2O5 to bulge after some ns. The Ta2O5 is disrupted in small particles after 50 ns.
The power spectral density (PSD) is a fundamental technique of random vibration fatigue providing an effective statistical characterization that can be processed by linear systems theory and load spectrum estimators. This lays the basis for a statistical-based fatigue assessment. While the PSD assembles a full stochastic characterization of stationary Gaussian loading, for loading subjected to changing operational, environmental, and excitational conditions, it provides no means of a fluctuating spectral density. Therefore, the PSD neither qualifies to characterize varying loads, nor reproduces comparable stress amplitudes to a referencing non-stationary excitation following a statistical-based stress analysis. Consequently, this paper employs the non-stationarity matrix to characterize the varying evolution of realistic loading and proposes a system of equations that decomposes this characterization into stationary Gaussian portions. The fundamental idea is to approximate realistic loading by abstracting a series of stationary segments, whose assembly in return embodies a full statistical characterization. The resulting quasi-stationary load definition better reflects the fatigue damage potential of realistic, non-stationary loading and allows to implement load spectrum estimators, ensuing computationally efficient and statistically robust structural lifetime predictions. Further, quasi-stationary load definitions are utilized to advance the concept of damage-equivalent statistical load definitions to be independent of a specific Miner exponent.
This paper addresses the use of higher-order spectra to study the non-Gaussian nature of random vibration loading. Since the power spectral density is only a full description for stationary Gaussian processes, specifying non-Gaussian random vibration loading requires a sophisticated statistical description. In recent research higher-order statistical moments such as skewness and kurtosis have been used to de_ne non-Gaussian properties of vibration loading. However, useful information contained in the spectral representation of these moments is neglected. This paper introduces the trispectrum as a tool for analyzing vibration loading. It is the spectral representation of the fourth-order moment and thus extends the information content of the kurtosis. For demonstration several common methods for generating non-Gaussian loading are reviewed and used to derive loads that reproduce the power spectral density and kurtosis of a real in-service loading. These loads are analyzed using Fatigue Damage Spectra and trispectra to relate structural response behavior to their non-Gaussian nature. The results suggest that the trispectrum is a valuable tool for analyzing and classifying non-Gaussian random loading.
Ferroelectric field- effect-transistors, based on Zirconia or Hafnia, have the potential to play an important roll in future non-volatile electronic memory devices. Artificial stabilization of the ferroelectric phase is necessary, since the ferroelectric phase is not the natural occurring phase of those materials. The intension of this work is to investigate the influence of II- and IV-valent dopants on the phase stability of Zirconia, using Ab Initio calculations based on density functional theory. Experimental results show ferroelectricity for dopant concentrations in the percent margins. Therefore the necessity to use large 96 atomic supercells arises, in which individual atoms can be removed or replaced with a dopant atom. The calculations were performed on the ICE1-segment of the Leibniz Rechenzentrum in Munich, using the program Abinit. Parallelization occurred, using up to 120 cores. The used exchange correlation method was PW92 at a cut off energy of 30 Ha, the atomic positions were optimized using the Broyden-Fletcher-Goldfarb-Shanno-Minimization-Method. Several different external constraints to optimize lattice parameters were investigated. The results show, that for the chosen dopant concentration the monoclinic phase is always favored. That implies that other effects like thermal energy or surface tension might also play a roll in stabilizing the ferroelectric phase. None the less dopants have a big impact on the total energy of the different phases, especially the tetragonal phase. Since in contrast to experimental results the dopants did not stabilize the ferroelectric phase in Zirconia, calculations including III-valent dopants as well as thermal and surface energies for both Zirconia and Hafnia are necessary.
Im Rahmen dieser Arbeit wird folglich ein Cavity-Ring-Down-Aufbau entworfen, installiert und in Betrieb genommen, um künftig auch werksintern kostengünstig und zeitnah die produzierten Optiken auf ihre Reflektivität hin untersuchen zu können. Im zweiten Schritt soll die Auswirkung der Substratrauigkeit auf den Herstellungsprozess der Spiegel und deren resultierende Reflektivitäten untersucht werden.
The MAGIC II telescope is, after MAGIC I, the second large diameter Cherenkov telescope build in La Palma. Its goal is to detect extraterrestrial gamma-radiation in the energy region from 30 GeV to 30 TeV. Additionally, MAGIC II has been build with the purpose of stereoscopic observation with MAGIC I. Cosmic particles, such as gamma-rays, protons and heavy particles, generate, when hitting the atmosphere, so-called extended air showers. Ultra-relativistic particles of these showers produce Cherenkov light that can be observed by ground-based telescopes during dark, clear nights. These measurements both provide information about the direction of the primary particle and its energy. From the shower image shape one can distinguish the very rare gamma-rays from the much larger hadronic background. The intensity of the Cherenkov radiation scales in first order with energy. Depending on the shower impact parameter the light flux is typically in the order of 100 photons/1 m2 for a 1 TeV shower and an impact value below about 120 m. For impact parameters above 120 m the intensity of the light flux is rapidly dropping as the light originates from the shower halo particles. Therefore, the optical performance as well as the size of the mirror for Cherenkovtelescopes is of great importance, since it is a key element in the detection chain. The optical parameters like the point spread function, the focused reßectivity and the focal length play a major role for recording high quality images. The mirror performance data have to be used for the Monte Carlo Simulation of the MAGIC telescope because it is impossible to test the telescope with calibrated cosmic ‘test sources’, which do not exist. Note that this is an important difference to particle physics experiments where one normally tests detector elements in test beams. Moreover, it is of great importance to assure the mirror’s resistance against the impact of the environmental, such as against dew, rain water or icing. Therefore, a number of measurements were performed to determine the relevant optical parameters and water tightness for the all-aluminium mirror elements of the MAGIC-II telescope. In this thesis the principles and the results of these measurements will be presented. Additionally, in order to improve the light detection efficiency for future telescope, one considered to use so-called Silicon Photo-Multipliers (SiPM) for Cherenkov light detection. One of the most critical parameters of these novel and still experimental semiconductor light detectors with internal gain is the generation of light in the avalanche gain amplification leading to optical crosstalk and preventing to operate SiPMs with high photon detection efficiency (PDE). As the operation parameters of these devices have also to be used for the Monte Carlo Simulation of future Cherenkov telescopes it is important to evaluate the optical crosstalk rate and its impact on the PDE in great detail. The detailed study of optical crosstalk is also part of this diploma thesis and the analysis and results of this study will be presented here.