70.00.00 CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
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- Topologischer Isolator (13)
- Molekularstrahlepitaxie (6)
- Quecksilbertellurid (6)
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- Center for Nanosystems Chemistry (CNC), Universität Würzburg (1)
- Institute of Physics and Center for Nanotechnology, University of Münster (1)
- Lehrstuhl für BioMolekulare Optik, Ludwig-Maximilians-Universität München (1)
- NanoOptics & Biophotonics Group, Experimental Physics 5, Universität Würzburg (1)
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- 614623 (1)
Um die Natur der Transportdynamik von Ladungsträgern auch auf mikroskopischen Längenskalen nicht-invasiv untersuchen zu können, wurde im ersten Schwerpunkt dieser Arbeit das PL- (Photolumineszenz-) Quenching (engl.: to quench: löschen; hier: strahlungslose Rekombination von Exzitonen) in einer organischen Dünnschicht durch die injizierten und akkumulierten Löcher in einer Transistorgeometrie analysiert. Diese Zusammenführung zweier Methoden - der elektrischen Charakterisierung von Dünnschichttransistoren und der Photolumineszenzspektroskopie - erfasst die Änderung des strahlenden Zerfalls von Exzitonen infolge der Wechselwirkung mit Ladungsträgern. Dadurch werden räumlich aufgelöste Informationen über die Ladungsverteilung und deren Spannungsabhängigkeit im Transistorkanal zugänglich. Durch den Vergleich mit den makroskopischen elektrischen Kenngrößen wie der Schwell- oder der Turn-On-Spannung kann die Funktionsweise der Transistoren damit detaillierter beschrieben werden, als es die Kenngrößen alleine ermöglichen. Außerdem wird die Quantifizierung dieser mikroskopischen Interaktionen möglich, welche beispielsweise als Verlustkanal in organischen Photovoltaikzellen und organicshen Leuchtdioden auftreten können. Die Abgrenzung zu anderen dissipativen Prozessen, wie beispielsweise der Exziton-Exziton Annihilation, Ladungsträgerrekombination, Triplett-Übergänge oder Rekombination an Störstellen oder metallischen Grenzflächen, erlaubt die detaillierte Analyse der Wechselwirkung von optisch angeregten Zuständen mit Elektronen und Löchern.
Im zweiten Schwerpunkt dieser Arbeit werden die Transporteigenschaften des Naphthalindiimids Cl2-NDI betrachtet, bei dem der molekulare Überlapp sowie die Reorganisationsenergie in derselben Größenordnung von etwa 0,1 eV liegen. Um experimentell auf den mikroskopischen Transport zu schließen, werden nach der Optimierung des Kristallwachstums Einkristalltransistoren hergestellt, mit Hilfe derer die Beweglichkeit entlang verschiedener kristallographischer Richtungen als Funktion der Temperatur gemessen werden kann. Die einkristalline Natur der Proben und die spezielle Transistorgeometrie ermöglichen die Analyse der räumlichen Anisotropie des Stromflusses. Der gemessene Beweglichkeitstensor wird daraufhin mit simulierten Tensoren auf der Basis von Levich-Jortner Raten verglichen, um auf den zentralen Ladungstransfermechanismus zu schließen.
Time and Spatially Resolved Photoluminescence Spectroscopy of Hot Excitons in Gallium Arsenide
(2015)
The present thesis investigates the impact of hot exciton effects on the low-temperature time and spatially resolved photoluminescence (PL) response of free excitons in high-purity gallium arsenide (GaAs). The work at hand extends available studies of hot carrier effects, which in bulk GaAs have up to now focused on hot electron populations. In crucial distinction from previous work, we extensively study the free exciton second LO-phonon replica. The benefit of this approach is twofold. First, the two LO phonon-assisted radiative recombination allows to circumvent the inherent interpretation ambiguities of the previously investigated free exciton zero-phonon line. Second, the recombination line shape of the second LO-phonon replica provides direct experimental access to the exciton temperature, thereby enabling the quantitative assessment of hot exciton effects.
In the first part of the thesis, we address the influence of transient cooling on the time evolution of an initially hot photocarrier ensemble. To this end, we investigate time-resolved photoluminescence (TRPL) signals detected on the free exciton second LO-phonon replica. Settling a long-standing question, we show by comparison with TRPL transients of the free exciton zero-phonon line that the slow free exciton photoluminescence rise following pulsed optical excitation is dominated by the slow buildup of a free exciton population and not by the relaxation of large K-vector excitons to the Brillouin zone center. To establish a quantitative picture of the delayed photoluminescence onset, we determine the cooling dynamics of the initially hot photocarrier cloud from a time-resolved line shape analysis of the second LO-phonon replica. We demonstrate that the Saha equation, which fundamentally describes the thermodynamic population balance between free excitons and the uncorrelated electron-hole plasma, directly translates the experimentally derived cooling curves into the time-dependent conversion of unbound electron-hole pairs into free excitons.
In the second part of the thesis, we establish the impact of hot exciton effects on low-temperature spatially resolved photoluminescence (SRPL) studies. Such experiments are widely used to investigate charge carrier and free exciton diffusion in semiconductors and semiconductor nanostructures. By SRPL spectroscopy of the second LO-phonon replica, we show that above-band gap focused laser excitation inevitably causes local heating in the carrier system, which crucially affects the diffusive expansion of a locally excited exciton packet. Undistorted free exciton diffusion profiles, which are correctly described by the commonly used formulation of the photocarrier diffusion equation, are only observed in the absence of spatial temperature gradients. At low sample temperatures, the reliable determination of free exciton diffusion coefficients from both continuous-wave and time-resolved SRPL spectroscopy requires strictly resonant optical excitation.
Using resonant laser excitation, we observe the dimensional crossover of free exciton diffusion in etched wire structures of a thin, effectively two-dimensional GaAs epilayer. When the lateral wire width falls below the diffusion length, the sample geometry becomes effectively one-dimensional. The exciton diffusion profile along the wire stripe is then consistently reproduced by the steady-state solution to the one-dimensional diffusion equation.
Finally, we demonstrate the formation of macroscopic free and bound exciton photoluminescence rings in bulk GaAs around a focused laser excitation spot. Both ring formation effects are due to pump-induced local heating in the exciton system. For a quantitative assessment of the mechanism underlying the free exciton ring formation, we directly determine the exciton temperature gradient from a spatially resolved line shape analysis of the free exciton second LO-phonon replica. We demonstrate that a pump-induced hot spot locally modifies the thermodynamic population balance between free excitons and unbound electron-hole pairs described by the Saha equation, which naturally explains the emergence of macroscopic free exciton ring structures.
In summary, we demonstrate that quantitative consideration of hot exciton effects provides a coherent picture both of the time-domain free exciton luminescence kinetics and of the distinct spatially resolved photoluminescence patterns developing under the influence of spatial photocarrier diffusion.
Feedback efficiency and training effects during alpha band modulation over the sensorimotor cortex
(2015)
Neural oscillations can be measured by electroencephalography (EEG) and these oscillations can be characterized by their frequency, amplitude and phase. The mechanistic properties of neural oscillations and their synchronization are able to explain various aspects of many cognitive functions such as motor control, memory, attention, information transfer across brain regions, segmentation of the sensory input and perception (Arnal and Giraud, 2012). The alpha band frequency is the dominant oscillation in the human brain. This oscillatory activity is found in the scalp EEG at frequencies around 8-13 Hz in all healthy adults (Makeig et al., 2002) and considerable interest has been generated in exploring EEG alpha oscillations with regard to their role in cognitive (Klimesch et al., 1993; Hanselmayr et al., 2005), sensorimotor (Birbaumer, 2006; Sauseng et al., 2009) and physiological (Lehmann, 1971; Niedermeyer, 1997; Kiyatkin, 2010) aspects of human life. The ability to voluntarily regulate the alpha amplitude can be learned with neurofeedback training and offers the possibility to control a brain-computer interface (BCI), a muscle independent interaction channel. BCI research is predominantly focused on the signal processing, the classification and the algorithms necessary to translate brain signals into control commands than on the person interacting with the technical system. The end-user must be properly trained to be able to successfully use the BCI and factors such as task instructions, training, and especially feedback can therefore play an important role in learning to control a BCI (Neumann and Kübler, 2003; Pfurtscheller et al., 2006, 2007; Allison and Neuper, 2010; Friedrich et al., 2012; Kaufmann et al., 2013; Lotte et al., 2013).
The main purpose of this thesis was to investigate how end-users can efficiently be trained to perform alpha band modulation recorded over their sensorimotor cortex. The herein presented work comprises three studies with healthy participants and participants with schizophrenia focusing on the effects of feedback and training time on cortical activation patterns and performance. In the first study, the application of a realistic visual feedback to support end-users in developing a concrete feeling of kinesthetic motor imagery was tested in 2D and 3D visualization modality during a single training session. Participants were able to elicit the typical event-related desynchronisation responses over sensorimotor cortex in both conditions but the most significant decrease in the alpha band power was obtained following the three-dimensional realistic visualization. The second study strengthen the hypothesis that an enriched visual feedback with information about the quality of the input signal supports an easier approach for motor imagery based BCI control and can help to enhance performance. Significantly better performance levels were measurable during five online training sessions in the groups with enriched feedback as compared to a conventional simple visual feedback group, without significant differences in performance between the unimodal (visual) and multimodal (auditory–visual) feedback modality. Furthermore, the last study, in which people with schizophrenia participated in multiple sessions with simple feedback, demonstrated that these patients can learn to voluntarily regulate their alpha band. Compared to the healthy group they required longer training times and could not achieve performance levels as high as the control group. Nonetheless, alpha neurofeedback training lead to a constant increase of the alpha resting power across all 20 training session.
To date only little is known about the effects of feedback and training time on BCI performance and cortical activation patterns. The presented work contributes to the evidence that healthy individuals can benefit from enriched feedback: A realistic presentation can support participants in getting a concrete feeling of motor imagery and enriched feedback, which instructs participants about the quality of their input signal can give support while learning to control the BCI. This thesis demonstrates that people with schizophrenia can learn to gain control of their alpha oscillations recorded over the sensorimotor cortex when participating in sufficient training sessions. In conclusion, this thesis improved current motor imagery BCI feedback protocols and enhanced our understanding of the interplay between feedback and BCI performance.
Topological insulators are electronic phases that insulate in the bulk and accommodate a peculiar, metallic edge liquid with a spin-dependent dispersion.
They are regarded to be of considerable future use in spintronics and for quantum computation.
Besides determining the intrinsic properties of this rather novel electronic phase, considering its combination with well-known physical systems can generate genuinely new physics.
In this thesis, we report on such combinations including topological insulators. Specifically, we analyze an attached Rashba impurity, a Kondo dot in the two channel setup, magnetic impurities on the surface of a strong three-dimensional topological insulator, the proximity coupling of the latter system to a superconductor, and hybrid systems consisting of a topological insulator and a semimetal.
Let us summarize our primary results.
Firstly, we determine an analytical formula for the Kondo cloud and describe its possible detection in current correlations far away from the Kondo region.
We thereby rely on and extend the method of refermionizable points.
Furthermore, we find a class of gapless topological superconductors and semimetals, which accommodate edge states that behave similarly to the ones of globally gapped topological phases. Unexpectedly, we also find edge states that change their chirality when affected by sufficiently strong disorder.
We regard the presented research helpful in future classifications and applications of systems containing topological insulators, of which we propose some examples.
This work sheds light on different aspects of the silicon vacancy in SiC:
(1) Defect creation via irradiation is shown both with electrons and neutrons. Optical properties have been determined: the excitation of the vacancy is most efficient at excitation wavelengths between 720nm and 800nm. The PL decay yields a characteristic excited state lifetime of (6.3±0.6)ns.
(2) Defect engineering, meaning the controlled creation of vacancies in SiC with varying neutron fluence. The defect density could be engineered over eight orders of magnitude. On the one hand, in the sample with highest emitter density, the huge PL signal could even be enhanced by factor of five via annealing mechanisms. On the other hand, in the low defect density samples, single defects with photostable room temperature NIR emission were doubtlessly proven. Their lifetime of around 7ns confirmed the value of the transient measurement.
(3) Also electrical excitation of the defects has been demonstrated in a SiC LED structure.
(4) The investigations revealed for the first time that silicon vacancies can even exist SiC nanocrystals down to sizes of about 60 nm. The defects in the nanocrystals show stable PL emission in the NIR and even magnetic resonance in the 600nm fraction.
In conclusion, this work ascertains on the one hand basic properties of the silicon vacancy in silicon carbide. On the other hand, proof-of-principle measurements test the potential for various defect-based applications of the vacancy in SiC, and confirm the feasibility of e.g. electrically driven single photon sources or nanosensing applications in the near future.
In oxidischen Heterostrukturen rufen Neuordnung von Ladung und Spin eine Vielzahl von unerwarteten physikalischen Eigenschaften hervor. Die Möglichkeit, Leitfähigkeit, Magnetismus oder auch Hochtemperatur-Supraleitung zu kontrollieren, machen diese künstlich hergestellten Materialien vor allem in Hinblick auf eine zukünftige Anwendung in der Mikroelektronik äußerst interessant. Dies erfordert jedoch ein grundsätzliches Verständnis für die zugrunde liegenden Mechanismen. Die vorliegende Doktorarbeit befasst sich mit photonengestützter Spektroskopie, die einen direkten Zugang zur elektronischen Struktur dieser Heterostruktursysteme ermöglicht. Ein weiteres Ziel ist es, geeignete spektroskopische Methoden zur Charakterisierung der vergrabenen Schichten zu etablieren.
Zwei prototypische oxidische Mehrschichtsysteme stehen im Zentrum der hier vorgestellten Untersuchungen. Das LaAlO3/SrTiO3-Heterostruktursystem weist ab einer kritischen LaAlO3-Filmdicke an der Grenzfläche ein zweidimensionales Elektronensystem mit hochmobilen Ladungsträgern auf. Als treibender Mechanismus wird die elektronische Rekonstruktion diskutiert. Im Rahmen dieser Arbeit wurde dieses zweidimensionale Elektronensystem mithilfe der Photoelektronenspektroskopie und der resonanten inelastischen Röntgenstreuung charakterisiert. Die daraus bestimmten Ladungsträgerdichten weisen im Vergleich mit Daten aus Transportmessungen auf eine Koexistenz von lokalisierten und mobilen Ladungsträgern an der Grenzfläche hin. Die Analyse von Rumpfniveau- und Valenzbandspektren zeigt, dass man zur Erklärung der experimentellen Resultate ein modifiziertes Bild der elektronischen Rekonstruktion benötigt, bei der Sauerstofffehlstellen an der LaAlO3-Oberfläche als Ladungsreservoir dienen könnten. Mithilfe der resonanten Photoelektronenspektroskopie war es möglich, die metallischen Zustände am chemischen Potential impulsaufgelöst zu spektroskopieren. So gelang es erstmals, die vergrabene Fermi-Fläche einer oxidischen Heterostruktur zu vermessen. Außerdem konnten Titan-artige Zustände identifiziert werden, die höchstwahrscheinlich durch Sauerstofffehlstellen im SrTiO3 lokalisiert sind. Diese werden als mögliche Quelle für den Ferromagnetismus interpretiert, der mit der supraleitenden Phase in der LaAlO3/SrTiO3-Heterostruktur koexistiert.
Bei dem anderen hier untersuchten Mehrschichtsystem handelt es sich um die LaNiO3-LaAlO3-Übergitterstruktur. Der Einbau des metallischen LaNiO3 in eine Heterostruktur ist aufgrund seiner Nähe zu einer korrelationsinduzierten isolierenden Phase hinsichtlich einer kontrollierten Ausbildung von neuartigen Phasen besonders interessant. In der Tat beobachtet man unterhalb einer LaNiO3-Schichtdicke von vier Einheitszellen einen kontinuierlichen Metall-Isolator-Übergang, der sich in den Valenzbandspektren durch einen Verlust an Quasiteilchenkohärenz äußert. Auch wenn die impulsaufgelösten
Daten am Fermi-Niveau durch Photoelektronenbeugung beeinflusst sind, so lässt sich dennoch eine Fermi-Fläche identifizieren. Ihre Topologie bietet die Möglichkeit eines Fermi-Flächen-Nestings mit der Ausbildung einer Spindichtewelle. Die Resultate unterstützen die Hinweise auf eine magnetische Ordnung im zweidimensionalen Grundzustand.
This thesis deals with quantum Monte Carlo simulations of correlated low dimensional electron systems. The correlation that we have in mind is always given by the Hubbard type electron electron interaction in various settings. To facilitate this task, we develop the necessary methods in the first part. We develop the continuous time interaction expansion quantum algorithm in a manner suitable for the treatment of effective and non-equilibrium problems. In the second part of this thesis we consider various applications of the algorithms. First we examine a correlated one-dimensional chain of electrons that is subject to some form of quench dynamics where we suddenly switch off the Hubbard interaction. We find the light-cone-like Lieb-Robinson bounds and forms of restricted equilibration subject to the conserved quantities. Then we consider a Hubbard chain subject to Rashba spin-orbit coupling in thermal equilibrium. This system could very well be realized on a surface with the help of metallic adatoms. We find that we can analytically connect the given model to a model without spin-orbit coupling. This link enabled us to interpret various results for the standard Hubbard model, such as the single-particle spectra, now in the context of the Hubbard model with Rashba spin-orbit interaction. And finally we have considered a magnetic impurity in a host consisting of a topological insulator. We find that the impurity still exhibits the same features as known from the single impurity Anderson model. Additionally we study the effects of the impurity in the bath and we find that in the parameter regime where the Kondo singlet is formed the edge state of the topological insulator is rerouted around the impurity.
In the course of this dissertation, we have presented the interest of using spectroscopic methods to unravel the physics of polymer semiconductors in photovoltaic applications. Applying photoluminescence and photoinduced absorption spectroscopy to the reference system P3HT:PCBM has enabled us to study the major steps of photocurrent generation in organic bulk heterojunctions, from excitons generation to charges extraction and loss mechanisms and thus to improve the understanding of those mechanisms.
The exciton binding energy, is the first obstacle to overcome for photocurrent generation in organic solar cell and the reason for the use of two materials, whose heterojunction act as a driving force for charge separation. We developed an original photoluminescence-detected field-induced exciton quenching method to investigate this energy. Absorption and photoluminescence spectra of pure P3HT show that, while both amorphous and crystalline domains participate in
absorption, the energy is then transferred to the crystalline domains, from where the photoluminescence is exclusively originating. The field dependence of this photoluminescence showed that an energy of no less than 420 meV is necessary to split excitons into non photon-emitting species. Comparing those results with energy levels obtained by absorption and photoelectron spectroscopies, confirmed that the formation of those species is only a first step toward dissociation into free charges. Indeed, photoemission spectroscopy and the onset
of photocurrent upon increasing the photon energy in a pure P3HT solar cell, concomitantly show that the energy level of a pair of free polarons is located 0.7 eV above the one of the exciton. The comprehensive analysis of those results originating from those different method enable us to draw a global picture of the states and energies involved in free polarons generation in pure material. This work has been widely acknowledged by the scientific community, published in Physical Review B in 2010 [1] and presented in national [2] and international [3] conferences.
The spectroscopy of excited states is used to detect the presence of wanted species (charges) and potentially unwanted neutral species upon photoexcitation. As such, it offers us the possibility to qualify the efficiency of charge generation and, if any, identify the competing processes and the generation of unwanted species. In the frame of the European Marie Curie Research Network SolarNType,[4] this possibility was used - in combination with morphological,
charge transport and devices characterizationsn - to study a number of new donor:acceptor blends. Thanks to those techniques, we were able to not only quantify the potential of those blends, but also to provide the chemist laboratories with a precious and detailed feedback on the strengths and weakness of the molecules, regarding charge generation, transport and extraction. The detailed study of terrylene-3,4:11,12-bis(dicarboximide) as electron acceptor for
solar cells application was published in the peer review journal Synthetic Metals and was chosen to illustrate the cover page of the issue [5].
Finally, in the last chapter, we have used time resolved photoinduced absorption to improve the understanding of the charge carrier loss mechanisms in P3HT:PCBM active layers. This comprehension is of prime importance because, the fact that this recombination is far weaker than expected from the Langevin theory, enable polarons to travel further without recombining and thus to build thicker and more efficient devices. A comprehensive analysis of steady-state
PIA spectra of pure P3HT, indicates that probing at 980 nm at a temperature between 140 and 250 K enables to monitor specifically polaron densities in both neat P3HT and P3HT:PCBM. Applying this finding to transient absorption enabled us to monitor, for the first time, the bimolecular recombination in pure P3HT, and to discover that - in sharp contrast with the blend - this recombination was in agreement with the Langevin theory. Moreover, it enables us to pinpoint the important role played by the existence of two materials and of energetical traps in the slow recombination and high recombination orders observed in the blend. This work has been published in the Journal of Applied Physics.[6]
Those new insights in the photophysics of polymer:fullerene photoactive layers could have a strong impact on the future developement of those materials. Consistent measurements of the binding energy of excitons and intermediate species, would enable to clarify the role played by excess thermal energy in interfacial states dissociation. Better understanding of blends
morphology and its influence on solar cells parameters and in particular on recombination could enable to reproduce the conditions of limited recombination on material systems offering some promising performances but with only limited active layer thicknesses. However, due to the number of parameters involved, further experimentation is required, before we can reach a quantitative modeling of bimolecular recombination.
[1] Deibel et al., Phys. Rev. B, 81:085202, 2010
[2] Gorenflot et al., Deutsche Physikalische Gesellschaft Frühjahrstagung 2010, CPP20:10, Regensburg, Germany, 2010
[3] Gorenflot et al., International Conference of Synthetic Metals, 7Ax:05, Kyoto, Japan, 2010
[4] Marie-Curie RTN "SolarNTyp" Contract No. MRTN-CT-2006-035533
[5] Gorenflot et al., Synth. Met., 161(23{24):2669-2676, 2012
[6] Gorenflot et al., J. Appl. Phys., 115(14):144502, 2014
This thesis describes the epitaxial growth of the Half-Heusler alloy NiMnSb by molecular beam epitaxy. Its structural and magnetic properties are controlled by tuning the composition and the resulting small deviation from stoichiometry. The magnetic in-plane anisotropy depends on the Mn concentration of the sample and can be controlled in both strength and orientation. This control of the magnetic anisotropy allows for growing NiMnSb layers of a given thickness and magnetic properties as requested for the design of NiMnSb-based devices. The growth and characterization of NiMnSb-ZnTe-NiMnSb heterostructures is presented - such heterostructures form an all-NiMnSb based spin-valve and are a promising basis for spin torque devices.
The present thesis “Hot spin carriers in cold semiconductors” investigates hot carrier effects in low-temperature photoinduced magneto-optical Kerr effect (MOKE) microscopy of electron spins in semiconductor heterostructures. Our studies reveal that the influence of hot photocarriers in magneto-optical pump-probe experiments is twofold.
First, it is commonly assumed that a measurement of the local Kerr rotation using an arbitrary probe wavelength maps the local electron spin polarization. This is the fundamental assumption that underlies the widely used two-color MOKE microscopy technique. Our continuous-wave (cw) spectroscopy experiments demonstrate that this assumption is not correct.
At low lattice temperatures the nonresonant spin excitation by the focused pump laser inevitably leads to a strong heating of the electron system. This heating, in turn, locally modifies the magneto-optical coefficient which links the experimentally observed Kerr rotation to the electron spin polarization. As a consequence, the spin-induced local Kerr rotation is augmented by spin-unrelated changes in the magneto-optical coefficient. A spatially resolved measurement of the Kerr rotation then does not correctly map the electron spin polarization profile.
We demonstrate different ways to overcome this limitation and to correctly measure the electron spin profile. For cw spectroscopy we show how the true local electron spin polarization can be obtained from a quantitative analysis of the full excitonic Kerr rotation spectrum. Alternatively, picosecond MOKE microscopy using a spectrally broad probe laser pulse mitigates hot-carrier effects on the magneto-optical spin detection and allows to directly observe the time-resolved expansion of optically excited electron spin packets in real-space.
Second, we show that hot photocarriers strongly modify the spin diffusion process. Owing to their high kinetic energy, hot carriers greatly enhance the electron spin diffusion coefficient with respect to the intrinsic value of the undisturbed system. Therefore, for steady-state excitation the spin diffusivity is strongly enhanced close to the pump spot center where hot electrons are present. Similarly, for short delays following pulsed excitation the high initial temperature of the electrons leads to a very fast initial expansion of the spin packet which gradually slows as the electrons cool down to the lattice temperature.
While few previous publications have recognized the possible influence of hot carriers on the electron spin transport properties, the present work is the first to directly observe and quantify such hot carrier contributions. We develop models which for steady-state and pulsed excitation quantitatively describe the experimentally observed electron spin diffusion. These models are capable of separating the intrinsic spin diffusivity from the hot electron contribution, and allow to obtain spin transport parameters of the undisturbed system.
We perform extensive cw and time-resolved spectroscopy studies of the lattice temperature dependence of the electron spin diffusion in bulk GaAs. Using our models we obtain a consistent set of parameters for the intrinsic temperature dependence of the electron spin diffusion coefficient and spin relaxation time and the hot carrier contributions which quantitatively describes all experimental observations. Our analysis unequivocally demonstrates that we have, as we believe for the first time, arrived at a coherent understanding of photoinduced low-temperature electron spin diffusion in bulk semiconductors.
Ausgehend von mittels Molekularstrahlepitaxie im InGaAs/InAlAs/InP Materialsystem gewachsenen Lasermedien wurden monochromatische Quantenkaskadenlaser für die GasSensorik mit Emission im mittleren Infrarot entworfen, hergestellt und charakterisiert. Vorrangige Ziele waren hierbei die Entwicklung von leistungsstarken monomodigen Lasern im langwelligen Spektralbereich um 14 µm, sowie von Bauteilen mit weiter und schneller spektraler Abstimmbarkeit.
Für den Entwurf der Laserstege wurde zunächst die zeitliche Entwicklung der Temperaturverteilung für verschiedene Varianten von Wellenleitern sowohl im gepulsten als auch im kontinuierlichen Betrieb simuliert. Anhand der berechneten thermischen Bauteilwiderstände konnten so geeignete Prozessparameter für die Herstellung der Laserstrukturen ermittelt werden
Zur Herstellung von monochromatischen DFB-Lasern auf Basis eines MesaWellenleiters mit Seitenwandgittern wurde ein Prozess entwickelt, der sich - im Vergleichzu gängigen Verfahren zur Strukturierung von DFB-Gittern - durch eine stark reduzierte Anzahl an Verfahrenschritten und eine schnelle und einfache Durchführbarkeit auszeichnet. Für Laser mit 4 mm Länge und 14 µm mittlerer Breite wurde eine Spitzenleistung über 200 mW bei einer externen Effizienz von 330 mW/A und einer Schwellstromdichte von 2,1 kA/cm^2 bei Raumtemperatur bestimmt.
DFB-Laser um 14 µm, welche - durch die große Wellenlänge bedingt – höhere Schwellstromdichten aufweisen, wurden dagegen auf Basis von nasschemisch geätzten Doppelkanal-Wellenleitern mit in die Oberseite des Steges geätzten Gittern und dickem Gold auf den Stegflanken hergestellt, um eine bessere laterale Wärmeabfuhr zu erreichen. Basierend auf der Analyse des Strahlprofils und des Emissionsspektrums war trotz der großen Stegbreite ausschließlich Betrieb auf der Grundmode zu beobachten. So konnte eine Spitzenleistung von 810 mW bei einer Schwellstromdichte von 4,3 kA/cm^2 bei Raumtemperatur erreicht werden.
Um eine größere spektrale Abstimmbarkeit zu erreichen als dies mit DFB-Lasern möglich ist, wurde ein Lasertyp auf Basis von zwei gekoppelten Fabry-P erot Kavitäten entworfen, hergestellt und untersucht. Mit diesem Konzept konnte über eine geringe Stromvariation ein Umschalten zwischen verschiedenen Resonanzen erreicht werden, was bei konstanter Temperatur der Wärmesenke um Raumtemperatur einen Abstimmbereich von 5,2 cm^−1 ermöglichte. Unter Einbeziehung einer Variation der Temperatur der Wärmesenke konnte monomodige Emission in einem Spektralbereich von 52 cm^−1 erreicht und die Tauglichkeit der Laser für die Gas-Sensorik anhand einer Absorptionsmessung an Ammoniak demonstriert werden.
Da die monomodige Spitzenleistung dieser Laser jedoch konzeptbedingt auf wenige mW beschränkt war, wurde für den Einsatz weit abstimmbarer Laser in der Spurengasanalytik im letzten Teil der Arbeit ein anderer Lasertyp mit flachgeätztem Bragg-Reflektor entwickelt. Durch sorgfältige Wahl der Gitterparameter und ein spezielles Puls-Schema wurde eine über 30 cm^−1 quasi-kontinuierlich abstimmbare, monomodige Emission erreicht. Die Stabilität und die spektrale Reinheit des Laserlichts mit einer Seitenmodunterdrückung von mehr als 30 dB konnte anhand von zeitaufgelösten Messungen des Abstimmvorgangs und durch ein Absorptionsexperiment mit Ethen belegt werden. Die erzielte spektrale Auflösung war durch die Messelektronik begrenzt und betrug 0,0073 cm^-1. Zudem ergab sich auch die Möglichkeit einer Analyse des thermischen Übersprechens, welche einen vernachlässigbaren Einfluss für den Pulsbetrieb der Laser zeigte und eine moderate Erwärmung benachbarter Segmente um 10% des für das vorsätzlich beheizte Segment gemessenen Wertes. Des Weiteren konnte dank der Möglichkeit zur unabhängigen Strominjektion in verschiedene Sektionen die Temperaturabhängigkeit von Verstärkung und Absorption im Resonator untersucht werden. Herausstechende Eigenschaften dieser Laser wie die Verringerung der gepulsten Chirprate im Vergleich zu DFB-Lasern um den Faktor 3 konnten anhand von systematischen Untersuchungen mit einer Vielzahl von Bauteilen analysiert und auf die zeitlicheTemperaturentwicklung bzw. die räumliche Temperaturverteilung im Lasersteg zurückgeführt werden. Die optische Spitzenleistung von 600 mW und externe Effizienzen bis 300mW/A sollten auch den Einsatz in der Spurengasanalyse erlauben, die hohe Geschwindigkeit mit der die Emissionswellenlänge variiert werden kann, überdies die Untersuchung der Reaktionskinetik in der Gasphase.
Recently a new state of matter was discovered in which the bulk insulating state in a material is accompanied by conducting surface or edge states. This new state of matter can be distinguished from a conventional insulator phase by the topological properties of its band structure which led to the name "topological insulators". Experimentally, topological insulator states are mostly found in systems characterized by a band inversion compared to conventional systems. In most topological insulator systems, this is caused by a combination of energetically close bands and spin orbit coupling. Such properties are found in systems with heavy elements like Hg and Bi. And indeed, the first experimental discovery of a topological insulator succeeded in HgTe quantum wells and later also in BiSb bulk systems.
Topological insulators are of large interest due to their unique properties: In 2-dimensional topological insulators one dimensional edge states form without the need of an external magnetic field (in contrast to the quantum Hall effect). These edge states feature a linear band dispersion, a so called Dirac dispersion. The quantum spin Hall states are helical edge states, which means they consist of counterpropagating oppositely spin polarized edge channels. They are therefore of great potential for spintronic applications as well as building blocks for new more exotic states like Majorana Fermions. 3-dimensional topological insulators feature 2-dimensional surface states with only one Dirac band (also called Dirac cone) on each surface and an interesting spin texture where spin and momentum are locked perpendicular to each other in the surface plane. This unique surface band structure is predicted to be able to host several exotic states like e.g. Majorana Fermions (in combination with superconductors) and magnetic monopole like excitations.
This PhD thesis will summarize the discovery of topological insulators and highlights the developments on their experimental observations. The work focuses on HgTe which is up to now the only topological insulator material where the expected properties are unambiguously demonstrated in transport experiments. In HgTe, the topological insulator properties arise from the inversion of the Gamma_6 and Gamma_8 bands. The band inversion in HgTe is due to a combination of a high spin orbit splitting in Te and large energy corrections (due to the mass-velocity term) to the energy levels in Hg. Bulk HgTe, however, is a semimetal, which means for the conversion into a topological insulator a band gap has to be opened. In two dimensions (HgTe quantum well structures) this is achieved via quantum confinement, which opens a band gap between the quantum well subbands. In three dimensions, strain is used to lift the degeneracy of the semimetallic Gamma_8 bands opening up a band gap.
The thesis is structured as follows:
- The first chapter of this thesis will give a brief overview on discoveries in the field of topological insulators. It focuses on works relevant to experimental results presented in the following chapters. This includes a short outline of the early predictions and a summary of important results concerning 2-dimensional topological insulators while the final section discusses observations concerning 3-dimensional topological insulators.
- The discovery of the quantum spin Hall effect in HgTe marked the first experimental observation of a topological insulator. Chapter 2 will focus on HgTe quantum wells and the quantum spin Hall effect.
Above a critical thickness, HgTe quantum wells are predicted to host the quantum spin Hall state, the signature of a 2-dimensional topological insulator. HgTe quantum wells exhibiting low carrier concentrations and at the same time high carrier mobilities are required to be able to measure the quantum spin Hall effect. The growth of such high quality HgTe quantum wells was one of the major goals for this work. Continuous optimization of the substrate preparation and growth conditions resulted in controlled carrier densities down to a few 10^10 cm^-2. At the same time, carrier mobilities exceeding 1 x 10^6 cm^2/Vs have been achieved, which provides mean free paths of several micrometers in the material. Thus the first experimental evidence for the existence of the quantum spin Hall edge states succeeded in transport experiments on microstructures: When the Fermi energy was located in the bulk band gap a residual quantized resistance of 2e^2/h was found. Further experiments focused on investigating the nature of transport in this regime. By non-local measurements the edge state character could be established. The measured non-local resistances corresponded well with predictions from the Landauer-Büttiker theory applied to transport in helical edge channels.
In a final set of experiments the spin polarization of the edge channels was investigated. Here, we could make use of the advantage that HgTe quantum well structures exhibit a large Rashba spin orbit splitting. In systems with a large Rashba spin orbit splitting a spin accumulation is expected to occur at the edge of the sample perpendicular to a current flow. This so-called spin Hall effect was then used as a spin injector and detector. Using split gate devices it was possible to bring spin Hall and quantum spin Hall state into direct contact, which enabled an all electrical detection of the spin polarization of the quantum spin Hall edge channels.
- HgTe as a 3-dimensional topological insulator will be presented in chapter 3. Straining the HgTe layer enables the observation of topological insulator behavior. It was found that strain can be easily implemented during growth by using CdTe substrates. CdTe has a slightly larger lattice constant than HgTe and therefore leads to tensile strain in the HgTe layer as long as the growth is pseudomorphic. Magnetotransport studies showed the emergence of quantum Hall transport with characteristic signatures of a Dirac type bandstructure. Thus, this result marks the first observation of the quantum Hall effect in the surface states of a 3-dimensional topological insulator.
Transport experiments on samples fitted with a top gate enabled the identification of contributions from individual surfaces. Furthermore, the surface state quantum Hall effect was found to be surprisingly stable, perturbations due to additional bulk transport could not be found, even at high carrier densities of the system.
- Chapters 4 - 6 serve as in depth overviews of selected works: Chapter 4 presents a detailed overview on the all electrical detection of the spin Hall effect in HgTe quantum wells. The detection of the spin polarization of the quantum spin Hall effect is shown in chapter 5 and chapter 6 gives a detailed overview on the quantum Hall effect originating from the topological surface state in strained bulk HgTe.
The investigations discussed in this thesis pioneered the experimental work on the transport properties of topological insulator systems. The understanding of the fundamental properties of topological insulators enables new experiments in which e.g. the inclusion of magnetic dopants or the interplay between topological insulator and superconductors can be investigated in detail.
This thesis presents the detailed development of the fabrication process and the first observations of artificial magnetic atoms from the II-VI diluted magnetic semiconductor alloy (Zn,Cd,Be,Mn)Se. In order to manufacture the vertical quantum dot device which exhibits artificial atom behavior a number of development steps are conducted. First, the II-VI heterostructure is adjusted for the linear transport regime. Second, state of the art vertical quantum dot fabrication techniques in the III-V material system are investigated regarding their portability to the II-VI heterostructure. And third, new approaches to the fabrication process are developed, taking into account the complexity of the heterostructure and its physical properties. Finally a multi-step fabrication process is presented, which is built up from electron beam and optical lithography, dry and wet etching and insulator deposition. This process allows for the processing of pillars with diameters down to 200 nm with an insulating dielectric and gate. Preliminary transport data on the fabricated vertical quantum dots are presendted confirming the magnetic nature of the resulting artificial atoms.
The discovery of the Giant Magneto Resistance (GMR) effect in 1988 by Albert Fert [Baib 88] and Peter Grünberg [Bina 89] led to a rapid development of the field of spintronics and progress in the information technology. Semiconductor based spintronics, which appeared later, offered a possibility to combine storage and processing in a single monolithic device. A direct result is reduced heat dissipation. The observation of the spin Seebeck effect by Ushida [Uchi 08] in 2008 launched an increased interest and encouraged research in the field of spin caloritronics. Spintronics is about the coupling of charge and spin transport. Spin caloritronics studies the interaction between heat and spin currents. In contrast to spintronics and its variety of applications, a particular spin-caloritronic device has not yet been demonstrated. However, many of the novel phenomena in spin caloritronics can be detected in most spintronic devices. Moreover, thermoelectric effects might have a significant influence on spintronic device operation. This will be of particular interest for this work. Additional knowledge on the principle of coupling between heat and spin currents uncovers an alternative way to control heat dissipation and promises new device functionalities.
This thesis aims to further extend the knowledge on thermoelectrics in materials with strong spin-orbit coupling, in this case the prototypical ferromagnetic semiconductor (Ga,Mn)As. The study is focused on the thermoelectric / thermomagnetic effects at the interface between a normal metal and the ferromagnetic (Ga,Mn)As. In such systems, the different interfaces provide a condition for minimal phonon drag contribution to the thermal effects. This suggests that only band contributions (a diffusion transport regime) to these effects will be measured.
Chapter 2 begins with an introduction on the properties of the studied material system, and basics on thermoelectrics and spin caloritronics. The characteristic anisotropies of the (Ga,Mn)As density of states (DOS) and the corresponding magnetic properties are described. The DOS and magnetic anisotropies have an impact on the transport prop- erties of the material and that results in effects like tunneling anisotropic magnetores- istance (TAMR) [Goul 04]. Some of these effects will be used later as a reference to the results from thermoelectric / thermomagnetic measurements. The Fingerprint tech- nique [Papp 07a] is also described. The method gives an opportunity to easily study the anisotropies of materials in different device geometries.
Chapter 3 continues with the experimental observation of the diffusion thermopower of (Ga,Mn)As / Si-doped GaAs tunnel junction. A device geometry for measuring the diffusion thermopower is proposed. It consists of a Si - doped GaAs heating channel with a Low Temperature (LT) GaAs / (Ga,Mn)As contact (junction) in the middle of the channel. A single Ti / Au contact is fabricated on the top of the junction. For transport characterization, the device is immersed in liquid He. A heating current technique is used to create a temperature difference by local heating of the electron system on the Si:GaAs side. An AC current at low frequency is sent through the channel and it heats the electron population in it, while the junction remains at liquid He temperature (experimentally con- firmed). A temperature difference arises between the heating channel and the (Ga,Mn)As contact. As a result, a thermal (Seebeck) voltage develops across the junction, which we call tunnelling anisotropic magneto thermopower (TAMT), similar to TAMR. TAMT is detected by means of a standard lock-in technique at double the heating current frequency (at 2f ). The Seebeck voltage is found to be linear with the temperature difference. That dependence suggests a diffusion transport regime. Lattice (phonon drag) contribution to the thermovoltage, which is usually highly nonlinear with temperature, is not observed.
The value of the Seebeck coefficient of the junction at 4.2 K is estimated to be 0.5 µV/K.
It is about three orders of magnitude smaller than the previously reported one [Pu 06]. Subsequently, the thermal voltage is studied in external magnetic fields. It is found that the thermopower is anisotropic with the magnetization direction. The anisotropy is explained with the anisotropies of the (Ga,Mn)As contact. Further, switching events are detected in the thermopower when the magnetic field is swept from negative to positive fields. The switchings remind of a spin valve signal and is similar to the results from previous experiments on spin injection using a (Ga,Mn)As contacts in a non-local detection scheme. That shows the importance of the thermoelectric effects and their possible contribution to the spin injection measurements. A polar plot of the collected switching fields for different magnetization angles reveals a biaxial anisotropy and resembles earlier TAMR measurements of (Ga,Mn)As tunnel junction. A simple cartoon model is introduced to describe and estimate the expected thermopower of the studied junction. The model yields a Fermi level inside of the (Ga,Mn)As valence band. Moreover, the model is found to be in good agreement with the experimental results.
The Nernst effect of a (Ga,Mn)As / GaAs tunnel junction is studied in Chapter 4. A modified device geometry is introduced for this purpose. Instead of a single contact on the top of the square junction, four small contacts are fabricated to detect the Nernst signal. A temperature difference is maintained by means of a heating current technique described in Chapter 3. A magnetic field is applied parallel to the device plane. A voltage drop across two opposite contacts is detected at 2f. It appears that a simple cosine function with a parameter the angle between the magnetization and the [100] crystal direction in the (Ga,Mn)As layer manages to describe this signal which is attributed to the anomalous Nernst effect (ANE) of the ferromagnetic contact. Its symmetry is different than the Seebeck effect of the junction. For the temperature range of the thermopower measurements the ANE coefficient has a linear dependence on the temperature difference (∆T). For higher ∆T, a nonlinear dependence is observed for the coefficient. The ANE coefficient is found to be several orders of magnitude smaller than any Nernst coefficient in the literature. Both the temperature difference and the size of the ANE coefficient require further studies and analysis. Switching events are present in the measured Nernst signal when the magnetic field is swept from positive to negative values. These switchings are related to the switching fields in the ferromagnetic (Ga,Mn)As. Usually, there are two states which are present in TAMR or AMR measurements - low and high resistance. Instead of that, the Nernst signal appears to have three states - high, middle and low thermomagnetic voltage. That behaviour is governed not only by the magnetization, but also by the characteristic of the Nernst geometry.
Chapter 5 summarizes the main observations of this thesis and contains ideas for future work and experiments.
Die vorliegende Arbeit beschreibt die Charakterisierung von Halbleiter-Quantenpunkten (QP) in unterschiedlichen Materialsystemen. Die hier dargelegten Untersuchungen wurden mit verschiedenen Methoden der optischen Spektroskopie durchgeführt. Zu Beginn der Arbeit werden theoretische Grundlagen von QP hinsichtlich ihrer elektronischen Struktur und statistischen Eigenschaften erläutert. Darüber hinaus wird näher auf die Physik von Solarzellen eingegangen, in dem die relevanten Gleichungen für die Beschreibung des Ladungsträgertransportes hergeleitet und diskutiert werden.
Darauf folgend werden die experimentelle Methoden erklärt, welche zur Charakterisierung der jeweiligen Proben dienten. Besonderes Augenmerk wird auf die Methode zur Messung des Zwei-Photonen-Absorptionsprozesses gelegt.
Der Abschnitt der experimentell gewonnenen Ergebnisse beginnt mit Untersuchungen an einzelnen, spektral isolierten InP QP, welche mit ultralangsamen Wachstumsraten hergestellt wurden. Aufgrund der sehr geringen Flächendichte konnten grundlegende physikalische Eigenschaften von QP ohne zusätzliche laterale Strukturierungen studiert werden. Mittels Messungen in Abhängigkeit der Anregungsleistung und Detektion in Abhängigkeit der Polarisation konnten die verschiedenen Lumineszenzlinien eines QP-Spektrums den jeweiligen exzitonischen Zuständen zugeordnet werden. Zusätzlich wurden die QP in einem externen Magnetfeld in Faraday-Konfiguration untersucht. Abschließend durchgeführte Autokorrelationsmessungen erlaubten die Untersuchung der zeitlichen Statistik der QP-Photonen. Es konnte die Emission einzelner Photonen nachgewiesen werden.
Anschließend folgen spektroskopische Untersuchungen von InP QP, welche mittels sequentiellen Wachstums hergestellt wurden. Anhand von Messungen in Abhängigkeit der Anregungsleistung und bestätigt durch zeitaufgelöste Messungen am QP-Ensemble wurde eine bimodale QP-Verteilung mit Typ-I und Typ-II Bandverlauf bestimmt. Zusätzlich konnten an einzelnen, spektral isolierten QP verschiedene Exziton-Zustände identifiziert werden, bevor abschließend Autokorrelationsmessungen die Emission einzelner Photonen demonstrierten.
Zur Steigerung der Auskoppeleffizienz der Photonen wurden InP QP in Mikrosäulenresonatoren, bestehend aus zwei Bragg-Spiegeln mit einer dazwischenliegenden GaInP Kavität, eingebettet. Anfangs wurde die Emission der Kavitätsmode von Strukturen mit unterschiedlichen lateralen Durchmessern charakterisiert. Mittels Temperaturverstimmung konnte die Energie eines einzelnen QP-Exzitons in Resonanz mit der Resonatormode gebracht werden. Im Regime der schwachen Wechselwirkung wurde eine signifikante Überhöhung der Lumineszenzintensität aufgrund des Purcell-Effektes gemessen. Zusätzlich wurde im Regime der schwachen Kopplung die Emission einzelner Photonen anhand von Korrelationsmessungen nachgewiesen. Im zweiten Schritt wurden die QP-Mikrosäulenresonatorstrukturen elektrisch angeregt. Nach einer grundlegenden Charakterisierung konnte auch hier mittels Temperaturverstimmung die Energie der Resonatormode mit der eines Exziton in Resonanz gebracht werden. Im Regime der schwachen Wechselwirkung stieg die Intensität der Lumineszenz aufgrund des Purcell-Effekts signifikant an. Zum Abschluss bestätigen Korrelationsmessungen den Nachweis der Emission einzelner Photonen.
In Kapitel 6 werden die Eigenschaften von InGaN QP genauer analysiert. Nitrid-Verbindungshalbleiter kristallieren vorzugsweise stabil in der Wurtzit-Kristallstruktur. Polare Kristallebenen mit fehlender Spiegelsymmetrie führen zu starken piezoelektrischen Feldern. Dies hat eine Lumineszenz mit ausgeprägter linearer Polarisation zur Folge hat. Diese Eigenschaft wurde mittels statistischen Untersuchungen näher betrachtet. Zusätzlich erlaubten Messungen in Abhängigkeit der Anregungsleistung die verschiedenen Exziton-Zustände eines QP zu identifizieren. Zudem wurde die Emission einzelner Photonen durch InGaN QP demonstriert, erstmals sogar bis zu einer Temperatur von 50 K.
Im abschliessenden Kapitel wird eine mögliche Anwendung von QP präsentiert, bei der Eigenschaften in Bauteilen gezielt ausgenutzt werden, um die Bandbreite der Photonenabsorption zu erhöhen. Das Konzept der Zwischenband-Solarzellen verspricht auch Photonen mit einer Energie kleiner der Bandlücke des umgebenden Materials aufnehmen zu können und somit den spektralen Absorptionsbereich zu erweitern. Für eine systematische Untersuchung wurden verschiedene Proben mit integrierten AlGaInAs QP hergestellt. Anhand der Strom-Spannungs-Kennlinien der jeweiligen Proben im Dunkeln und unter Beleuchtung konnten wichtige Solarzellenparameter bestimmt werden. Spektrale Messungen liefern Informationen über die externe Quanteneffizienz der Proben. Entscheidend für den experimentellen Nachweis des Funktionsprinzips der Zwischenband-Solarzellen ist die Messung der Zwei-Photonen-Absorption für zwei Photonen mit jeweils kleineren Energien als der Bandlücke des umgebenden Materials.
Die vorliegende Arbeit beschäftigt sich mit der Licht-Materie-Wechselwirkung in Quantenpunkt-Mikroresonatoren und deren vertikalen und lateralen Emissionseigenschaften. Quantenpunkte sind nanoskopische Strukturen, in denen die Beweglichkeit der Ladungsträger unterhalb der de-Broglie-Wellenlänge eingeschränkt ist, wodurch die elektronische Zustandsdichte diskrete Werte annimmt. Sie werden daher auch als künstliche Atome bezeichnet. Um die Emissionseigenschaften der Quantenpunkte zu modifizieren, werden sie im Rahmen dieser Arbeit als aktive Schicht in Mikrosäulenresonatoren eingebracht. Diese bestehen aus einer GaAs lambda-Kavität, die zwischen zwei Braggspiegeln aus alternierenden GaAs und AlAs Schichten eingefasst ist. Diese Resonatoren bieten sowohl eine vertikale Emission über Fabry-Perot Moden, als auch eine laterale Emission über Flustergaleriemoden. Die Licht-Materie-Wechselwirkung zwischen den Resonatormoden und lokalisierten Ladungsträgern in den Quantenpunkten, genannt Exzitonen, kann in zwei Regime unterteilt werden. Im Regime der starken Kopplung wird der spontane Emissionsprozess in einem Quantenpunkt reversibel und das emittierte Photon kann wieder durch den Quantenpunkt absorbiert werden. Die theoretische Beschreibung der Kopplung eines Exzitons an die Resonatormode erfolgt über das Jaynes-Cummings Modell und kann im Tavis-Cummings Modell auf mehrere Emitter erweitert werden. Ist die Dämpfung des Systems zu gross, so befindet man sich im Regime der schwachen Kopplung, in dem die Emissionsrate des Quantenpunkts durch den Purcell-Effekt erhöht werden kann. In diesem Regime können Mikrolaser mit hohen Einkopplungsraten der spontanen Emission in die Resonatormode und niedrigen Schwellpumpströmen realisiert werden. Zur Charakterisierung der Proben werden vor allem die Methoden der Mikro-Elektrolumineszenz und der Photonenkorrelationsmessungen eingesetzt.
In the field of organic photovoltaics, one of the most intensely researched topics to date is the charge carrier photogeneration in organic bulk heterojunction solar cells whose thorough understanding is crucial for achieving higher power conversion efficiencies. In particular, the mechanism of singlet exciton dissociation at the polymer–fullerene interface is still controversially debated.
This work addresses the dissociation pathway via relaxed charge transfer states (CTS) by investigating its field dependence for reference material systems consisting of MDMO-PPV and one of the fullerene derivatives PC61BM, bisPCBM and PC71BM. Field dependent photoluminescence (PL(F)) and transient absorption (TA(F)) measurements give insight into the recombination of charge transfer excitons (CTE) and the generation of polarons, respectively. Optically detected magnetic resonance and atomic force microscopy are used to characterize the morphology of the samples.
The comparison of the experimental field dependent exciton recombination recorded by PL(F) and the theoretical exciton dissociation probability given by the Onsager–Braun model yields the exciton binding energy as one of the key parameters determining the dissociation efficiency. The binding energies of both the singlet exciton in neat MDMO-PPV and the CTE in MDMO-PPV:PC61BM 1:1 are extracted, the latter turning out to be significantly reduced with respect to the one of the singlet exciton.
Based on these results, the field dependence of CTE dissociation is evaluated for MDMO-PPV:PC61BM blends with varying fullerene loads by PL(F) and TA(F). For higher PC61BM contents, the CTE binding energies decrease notably. This behavior is ascribed to a larger effective dielectric constant for well-intermixed blends and to an interplay between dielectric constant and CTE delocalization length for phase separated morphologies, emphasizing the importance of high dielectric constants for the charge carrier photogeneration process.
Finally, the CTE binding energies are determined for MDMO-PPV blends with different fullerene derivatives, focusing on the influence of the acceptor LUMO energy. Here, the experimental results suggest the latter having no or at least no significant impact on the binding energy of the CTE. Variations of this binding energy are rather related to different trap levels in the acceptors which seem to be involved in CTS formation.
Elektrooptische Transporteigenschaften und stochastisch aktivierte Prozesse Resonanter Tunneldioden
(2012)
Im Rahmen der vorliegenden Arbeit wurden elektrooptische Transporteigenschaften und stochastisch aktivierte Prozesse Resonanter Tunneldioden (RTDs) bei Raumtemperatur untersucht. Die RTDs wurden auf dem III-V Halbleitermaterialsystem AlGaAs/GaAs durch Molekularstrahlepitaxie, Elektronenstrahllithographie und trockenchemischen Ätztechniken hergestellt. Im Bereich des negativen differentiellen Leitwerts konnte bistabi-les Schalten und hierbei stochastisch aktivierte Dynamik nichtlinearer Systeme untersucht werden. Die Flächenabhängigkeit der Ätzrate konnte ausgenutzt werden, um RTDs mit einem Stamm und zwei Transportästen zu realisieren, welche hinsichtlich ihrer optischen und elektrischen Eigenschaften untersucht wurden. Im ersten experimentellen Abschnitt 3.1 werden die elektrischen Transporteigenschaften Resonanter Tunneldioden bei Raum-temperatur und die Flächenabhängigkeit des kohärenten und nicht-kohärenten Elektronen-transports analysiert. Die Realisierung universeller logischer Gatter (NOR und NAND) und deren Rekonfigurierbarkeit durch einen externen Kontrollparameter wird in Abschnitt 3.2 gezeigt. In Abschnitt 3.3 wird die Lichtsensitivität Resonanter Tunneldioden als Photode-tektoren für den sichtbaren Wellenlängenbereich und in Abschnitt 3.4 für die Telekommu-nikationswellenlänge bei λ = 1,3 µm demonstriert.