14 Externe wissenschaftliche Einrichtungen
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Item Open Access Towards spin injection into silicon(2007) Dash, Saroj Prasad; Carstanjen, Heinz Dieter (Prof. Dr.)The efficient spin injection into semiconductors could pave the way to a new generation of electronics devices such as spin memories, spin transistors, and spin quantum computers. The most important semiconductor for industrial application, Si has been studied for the purpose of spin injection extensively in this thesis. Three different concepts for spin injection into Si have been addressed: (1) spin injection through a ferromagnet-Si Schottky contact, (2) spin injection using MgO tunnel barriers in between the ferromagnet and Si, and (3) spin injection from Mn-doped Si (DMS) as spin aligner. (1) FM-Si Schottky contact for spin injection: In a heterostructure of a ferromagnetic thin film on a Si substrate, any structural disorder at the interface would drastically reduce the spin polarization at the interface and, hence, the spin injection efficiency. To be able to improve the interface qualities one needs to understand the atomic processes involved in the formation of such silicide phases. In order to obtain more detailed insight into the formation of such silicide phases the initial stages of growth of Co and Fe were studied in situ by HRBS with monolayer depth resolution. As understood, it was important to prohibit the in-diffusion of Co into interstitial sites at the initial stages of growth and the out-diffusion of Si atoms in the latter stages. So in order to control and improve the interface, equilibrium growth conditions were followed (i) by lowering the growth temperature and (ii) by surfactant-mediated growth. Low temperature growth of Co on Si (100): Already at very low coverage Co diffusion into the bulk Si has been observed. The amount of in-diffused Co is, however, less than at room temperature. In contradiction to room temperature growth, Co atoms form layers of pure Co on top of the Si surface already at very low coverage. Every second Si layer, starting with the first Si layer, is Co depleted. This leads to an oscillatory Co distribution in the Si lattice which is preserved up to higher coverages (1.3 ML). Surfactant-mediated growth of Co on Si (100) : The lower surface free energy of Sb in comparison to Co and Si, makes it a potential candidate for surfactant mediated growth. By the use of one monolayer of Sb adsorbed on a Si (100) surface, Co-Si intermixing at the interface is strongly reduced in comparison to the interface without Sb as surfactant. The improved interface quality with Sb-mediated growth is also reflected in magnetic measurements. Co with Sb-mediated growth shows a higher magnetic moment. It was shown that simple solutions can reduce the FM-Si inter diffusion at the interface and improve the interface quality. However these non-equilibrium growth conditions could not stop the silicide formation completely. (2) MgO tunnel barrier for spin injection into Si: On the other hand, using an ultra-thin tunnel barrier between FM and Si will have three advantages: (i) form a chemical barrier between the FM and Si, (ii) circumvent the conductivity mismatch problem, and (iii) in addition act as a spin filter. The fabrication and characterization of ultra-thin crystalline MgO tunnel barriers on Si (100) was presented. Some of the important properties required for tunnel barriers on Si have been addressed. Ultra-thin stoichiometric MgO tunnel barriers with sharp interface with Si (100), very homogeneous, without pin-holes, and crystalline in structure could be fabricated by reactive molecular beam epitaxy. Co and Fe on an ultra thin MgO tunnel barrier were found to have island-like growth with a rough surface. Ultra-thin Co and Fe films are found to be thermally quite stable up to 450 °C. (3) Mn doped Si for spin injection: For spin injection purpose, instead of contacting the Si with a ferromagnetic metal, the contact could be made with another semiconductor, one with ferromagnetic properties. This solves the conductivity mismatch problem by ensuring that the resistivities of the materials on both side of the interface are comparable in magnitude. Si-based diluted magnetic semiconductor samples were prepared by doping Si with Mn by two different methods i) by Mn ion implantation and ii) by in-diffusion of Mn atoms (solid state growth). In the case of implanted samples, Mn atoms do not substitute Si sites. The implanted samples show room temperature ferromagnetism as measured by a SQUID magnetometer. The magnetic moment per Mn atom is found to decrease with increasing implantation dose. It has been observed that the implanted samples show carrier mediated ferromagnetism and, more importantly, mediated by both holes and electrons in contrast to statements in the literature. Solid state growth of Mn doped Si : For evaporation of Mn on Si (100), Mn atoms diffuse deep into the Si bulk already at room temperature, even for very low coverage (0.25 ML) with an oscillatory concentration depth profile as observed by HRBS with monolayer depth resolution. This results in natural MnxSi1-x/Si digital layers on the surface. Surprisingly, the samples prepared by this solid state diffusion process show room-temperature ferromagnetism having a magnetic moment of 1.8 µB per Mn atom, which is much higher than that of the ion-implanted samples. In contrast to ion-implanted samples the ferromagnetism in these samples does not show any carrier mediation.Item Open Access Resonante magnetische Röntgenuntersuchungen an einem Co/Cu/Co-Schichtsystem und an Platinlegierungen(2006) Grüner, Uwe; Schütz, Gisela (Prof. Dr.)Im Rahmen dieser Arbeit wurde das magnetische Tiefenprofil eines Co/Cu/Co-Schichtsystem mit Hilfe der resonanten magnetischen Röntgenreflektometrie (XRMR) untersucht. Dazu wurde als Funktion des Einfallswinkels das Asymmetrieverhältnis an der CoL3- und an der CuL3-Kante gemessen. Die zugehörigen resonanten optischen Konstanten wurden über ein gesondertes XMCD-Experiment ermittelt. Mit Hilfe eines neu entwickelten Programm auf der Basis des Parratt-Formalismus konnten die gemessenen Asymmetrien simuliert und quantitative Änderungen an den Grenzflächen der Kobaltschichten sowie induzierte magnetische Effekte im Kupfer ermittelt werden. Weiterhin wurden XMCD-Messungen an der PtL3-Kante durchgeführt. Mit Hilfe einer weiterentwickelten, auf digitaler Lock-In-Technik basierenden Methode mit einem Phasenschieber wurden drei verschiedene Platinlegierungen untersucht und der induzierte Magnetismus im Platin quantitativ bestimmt.Item Open Access Kritische Phänomene auf chemisch strukturierten Substraten(2006) Sprenger, Monika; Dietrich, Siegfried (Prof. Dr.)Chemisch strukturierte Substrate haben zunehmend an Bedeutung gewonnen seit es möglich ist, Oberflächen im Bereich von Mikrometern und darunter zu strukturieren. Auf diesen kleinen Skalen wird die Wechselwirkung der Flüssigkeiten mit dem Substrat wichtig und eine chemische Strukturierung der Substrate verursacht eine reiche Grenzflächenstruktur, die von den molekularen Details des lokalen Kraftfeldes anhängt. Konzentriert man sich jedoch auf das Gebiet um den kritischen Punkt eines Phasenübergangs zweiter Ordnung, werden die molekularen Details unbedeutend und das System zeigt ein universelles Verhalten, das durch kritische Exponenten, nicht-universelle Amplituden und universelle Skalenfunktionen beschrieben wird. Systeme mit kritischen Punkten werden bezüglich ihres Bulk-Verhaltens klassifiziert und Universalitätsklassen zugeordnet. Bei Systemen, die durch ein Substrat oder eine freie Oberfläche begrenzt werden, spalten die Universalitätsklassen in Oberflächen-Universalitätsklassen bezüglich des kritischen Verhaltens an der Oberfläche auf. Physikalisch unterschiedliche Systeme können zur selben Universalitätsklasse gehören: einkomponentige Flüssigkeiten in der Nähe ihres kritischen Punktes zwischen Flüssigkeit und Gas gehören ebenso wie binäre Flüssigkeitsmischungen nahe ihres kritischen Punktes der Entmischung - die in dieser Arbeit betrachtet werden - und uniaxiale Ferromagnete nahe der Curie-Temperatur zur Ising-Universalitätsklasse. Die Universalitätsklassen werden durch die Reichweite der Wechselwirkung, die räumliche Dimension des Systems und die Dimension des Ordnungsparameters bestimmt. Für eine binäre Flüssigkeitsmischung lässt sich der Ordnungsparameter, der den Grad der Ordnung im System beschreibt, entweder als Differenz der Konzentrationen der beiden Flüssigkeiten oder als Konzentration einer der Flüssigkeiten minus ihrer Konzentration am kritischen Entmischungspunkt definieren. Das Thema dieser Arbeit sind die kritischen Phänomene, die auftreten, wenn eine binäre Flüssigkeitsmischung, die sich in der Umgebung ihres kritischen Entmischungspunktes befindet, mit einem topologisch flachen, chemisch strukturierten Substrat in Kontakt gebracht wird. Dabei verursacht der chemische Kontrast unterschiedliche lokale Präferenzen für die beiden Spezies der binären Flüssigkeitsmischung. In der vorliegenden Arbeit werden drei verschiedene Typen von chemisch strukturierten Substraten betrachtet: eine chemische Stufe (wichtig für das Verständnis von lokalen Eigenschaften einer Flüssigkeit an der Grenze von chemischen Streifen), ein einzelner chemischer Streifen (das einfachste chemische Muster auf einer Oberfläche) und ein periodisches Streifenmuster (als Beispiel für die Adsorption an heterogenen Oberflächen). Die Ordnungsparameterprofile und ihre Temperaturabhängigkeit sind durch universelle Skalenfunktionen gegeben, die im Rahmen der Mean-Field-Theorie berechnet werden. Die Skalenfunktionen und der Einfluss der chemischen Streifen werden in der Arbeit eingehend untersucht. Wird eine Flüssigkeit, die von zwei Substraten eingeschlossen wird, in die Nähe ihres kritischen Punkts gebracht, entsteht aufgrund der Randbedingungen, die das Spektrum der kritischen Fluktuationen des Ordnungsparameters einschränken, eine auf die Substrate wirkende effektive Kraft ("kritische Casimir-Kraft"). In dieser Arbeit werden die singulären Beiträge der effektiven Kraft untersucht, die auf chemisch inhomogene Substrate wirken, welche binäre Flüssigkeitsmischungen begrenzen. Es werden vier grundlegende Konfigurationen zweier geometrisch flachen, parallelen Substrate mit periodischen chemischen Mustern aus Streifen mit positiven und Streifen mit negativen Oberflächenfeldern betrachtet: zwei Substrate mit den gleichen Streifenmustern (d.h. ein positiver Streifen liegt gegenüber einem positiven Streifen), zwei Substrate mit entgegengesetzten Streifenmustern (d.h. ein positiver Streifen liegt gegenüber einem negativen Streifen), ein strukturiertes und ein homogenes Substrat und abschließend zwei Substrate mit den gleichen Streifenmustern, die aber gegeneinander verschoben sind (d.h. ein positiver Streifen liegt teilweise einem positiven und teilweise einem negativen Streifen gegenüber). Das universelle Verhalten der Ordnungsparameterprofile und der effektiven Kräfte, die auf die Substrate wirken, wird durch universelle Skalenfunktionen beschrieben. Die Skalenfunktionen der Ordnungsparameterprofile werden im Rahmen der Mean-Field-Theorie numerisch berechnet und daraus mittels des Stress-Tensors die Kräfte zwischen den Substraten abgeleitet. Die Abhängigkeit der Skalenfunktionen der Kräfte von der Distanz zwischen den Substraten, von den Streifenbreiten und - im Fall des verschobenen Streifenmusters - von der relativen Verschiebung wird untersucht. Es werden verallgemeinerte Casimir-Amplituden definiert und ihre Abhängigkeit von der chemischen Strukturierung der Substrate betrachtet.Item Open Access Density profiles of ionic liquids at a hard wall(2008) Schramm, Sebastian M.In this work a high energy x-ray reflectivity study of deeply buried interfaces between room temperature ionic liquids (RTILs) and a sapphire hard wall is reported. For the first time the interfacial structure was obtained with molecular resolution. The experiments have been carried out at beamline ID15A (ESRF, Grenoble) using the HEMD (High Energy Micro Diffraction) instrument. The thorough analysis of the experimental reflectivities gives clear evidence of a pronounced molecular layering at the RTIL-solid interface. The periodicity of the molecular layering corresponds to correlation distances in the bulk liquid RTILs. The values of the surface tension seem to be unrelated to the interfacial structure. RTILs are molten salts consisting solely of ions with a melting point below 100 °C. Most RTILs are composed of relatively large (polyatomic) organic cations and inorganic anions. In the last few years the interest in them experienced an enormous growth. Their unique and useful properties like non-volatility, low melting point, and a wide electrochemical window render them suitable for a wide range of applications, i.e. as green solvents, or as electrolytes in a variety of electrochemical processes. In most of the applications of RTILs the RTIL-solid interface plays a crucial role. Four different RTILs were studied. Two of them share the same cation, 1-butyl-3-methylimidazolium, with the most widely used and extensively studied anions, tetrafluoroborate and hexafluorophosphate. The other two RTILs fall into a more recent class of RTILs with higher electrochemical stability. They share the same anion, bis(trifluoromethylsulfonyl)imide, and a pyrrolidinium-based and an imidazolium-based cation, respectively. The systematic exchange of the ion types within these four RTILs revealed a distinct impact of the ion pair on the interfacial behavior. Sapphire wafers with a (0001) surface served as a model system for a hard wall. Further information on interactions within the probed systems was gained by measurements of the interfacial tensions with air and n-hexane for all four RTILs at ambient condition. The surface tension of the RTILs is higher than the one for n-hexane but still smaller than the one for water. The measurements revealed that small changes in the nature of the ions have only a minor impact on the value of the surface and interfacial tension. In order to correlate the results for the interfacial structures with the bulk properties of the RTILs, bulk liquid x-ray scattering experiments were carried out at all four RTILs. These measurements revealed the presence of significant spatial correlations.Item Open Access Ab-initio modeling of ultrafast demagnetization after laser irradiation in nickel, iron and cobalt(2013) Illg, Christian Michael; Fähnle, Manfred (Prof. Dr.)This work deals with ultrafast demagnetization within few hundred femtoseconds after laser pulse irradiation in nickel, iron and face-centered cubic (fcc) cobalt. It is examined with ab-initio density-functional theory and physical modeling whether the electron-phonon spin-flip scattering can be considered as underlying mechanism for ultrafast demagnetization.Item Open Access Ultrafast spectroscopy of single quantum dots(2012) Wolpert, Christian; Lippitz, Markus (Juniorprofessor Dr.)In this thesis, the coherent interaction of single semiconductor quantum dots and ultrafast optical pulses is studied. Under certain conditions, localized exciton transitions in quantum dots can be seen as semi-isolated two-level systems. While this description is sufficient for the explanation of some observations in coherent experiments, it is sometimes necessary to explicitly consider coupling of the discreet quantum states confined to the dot with the environment. We start out from simple, classical examples of coherent spectroscopy and then turn towards experiments where the interaction with the vicinity of the dot becomes an important factor. First, a novel method for transient differential reflectivity spectroscopy of single quantum systems is introduced. It is a pure far-field optical technique which does not require any sophisticated sample preparation steps which makes it applicable to a broad range of structures. Pump pulses excite the sample structure and probe pulses read out the pump-induced changes in the system after a variable delay time. In the case of a single dipole, the signal is given in the form of the spectral inteferogram between the backscattered wave from the particle and the probe light which is reflected at the sample surface. This form of homodyne detection amplifies the weak scattered wave from the particle and thus makes this kind of spectroscopy for single quantum dots feasible. In the remainder of this thesis our spectroscopic method is applied to either characterize the coherent properties of single quantum dots, to prepare and read-out a desired quantum state or to deliberately manipulate them. Coherence times and oscillator strengths are determined for localized exciton transitions. Arbitrary population states can be written by driving coherent population oscillations using resonant pulses, while entangled superpositions of two exciton states in a single dot are investigated by quantum beats on transient differential spectra. We finally exploit the interaction between the dot and a nearby absorbing layer to switch the dot's absorption spectrum on ultrafast timescales via light-induced transient electric fields.Item Open Access Cohesive properties of bcc and fcc rubidium from ab initio pseudopotentials(1985) Maysenhölder, Waldemar; Louie, Steven G.; Cohen, Marvin L.Total-energy calculations have been performed for Rb at zero temperature using a self-consistent ab initio pseudopotential approach within a local-density-functional scheme. The energy difference between fcc and bcc Rb, and the energy barrier between these structures, are found to be extremely small near the equilibrium volume. Agreement of the calculated cohesive properties of bcc Rb with experimental values is good in view of the softness of the material. A transition from bcc to fcc has been calculated to occur at a pressure of about 52 kbar for T=0 K, which compares favorably with the observed value of 70 kbar for this transition at room temperature.Item Open Access Spin-echo resolved neutron scattering from self-organised polymer interfaces(2010) Nülle, Max; Dosch, Helmut (Prof. Dr.)This thesis focused on two main objectives: First, the clarification of the prospects of the spin-echo resolved grazing incidence neutron scattering method (SERGIS) for the investigation of buried interfaces. And second, the investigation of the self-organisation (i.e. microphase separation and dewetting) of ultrathin poly(styrene-block-isoprene) diblock copolymer films on silicon substrates by means of SERGIS and complementary techniques. SERGIS is a novel neutron scattering technique which was implemented and further developed at the new neutron / x-ray reflectometer N-REX+ at the FRM II (Garching, Germany). In contrast to conventional small-angle scattering methods, SERGIS characterises the lateral structure and morphology of interfaces and thin-film systems in real space. The technique uses a polarised primary beam, and the measured quantity is the integral polarisation of the scattered beam. By decoupling the measurement resolution and the beam divergence (in a first approximation), SERGIS aims at a good resolution and a good measurement statistics simultaneously. As a first systematic application of SERGIS to a real physical problem, the dewetting and internal structure of ultrathin poly(styrene-block-isoprene) diblock copolymer films were studied by means of SERGIS and complementary surface sensitive techniques, namely neutron and x ray reflectivity and atomic force microscopy (AFM).Item Open Access Organic solar cells : correlation between molecular structure, morphology and device performance(2010) Bruder, Ingmar; Weis, Jürgen (Prof. Dr.)The development of efficient organic solar cells could be one approach to provide mankind with cheap, sustainable and ecofriendly energy. The introduction of bulk heterojunction and tandem device architectures led recently to devices with power conversion efficiencies close or even higher than n = 6%, showing the potential of organic photovoltaics. Nevertheless, to compete for the foreseeable future to inorganic solar cell technologies, the power conversion efficiencies of organic solar cells have to rise further in the range of 10 % into 15 %. Since the functioning of organic photovoltaics is based on a complex interplay of the electronic properties of its molecular components, it is desirable for an efficient evolution, to identify structural and energetical key characteristics of the molecular components that can lead to efficiency gains. Furthermore, there are virtually no limits for the synthesis of new photoactive materials for the use in organic photovoltaics. Therefore, it is crucial for the device fabrication as well as under a chemical point of view, to narrow potentially promissing classes of molecules and their derivatives under certain physical criteria. One aim of this study was to find and identify so far unknown design criteria for molecules providing high efficiencies in organic solar cells. Thus, the question was raised: What is the physical cause for the differing performance of various metal-phthalocyanines (MPc's with M = Zn, Cu, Ni, Fe) in organic solar cells. Therefore, MPc/C60 based bilayer heterojunction solar cells were fabricated showing a clear dependence of the optimal layer thickness and overall performance on the employed MPc material. Initially, the origin of these differences were explored through structural analysises by AFM and high resolution XRPD measurements on powder and evaporated thin films. The optical properties of the metal phthalocyanines were investigated by solidstate fluorescence and absorption measurements. The lowest excited states of the MPc series were explored by correlated multi-reference ab inito calculations. A high open circuit voltage Voc of a solar cell is a prerequisite for high efficiencies. Unfortunately, the Voc of small molecule based organic solar cells is usually considerably lower than the HOMO-LUMO offset of the device, which determines the theoretical maximum of the Voc in a first approximation. Thus, the question was investigated: What causes the difference between the possible open-circuit voltage and the actual measured voltage and how can this difference be reduced? To answer this question, heterojunction solar cells were produced containing ZnPc or one of the novel synthesized Phenyl-ZnPc, Naphtyl-ZnPc or Anthracenyl-ZnPc as p-conducting and C60 as n-conducting organic layers. By adding the respective aryl substituents to the ZnPc core, the polarizability of the molecules was successively increased. Concurrently, an increase of the Voc from 550 mV to 790 mV by using the highly polarizable Anthracenyl-ZnPc instead of ZnPc was achieved. Quantum mechanical calculations, simulating the charge separation mechanism at the DA-interface of Phenyl-ZnPc/C60 and Naphtyl-ZnPc/C60 showed, that the interplay between characteristic packing and polarization effects could lead to considerably different Coulomb interactions of the electron-hole pairs at the DA-interface. The control of the conduction type and Fermi-level of semiconductors is crucial for the realization of all optoelectronic devices. In inorganic as well as in organic devices this can be achieved by defined doping of appropriate areas within the device. Thus, it has been investigated, how the molecular structure of a dopant should be in order to reduce its diffusivity and increase the evaporation temperature to allow a more efficient processing of the compound. As a result, the novel p-dopant 2,3-di(N-phthalimido)-5,6-dicyano-1,4-benzoquinone (BAPD) was synthesized and compared to the state-of-the-art dopant F4TCNQ. In addition to basic and applied physical questions, I worked on the development of new, efficient solar cell architectures during my PhD thesis. In the course of this work it could be shown, that an efficient organic tandem cell can be prepared from a solid state dye-sensitized solar cell combined with a vacuum-deposited bulk heterojunction solar cell. The complementary absorption of the dyes, as well as an adequate serial connection of both subcells, leads to a high power conversion efficiency of n = (6.0±0.1)% under simulated 100 mW/cm2 AM 1.5 illumination.Item Open Access Spatio-temporal and polarisation dynamics of semiconductor microcavity lasers(2004) Hamm, Joachim; Hess, Ortwin (Prof. Dr.)Microcavity semiconductor lasers are known for their inherent tight coupling between active material and light-field. The dynamic interaction between the carrier and the photon subsystems is influenced equally strong by both, the dynamics of carriers within the quantum-well and the intra-cavity light-field dynamics. In this work, we develop theoretical models and investigate the nonlinear spatio-temporal behaviour of two prominent types of microcavity lasers, the vertical-cavity surface-emitting laser (VCSEL) and the vertical extended cavity surface-emitting laser (VECSEL). Today's aim to build faster and more powerful semiconductor laser devices goes hand in hand with a miniaturisation of the semiconductor laser structures down to the nanometerscale. Difficult even for simple bulk semiconductor devices, the even tighter coupling of the carrier and light-field sub-systems with respect to time- and length-scales disallow a separate dynamical treatment of the physical processes which take place within such novel microcavity semiconductor lasers. Due to their flexibility and their physical nature, time-domain simulations constitute an appropriate tool for targeting the entangled dynamics within the cavity, the structure and the active quantum-wells. We predict that along with the technological progress of microcavity semiconductor lasers and the availability of inexpensive computing power time-domain methods will gain more importance and constitute a valuable tool to analyse the optical and electronic properties of these devices.