@phdthesis{Mahapatra2007, author = {Mahapatra, Suddhasatta}, title = {Formation and Properties of Epitaxial CdSe/ZnSe Quantum Dots : Conventional Molecular Beam Epitaxy and Related Techniques}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-32831}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2007}, abstract = {Albeit of high technological import, epitaxial self-assembly of CdSe/ZnSe QDs is non-trivial and still not clearly understood. The origin and attributes of these QDs are significantly different from those of their III-V and group-IV counterparts. For III-V and group-IV heterosystems, QD-formation is assigned to the Stranski Krastanow (SK) transition, wherein elastic relaxation of misfit strain leads to the formation of coherent three-dimensional (3D) islands, from a supercritically strained two-dimensional (2D) epilayer. Unfortunately, this phenomenon is inconspicuous for the CdSe/ZnSe heterosystem. Well-defined 3D islands are not readily formed in conventional molecular beam epitaxial (MBE) growth of CdSe on ZnSe. Consequently, several alternative approaches have been adopted to induce/enhance formation of QDs. This thesis systematically investigates three such alternative approaches, along with conventional MBE, with emphasis on the formation-mechanism of QDs, and optimization of their morphological and optical attributes. It is shown here that no distinct 3D islands are formed in MBE growth of CdSe on ZnSe. The surface of the CdSe layer represents a rough 2D layer, characterized by a dense array of shallow (<1nm) abutting mounds. In capped samples, the CdSe deposit forms an inhomogeneous CdZnSe quantum well (QW)-like structure. This ternary QW consists of local Cd-rich inclusions, which confine excitons three-dimensionally, and act as QDs. The density of such QDs is very high (~ 1012 cm-2). The QDs defined by the composition inhomogeneities of the CdZnSe QW presumably originate from the shallow mounds of the uncapped CdSe surface. By a technique wherein a CdSe layer is grown at a low temperature (TG = 230 °C) and subsequently annealed at a significantly higher temperature (TA =310 °C), tiny but distinct 3D islands are formed. In this work, the mechanism underlying the formation of these islands is reported. While the CdSe deposit forms a quasi-two-dimensional (quasi-2D) layer at TG = 230 °C, subsequent annealing at TA = 310 °C results in a thermally activated "up-climb" of adatoms onto two-dimensional clusters (or precursors) and concomitant nucleation of 3D islands. The areal density of QDs, achieved by this technique, is at least a decade lower than that typical for conventional MBE growth. It is demonstrated that further reduction is possible by delaying the temperature ramp-up to TA. In the second technique, formation of distinct islands is demonstrated by deposition of amorphous selenium (a-Se) onto a 2D CdSe epilayer at room temperature and its subsequent desorption at a higher temperature (TD = 230 °C). Albeit the self-assembled islands are large, they are severely truncated during subsequent capping with ZnSe, presumably due to segregation of Cd and Zn-alloying of the islands. The segregation phenomenon is analyzed in this work and correlated to the optical properties of the QDs. Additionally, very distinct vertical correlation of QDs in QD-superlattices, wherein the first QD-layer is grown by this technique and the subsequent ones by migration enhanced epitaxy (MEE), is reported. The process steps of the third variant technique, developed in course of this work, are very similar to those of the previous one-the only alteration being the substitution of selenium with tellurium as the cap-forming-material. This leads not only to large alteration of the morphological and optical attributes of the QDs, but also to formation of unique self-assembled island-patterns. Oriented dashes, straight and buckled chains of islands, and aligned island-pairs are formed, depending on the thickness of the Te-cap layer. The islands are partially alloyed with Te and emit luminescence at very low energies (down to 1.7 eV at room temperature). The Te cap layer undergoes (poly)crystallization during temperature ramp-up (from room temperature to TD) for desorption. Here, it is shown that the self-assembled patterns of the island-ensembles are determined by the pattern of the grain boundaries of the polycrystalline Te layer. Based on an understanding of the mechanism of pattern formation, a simple and "clean" method for controlled positioning of individual QDs and QD-based extended nanostructures, is proposed in this work. The studies carried out in the framework of this thesis provide not only a deeper insight into the microscopic processes governing the heteroepitaxial self-assembly of CdSe/ZnSe(001) QDs, but also concrete approaches to achieve, optimize, and control several technologically-important features of QD-ensembles. Reduction and control of QD-areal-density, pronounced vertical correlation of distinctly-defined QDs in QD-superlattices, and self-assembly of QD-based extended structures, as demonstrated in this work, might turn out to be beneficial for envisioned applications in information-, and communication-technologies.}, subject = {Nanostruktur}, language = {en} } @phdthesis{Scheibner2007, author = {Scheibner, Ralf}, title = {Thermoelectric Properties of Few-Electron Quantum Dots}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-26699}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2007}, abstract = {This thesis presents an experimental study of the thermoelectrical properties of semiconductor quantum dots (QD). The measurements give information about the interplay between first order tunneling and macroscopic quantum tunneling transport effects in the presence of thermal gradients by the direct comparison of the thermoelectric response and the energy spectrum of the QD. The aim of the thesis is to contribute to the understanding of the charge and spin transport in few-electron quantum dots with respect to potential applications in future quantum computing devices. It also gives new insight into the field of low temperature thermoelectricity. The investigated QDs were defined electrostatically in a two dimensional electron gas (2DEG) formed with a GaAs/(Al,Ga)As heterostructure by means of metallic gate electrodes on top of the heterostructure. Negative voltages with respect to the potential of the 2DEG applied to the gate electrodes were used to deplete the electron gas below them and to form an isolated island of electron gas in the 2DEG which contains a few ten electrons. This QD was electrically connected to the 2DEG via two tunneling barriers. A special electron heating technique was used to create a temperature difference between the two connecting reservoirs across the QD. The resulting thermoelectric voltage was used to study the charge and spin transport processes with respect to the discrete energy spectrum and the magnetic properties of the QD. Such a two dimensional island usually exhibits a discrete energy spectrum, which is comparable to that of atoms. At temperatures below a few degrees Kelvin, the electrostatic charging energy of the QDs exceeds the thermal activation energy of the electrons in the leads, and the transport of electrons through the QD is dominated by electron-electron interaction effects. The measurements clarify the overall line shape of thermopower oscillations and the observed fine structure as well as additional spin effects in the thermoelectrical transport. The observations demonstrate that it is possible to control and optimize the strength and direction of the electronic heat flow on the scale of a single impurity and create spin-correlated thermoelectric transport in nanostructures, where the experimenter has a close control of the exact transport conditions. The results support the assumption that the performance of thermoelectric devices can be enhanced by the adjustment of the QD energy levels and by exploiting the properties of the spin-correlated charge transport via localized, spin-degenerate impurity states. Within this context, spin entropy has been identified as a driving force for the thermoelectric transport in the spin-correlated transport regime in addition to the kinetic contributions. Fundamental considerations, which are based on simple model assumptions, suggest that spin entropy plays an important role in the presence of charge valence fluctuations in the QD. The presented model gives an adequate starting point for future quantitative analysis of the thermoelectricity in the spin-correlated transport regime. These future studies might cover the physics in the limit of single electron QDs or the physics of more complex structures such as QD molecules as well as QD chains. In particular, it should be noted that the experimental investigations of the thermopower of few-electron QDs address questions concerning the entropy transport and entropy production with respect to single-bit information processing operations. These questions are of fundamental physical interest due to their close connection to the problem of minimal energy requirements in communication, and thus ultimately to the so called "Maxwell's demon" with respect to the second law of thermodynamics.}, subject = {Quantenpunkt}, language = {en} } @phdthesis{Pappert2007, author = {Pappert, Katrin}, title = {Anisotropies in (Ga,Mn)As - Measurement, Control and Application in Novel Devices}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-23370}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2007}, abstract = {Ferromagnetic semiconductors (FS) promise the integration of magnetic memory functionalities and semiconductor information processing into the same material system. The prototypical FS (Ga,Mn)As has become the focus of semiconductor spintronics research over the past years. The spin-orbit mediated coupling of magnetic and semiconductor properties in this material gives rise to many novel transport-related phenomena which can be harnessed for device applications. In this thesis we address challenges faced in the development of an all-semiconductor memory architecture. A starting point for information storage in FS is the knowledge of their detailed magnetic anisotropy. The first part of this thesis concentrates on the investigation of the magnetization behaviour in compressively strained (Ga,Mn)As by electrical means. The angle between current and magnetization is monitored in magnetoresistance(MR) measurements along many in-plane directions using the Anisotropic MR(AMR) or Planar Hall effect(PHE). It is shown, that a full angular set of such measurements displayed in a color coded resistance polar plot can be used to identify and quantitatively determine the symmetry components of the magnetic anisotropy of (Ga,Mn)As at 4 K. We compile such "anisotropy fingerprints" for many (Ga,Mn)As layers from Wuerzburg and other laboratories and find the presence of three symmetry terms in all layers. The biaxial anisotropy term with easy axes along the [100] and [010] crystal direction dominates the magnetic behaviour. An additional uniaxial term with an anisotropy constant of ~10\% of the biaxial one has its easy axis along either of the two <110> directions. A second contribution of uniaxial symmetry with easy axis along one of the biaxial easy axes has a strength of only ~1\% of the biaxial anisotropy and is therefore barely visible in standard SQUID measurements. An all-electrical writing scheme would be desirable for commercialization. We report on a current assisted magnetization manipulation experiment in a lateral (Ga,Mn)As nanodevice at 4 K (far below Tc). Reading out the large resistance signal from DW that are confined in nanoconstrictions, we demonstrate the current assisted magnetization switching of a small central island through a hole mediated spin transfer from the adjacent leads. One possible non-perturbative read-out scheme for FS memory devices could be the recently discovered Tunneling Anisotropic MagnetoResistance (TAMR) effect. Here we clarify the origin of the large amplification of the TAMR amplitude in a device with an epitaxial GaAs tunnel barrier at low temperatures. We prove with the help of density of states spectroscopy that a thin (Ga,Mn)As injector layer undergoes a metal insulator transition upon a change of the magnetization direction in the layer plane. The two states can be distinguished by their typical power law behaviour in the measured conductance vs voltage tunneling spectra. While all hereto demonstrated (Ga,Mn)As devices inherited their anisotropic magnetic properties from their parent FS layer, more sophisticated FS architectures will require locally defined FS elements of different magnetic anisotropy on the same wafer. We show that shape anisotropy is not applicable in FS because of their low volume magnetization. We present a method to lithographically engineer the magnetic anisotropy of (Ga,Mn)As by submicron patterning. Anisotropic strain relaxation in submicron bar structures (nanobars) and the related deformation of the crystal lattice introduce a new uniaxial anisotropy term in the energy equation. We demonstrate by both SQUID and transport investigations that this lithographically induced uniaxial anisotropy overwrites the intrinsic biaxial anisotropy at all temperatures up to Tc. The final section of the thesis combines all the above into a novel device scheme. We use anisotropy engineering to fabricate two orthogonal, magnetically uniaxial, nanobars which are electrically connected through a constriction. We find that the constriction resistance depends on the relative orientation of the nanobar magnetizations, which can be written by an in-plane magnetic field. This effect can be explained with the AMR effect in connection with the field line patterns in the respective states. The device offers a novel non-volatile information storage scheme and a corresponding non-perturbative read-out method. The read out signal is shown to increase drastically in samples with partly depleted constriction region. This could be shown to originate in a magnetization direction driven metal insulator transition of the material in the constriction region.}, subject = {Anisotropie}, language = {en} } @phdthesis{Koenig2007, author = {K{\"o}nig, Markus}, title = {Spin-related transport phenomena in HgTe-based quantum well structures}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-27301}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2007}, abstract = {Within the scope of this thesis, spin related transport phenomena have been investigated in HgTe/HgCdTe quantum well structures. This material exhibits peculiar band structure properties, which result in a strong spin-orbit interaction of the Rashba type. An inverted band structure, i.e., a reversed ordering of the energy states in comparison to common semiconductors, is obtained for quantum well layers above a critical thickness. Furthermore, the band structure properties can be controlled in the experiments by moderate gate voltages. Most prominently, the type of carriers in HgTe quantum wells can be changed from n to p due to the narrow energy gap. Along with the inverted band structure, this unique transition is the basis for the demonstration of the Quantum Spin Hall state, which is characterized by the existence of two one-dimensional spin-polarized edge states propagating in opposite directions, while the Fermi level in the bulk is in the energy gap. Since elastic scattering is suppressed by time reversal symmetry, a quantized conductance for charge and spin transport is predicted. Our experiments provide the first experimental demonstration of the QSH state. For samples with characteristic dimensions below the inelastic mean free path, charge conductance close to the expected value of 2e^2/h has been observed. Strong indication for the edge state transport was found in the experiments as well. For large samples, potential fluctuations lead to the appearance of local n-conducting regions which are considered to be the dominant source of backscattering. When time reversal symmetry is broken in a magnetic field, elastic scattering becomes possible and conductance is significantly suppressed. The suppression relies on a dominant orbital effect in a perpendicular field and a smaller Zeeman-like effect present for any field direction. For large perpendicular fields, a re-entrant quantum Hall state appears. This unique property is directly related to the non-trivial QSH insulator state. While clear evidence for the properties of charge transport was provided, the spin properties could not be addressed. This might be the goal of future experiments. In another set of experiments, the intrinsic spin Hall effect was studied. Its investigation was motivated by the possibility to create and to detect pure spin currents and spin accumulation. A non-local charging attributed to the SHE has been observed in a p-type H-shaped structure with large SO interaction, providing the first purely electrical demonstration of the SHE in a semiconductor system. A possibly more direct way to study the spin Hall effects opens up when the spin properties of the QSH edge states are taken into account. Then, the QSH edge states can be used either as an injector or a detector of spin polarization, depending on the actual configuration of the device. The experimental results indicate the existence of both intrinsic SHE and the inverse SHE independently of each other. If a spin-polarized current is injected from the QSH states into a region with Rashba SO interaction, the precession of the spin can been observed via the SHE. Both the spin injection and precession might be used for the realization of a spin-FET similar to the one proposed by Datta and Das. Another approach for the realization of a spin-based FET relies on a spin-interference device, in which the transmission is controlled via the Aharonov-Casher phase and the Berry phase, both due to the SO interaction. In the presented experiments, ring structures with tuneable SO coupling were studied. A complex interference pattern is observed as a function of external magnetic field and gate voltage. The dependence on the Rashba splitting is attributed to the Aharonov-Casher phase, whereas effects due to the Berry phase remain unresolved. This interpretation is confirmed by theoretical calculations, where multi-channel transport through the device has been assumed in agreement with the experimental results. Thus, our experiments provide the first direct observation of the AC effect in semiconductor structures. In conclusion, HgTe quantum well structures have proven to be an excellent template for studying spin-related transport phenomena: The QSHE relies on the peculiar band structure of the material and the existence of both the SHE and the AC effect is a consequence of the substantial spin-orbit interaction. While convincing results have been obtained for the various effects, several questions can not be fully answered yet. Some of them may be addressed by more extensive studies on devices already available. Other issues, however, ask, e.g., for further advances in sample fabrication or new approaches by different measurements techniques. Thus, future experiments may provide new, compelling insights for both the effects discussed in this thesis and, more generally, other spin-orbit related transport properties.}, subject = {Spin-Bahn-Wechselwirkung}, language = {en} } @phdthesis{Winterfeldt2006, author = {Winterfeldt, Carsten}, title = {Generation and control of high-harmonic radiation}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-20309}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2006}, abstract = {High-harmonic generation provides a powerful source of ultrashort coherent radiation in the XUV and soft-x-ray range, which also allows for the production of attosecond light pulses. Based on the unique properties of this new radiation it is now possible to perform time-resolved spectroscopy at high excitation energies, from which a wide field of seminal discoveries can be expected. Since the exploration and observation of the corresponding processes in turn are accompanied by the desire to control them, this work deals with new ways to manipulate and characterize the properties of these high-harmonic-based soft-x-ray pulses. After introductory remarks this work first presents a comprehensive overview over recent developments and achievements on the field of the control of high-harmonic radiation in order to classify the experimental results obtained in this work. These results include the control of high-harmonic radiation both by temporally shaping and by manipulating the spatial properties of the fundamental laser pulses. In addition, the influence of the conversion medium and of the setup geometry (gas jet, gas-filled hollow fiber) was investigated. Using adaptive temporal pulse shaping of the driving laser pulse by a deformable mirror, this work demonstrates the complete control over the XUV spectrum of high harmonics. Based on a closed-loop optimization setup incorporating an evolutionary algorithm, it is possible to generate arbitrarily shaped spectra of coherent soft-x-ray radiation in a gas-filled hollow fiber. Both the enhancement and suppression of narrowband high-harmonic emission in a selected wavelength region as well as the enhancement of coherent soft-x-ray radiation over a selectable extended range of harmonics (multiple harmonics) can be achieved. Since simulations that do not take into account spatial properties such as propagation effects inside a hollow fiber cannot reproduce the experimentally observed high contrast ratios between adjacent harmonics, a feedback-controlled adaptive two-dimensional spatial pulse shaper was set up to examine selective fiber mode excitation and the optimization of high-harmonic radiation in such a geometry. It is demonstrated that different fiber modes contribute to harmonic generation and make the high extent of control possible. These results resolve the long-standing issue about the controllability of high-harmonic generation in free-focusing geometries such as gas jets as compared to geometries where the laser is guided. Temporal pulse shaping alone is not sufficient. It was possible to extend the cutoff position of harmonics generated in a gas jet, however, selectivity cannot be achieved. The modifications of the high-harmonic spectrum have direct implications for the time structure of the harmonic radiation, including the possibility for temporal pulse shaping on an attosecond time scale. To this end, known methods for the temporal characterization of optical pulses and high-harmonic pulses (determination of the harmonic chirp on femtosecond and attosecond time scales) were introduced. The experimental progress in this work comprises the demonstration of different setups that are in principle suitable to determine the time structure of shaped harmonic pulses based on two-photon two-color ionization cross-correlation techniques. Photoelectron spectra of different noble gases generated by photoionization with high-harmonic radiation reproduce the spin-orbit splitting of the valence electrons and prove the satisfactory resolution of our electron time-of-flight spectrometer for the temporal characterization of high harmonics. Unfortunately no positive results for this part could be achieved so far, which can probably be attributed mainly to the lack of the focusability of the high harmonics and to the low available power of our laser system. However, we have shown that shaping the high-harmonic radiation in the spectral domain must result in modifications of the time structure on an attosecond time scale. Therefore this constitutes the first steps towards building an attosecond pulse shaper in the soft-x-ray domain. Together with the ultrashort time resolution, high harmonics open great possibilities in the field of time-resolved soft-x-ray spectroscopy, for example of inner-shell transitions. Tailored high-harmonic spectra as generated in this work and shaped attosecond pulses will represent a multifunctional toolbox for this kind of research.}, subject = {Frequenzvervielfachung}, language = {en} } @phdthesis{Bach2006, author = {Bach, Peter}, title = {Growth and characterization of NiMnSb-based heterostructures}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-17771}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2006}, abstract = {In this work heterostructures based on the half-Heusler alloy NiMnSb have been fabricated and characterized. NiMnSb is a member of the half-metallic ferromagnets, which exhibit an electron spin-polarization of 100\% at the Fermi-level. For fabrication of these structures InP substrates with surface orientations of (001),(111)A and (111)B have been used. The small lattice mismatch of NiMnSb to InP allows for pseudomorphic layers, the (111) orientation additionally makes the formation of a half-metallic interface possible. For the growth on InP(001), procedures for the substrate preparation, growth of the lattice matched (In,Ga)As buffer layer and of the NiMnSb layer have been developed. The effect of flux-ratios and substrate temperatures on the MBE growth of the buffer as well as of the NiMnSb layer have been investigated and the optimum conditions have been pointed out. NiMnSb grows in the layer-by-layer Frank-van der Merwe growth mode, which can be seen by the intensity oscillations of the RHEED specular spot during growth. RHEED and LEED measurements show a flat surface and a well-defined surface reconstruction. High resolution x-ray measurements support this statement, additionally they show a high crystalline quality. Measurements of the lateral and the vertical lattice constant of NiMnSb films on (001) oriented substrates show that layers above a thickness of 20nm exhibit a pseudomorphic as well as a relaxed part in the same layer. Whereas layers around 40nm show partly relaxed partitions, these partitions are totally relaxed for layers above 100nm. However, even these layers still have a pseudomorphic part. Depth-dependent x-ray diffraction experiments prove that the relaxed part of the samples is always on top of the pseudomorphic part. The formation and propagation of defects in these layers has been investigated by TEM. The defects nucleate early during growth and spread until they form a defect network at a thickness of about 40nm. These defects are not typical misfit dislocations but rather antiphase boundaries which evolve in the Mn/Sb sublattice of the NiMnSb system. Dependent on the thickness of the NiMnSb films different magnetic anisotropies can be found. For layers up to 15nm and above 25nm a clear uniaxial anisotropy can be determined, while the layers with thicknesses in between show a fourfold anisotropy. Notably the easy axis for the thin layers is perpendicular to the easy axis observed for the thick layers. Thin NiMnSb layers show a very good magnetic homogeneity, as can be seen by the very small FMR linewidth of 20Oe at 24GHz. However, the increase of the linewidth with increasing thickness shows that the extrinsic damping gets larger for thicker samples which is a clear indication for magnetic inhomogeneities introduced by crystalline defects. Also, the magnetic moment of thick NiMnSb is reduced compared to the theoretically expected value. If a antiferromagnetic material is deposited on top of the NiMnSb, a clear exchange biasing of the NiMnSb layer can be observed. In a further step the epitaxial layers of the semiconductor ZnTe have been grown on these NiMnSb layers, which enables the fabrication of NiMnSb/ZnTe/NiMnSb TMR structures. These heterostructures are single crystalline and exhibit a low surface and interface roughness as measured by x-ray reflectivity. Magnetic measurements of the hysteresis curves prove that both NiMnSb layers in these heterostructures can switch separately, which is a necessary requirement for TMR applications. If a NiMn antiferromagnet is deposited on top of this structure, the upper NiMnSb layer is exchange biased by the antiferromagnet, while the lower one is left unaffected. Furthermore the growth of NiMnSb on (111) oriented substrates has been investigated. For these experiments, InP substrates with a surface orientation of (111)A and (111)B were used, which were miscut by 1 to 2° from the exact orientation to allow for smoother surfaces during growth. Both the (In, Ga)As buffer as well as the NiMnSb layer show well defined surface reconstructions during growth. X-ray diffraction experiments prove the single crystalline structure of the samples. However, neither for the growth on (111)A nor on (111)B a perfectly smooth surface could be obtained during growth, which can be attributed to the formation of pyramid-like facets evolving as a result of the atomic configuration at the surface. A similar relaxation behavior as NiMnSb layers on (001) oriented InP could not be observed. RHEED and x-ray diffraction measurements show that above a thickness of about 10nm the NiMnSb layer begins to relax, but remnants of pseudomorphic parts could not be found. Magnetic measurements show that the misorientation of the substrate crystal has a strong influence on the magnetic anisotropies of NiMnSb(111) samples. In all cases a uniaxial anisotropy could be observed. The easy axis is always aligned parallel to the direction of the miscut of the substrate.}, subject = {Nickelverbindungen}, language = {en} } @phdthesis{Slobodskyy2006, author = {Slobodskyy, Taras}, title = {Semimagnetic heterostructures for spintronics}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-21011}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2006}, abstract = {F{\"u}r zuk{\"u}nftige Technologien ist die Erforschung von der verwendeten Teilchen n{\"o}tig. Spintronik ist ein modernes Gebiet der Physik, welches neben der Ladung auch die Spineigenschaften als zus¨atzlichen Freiheitsgrad nutzbar macht. Der "conductivity mismatch" stellt ein fundamentales Problem f{\"u}r elektrische Spininjektion aus einem ferromagnetischem Metal in einen diffusiven Halbleiter dar. Daher m{\"u}ssen andere Methoden f{\"u}r die Injektion spin-polarisierter Ladungstr{\"a}ger benutzt werden. Mit einem Tunnelkontakt ist es m{\"o}glich, eine hoch spin-polarisierte, Raumtemperatur Tunnel-Injektion zu erzielen. Wir benutzten einen neuen Ansatz und verwendeten magnetische RTDs zur Spinmanipulation. In dieser Arbeit wurden die Eigenschaften von magnetischen, resonanten Tunneldioden (RTDs) aus rheinen II-VI-Halbleitern in ihrer Verwendung f{\"u}r die Spintronik beschrieben. Wachstumsbedingungen wurden optimiert, um das Peak-to-Valley-Verh{\"a}ltnis zu vergr{\"o}ßern. Das Design der RTDs wurde optimiert, um spinbezogene Transporteffekte beobachten zu k{\"o}nen. Mit einem externen Magnetfeld war Spinmanipulation m{\"o}glich. Selbstorganisierte CdSe Quanten-Strukturen wurden hergestelt und mit optischen Techniken untersucht. Sie w{\"u}rden in (Zn,Be)Se Tunnelbarrieren eingebettet, so dass ihre Eigenschaften durch resonantes Tunneln zug{\"a}nglich wurden.}, subject = {Heterostruktur-Bauelement}, language = {en} } @phdthesis{Walter2006, author = {Walter, Dominik}, title = {Adaptive Control of Ultrashort Laser Pulses for High-Harmonic Generation}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-21975}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2006}, abstract = {The generation of high harmonics is an ideal method to convert frequencies of the infrared- or visible range into the soft x-ray range. This process demands high laser intensities that are nowadays supplied by femtosecond laser systems. As the temporal and spatial coherence properties of the laser are transferred during the conversion process, the generated high harmonics will propagate as a beam with high peak-brightness. Under ideal conditions the generation of soft-x-ray pulses shorter than one femtosecond is possible. These properties are exploited in many applications like time-resolved x-ray spectroscopy. The topic of this thesis is the generation and optimization of high harmonics. A variety of conversion setups is investigated (jet of noble gas atoms, gas-filled hollow-fiber, water microdroplets) and theoretical models present ideas to further enhance the conversion efficiency (using excited atoms or aligned molecules). In different setups the peak intensity of the fundamental laser pulses is increased by spectral broadening and subsequent temporal compression. This is achieved with the help of pulse shaping devices that can modify the spectral phase and therefore also the temporal intensity distribution of laser pulses. These pulse shaping devices are controlled by an evolutionary algorithm. With this setup not only adaptive compression of laser pulses is possible, but also the engineering of specific laser pulse shapes to optimize an experimental output. This setup was used to influence the process of high harmonic generation. It is demonstrated that the spectral distribution of the generated soft-x-ray radiation can be controlled by temporal pulse shaping. This method to tailor high harmonics is complemented by spatial shaping techniques. These findings demonstrate the realization of a tunable source of soft-x-ray radiation.}, subject = {Frequenzvervielfachung}, language = {en} } @phdthesis{Breuer2006, author = {Breuer, Felix}, title = {Development and Applications of Efficient Strategies for Parallel Magnetic Resonance Imaging}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-20683}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2006}, abstract = {Virtually all existing MRI applications require both a high spatial and high temporal resolution for optimum detection and classification of the state of disease. The main strategy to meet the increasing demands of advanced diagnostic imaging applications has been the steady improvement of gradient systems, which provide increased gradient strengths and faster switching times. Rapid imaging techniques and the advances in gradient performance have significantly reduced acquisition times from about an hour to several minutes or seconds. In order to further increase imaging speed, much higher gradient strengths and much faster switching times are required which are technically challenging to provide. In addition to significant hardware costs, peripheral neuro-stimulations and the surpassing of admissable acoustic noise levels may occur. Today's whole body gradient systems already operate just below the allowed safety levels. For these reasons, alternative strategies are needed to bypass these limitations. The greatest progress in further increasing imaging speed has been the development of multi-coil arrays and the advent of partially parallel acquisition (PPA) techniques in the late 1990's. Within the last years, parallel imaging methods have become commercially available,and are therefore ready for broad clinical use. The basic feature of parallel imaging is a scan time reduction, applicable to nearly any available MRI method, while maintaining the contrast behavior without requiring higher gradient system performance. PPA operates by allowing an array of receiver surface coils, positioned around the object under investigation, to partially replace time-consuming spatial encoding which normally is performed by switching magnetic field gradients. Using this strategy, spatial resolution can be improved given a specific imaging time, or scan times can be reduced at a given spatial resolution. Furthermore, in some cases, PPA can even be used to reduce image artifacts. Unfortunately, parallel imaging is associated with a loss in signal-to-noise ratio (SNR) and therefore is limited to applications which do not already operate at the SNR limit. An additional limitation is the fact that the coil array must provide sufficient sensitivity variations throughout the object under investigation in order to offer enough spatial encoding capacity. This doctoral thesis exhibits an overview of my research on the topic of efficient parallel imaging strategies. Based on existing parallel acquisition and reconstruction strategies, such as SENSE and GRAPPA, new concepts have been developed and transferred to potential clinical applications.}, subject = {NMR-Bildgebung}, language = {en} } @phdthesis{Ruester2005, author = {R{\"u}ster, Christian}, title = {Magnetotransport effects in lateral and vertical ferromagnetic semiconductor junctions}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-15554}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2005}, abstract = {This work is an investigation of giant magnetoresistance (GMR), tunneling magnetoresistance (TMR) and tunneling anisotropic magnetoresistance (TAMR)effects in (Ga,Mn) based ferromagnetic semiconductor junctions. Detailed results are published in the following articles: [1] C. R{\"u}ster, T. Borzenko, C. Gould, G. Schmidt, L.W. Molenkamp, X. Liu, T.J.Wojtowicz, J.K. Furdyna, Z.G. Yu and M. Flatt´e, Very Large Magnetoresistance in Lateral Ferromagnetic (Ga,Mn)As Wires with Nanoconstrictions, Physical Review Letters 91, 216602 (2003). [2] C. Gould, C. R{\"u}ster, T. Jungwirth, E. Girgis, G.M. Schott, R. Giraud, K. Brunner, G. Schmidt and L.W. Molenkamp, Tunneling Anisotropic Magnetoresistance: A Spin-Valve-Like Tunnel Magnetoresistance Using a Single Magnetic Layer, Physical Review Letters 93, 117203 (2004). [3] C. R{\"u}ster, C. Gould, T. Jungwirth, J. Sinova, G.M. Schott, R. Giraud, K. Brunner, G. Schmidt and L.W. Molenkamp, Very Large Tunneling Anisotropic Magnetoresistance of a (Ga,Mn)As/GaAs/(Ga,Mn)As Stack, Physical Review Letters 94, 027203 (2005). [4] C. R{\"u}ster and C. Gould, T. Jungwirth, E. Girgis, G.M. Schott, R. Giraud, K. Brunner, G. Schmidt and L.W. Molenkamp, Tunneling anisotropic magnetoresistance: Creating a spin-valve-like signal using a single ferromagnetic semiconductor layer, Journal of Applied Physics 97, 10C506 (2005).}, subject = {Galliumarsenid}, language = {en} } @phdthesis{Daumer2005, author = {Daumer, Volker}, title = {Phase coherent transport phenomena in HgTe quantum well structures}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-15538}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2005}, abstract = {Although spintronics has aroused increasing interest, much fundamental research has to be done. One important issue is the control over the electronic spin. Therefore, spin and phase coherent transport are very important phenomena. This thesis describes experiments with mercury based quantum well structures. This narrow gap material provides a very good template to study spin related effects. It exhibits large Zeeman spin splitting and Rashba spin-orbit splitting. The latter is at least four to five times larger than in III-V semiconductors. Initially a short review on the transport theory was presented. The main focus as on quantisation effects that are important to understand the related experiments. Thus, Shubnikov-de Haas and the quantum Hall effect have been analysed. Due to the first fabrication of nanostructures on Hg-based quantum well samples, the observation of ballistic transport effects could be expected. Hence, the Landauer-B¨uttiker theory has been introduced which gives the theoretical background to understand such effects. With respect to the main topic of this thesis, phase coherence has been introduced in detail. Experiments, where coherence effects could be observed, have been explained theoretically. Here, possible measurement setups have been discussed, e.g., a ring shaped structure to investigate the Aharonov-Bohm and related effects. Due to the fact, that all experiments, described in this thesis, were performed on Hg-based samples, the exceptional position of such samples among the \&\#147;classical\&\#148; semiconductors has been clarified. Hg1-xMnx Te quantum wells are type-III QWs in contrast to the type-I QWs formed by e.g., GaAs/AlGaAs heterostructures. With a well width of more than 6 nm and a manganese content of less than 7\% they exhibit an inverted band alignment. Band structure calculations based on self consistent Hartree calculations have been presented. The common description of a diluted magnetic semiconductor with the Brillouin function has been introduced and the experiments to obtain the empiric parameters T0 and S0 have been presented. Rashba spin-orbit splitting and giant Zeeman splitting have been explained theoretically and the magnetic ordering of a spin glass as well as the relevant interactions therein have been discussed. The next chapter describes the first realisation of nanostructures on Hg-based heterostructures. Several material specific problems have been solved, but the unique features of this material system mentioned above justify the effort. Interesting new insight could be found and will be found with these structures. Onto a series of QW samples, cross-shaped structures with several lead widths have been patterned. With the non-local resistance measurement setup, evidence for quasiballistic transport was demonstrated in cross-shaped structures with lead widths down to 0.45 mm. The non-local bend resistance and a regime of rebound trajectories as well as the anomalous Hall effect could be identified. Monte-Carlo simulations of the classical electron trajectories have been performed. A good agreement with the experimental data has been achieved by taking a random scattering process into account. Encouraged by this success the technology has been improved and ring-shaped structures with radii down to 1 mm have been fabricated. Low temperature (below 100 mK), four terminal resistance measurements exhibit clear Aharonov-Bohm oscillations. The period of the oscillations agrees very well with a calculation that takes only the sample geometry into account. One goal using such a structure is the experimental prove of the spin-orbit Berry phase. Therefore an additional Shottky gate on top of the ring was needed. With this structure evidence for the Aharonov-Casher effect was observed. Here, a perpendicular applied electric field causes analogous oscillations as does the magnetic field in the AB effect. A subsequent change in the Rashba SO splitting due to several applied gate voltages while measuring the AB effect should reveal the SO Berry phase. Although initially evidence of a phase change was detected, a clear proof for the direct measurement of the SO Berry phase could not be found. In the future, with an advanced sample structure, e.g., with an additional Hall bar next to the ring, which permits a synchronous measurement of the Rashba splitting, it might be possible to measure the SO Berry phase directly. In manganese doped HgTe QWs two different effects simultaneously cause spin splitting: the giant Zeeman and the Rashba effect. By analysing the Shubnikovde Haas oscillations and the node positions of their beating pattern, it has been possible to separate these two effects. Whereas the Rashba effect can be identified by its dependence on the structure inversion asymmetry, varied by the applied gate voltage, the giant Zeeman splitting is extracted from its strong temperature dependence, because Rashba splitting is temperature independent. The analysis revealed, that the Rashba splitting is larger than or comparable to the giant Zeeman splitting even at moderately high magnetic fields. In an extraordinary HgMnTe QW sample, that exhibits the n= 1 quantum Hall plateau from less than 1 T up to 28 T, the anomalous Hall effect could be excluded. Intense studies on the temperature dependence of the QHE as well as band structure calculations have revealed this extraordinary behaviour to be an ordinary band structure effect of this system. In a series of mesoscopic structures on nonmagnetic and magnetic QWs, an investigation of the universal conductance uctuations have been carried out. In the}, subject = {Quecksilbertellurid}, language = {en} } @phdthesis{Slobodskyy2005, author = {Slobodskyy, Anatoliy}, title = {Diluted magnetic semiconductor Resonant Tunneling Structures for spin manipulation}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-18263}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2005}, abstract = {In this work we investigate magnetic resonant tunneling diode (RTD) structures for spin manipulation. All-II-VI semiconductor RTD structures based on [Zn,Be]Se are grown by molecular beam epitaxy. We observe a strong, magnetic field induced, splitting of the resonance peaks in the I-V characteristics of RTDs with [Zn,Mn]Se diluted magnetic semiconductors (DMS) quantum well. The splitting saturates at high fields and has strong temperature dependence. A phonon replica of the resonance is also observed and has similar behaviour to the peak. We develop a model based on the giant Zeeman splitting of the spin levels in the DMS quantum well in order to explain the magnetic field induced behaviour of the resonance.}, subject = {Resonanz-Tunneldiode}, language = {en} } @phdthesis{Joshi2005, author = {Joshi, Sanjeev}, title = {Preparation and characterization of CdS nanoparticles}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-13395}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2005}, abstract = {Zusammenfassung CdS-Nanoteilchen mit Gr{\"o}ßen zwischen 1.1 und 4.2 nm wurden in {\"A}thanol und mit Thioglycerol (TG)-H{\"u}lle synthetisiert. Es wurde gezeigt, dass die nass-chemische Synthese ohne Wasser und die Verwendung von TG als H{\"u}lle folgende Vorteile bieten: Es konnten kleinere Teilchen hergestellt und eine schmalere Gr{\"o}ßenverteilung erzielt werden. Zus{\"a}tzlich wird dem Altern der Teilchen vorgebeugt, und die Ergebnisse sind besser reproduzierbar. Hochaufgel{\"o}ste Photoemissions-Messungen an kleinen CdS-Teilchen (1.1, 1.4, 1.7, 1.8; 1.8 nm mit Glutathion-H{\"u}lle) ergaben Beitr{\"a}ge von f{\"u}nf verschiedenen Schwefelatom-Typen zum S 2p-Gesamt Signal. Außerdem wurde beobachtet, dass Nanoteilchen unterschiedlicher Gr{\"o}ße und/oder mit unterschiedlichen H{\"u}llen-Substanzen verschiedene Photoemissionsspektren zeigen und verschieden starke Strahlensch{\"a}den aufweisen. Bei den 1.4 nm großen CdS-Teilchen entsprechen die Komponenten des S 2p-Signals entweder Schwefelatomen mit unterschiedlichen Cd-Nachbarn, Thiol-Schwefelatomen oder teilweise oxidiertem Schwefel. Die jeweilige Zuweisung der Schwefeltypen erfolgte {\"u}ber Intensit{\"a}ts-{\"A}nderungen der einzelnen S 2p-Komponenten als Funktion der Photonenenergie und des Strahlenschadens. Die Daten der 1.4 nm großen CdS-Teilchen wurden mit PES-Intensit{\"a}ts-Rechnungen verglichen, die auf einem neuen Strukturmodell-Ansatz basieren. Von den drei verwendeten CdS-Strukturmodellen konnte nur ein Modell mit 33 S-Atomen die Variation der experimentellen Intensit{\"a}ten richtig wieder geben. Modelle von gr{\"o}ßeren Nanoteilchen mit beispielsweise 53 S-Atomen zeigen Abweichungen von den experimentellen Daten der 1.4 nm-Teilchen. Auf diese Weise kann indirekt auf die Gr{\"o}ße der gemessenen Teilchen geschlossen werden. Die Intensit{\"a}tsrechnungen wurden zum einen „per Hand" zur groben Absch{\"a}tzung durchgef{\"u}hrt, zum anderen wurden exaktere Berechnungen mit einem von L. Weinhardt und O. Fuchs entwickelten Programm angestellt. Diese best{\"a}tigen die Ergebnisse der Absch{\"a}tzung. Zudem wurde festgestellt, dass die inelastische freie Wegl{\"a}nge \&\#955; keinen signifikanten Einfluss auf die Modellrechnungen hat. Die gemessenen Intensit{\"a}ts-{\"A}nderungen konnten zwar mit mehreren leicht verchiedenen Modellen erkl{\"a}rt werden, allerdings f{\"u}hrte nur ein kugelf{\"o}rmiges Teilchen-Modell auch zu den richtigen Intensit{\"a}tsverh{\"a}ltnissen der einzelnen S 2p-Komponenten. Weiterhin konnte beobachtet werden, dass die elektronische Bandl{\"u}cke gr{\"o}ßer ist als die optische Bandl{\"u}cke. Bei den PES-Messungen wurden einige wichtige Einfl{\"u}sse sichtbar. So spielen strahlenbedingte Effekte eine große Rolle. Kenntnisse {\"u}ber die Zeitskala solcher Effekte erm{\"o}glichen PES-Aufnahmen mit guter Signal-Qualit{\"a}t und erlauben eine Extraploation zur Situation ohne Strahlenschaden. Auch die D{\"u}nnschicht-Pr{\"a}paration beeinflusst die Spektren. Beispielsweise zeigten mit Elektrophorese hergestellte Filme Hinweise auf Agglomeration. Schichten, die per Tropfen-Deposition erzeugt wurden, weisen spektrale {\"A}nderungen am Rand der Probe auf, und Filme aus Nanoteilchen-Pulver waren nicht homogen. Mikro-Raman Experimente, die in Kollaboration mit Dr. M. Schmitt und Prof. W. Kiefer durchgef{\"u}hrt wurden, ließen große Unterschiede in den Spektren von Nanoteilchen und TG in L{\"o}sung erkennen. Dies wurde vor allem auf das Fehlen von S - H -Bindungen zur{\"u}ckgef{\"u}hrt und zeigt damit, dass alle TG-Molek{\"u}le verwertet oder ausgewaschen wurden.}, subject = {Cadmiumsulfid}, language = {en} } @phdthesis{Mahnkopf2005, author = {Mahnkopf, Sven}, title = {Photonic crystal based widely tunable laser diodes and integrated optoelectronic components}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-13860}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2005}, abstract = {In a first aspect of this work, the development of photonic crystal based widely tunable laser diodes and their monolithic integration with photonic crystal based passive waveguide and coupler structures is explored theoretically and experimentally. In these devices, the photonic crystal is operated in the photonic bandgap which can be used for the realization of effective reflectors and waveguide structures. Such tunable light sources are of great interest for the development of optical network systems that are based on wavelength division multiplexing. In a second aspect of this work, the operation of a photonic crystal block near the photonic band edge is investigated with respect to the so-called superprism effect. After a few introductory remarks that serve to motivate this work, chapter 3 recapitulates some aspects of semiconductor lasers and photonic crystals that are essential for the understanding of this work so that the reader should be readily equipped with the tools to appreciate the results presented in this work.}, subject = {Laserdiode}, language = {en} } @phdthesis{Papastathopoulos2005, author = {Papastathopoulos, Evangelos}, title = {Adaptive control of electronic excitation utilizing ultrafast laser pulses}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-12533}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2005}, abstract = {The subject of this work has been the investigation of dynamical processes that occur during and after the interaction of matter with pulses of femtosecond laser radiation. The experiments presented here were performed in the gas phase and involve one atomic and several model molecular systems. Absorption of femtosecond laser radiation by these systems induces an electronic excitation, and subsequently their ionization, photofragmentation or isomerization. The specific adjustment of the excitation laser field properties offers the possibility to manipulate the induced electronic excitation and to influence the formation of the associated photoproducts. From the perspective of the employed spectroscopic methods, the development of photoelectron spectroscopy and its implementation in laser control experiments has been of particular interest in this thesis. This technique allows for a most direct and intuitive observation of electronic excitation dynamics in atomic as well as in complex polyatomic molecular systems. The propagation of an intermediate electronic transient state, associated to the formation of a particular photoproduct, can be interrogated by means of its correlation to a specific state of the atomic or molecular continuum. Such correlations involve the autoionization of the transient state, or by means of a second probe laser field, a structural correlation, as summarized by the Koopman's theorem (section 2.4.1). The technique of adaptive femtosecond quantum control has been the subject of development in our group for many years. The basic method, by which the temporal profile of near-infrared laser pulses at a central wavelength of 800 nm, can be adjusted, is a programmable femtosecond pulse-shaper that comprises of a zero dispersion compressor and a commercial liquid crystal modulator (LCD). This experimental arrangement was realized prior to this thesis and served as a starting point to extend the pulse-shaping technique to the ultraviolet spectral region. This technological development was realized for the purposes of the experiments presented in Chapter 5. It involves a combination of the LCD-pulse-shaper with frequency up-conversion techniques on the basis of producing specifically modulated laser pulses of central wavelength 266 nm. Furthermore, the optical method X-FROG had to be developed in order to characterize the often complex structure of generated ultraviolet pulses. In the adaptive control experiments presented in this work, the generated femtosecond laser pulses could be automatically adjusted by means of specifically addressing the 128 independent voltage parameters of the programmable liquid-crystal modulator. Additionally a machine learning algorithm was employed for the cause of defining laser pulse-shapes that delivered the desired (optimal) outcome in the investigated laser interaction processes. In Chapter 4, the technique of feedback-controlled femtosecond pulse shaping was combined with time-of-flight mass spectroscopy as well as photoelectron spectroscopy in order to investigate the multiphoton double ionization of atomic calcium. A pronounced absolute enhancement of the double ionization yield was obtained with optimized femtosecond laser pulses. On the basis of the measured photoelectron spectra and of the electron optimization experiments, a non-sequential process was found, which plays an important role in the formation of doubly charged Calcium ions. Then in Chapter 5, the dynamics following the pp* excitation of ethylene-like molecules were investigated. In this context, the model molecule stilbene was studied by means of femtosecond photoelectron spectroscopy. Due to the simplicity of its chemical structure, stilebene is one of the most famous models used in experimental as well as theoretical studies of isomerization dynamics. From the time-resolved experiments described in that chapter, new spectroscopic data involving the second excited electronic state S2 of the molecule were acquired. The second ethylenic product was the molecule tetrakis (dimethylamino) ethylene (TDMAE). Due to the presence of numerous lone pair electrons on the four dimethylamino groups, TDMAE exhibits a much more complex structure than stilbene. Nevertheless, previously reported studies on the dynamics of TDMAE provided vital information for planning and conducting a successful optimisation control experiment of the wavepacket propagation upon the (pp*) S1 excited potential surface of the molecule. Finally, in Chapter 6 the possibility of employing femtosecond laser pulses as an alternative method for activating a metallocene molecular catalyst was addressed. By means of an adaptive laser control scheme, an optimization experiment was realized. There, the target was the selective cleavage of one methyl-ligand of the model catalyst (Cp)^2Zr(CH3)^2, which induces a catalytic coordination position on the molecule. The spectroscopic studies presented in that chapter were performed in collaboration to the company BASF A.G. and constitute a proof-of principle attempt for a commercial application of the adaptive femtosecond quantum control technique.}, subject = {Ultrakurzer Lichtimpuls}, language = {en} } @phdthesis{Pfeifer2004, author = {Pfeifer, Thomas}, title = {Adaptive control of coherent soft X-rays}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-9854}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2004}, abstract = {The availability of coherent soft x-rays through the nonlinear optical process of high-harmonic generation allows for the monitoring of the fastest events ever observed in the laboratory. The attosecond pulses produced are the fundamental tool for the time-resolved study of electron motion in atoms, molecules, clusters, liquids and solids in the future. However, in order to exploit the full potential of this new tool it is necessary to control the coherent soft x-ray spectra and to enhance the efficiency of conversion from laser light to the soft x-ray region in the harmonic-generation process. This work developed a comprehensive approach towards the optimization of the harmonic generation process. As this process represents a fundamental example of \emph{light}--\emph{matter} interaction there are two ways of controlling it: Shaping the generating laser \emph{light} and designing ideal states of \emph{matter} for the conversion medium. Either of these approaches was closely examined. In addition, going far beyond simply enhancing the conversion process it could be shown that the qualitative spectral response of the process can be modified by shaping the driving laser pulse. This opens the door to a completely new field of research: Optimal quantum control in the attosecond soft x-ray region---the realm of electron dynamics. In the same way as it is possible to control molecular or lattice vibrational dynamics with adaptively shaped femtosecond laser pulses these days, it will now be feasible to perform real-time manipulation of tightly bound electron motion with adaptively shaped attosecond light fields. The last part of this work demonstrated the capability of the herein developed technique of coherent soft-x-ray spectral shaping, where a measured experimental feedback was used to perform a closed-loop optimization of the interaction of shaped soft x-ray light with a sulfur hexafluoride molecule to arrive at different control objectives. For the optimization of the high-harmonic-generation process by engineering the conversion medium, both the gas phase and the liquid phase were explored both in experiment and theory. Molecular media were demonstrated to behave more efficiently than commonly used atomic targets when elliptically polarized driving laser pulses are applied. Theory predicted enhancement of harmonic generation for linearly polarized driving fields when the internuclear distance is increased. Reasons for this are identified as the increased overlap of the returning electron wavefunction due to molecular geometry and the control over the delocalization of the initial electronic state leading to less quantum-mechanical spreading of the electron wavepacket during continuum propagation. A new experimental scheme has been worked out, using the method of molecular wavepacket generation as a tool to enhance the harmonic conversion efficiency in `pump--drive' schemes. The latter was then experimentally implemented in the study of high-harmonic generation from water microdroplets. A transition between the dominant laser--soft-x-ray conversion mechanisms could be observed, identifying plasma-breakdown as the fundamental limit of high-density high-harmonic generation. Harmonics up to the 27th order were observed for optimally laser-prepared water droplets. To control the high-harmonic generation process by the application of shaped laser light fields a laser-pulse shaper based on a deformable membrane mirror was built. Pulse-shape optimization resulted in increased high-harmonic generation efficiency --- but more importantly the qualitative shape of the spectral response could be significantly modified for high-harmonic generation in waveguides. By adaptive optimization employing closed-loop strategies it was possible to selectively generate narrow (single harmonics) and broad bands of harmonic emission. Tunability could be demonstrated both for single harmonic orders and larger regions of several harmonics. Whereas any previous experiment reported to date always produced a plateau of equally intense harmonics, it has been possible to demonstrate ``untypical'' harmonic soft x-ray spectra exhibiting ``switched-off'' harmonic orders. The high degree of controllability paves the way for quantum control experiments in the soft x-ray spectral region. It was also demonstrated that the degree of control over the soft x-ray shape depends on the high-harmonic generation geometry. Experiments performed in the gas jet could not change the relative emission strengths of neighboring harmonic orders. In the waveguide geometry, the relative harmonic yield of neighboring orders could be modified at high contrast ratios. A simulation based solely on the single atom response could not reproduce the experimentally observed contrast ratios, pointing to the importance of propagation (phase matching) effects as a reason for the high degree of controllability observed in capillaries, answering long-standing debates in the field. A prototype experiment was presented demonstrating the versatility of the developed soft x-ray shaping technique for quantum control in this hitherto unexplored wavelength region. Shaped high-harmonic spectra were again used in an adaptive feedback loop experiment to control the gas-phase photodissociation reaction of SF\$_6\$ molecules. A time-of-flight mass spectrometer was used for the detection of the ionic fragments. The branching ratios of particular fragmentation channels could be varied by optimally shaped soft x-ray light fields. Although in one case only slight changes of the branching ratio were possible, an optimal solution was found, proving the sufficient technical stability of this unique coherent soft-x-ray shaping method for future applications in optimal control. Active shaping of the spectral amplitude in coherent spectral regions of \$\sim\$10~eV bandwidth was shown to directly correspond to shaping the temporal features of the emerging soft x-ray pulses on sub-femtosecond time scales. This can be understood by the dualism of frequency and time with the Fourier transformation acting as translator. A quantum-mechanical simulation was used to clarify the magnitude of temporal control over the shape of the attosecond pulses produced in the high-harmonic-generation process. In conjunction with the experimental results, the first attosecond time-scale pulse shaper could thus be demonstrated in this work. The availability of femtosecond pulse shapers opened the field of adaptive femtosecond quantum control. The milestone idea of closed-loop feedback control to be implemented experimentally was expressed by Judson and Rabitz in their seminal work titled ``Teaching lasers to control molecules''. This present work extends and turns around this statement. Two fundamentally new achievements can now be added, which are ``Teaching molecules to control laser light conversion'' and ``Teaching lasers to control coherent soft x-ray light''. The original idea thus enabled the leap from femtosecond control of molecular dynamics into the new field of attosecond control of electron motion to be explored in the future. The \emph{closed}-loop approach could really \emph{open} the door towards fascinating new perspectives in science. Coming back to the introduction in order to close the loop, let us reconsider the analogy to the general chemical reaction. Photonic reaction control was presented by designing and engineering effective media (catalysts) and controlling the preparation of educt photons within the shaped laser pulses to selectively produce desired photonic target states in the soft x-ray spectral region. These newly synthesized target states in turn could be shown to be effective in the control of chemical reactions. The next step to be accomplished will be the control of sub-femtosecond time-scale electronic reactions with adaptively controlled coherent soft x-ray photon bunches. To that end a time-of-flight high-energy photoelectron spectrometer has recently been built, which will now allow to directly monitor electronic dynamics in atomic, molecular or solid state systems. Fundamentally new insights and applications of the nonlinear interaction of shaped attosecond soft x-ray pulses with matter can be expected from these experiments.}, subject = {Ultrakurzer Lichtimpuls}, language = {en} } @phdthesis{Henn2004, author = {Henn, Julian}, title = {The electron density : a bridge between exact quantum mechanics and fuzzy chemical concepts}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-9003}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2004}, abstract = {Summary The nature of the chemical bond is a topic under constant debate. What is known about individual molecular properties and functional groups is often taught and rationalized by explaining Lewis structures, which, in turn, make extensive use of the valence concept. The valence concept distinguishes between electrons, which do not participate in chemical interactions (core electrons) and those, which do (single, double, triple bonds, lone-pair electrons, etc.). Additionally, individual electrons are assigned to atomic centers. The valence concept is of paramount success: It allows the successful planning of chemical syntheses and analyses, it explains the behavior of individual functional groups, and, moreover, it provides the "language" to think of and talk about molecular structure and chemical interactions. The resounding success of the valence concept may be misleading to forget its approximative character. On the other hand, quantum mechanics provide in principle a quantitative description of all chemical phenomena, but there is no discrimination between electrons in quantum mechanics. From the quantum mechanical point of view there are only indistinguishable electrons in the field of the nuclei, i.e., it is impossible to assign a given electron to a particular center or to ascribe a particular purpose to individual electrons. The concept of indistinguishability of micro particles is founded on the Heisenberg uncertainty relation, which states, that wavepackets diverge in the 6N dimensional phase space, such that individual trajectories can not be identified. Hence it is a deep-rooted and approved physical concept. As an introduction to the present work density partitioning schemes were discussed, which divide the total molecular density into chemically meaningful areas. These partitioning schemes are intimately related to either the concepts of bound atoms in a molecule (as in the Atoms In Molecules theory (AIM) according to Bader or as in the Hirshfeld partitioning scheme) or to the concept of chemical structure in the sense of Lewis structures, which divide the total molecular density into core and valence density, where the valence density is split up again into bonding and non-bonding electron densities. Examples are early and recent loge theories, the topological analysis by means of the Electron Localization Function (ELF), and the Natural Bond Orbital (NBO) approach. Of these partitioning schemes, the theories according to Bader (AIM), to Becke and Edgecomb (ELF) and according to Weinhold (NBO and Natural Resonance Theory, NRT), respectively, were reviewed in detail critically. Points of criticism were explicated for each of the mentioned theories. Since theoretically derived electron densities are to be compared to experimentally derived densities, a brief introduction into the theory of X-ray di®raction experiments was given and the multipole formalism was introduced. The procedure of density refinement was briefly discussed. Various suggestions for improvements were developed: One strategy would be the employment of model parameters, which are to a maximum degree mutually orthogonal, with the object of minimizing correlations among the model parameters, e.g., to introduce nodal planes into the radial functions of the multipole model. A further suggestion involves the guidance of the iterative refinement procedure by an extremum principle, which states, that when di®erent solutions to the least squares minimization problem are available with about the same statistical measures of quality and with about the same residual density, then the solution is to prefer, which yields a minimum density at the bond critical point (BCP) and a maximum polarity in terms of the ratio of distances between the BCP and the nuclei. This suggestion is based on the well known fact, that the bond polarity (in terms of the ratio of distances between the BCP and the respective nuclei) is underestimated in the experiment. Another suggestion for including physical constraints is the explicit consideration of the virial theorem, e.g., by evaluating the integration of the Laplacian over the entire atomic basins and comparing this value to zero and to the value obtained from the integration of the electron gradient field over the atomic surface. The next suggestion was to explicitly use the electrostatic theorem of Feynman (often also denoted as Hellmann-Feynman theorem), which states, that the forces onto the nuclei can be calculated from the purely classical electrostatic forces of the electron distribution and the nuclei distribution. For a stationary system, these forces must add to zero. This also provides an internal quality criterion of the density model. This can be performed in an iterative way during the refinement procedure or as a test of the final result. The use of the electrostatic theorem is expected to reduce significantly correlations among static density parameters and parameters describing vibrations, since it is a valuable tool to discriminate between physically reasonable and artificial static electron densities. All of these mentioned suggestions can be applied as internal quality criteria. The last suggestion is based on the idea to initiate the experimental refinement with a set of model parameters, which is, as much as possible close to the final solution. This can be achieved by performing periodic boundary conditions calculations, from which theoretically created files are obtained, which contain the Miller indices (h, k, l) and the respective intensity I. This file is used for a model parameter estimation (refinement), which excludes vibrations. The resulting parameters can be used for the experimental refinement, where, in a first step, the density parameters are fixed to determine the parameters describing vibrations. For a fine tuning, again the electrostatic theorem and the other above mentioned suggestions could be applied. Theoretical predictions should not be biased by the method of computation. Therefore the dependence of the density analyzing tools on the level of calculation (method of calculation/basis set) and on the substituents in complex chemical bonding situations were evaluated in the second part of the present work. A number of compounds containing formal single and double sulfur nitrogen bonds was investigated. For these compounds, experimental data were also available. The calculated data were compared internally and with the experimental results. The internal comparison was drawn with regard to questions of convergency as well as with regard to questions of consistency: The resulting molecular properties from NBO/NRT analyses were found to be very stable, when the geometries were optimized at the respective level of theory. This stability is valid for variations in the methods of calculation as well as for variations in the basis set. Only the individual resonance weights of the contributing Natural Lewis Structures differed considerably depending on the level of calculation and depending on the substituents. However, the deviations were in both cases to a large extent within a limit which preserves the descending order of the leading resonance structure weights. The resulting bond orders, i.e., the total, covalent and ionic bond order from NRT calculations, were not affected by the shift in the resonance weights. The analysis of the bond topological parameters resulted in a discrimination between insensitive parameters and sensitive parameters. The stable parameters do neither depend strongly on the method of calculation nor on the basis set. Only minor variation occurs in the numerical values of these parameters, when the level of calculation is changed or even when other functional groups (H, Me, or tBu) are employed, as long as the methods of calculation do not drop considerably below a standard level. The bond descriptors of the sulfur nitrogen bonds were found to be also stable with respect to the functional groups R = H, R = Me, and R = tBu. Stable parameters are the bond distance, the density at the bond critical point (BCP) and the ratio of distances between the BCP and the nuclei A and B, which varies clearly when considering the formal bond type. For very small basis sets like the 3-21G basis set, this characteristic stability collapses. The sensitive parameters are based on the second derivatives of the density with respect to the coordinates. This is in accordance with the well known fact, that the total second derivative of the density with respect to the coordinates is a strongly oscillating function with positive as well as negative values. A profound deviation has to be anticipated as a consequence of strong oscillations. lambda3, which describes the local charge depletion in the direction of the interaction line, is the most varying parameter. A detailed analysis revealed that the position of the BCP in the rampant edge of the Laplacian distribution is responsible for the sensitivity of the numerical value of lambda3 in formal double bonds. Since the slope of the Laplacian assumes very high values in its rampant edge, a tiny displacement of the BCP leads already to a considerable change in lambda3. This instability is not a failure of the underlying theory, but it yields de facto to a considerable dependence of sensitive bond topological properties on the method of calculation and on the applied basis sets. Since the total second derivative is important to judge on the nature of the bond in the AIM theory (closed shell interactions versus shared interactions), the changes in lambda3 can lead to differing chemical interpretations. The comparison of theoretically derived bond topological properties of various sulfur nitrogen bonds provides the possibility to measure the self consistency of this data set. All data sets clearly exhibit a linear correlation between the bond distances and the density at the BCP on one hand and between the bond distances and the Laplacian values at the BCP on the other hand. These correlations were almost independent of the basis set size. In this context, the linear regression has to be regarded exclusively as a descriptive statistics tool. There is no correlation anticipated a priori. The formal bond type was found to be readily deducible from the theoretically obtained bond topological descriptors of the model systems. In this sense, the bond topological properties are self consistent despite of the numerical sensitivity of the derivatives, as exemplified above. Often, calculations are performed with the experimentally derived equilibrium geometries and not with optimized ones. Applying this approach, the computationally costly geometry optimizations are saved. Following this approach the bond topological properties were calculated using very flexible basis sets and employing the fixed experimental geometry (which, of course, includes the application of tBu groups). Regression coe±cients similar to those from optimized geometries were obtained for correlations between bond distances and the densities at the BCP as well as for the correlation between bond distances and the Laplacian at the BCP, i.e. the approach is valid. However, the data points scattered less and the coe±cient of correlation was clearly increased when geometry optimizations were performed beforehand. The comparison between data obtained from theory and experiment revealed fundamental discrepancies: In the data set of bond topological parameters from the experiment, the behavior of only 2 out of 3 insensitive parameters was comparable to the behavior of the theoretically obtained values, i.e. theoretical and experimental bond distances as well as theoretical and experimental densities at the BCP correlate. From the theoretically obtained data it was easy to deduce the formal bond type from the position of the BCP, since it changed in a systematic manner. The respective experimentally obtained values were almost constant and did not change systematically. For the SN bonds containing compounds, the total second derivative assumes exclusively negative values in the experiment. Due to the different internal behavior, experimentally and theoretically sensitive bond topological values could not be compared directly. The qualitative agreement in the Laplacian distribution, however, was excellent. In the third and last part of this work, the application to chemical systems follows. Formal hypervalent molecules, i.e. molecules where some atoms are considered to hold more than 8 electrons in their valence shell, were investigated. These were compounds containing sulfur nitrogen bonds (H(NtBu)2SMe, H2C{S(NtBu)2(NHtBu)}2, S(NtBu)2 and S(NtBu)3) and a highly coordinated silicon compound. The set of sulfur nitrogen compounds also contained a textbook example for valence expansion, the sulfur triimide. For these molecules, experimental reference values were available from high resolution X-ray experiments. The experimental results were in the case of the sulfur triimide not unique. Furthermore, from the experimental bond topological data no definite conclusion about the formal bonding type could be drawn. The situation of sulfur nitrogen bonds in the above mentioned set of molecules was analyzed in terms of a geometry discussion and by means of a topological analysis. The methyl-substituted isolated molecules served as model compounds. For the interpretation of the bonding situation additional NBO/NRT calculations were preformed for the sulfur nitrogen compounds and an ELF calculation and analysis was performed for the silicon compound. The ELF analysis included not only the presentation and discussion of the ELF-isosurfaces (eta = 0.85), but also the investigation of populations of disynaptic valence basins and the percentage contributions to these populations of the individual atoms when the disynaptic valence basins are split into atomic contributions according to Bader's partitioning scheme. The question of chemical interest was whether hypervalency is present in the set of molecules or not. In the first case the octet rule would be violated, in the second case Pauling's verdict would be violated. While the concept of hypervalency is well established in chemistry, the violation of Pauling's verdict is not. The quantitative numbers of the sensitive bond topological values from theory and experiment were not comparable, since no systematic relationship between the experimentally and theoretically determined sensitive bond descriptors was found. However, the insensitive parameters are in good agreement and the qualitative Laplacian distribution is, with few exceptions, in excellent agreement. The formal bonding type was deduced from experimental and theoretical topological data by considering the number and shape of valence shell charge concentrations in proximity to the sulfur and nitrogen centers. The results from NBO/NRT calculations confirmed the findings. All employed density analyzing tools AIM, ELF and NBO/NRT coincided in describing the bonding situation in the formally hypervalent molecules as highly polar. A comparison and analysis of experimentally and theoretically derived electron densities led consistently to the result, that regarding this set of molecules, hypervalency has to be excluded unequivocally.}, subject = {Elektronendichtebestimmung}, language = {en} } @phdthesis{Krampert2004, author = {Krampert, Gerhard}, title = {Femtosecond quantum control and adaptive polarization pulse shaping}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-10304}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2004}, abstract = {Adaptive Femtosekunden-Quantenkontrolle hat sich in den letzten Jahren als eine sehr erfolgreiche Methode in vielen wissenschaftlichen Gebieten wie Physik, Chemie oder Biologie erwiesen. Eine Vielzahl von Quantensystemen und insbesondere Molek{\"u}le, die eine chemische Reaktion durchlaufen, sind durch speziell geformte, Femtosekunden-Laserimpulse kontrolliert worden. Diese Methode erlaubt es, nicht nur das Quantensystem zu beobachten, sondern einen Schritt weiterzugehen und aktive Kontrolle {\"u}ber quantenmechanische Dynamik zu erlangen. In diesem Schema werden Interferenzph{\"a}nomene im Zeit- und Frequenzraum benutzt, um Selektivit{\"a}t zum Beispiel in einer chemischen Reaktion zu erhalten. Die dazu benutzten, speziell geformten Femtosekunden-Laserimpulse waren bislang nur linear polarisiert. Deshalb konnten sie nur die skalaren Eigenschaften der Licht - Materie - Wechselwirkung ausnutzen und haben so den vektoriellen Charakter des elektrischen Dipolmoments \$\vec{\mu}\$ und des elektrischen Lichtfeldes \$\vec{E}(t)\$ vernachl{\"a}ssigt. Im besonderen in der Quantenkontrolle von chemischen Reaktionen ist das untersuchte System, die Molek{\"u}le, dreidimensional und zeigt komplexe raumzeitliche Dynamik. Mit der Hilfe von polarisations-geformten Laserimpulsen ist man jetzt in der Lage dieser Dynamik, sowohl in der Zeit als auch in der r{\"a}umlichen Richtung zu folgen. Deshalb kann nun ein neues Niveau an Kontrolle in quanten-mechanischen Systemen erreicht werden. In dieser Arbeit konnte die Erzeugung von polarisations-geformten Laserimpulsen in einem optischen Aufbau verwirklicht werden. Dieser Aufbau erfordert keine interferometrische Stabilit{\"a}t, da beide Polarisationskomponenten demgleichen Strahlweg folgen. Zwei-Kanal spektrale Interferometrie wurde eingesetzt, um die Laserimpulse experimentell vollst{\"a}ndig zu charakterisieren. Um den zeitabh{\"a}ngigen Polarisationszustand dieser Pulse exakt zu beschreiben, wurde eine mathematische Darstellung entwickelt und angewandt. Die Ver{\"a}nderungen des Polarisationszustandes durch optische Elemente wurde untersucht und einige L{\"o}sungen wurden aufgezeigt, um diese Ver{\"a}nderungen zu minimieren. Der Jones Matrix Formalismus wurde dazu benutzt, alle Verzerrungen des Polarisationszustandes zwischen dem Impulsformer und dem Ort des Experiments zu ber{\"u}cksichtigen. Zugleich k{\"o}nnen die Jones Matrizen zu einer vollst{\"a}ndigen Charakterisierung der erzeugten Laserimpulse verwendet werden. Dabei wurden experimentell kalibrierte Matrizen eingesetzt. Adaptive Polarisations-Impulsformung konnte in einem rein optischen Demonstrationsexperiment gezeigt werden. Dabei wurde die computergesteuerte Polarisationsformung mit einer Lernschleife und einem experimentellen R{\"u}ckkopplungssignal kombiniert. Durch diesen selbstlernenden Algorithmus konnte der ben{\"o}tigte, linear polarisierte Laserimpuls mit m{\"o}glichst kleiner Impulsdauer gefunden werden, der f{\"u}r die effektive Erzeugung der zweiten Harmonischen in einem nichtlinearen optischen Kristall am besten geeignet ist. Durch diese R{\"u}ckkopplungsschleife war es m{\"o}glich auch noch kompliziertere Polarisationsverzerrungen, die durch eine Wellenplatte f{\"u}r eine falsche Wellenl{\"a}nge verursacht wurden, r{\"u}ckg{\"a}ngig zu machen. Die zus{\"a}tzliche Verformung der spektralen Phase durch Materialdispersion in einem 10~cm langen Glasblock konnte ebenfalls automatisch kompensiert werden. Nach diesen optischen Demonstrationsexperimenten wurde ultraschnelle Polarisationsformung angewandt, um ein Quantensystem zu kontrollieren. Die Polarisationsabh{\"a}ngigkeit der Multi-Photonen Ionisation von Kaliumdimeren konnte in einer Anrege-Abtast Messung nachgewiesen werden. Diese Abh{\"a}ngigkeit wurde dann in einem adaptiven Polarisationsformungsexperiment in einer sehr viel allgemeineren Art ausgenutzt. Statt nur einem Anrege- und Abtastlaserimpuls mit jeweils unterschiedlicher Polarisation zu benutzen, wurde der zeitabh{\"a}ngige Polarisationszustand eines geformtem Laserimpulses benutzt, um die Ionisation zu maximieren. Anstelle von einer nur quantitativen Verbesserung konnte eine qualitativ neue Art von Kontrolle {\"u}ber Quantensysteme demonstriert werden. Diese Polarisationskontrolle ist anwendbar selbst bei zuf{\"a}llig ausgerichteten Molek{\"u}len. Durch diese M{\"o}glichkeit, auf Ausrichtung der Molek{\"u}le zu verzichten, konnte mit einem wesentlich vereinfachten experimentellen Aufbau gearbeitet werden. {\"U}ber diese Polarisationskontrollexperimente hinaus wurden auch die dreidimensionalen Aspekte der Dynamik von Molek{\"u}len erforscht und kontrolliert. Die \textit{cis-trans} Photoisomerisierungsreaktion von 3,3\$'\$-Diethyl-2,2\$'\$-Thiacyanin Iodid (NK88) wurde in der fl{\"u}ssigen Phase mit transienter Absorptionsspektroskopie untersucht. Die Isomerisierungsausbeute konnte sowohl erh{\"o}ht als auch erniedrigt werden durch den Einsatz geformter Femtosekunden-Laserimpulse mit einer Zentralwellenl{\"a}nge von 400~nm, die sowohl in spektraler Phase als auch Amplitude moduliert waren. Dieses Experiment zeigt die M{\"o}glichkeit, die koh{\"a}rente Bewegung großer molekularer Gruppen durch Laserimpulse gezielt zu beeinflussen. Diese Modifikation der molekularen Geometrie kann als erster Schritt angesehen werden, kontrollierte Stereochemie zu verwirklichen. Insbesondere da im ersten Teil dieser Arbeit die Kontrolle von Molek{\"u}len mit Polarisations-geformten Impulsen gezeigt werden konnte, ist der Weg geebnet zu einer Umwandlung von einem chiralen Enantiomer in das andere, da theoretische Modelle dieser Umwandlung polarisations-geformte Laserimpulse ben{\"o}tigen. Außer diesen faszinierenden Anwendungen der Polarisationsformung sollte es nun m{\"o}glich sein den Wellenl{\"a}ngenbereich der polarisations-geformten Laserimpulse auszuweiten. Sowohl Erzeugung der zweiten Harmonischen um in den ultravioletten Bereich zu kommen als auch optische Gleichrichtung von {\"a}ußerst kurzen Femtosekunden-Impulsen um den mittleren infrarot Bereich abzudecken sind M{\"o}glichkeiten, den Wellenl{\"a}ngenbereich von polarisations-geformten Laserimpulsen zu erweitern. Mit diesen neuen Wellenl{\"a}ngen tut sich eine Vielzahl an neuen M{\"o}glichkeiten auf, Polarisationsformung f{\"u}r die Kontrolle von quantenmechanischen Systemen einzusetzen.}, subject = {Ultrakurzer Lichtimpuls}, language = {en} } @phdthesis{Schallenberg2004, author = {Schallenberg, Timo}, title = {Shadow mask assisted heteroepitaxy of compound semiconductor nanostructures}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-10290}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2004}, abstract = {Shadow Mask assisted Molecular Beam Epitaxy (SMMBE) is a technique enabling selected area epitaxy of semiconductor heterostructures through shadow masks. The objective of this work was the development of the SMMBE technique for the reliable fabrication of compound semiconductor nanostructures of high structural and optical quality. In order to accomplish this, technological processes have been developed and optimized. This, in combination with model calculations of the basic kinetic growth processes has enabled the fabrication of high quality quantum structures. A high spatial precision and control of the incidence regions of the molecular beams during the SMMBE process are required for the fabrication of nanostructures. One of the technological developments to this effect, which has substantially enhanced the versatility of SMMBE, is the introduction of a new type of freestanding shadow masks: Growth through such a mask with different incidence angles of the molecular beams is equivalent to employing different mechanical masks, but is much more accurate since the precision of mechanical alignment is limited. A consistent model has been developed, which successfully explains the growth dynamics of molecular beam epitaxy through shadow masks. The redistribution of molecular fluxes under shadow masks may affect the growth rates on selected areas of the substrate drastically. In the case of compound semiconductors, reactions between the constituent species play important roles in controlling the growth rates as a function of the growth parameters. The predictions of the model regarding the growth of II-VI and III-V compounds have been tested experimentally and the dependence of the growth rates on the growth parameters has been verified. Moreover, it has been shown, that selected area epitaxy of II-VI and III-V compounds are governed by different surface kinetics. Coexisting secondary fluxes of both constituent species and the apparent non-existence of surface diffusion are characteristic for SMMBE of II-VI compounds. In contrast, III-V SMMBE is governed by the interplay between secondary group-V flux and the surface migration of group-III adatoms. In addition to the basic surface kinetic processes described by the model, the roles of orientation and strain-dependent growth dynamics, partial shadow, and material deposition on the mask (closure of apertures) have been discussed. The resulting advanced understanding of the growth dynamics (model and basic experiments) in combination with the implementation of technical improvements has enabled the development and application of a number of different processes for the fabrication of both II-VI and III-V nanostructures. In addition to specific material properties, various other phenomena have been exploited, e.g., self-organization. It has been shown that, e.g., single quantum dots and quantum wires can be reliably grown. Investigations performed on the SMMBE nanostructures have demonstrated the high positional and dimensional precision of the SMMBE technique. Bright cathodoluminescence demonstrates that the resulting quantum structures are of high structural and optical quality. In addition to these results, which demonstrate SMMBE as a prospective nanofabrication technique, the limitations of the method have also been discussed, and various approaches to overcome them have been suggested. Moreover, propositions for the fabrication of complex quantum devices by the multiple application of a stationary shadow mask have been put forward. In addition to selected area growth, the shadow masks can assist in etching, doping, and in situ contact definition in nanoscale selected areas. Due to the high precision and control over the dimensions and positions of the grown structures, which at the same time are of excellent chemical, crystal, and optical quality, SMMBE provides an interesting perspective for the fabrication of complex quantum devices from II-VI and III-V semiconductors.}, subject = {Verbindungshalbleiter}, language = {en} } @phdthesis{Schoell2003, author = {Sch{\"o}ll, Achim}, title = {High-resolution investigation of the electronic structure of organic thin films}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-10809}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2003}, abstract = {Die vorliegende Arbeit befasst sich mit der elektronischen Struktur organischer D{\"u}nnfilme. Eine zentrale Frage dabei ist der Einfluss der Wechselwirkung zwischen den Molek{\"u}len in der kondensierten Phase und der Wechselwirkung an metall-organischen Grenzfl{\"a}chen auf die elektronischen Eigenschaften. Dazu wurden die experimentellen Methoden Photoelektronenspektroskopie (PES) und R{\"o}ntgenabsorptionsspektroskopie (NEXAFS) mit h{\"o}chster Energieaufl{\"o}sung angewandt. Zus{\"a}tzlich wurden ab initio Rechnungen zur theoretischen Simulation von NEXAFS Spektren durchgef{\"u}hrt. Haupts{\"a}chlich wurden d{\"u}nne, vakuumsublimierte Filme aromatischer Modellmolek{\"u}le mit sauerstoffhaltigen funktionellen Gruppen (NTCDA, PTCDA, NDCA, BPDCA und ANQ) auf Ag(111) Oberfl{\"a}chen untersucht. Die ausgew{\"a}hlten Molek{\"u}le besitzen wegen ihrer großen delokalisierten p-Elektronensysteme sehr interessante Eigenschaften f{\"u}r die Anwendung in elektronischen Bauelementen. Dank der hohen Energieaufl{\"o}sung von Synchrotronstrahlungsquellen der dritten Generation war es erstmals m{\"o}glich, die Schwingungsfeinstruktur in den NEXAFS Spektren dieser kondensierten großen Molek{\"u}le sichtbar zu machen. Der Vergleich der Daten verschiedener Molek{\"u}le liefert dabei interessante Einblicke in den Kopplungmechanismus zwischen dem elektronischen {\"U}bergang und der Schwingungsanregung. Obwohl die Molek{\"u}le eine Vielzahl verschiedener Schwingungsmoden besitzen, kann man in deren NEXAFS Spektren beobachten, dass die elektronischen {\"U}berg{\"a}nge jeweils an haupts{\"a}chlich eine Schwingungsmode koppeln. Die hochaufgel{\"o}sten XPS Spektren der Molek{\"u}le NTCDA, PTCDA, NDCA, BPDCA und ANQ zeigen bestimmte systematische Unterschiede, so dass diese Spektren als Fingerabdruck f{\"u}r die jeweilige Substanz verwendet werden k{\"o}nnen. Durch die vergleichende Auswertung der Spektren konnten die 1s Bindungsenergien aller chemisch unterschiedlichen Kohlenstoff- und Sauerstoffatome bestimmt werden. Zus{\"a}tzliche Strukturen in den Spektren k{\"o}nnen shake-up Satelliten zugeschrieben werden. Die f{\"u}nf Molek{\"u}le stellen ein ideales Modellsystem dar, um fundamentale Aspekte der Rumpfelektronenspektroskopie zu untersuchen, wie Anfangs- und Endzustandseffekte und Satelliten, die durch die intramolekulare und intermolekulare Elektronendichteverteilung im Grund- und rumpfionisierten Zustand beeinflusst werden. Ein wichtiger Punkt dieser Dissertation sind spektroskopische Untersuchungen strukturell unterschiedlicher NTCDA Monolagenphasen auf Ag(111), deren Existenz aus vorangegangenen Arbeiten bekannt ist. Deutliche Unterschiede in der elektronischen Struktur der verschiedenen Phasen, die auf die Metall-Adsorbat Wechselwirkung zur{\"u}ckzuf{\"u}hren sind, konnten sowohl mittels XPS als auch mittels NEXAFS aufgezeigt werden. Sowohl f{\"u}r die komprimierte also auch f{\"u}r die relaxierte NTCDA Monolage kann die Bindung ans Substrat als schwach chemisorptiv charakterisiert werden, was eindeutig aus der Analyse der Satellitenstrukturen in den O 1s und C 1s XPS Spektren hervorgeht, die durch die dynamische Abschirmung durch Ladungstransfer vom Substrat erzeugt werden. Die NEXAFS Daten zeigen konsistent eine teilweise Besetzung des NTCDA LUMOs. Sowohl f{\"u}r die komprimierte als auch f{\"u}r die relaxierte NTCDA Monolage finden hochinteressante Phasen{\"u}berg{\"a}nge in ungeordnete Tieftemperaturphasen beim Abk{\"u}hlen auf 160 K statt. Dabei wird die Adsorbat-Substrat Wechselwirkung st{\"a}rker und das LUMO wird vollst{\"a}ndig besetzt. Dies kann in den NEXAFS Spektren anhand des Verschwindens der zugh{\"o}rigen {\"U}berg{\"a}nge beobachtet werden. Die XPS Spektren zeigen gleichzeitig eine deutliche Abnahme der Intensit{\"a}t schlecht abgeschirmter Photoemissionszust{\"a}nde, was auf die nun effektivere Ladungstransferabschirmung zur{\"u}ckzuf{\"u}hren ist. F{\"u}r den Phasen{\"u}bergang der relaxierten Monolage konnte mittels temperaturabh{\"a}ngiger NEXAFS Messungen eindeutig ein Hystereseverhalten gezeigt und die Hysteresekurve bestimmt werden. Die Hysterese betr{\"a}gt etwa 20 K. Des weiteren wurde aus SPA-LEED Messungen die Aktivierungsenergie f{\"u}r den Phasen{\"u}bergang der relaxierten Monolage beim Abk{\"u}hlen auf ca. 60 meV bestimmt. Schließlich wurden NEXAFS Untersuchungen an Poly{\"a}thylenproben mit verschiedenem Komonomergehalt durchgef{\"u}hrt. Unterschiede in den Absorptionsspektren von Proben mit unterschiedlichem Komonomeranteil konnten eindeutig auf die unterschiedliche Kristallinit{\"a}t der Proben zur{\"u}ckgef{\"u}hrt werden, indem eine hochkristalline Probe in situ bis zur Schmelztemperatur geheizt wurde. Ab initio Rechnungen an einer Modelmatrix aus Butanmolek{\"u}len zeigen, dass die Spektren von kristallinem und amorphem Poly{\"a}thylen aufgrund der intermolekularen Wechselwirkung deutliche Unterschiede haupts{\"a}chlich f{\"u}r Resonanzen mit starkem Rydberg Charakter aufweisen. Damit lassen sich die Unterschiede in den Poly{\"a}thylenspektren durch die {\"U}berlagerung der Signaturen der kristallinen und amorphen Anteile erkl{\"a}ren, die je nach Kristallinit{\"a}t der Probe in unterschiedlichen Verh{\"a}ltnissen vorliegen.}, subject = {D{\"u}nne Schicht}, language = {en} } @article{RoeschAtzmuellerSchaacketal.1994, author = {R{\"o}sch, M. and Atzm{\"u}ller, R. and Schaack, G. and Becker, Charles R.}, title = {Resonant Raman scattering in a zero-gap semiconductor: Interference effects and deformation potentials at the E\(_1\) and E\(_1\) + \(\Delta_1\) gaps of HgTe}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-38062}, year = {1994}, abstract = {No abstract available}, language = {en} } @article{ZhangBeckerZhangetal.1994, author = {Zhang, X. F. and Becker, Charles R. and Zhang, H. and He, L. and Landwehr, G.}, title = {Investigation of a short period (001) HgTe-Hg\(_{0.6}\)Cd\(_{0.4}\)Te superlattice by transmission electron microscopy}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-38029}, year = {1994}, abstract = {No abstract available}, language = {en} } @article{EinfeldtHeinkeBehringeretal.1994, author = {Einfeldt, S. and Heinke, H. and Behringer, M. and Becker, Charles R. and Kurtz, E. and Hommel, D. and Landwehr, G.}, title = {The growth of HgSe by molecular beam epitaxy for ohmic contacts to p-ZnSe}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-38001}, year = {1994}, abstract = {The structural properties of HgSe grown by molecular beam epitaxy (MBE) are investigated for different lattice mismatches to the substrate and various growth conditions. The growth rate is shown to depend strongly on the growth temperature above lOO°C as well as on the Hg/Se flux ratio. It has been found that the crystalline perfection and the electrical properties are mainly determined by the layer thickness, especially for the growth on highly lattice mismatched substrates. Changes in the surface morphology are related to growth parameters. Differences between the electrical behavior of MBE-grown and bulk HgSe are discussed. The electrical properties of HgSe contacts on p-ZnSe are investigated as a function of different annealing procedures.}, language = {en} } @article{SchikoraHausleitnerEinfeldtetal.1994, author = {Schikora, D. and Hausleitner, H. and Einfeldt, S. and Becker, Charles R. and Widmer, T. and Giftige, C. and Lischka, K. and von Ortenburg, M. and Landwehr, G.}, title = {Epitaxial overgrowth of II-VI compounds on patterned substrates}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37985}, year = {1994}, abstract = {The selected area epitaxial overgrowth of narrow gap HgTe as well as wide gap CdTe and ZnTe on CdTe/GaAs substrates, which had been structured by dry etching techniques, has been investigated. A plasma etching process using a barrel reactor with CH\(_4\)-CH\(_2\) gases has been employed to prepare stripes with a width of about 1 μm with anisotropic as well as isotropic etching profiles. It has been found, that the selected area HgTe overgrowth takes place with a high local selectivity to the low index planes of the patterned surface. In contrast, the selected area overgrowth of the wide gap CdTe and ZnTe is controlled by anisotropic growth kinetics provided that the substrate temperature is not lower than 220°C and the starting surface consists of well developed low index crystallographic planes.}, language = {en} } @article{ToenniesBacherForcheletal.1994, author = {T{\"o}nnies, D. and Bacher, G. and Forchel, Alfred and Waag, A. and Litz, Th. and Hommel, D. and Becker, Charles R. and Landwehr, G. and Heuken, M. and Scholl, M.}, title = {Optical study of interdiffusion in CdTe and ZnSe based quantum wells}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37750}, year = {1994}, abstract = {No abstract available}, language = {en} } @inproceedings{BoegeSchaeferShanjiaetal.1994, author = {Boege, P. and Sch{\"a}fer, H. and Shanjia, Xu and Xinzhang, Wu and Einfeldt, S. and Becker, Charles R. and Hommel, D. and Geick, R.}, title = {Improved conductivity-measurement of semiconductor epitaxial layers by means of the contactless microwave method}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37763}, year = {1994}, abstract = {Measurements and calculations of the scattering-characteristics of stratified lossy dielectric blocks completely filling a waveguide cross section are presented. The method is used for contactless conductivity measurements of MBE-grown II-VI semiconductor layers.}, subject = {Millimeterwelle}, language = {en} } @article{XuShengGreineretal.1994, author = {Xu, Shanjia and Sheng, Xinqing and Greiner, P. and Becker, Charles R. and Geick, R.}, title = {High-order finite-element analysis of scattering properties of II-VI semiconductor materials}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-86283}, year = {1994}, abstract = {The sattering characteristics ot the n-VI semiconductors were analyzed by a method which combines the second-order finite-element method with the rigorous mode matching procedure. The method avolds the difficulty of solving the complex transcendental equation introduced in the multimode network method and calculates all the eigenvalues and eigenfunctions simultaneously which are needed for the mode matching treatment in the longitudinal direction. As a result, the whole solution procedure is significantly simplified. A comparison is given between the experimental data and the calculated results obtained with this analysis and tbe network method. Very good agreement has been achieved, the accuracy and efficiency of the present method are thus verified.}, subject = {Halbleiter}, language = {en} } @article{BeckerHeEinfeldtetal.1993, author = {Becker, Charles R. and He, L. and Einfeldt, S. and Wu, Y. S. and L{\´e}rondel, G. and Heinke, H. and Oehling, S. and Bicknell-Tassius, R. N. and Landwehr, G.}, title = {Molecular beam epitaxial growth and characterization of (100) HgSe on GaAs}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-50947}, year = {1993}, abstract = {In this paper, we present results on the first MBE growth of HgSe. The influence of the GaAs substrate temperature as well as the Hg and Se fluxes on the growth and the electrical properties has been investigated. It has been found that the growth rate is very low at substrate temperatures above 120°C. At 120°C and at lower temperatures, the growth rate is appreciably higher. The sticking coefficient of Se seems to depend inversely on the Hg/Se flux ratio. Epitaxial growth could be maintained at 70°C with Hg/Se flux ratios between lOO and ISO, and at 160°C between 280 and 450. The electron mobilities of these HgSe epilayers at room temperature decrease from a maximum value of 8.2 x 10^3 cm2 /V' s with increasing electron concentration. The concentration was found to be between 6xlO^17 and 1.6x10^19 cm- 3 at room temperature. Rocking curves from X-ray diffraction measurements of the better epilayers have a full width at half maximum of 5S0 arc sec.}, subject = {Physik}, language = {en} } @article{BeckerLatussekHeinkeetal.1993, author = {Becker, Charles, R. and Latussek, V. and Heinke, H. and Regnet, M. M. and Goschenhofer, F. and Einfeldt, S. and He, L. and Bangert, E. and Kraus, M. M. and Landwehr, G.}, title = {Molecular beam epitaxial growth and characterization of (001) Hg\(_{1-x}\) Cd\(_x\) Te-HgTe superlattices}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-50959}, year = {1993}, abstract = {The molecular beam epitaxially growth of (001) Hg\(_{1-x}\) Cd\(_z\) Te-HgTe superlattices has been systematically investigated. The well width as well as the period were determined directly by X-ray diffraction. This was accomphshed for the well width by exploiting the high reflectivity from HgTe and the low reflectivity from CdTe for the (002) Bragg reflection. Knowing the well and barrier thicknesses we have been able to set an upper limit on the aver~ge composition of the barriers, Xl, by annealing the superlattice and then measuring the composition of the. resultmg alloy. Xb was shown to decrease exponentially with decreasing barrier width. Xb is appreciably smaller m. narrow barriers due to the increased significance of interdiffusion in the Hg\(_{1-x}\)Cd\(_x\) Te/HgTe interface in narrow barriers. The experimentally determined optical absorption coefficient for these superlattices is compared WIth theoretical calculations. The absorption coefficient was determined from transmission and reflection spectra at 300, 77 and 5 K. Using the thickness and composition of the barriers and wells, and an interface width due to interdiffusion, the complex refractive index is calculated and compared with the experimental absorption coefficient. The envelope function method based on an 8 x 8 second order k . p band model was used to calculate the superlattice states. These results when inserted into Kubo's formula, yield the dynamic conductivity for interband transitions. The experimental and theoretical values for the absorption coefficient using no adjustable parameters are in good agreement for most of the investigated superlattices. Furthermore the agreement for the higher energetic interband transitions is much worse if values for the barrier composition, which are appreciably different than the experimentally determined values, are used. The infrared photoluminescence was investigated at temperatures from 4.2 to 300 K. Pronounced photoluminescence was observed for all superlattices in this temperature range.}, subject = {Physik}, language = {en} } @article{ShanjiaXinzhangBoegeetal.1993, author = {Shanjia, Xu and Xinzhang, Wu and Boege, P. and Sch{\"a}fer, H. and Becker, Charles R. and Geick, R.}, title = {Scattering characteristics of 3-D discontinuity consisting of semiconductor sample filled in waveguide with gaps}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37964}, year = {1993}, abstract = {No abstract available}, language = {en} } @article{WaagHeinkeScholletal.1993, author = {Waag, A. and Heinke, H. and Scholl, S. and Becker, Charles R. and Landwehr, G.}, title = {Growth of MgTe and Cd\(_{1-x}\)Mg\(_x\)Te thin films by molecular beam epitaxy}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37917}, year = {1993}, abstract = {We report on the growth of the compound semiconductor MgTe as weil as the ternary alloy Cd\(_{l-x}\)Mg, Te by molecular hcam cpitaxy. This is to our knowkdgc thc first time that this material has heen grown by any epitaxial technique. Bulk MgTe, which is hygroscopic, has a band gap of 3.0 eV and crystallizcs usually in thc wurtzite structure. Pseudomorphic films were grown on zincblende Cd Te suhstrates for a MgTe thickness helow a critical layer thickncss of approximately 500 nm. In addition, Cd\(_{l_x}\),Mg\(_x\)Te epilayers were grown with a Mg concentration between 0 and 68\%, which corresponds to a band gap betwcen 1.5 and 2.5 eV at room temperature. The crystalline quality of thc layers is comparabk to CdTc thin films as long as they are fully strained. The lauice constant of zincblende MgTe is slightly smaller than that of CdTe, and the lattice mismatch is as low as O.7\%. In addition highly n-type CdMgTe layers were fabricatcd by hromine doping. The tunability of the band gap as weil as the rather good laUice match with CdTc makes the matcrial interesting for optoelectronic device applications for the entire visible range.}, language = {en} } @article{HerrmannHappMoellmannetal.1993, author = {Herrmann, K. H. and Happ, M. and M{\"o}llmann, K.-P. and Tomm, J. W. and Becker, Charles R. and Kraus, M. M. and Yuan, S. and Landwehr, G.}, title = {A new model for the absorption coefficient of narrow gap (Hg,Cd)Te that simultaneously considers band tails and band filling}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37894}, year = {1993}, abstract = {A semiempirical model is presented that correlates the broadening of the absorption edge with both transitions below the energy gap and with transitions by the Kane band model. This model correctly fits both the absorption and luminescence spectra of narrow-gap (Hg,Cd)Te samples that have been grown by the traveling heater method as well as by molecular-beam epitaxy. The accuracy of the band-gap determination is enhanced by this model.}, language = {en} } @article{WuBeckerWaagetal.1993, author = {Wu, Y. S. and Becker, Charles R. and Waag, A. and von Schierstedt, K. and Bicknell-Tassius, R. N. and Landwehr, G.}, title = {Surface sublimation of zinc blende CdTe}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37829}, year = {1993}, abstract = {The surface sublimation of Cd and Te atoms from the zinc blende (111)A CdTe surface has been investigated in detail by reflection high energy electron diffraction and x-ray photoelectron spectroscopy. These experiments verify that Te is much easier to evaporate than Cd. The experimental value for the Te activation energy from a Te stabilized (111)A CdTe surface is 1.41 ±0.1O eV, which is apparently inconsistent with recent theoretical results.}, language = {en} } @article{HeBeckerBicknellTassiusetal.1993, author = {He, L. and Becker, Charles R. and Bicknell-Tassius, R. N. and Scholl, S. and Landwehr, G.}, title = {Molecular beam epitaxial growth of (100) Hg\(_{0.8}\)Cd\(_{0.2}\)Te on Cd\(_{0.96}\)Zn\(_{0.04}\)Te}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-38044}, year = {1993}, abstract = {No abstract available}, language = {en} } @article{KrausBeckerScholletal.1993, author = {Kraus, M. M. and Becker, Charles R. and Scholl, S. and Wu, Y. S. and Yuan, S. and Landwehr, G.}, title = {Infrared photoluminescence on molecular beam epitaxially grown Hg\(_{1-x}\)Cd\(_x\)Te layers}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-38053}, year = {1993}, abstract = {No abstract available}, language = {en} } @article{WuBeckerWaagetal.1993, author = {Wu, Y. S. and Becker, Charles R. and Waag, A. and Bicknell-Tassius, R. N. and Landwehr, G.}, title = {Removal of oxygen and reduction of carbon contamination on (100) Cd\(_{0.96}\)Zn\(_{0.04}\)Te}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-38014}, year = {1993}, abstract = {No abstract available}, language = {en} } @article{HeBeckerBicknellTassiusetal.1993, author = {He, L. and Becker, Charles R. and Bicknell-Tassius, R. N. and Scholl, S. and Landwehr, G.}, title = {Molecular beam epitaxial growth and evaluation of intrinsic and extrinsically doped (100) Hg\(_{0.8}\)Cd\(_{0.2}\)Te on (100) Cd\(_{0.96}\)Zn\(_{0.04}\)Te}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37885}, year = {1993}, abstract = {No abstract available.}, language = {en} } @article{WuBeckerWaagetal.1993, author = {Wu, Y. S. and Becker, Charles R. and Waag, A. and Schmiedl, R. and Einfeldt, S. and Landwehr, G.}, title = {Oxygen on the (100) CdTe surface}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37869}, year = {1993}, abstract = {We have investigated oxygen on CdTe substrates by means of x-ray photoelectron spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED). A Te oxide layer that was at least 15 A thick was found on the surface of as-delivered CdTe substrates that were mechanically polished. This oxide is not easily evaporated at temperatures lower than 350°C. Furthermore, heating in air, which further oxidizes the CdTe layer, should be avoided. Etching with HCI acid (15\% HCl) for at least 20 s and then rinsing with de-ionized water reduces the Te oxide layer on the surface down to 4\% of a monoatomic layer. However, according to XPS measurements of the 0 Is peak, 20\%-30\% of a monoatomic layer of oxygen remains on the surface, which can be eliminated by heating at temperatures ranging between 300 and 340 cC. The RHEED patterns for a molecular beam epitaxially (MBE)-grown CdTe film on a (lOO) CdTe substrate with approximately one monoatomic layer of oxidized Te on the surface lose the characteristics of the normal RHEED pattems for a MBE-grown CdTe film on an oxygen-free CdTe substrate.}, language = {en} } @article{BeckerHeRegnetetal.1993, author = {Becker, Charles R. and He, L. and Regnet, M. M. and Kraus, M.M. and Wu, Y. S. and Landwehr, G. and Zhang, X. F. and Zhang, H.}, title = {The growth and structure of short period (001) Hg\(_{1-x}\)Cd\(_x\)Te-HgTe superlattices}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37858}, year = {1993}, abstract = {Molecular beam epitaxially grown short period (001) Hg\(_{1_x}\)Cd\(_x\)Te-HgTe superlattices have been systematically investigated. Several narrow well widths were chosen, e.g., 30, 35 and 40 {\AA}, and the barrier widths were varied between 24 and 90 {\AA} for a particular well width. Both the well width and the total period were determined directly by means of x-ray diffraction. The well width was determined by exploiting the high reflectivity from HgTe and the low reflectivity from CdTe for the (002) Bragg reflection. Knowing the well and barrier widths we have been able to set an upper limit on the average Cd concentration of the barriers, \(\overline x_b\), by annealing several superlattices and then measuring the composition of the resulting alloy. \(\overline x_b\) was shown to decrease exponentially with decreasing barrier width. The structure of a very short period superlattice, i.e., 31.4 {\AA}, was also investigated by transmission electron microscopy, corroborating the x-ray diffraction results.}, language = {en} } @article{QiuHeLietal.1993, author = {Qiu, Yueming and He, Li and Li, Jie and Yuan, Shixin and Becker, Charles R. and Landwehr, G.}, title = {Infrared photoconductor fabricated with HgTe/CdTe superlattice grown by molecular beam epitaxy}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37772}, year = {1993}, abstract = {An infrared photoconductor fabricated with a HgTe/CdTe superlattice grown on a GaAs substrate by molecular beam epitaxy is described here for the first time. The growth procedure, device fabrication, and measurement results are described. The results show that the device has relatively high uniformity and 1000 K black-body detectivity 2.4 X 10\(^9\) cm Hz\(^{1/2}\) W\(^{-1}\) . The photoconductivity decay method was used for determining carrier lifetime of the HgTe/CdTe superlattice, the measured lifetime is 12\(\mu\)s at 77 K, which is the longest lifetime ever reported for HgTe/CdTe superlattices and we believe that the increase of lifetime is mainly due to the reduction of dimensions.}, language = {en} } @article{GreinerPolignoneBeckeretal.1992, author = {Greiner, P. and Polignone, L. and Becker, Charles R. and Geick, R.}, title = {Contactless measurement of the conductivity of II-VI epitaxial layers by means of the partially filled waveguide method}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37838}, year = {1992}, abstract = {We report the contactless determination of the conductivity, the mobility and the carrier concentration of II-VI semiconductors by means of the technique of the partially filled waveguide at a microwave frequency of 9 GHz. The samples are CdHgTe epitaxial layers, grown on CdZnTe substrates by molecular beam epitaxy. The conductivity is determined from the transmission coefficient of the sample in the partially filled waveguide. For the analysis of the experimental data, the complex transmission coefficient is calculated by a rigorous multi-mode matching procedure. By varying the conductivity of the sample, we obtain an optimum fit of the calculated data to the experimental results. Comparison with conductivity data determined by the van der Pauw method shows that our method allows to measure the conductivity with good accuracy. The behaviour of the transmission coefficient of the sample is discussed in dependence on the layer conductivity, the layer thickness and the dielectric constant of the substrate. The calculations require to consider in detail the distribution of the electromagnetic fields in the sample region. The usual assumption of a hardly disturbed TE\(_{10}\) mode cannot be used in our case. By applying a magnetic field in extraordinary Voigt configuration. galvanomagnetic measurements have been carried out which yield the mobility and thus the carrier concentration. These results are also in good agreement with van der Pauw transport measurements.}, language = {en} } @article{ShanjiaXinzhangGreineretal.1992, author = {Shanjia, Xu and Xinzhang, Wu and Greiner, P. and Becker, Charles R. and Geick, R.}, title = {Microwave transmission and reflection of stratified lossy dielectric segments partially filled waveguide}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37958}, year = {1992}, abstract = {The microwave transmission and reßection is evaluated for stratified lossy dielectric segments partially filling the rectangular waveguide by the method which combines the multimode network theory with the rigorous mode matching procedure. As an example, we investigate in detail the microwave scattering properties of II-VI-epitaxial layer on a lossy dielectric substrate inserted in the rectangular waveguide. The experimental data verify the accuracy and the effectiveness of the present method. Extensive numerical results are presented to establish useful guidelines for the contactless microwave measurement of the conductivity of the epitaxiallayer.}, language = {en} } @article{WuBeckerWaagetal.1992, author = {Wu, Y. S. and Becker, Charles R. and Waag, A. and Bicknell-Tassius, R. N. and Landwehr, G.}, title = {Thermal effects on (100) CdZnTe substrates as studied by x-ray photoelectron spectroscopy and reflection high energy electron diffraction}, issn = {0003-6951}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37801}, year = {1992}, abstract = {The influence of different CdZnTe substrate treatments prior to II-VI molecular beam epitaxial growth on surface stoichiometry, oxygen, and carbon contamination has been studied using x-ray photoelectron spectroscopy and reflection high energy electron diffraction. Heating the substrate at 300 °C can eliminate oxygen contamination, but cannot completely remove carbon from the surface. Heating at higher temperatures decreases the carbon contamination only slightly, while increasing the Zn-Cd ratio on the surface considerably. The magnitude of the latter effect is surprising and is crucial when one is using lattice matched CdZnTe (Zn 4\%) substrates.}, language = {en} } @article{KrausRegnetBeckeretal.1992, author = {Kraus, M. M. and Regnet, M. M. and Becker, Charles R. and Bicknell-Tassius, R. N. and Landwehr, G.}, title = {Comparison of band structure calculations and photoluminescence experiments on HgTe/CdTe superlattices grown by molecular beam epitaxy}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37878}, year = {1992}, abstract = {We have grown HgTe/CdTe superlattices by molecular beam epitaxy; barrier thicknesses were in the range from 15 to 91 {\AA} and the well thickness was maintained at a constant value of 30 {\AA}. The infrared photoluminescence was investigated by means of Fourier transform infrared spectroscopy in the temperature range from 4.2 to 300 K. All superlattices showed pronounced photoluminescence at temperatures up to 300 K. To gain more detailed insight into the band structure of the HgTe/CdTe superlattices, band structure calculations were performed. The concept of the envelope function approximation was followed. Employing the transfer matrix method, the calculations were completed taking into account an eight band k·p model. An important parameter in these calculations is the natural valence band offset (VBO) between the well and barrier materials. As a general trend, the value for the direct gap decreases with increasing VBO. The experimentally determined energies of the band gap are in reasonable agreement with the values obtained by the theoretical calculations. A comparison between theory and experiment shows that the observed transition energies are closer to calculations employing a large offset (350 meV) as opposed to a small VBO (40 meV).}, language = {en} } @inproceedings{BicknellTassiusSchollBeckeretal.1992, author = {Bicknell-Tassius, R. N. and Scholl, S. and Becker, Charles R. and Landwehr, G.}, title = {High magnetic field transport in II-VI heterostructures}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37797}, year = {1992}, abstract = {In the present work we report the results of magneto-transport measurements on some Hg-based li-VI semiconductor epitaxiallayers grown by molecular beam epitaxy. The transport measurement were carried out at temperatures in the range 0.4 - 4.2 K in magnetic fields up to 10.0 T. Further, we point out the necessity of using multicarrier models for data interpretation and show finally some Shubnikov-de-Haas results on sampies with high mobility carners.}, language = {en} } @article{SchubertKrausKenkliesetal.1992, author = {Schubert, D.W. and Kraus, M.M and Kenklies, R. and Becker, Charles R. and Bicknell-Tassius, R.N.}, title = {Composition and wavelength dependence of the refractive index in Cd\(_{1-x}\)Mn\(_x\)Te epitaxial layers}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37716}, year = {1992}, abstract = {We have investigated Cd\(_{1-x}\)Mn\(_x\)Te thin films with Mn concentrations of x=0.12, 0.18, 0.30, 0.52, and 0.70. These single crystal layers were grown by molecular beam epitaxy on [001] CdTe substrates. The real part of the refractive index, n, was determined below the band-gap Eo in the range of 0.5-2.5 eV at T=300 K. The parallel reOectivity was measured near the Brewster angle at the YAG laser wavelength of 1.064 J.Lm (hv= 1.165 eV). Combining these results with the optical pathlength results (nd) of reOection measurements in a Fourier spectrometer we have determined n(x,v) over a wide spectral range by utilizing a three parameter fit. The accuracy of these results for n should improve waveguide designs based on this material.}, language = {en} } @article{BeckerWuWaagetal.1991, author = {Becker, Charles R. and Wu, Y. S. and Waag, A. and Kraus, M. M. and Landwehr, G.}, title = {The orientation independence of the CdTe-HgTe valence band offset as determined by x-ray photoelectron spectroscopy}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30784}, year = {1991}, abstract = {No abstract available}, language = {en} } @article{WuBeckerWaagetal.1991, author = {Wu, Y.S. and Becker, Charles R. and Waag, A. and Kraus, M. M. and Bicknell-Tassius, R. N. and Landwehr, G.}, title = {Correlation of the Cd-to-Te ratio on CdTe surfaces with the surface structure}, isbn = {0163-1829}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37789}, year = {1991}, abstract = {We report here that reconstruction on (100), (1lIlA, and (1l1lB CdTe surfaces is either C(2X2), (2X2), and (l X I) or (2X I), (l X I), and (l X I) when they are Cd or Te stabilized, respectively. There is a mixed region between Cd and Te stabilization in which the reflected high-energy electron-diffraction (RHEED) patterns contain characteristics of both Cd- and Te-stabilized surfaces. We have also found that the Cd-to-Te ratio of the x-ray photoelectron intensities of their 3d\(_{3/ 2}\) core levels is about 20\% larger for a Cd-stabilized (1lIlA, (1lIlB, or (100) CdTe surface than for a Te-stabilized one. According to a simple model calculation, which was normalized by means of the photoelectron intensity ratio of a Cd-stabilized (lll)A and aTe-stabilized (1l1lB CdTe surface, the experimental data for CdTe surfaces can be explained by a linear dependence of the photoelectron-intensity ratio on the fraction of Cd in the uppermost monatomic layer. This surface composition can be correlated with the surface structure, i.e., the corresponding RHEED patterns. This correlation can in turn be employed to determine Te and Cd evaporation rates. The Te reevaporation rate is increasingly slower for the Te-stabilized (Ill) A, (l1l)B, and (100) surfaces, while the opposite is true for Cd from Cd-stabilized (Ill) A and (Ill)B surfaces. In addition, Te is much more easily evaporated from all the investigated surfaces than is Cd, if the substrate is kept at normal molecular-beam-epitaxy growth temperatures ranging from 2oo·C to 300 ·C.}, subject = {Festk{\"o}rperphysik}, language = {en} } @article{WuBeckerWaagetal.1991, author = {Wu, Y. S. and Becker, Charles R. and Waag, A. and Bicknell-Tassius, R. N. and Landwehr, G.}, title = {The effects of laser illumination and high energy electrons on molecular-beam epitaxial growth of CdTe}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30795}, year = {1991}, abstract = {We report the results of a detailed investigation on the Te-stabilized (2 x 1) and the Cdstabilized c( 2 X 2) surfaces of ( 100) CdTe substrates. The investigation demonstrates for the first time that both laser illumination and, to a greater extent, high-energy electron irradiation increase the Te desorption and reduce the Cd desorption from ( 100) CdTe surfaces. Thus it is possible by choosing the proper growth temperature and photon or electron fluxes to change the surface reconstruction from the normally Te-stabilized to a Cd-stabilized phase.}, language = {en} } @inproceedings{HodgsonReuschKjoellerstroemetal.1989, author = {Hodgson, Barbara and Reusch, Wolfgang and Kj{\"o}llerstr{\"o}m, Bengt and Velarde, Manuel}, title = {Non-Formal Education}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30620}, year = {1989}, abstract = {No abstract available}, language = {en} } @inproceedings{DegerReuschLuchner1989, author = {Deger, H. and Reusch, Wolfgang and Luchner, K.}, title = {Microscopic Observation of Crystal Growth and Phase Transitions}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-33001}, year = {1989}, abstract = {No abstract available}, language = {en} } @article{TackeSchuberthBeckeretal.1982, author = {Tacke, M. and Schuberth, W. and Becker, Charles R. and Haas, L. D.}, title = {The dielectric constant of PbTe at 4.2 K and \(\tilde ν\)=84.15 cm\(^{-1}\), 96.97 cm\(^{-1}\), 103.60 cm\(^{-1}\)}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30821}, year = {1982}, abstract = {The dielectric constant of a PbTe epitaxial layer has been measured by surface wave spectroscopy using an optically pumped far-infrared laser and the technique of attenuated total reflection.}, language = {en} } @article{GeisGeickBeckeretal.1977, author = {Geis, G. and Geick, R. and Becker, Charles R. and Wagner, V.}, title = {Antiferromagnetic resonance in CoO/NiO mixed crystals}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-31239}, year = {1977}, abstract = {We have studied the lowest magnetic excitation of Ni\(_{1-x}\)Co\(_x\)O mixed crystals for 0.94 \(\leq\) x \(\leq\) 1. Together with previous results for 0.02 \(\leq\) x \(\leq\) 0.07 and neutron data for x = 0.14 and x = 0.30, the results are discussed by means of a model, especially the variation of AFMR frequency and preferred spin direction with Co concentration x.}, language = {en} } @article{KlitzingBecker1976, author = {Klitzing, K. von and Becker, Charles R.}, title = {Far infrared photoconductivity of residual acceptors in pure tellurium}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-31278}, year = {1976}, abstract = {The photoconductivity of both undeformed and deformed Te samples has been investigated at liquid He temperature by means of a Fourier spectrometer. Three peaks were usually found in the spectra of undeformed samples at 11, 24 and 46 cm\(^{-1}\). These are shown to be due to three different chemical impurities. The deformed samples are characterized by additional structure at higher frequencies.}, language = {en} } @misc{KoehlerBecker1976, author = {K{\"o}hler, H. and Becker, Charles R.}, title = {Infrared-active lattice vibrations in SnSe\(_2\)}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30838}, year = {1976}, abstract = {No abstract available}, language = {en} } @article{BeckerLauGEICKetal.1975, author = {Becker, Charles R. and Lau, Ph. and GEICK, R. and Wagner, V.}, title = {Antiferromagnetic Resonance in NiO:Co\(^{2+}\) and NiO:Fe\(^{2+}\)}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30772}, year = {1975}, abstract = {No abstract available.}, language = {en} } @article{KoehlerBecker1974, author = {K{\"o}hler, H. and Becker, Charles R.}, title = {Optically active lattice vibrations in Bi\(_2\)Se\(_3\)}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-31296}, year = {1974}, abstract = {The preparation of Bi2Sea single crystals with low free carrier densities allowed an investigation of the lattice vibrations to be carried out from the reflectivity of cleavage planes at nearly normal incidence of the radiation (E ~ c). The experimental results ean be explained with two classical oscillators, whose eigenfrequencies oceur at 92 and 69.5 cm-I. The static dielectric constant was determined to be 100 ± 10 for E ~ c.}, language = {en} } @misc{DeciusBeckerFredericks1973, author = {Decius, J. C. and Becker, Charles R. and Fredericks, W. J.}, title = {Erratum: Force constants of the metaborate ion in alkali halides}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37992}, year = {1973}, abstract = {Erratum for the article: Decius, J. C. ; Becker, Charles R. ; Fredericks, W. J. : Force constants of the metaborate ion in alkali halides, In: THE JOURNAL OF CHEMICAL PHYSICS (1972), 56, 10, 5189-5190.}, language = {en} } @misc{UnkelbachBeckerKoehleretal.1973, author = {Unkelbach, K. H. and Becker, Charles R. and K{\"o}hler, H. and Middendorff, A. V.}, title = {Optical Phonons of Bi\(_2\)Te\(_3\)}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30809}, year = {1973}, abstract = {No abstract available}, language = {en} } @article{DeciusBeckerFredericks1972, author = {Decius, J. C. and Becker, Charles R. and Fredericks, W. J.}, title = {Force constants of the metaborate ion in alkali halides}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-50935}, year = {1972}, abstract = {No abstract available}, subject = {Physik}, language = {en} } @article{BeckerMartin1972, author = {Becker, Charles R. and Martin, T. P.}, title = {Infrared absorption by Impurity-pair resonant modes in NaCl:F}, isbn = {1098-0121}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37840}, year = {1972}, abstract = {New resonant-mode infrared absorption lines have been observed in NaCl with high concentrations of fluorine impurities. The quadratic concentration dependence of the strength of these lines indicates that they are due to pairs of fluorine impurities. At the resonant frequencies, the motion of some host ions appears to be as important as the motion of the impurities themselves.}, subject = {Festk{\"o}rperphysik}, language = {en} } @misc{IshigamaBeckerMartinetal.1972, author = {Ishigama, M. and Becker, Charles R. and Martin, T. P. and Prettl, W.}, title = {Impurity-pair mode in NaCl:KF}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-31255}, year = {1972}, abstract = {No abstract available}, language = {en} } @article{Becker1971, author = {Becker, Charles R.}, title = {Evidence for a quasi-phonon gap in CsCl}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30767}, year = {1971}, abstract = {The observed impurity induced far-infrared absorption in CsCl : Rb\(^+\) and CsCl : K\(^+\) is compared with a calculated density of acoustic phonon states in CsCl. The absorption due to CsCl : Rb\(^+\) displays a minimum between the acoustic and optic phonon bands. A narrow line is observed in CsCl: K\(^+\) at 85.8 cm\(^{-1}\) which falls in this quasi-phonon gap.}, language = {en} } @article{BeckerNath1970, author = {Becker, Charles R. and Nath, G.}, title = {Optical properties of LiIO\(_3\) in the far infrared}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37905}, year = {1970}, abstract = {No abstract available}, language = {en} } @article{Becker1970, author = {Becker, Charles R.}, title = {Impurity-induced absorption in or near the phonon gap of KI}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-30757}, year = {1970}, abstract = {In or near the phonon gap of KI a single weak line due to OW (OD\(^-\)) at 69.7 (69.3) cm\(^{-1}\) has been observed. A second and much stronger line at 94.1 cm\(^{-1}\) was shown not to be related to OH\(^-\), but instead is thought to be due to CO\(_3\).}, language = {en} }