D. Geiger

Technische Universität Dresden, Dresden, Saxony, Germany

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Publications (21)35.36 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Electron holography has been shown to allow a posteriori aberration correction. Therefore, an aberration corrector in the transmission electron microscope does not seem to be needed with electron holography to achieve atomic lateral resolution. However, to reach a signal resolution sufficient for detecting single light atoms and very small interatomic fields, the aberration corrector has turned out to be very helpful. The basic reason is the optimized use of the limited number of “coherent” electrons that are provided by the electron source, as described by the brightness. Finally, quantitative interpretation of atomic structures benefits from the holographic facilities of fine-tuning of the aberration coefficients a posteriori and from evaluating both amplitude and phase.
    Microscopy and Microanalysis 07/2010; 16(04):434 - 440. · 1.76 Impact Factor
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    ABSTRACT: The aim of the study was the identification of gamma' and gamma'' strengthening precipitates in a commercial nickel-base superalloy Inconel 718 (Ni-19Fe-18Cr-5Nb-3Mo-1Ti-0.5Al-0.04C, wt %) using TEM dark-field, HRTEM, electron holography and electron tomography imaging. To identify gamma' and gamma'' nanoparticles unambiguously, a systematic analysis of experimental and theoretical diffraction patterns were performed. Using HRTEM method it was possible to analyse small areas of precipitates appearance. Electron holography and electron tomography techniques show new possibilities of visualization of gamma' and gamma'' nanoparticles. The analysis by means of different complementary TEM methods showed that gamma'' particles exhibit a shape of thin plates, while gamma' phase precipitates are almost spherical.
    Journal of Microscopy 11/2009; 236(2):149-57. · 1.63 Impact Factor
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    ABSTRACT: The defect structure of Ge(111) epilayers grown by molecular beam epitaxy on cubic Pr <sub>2</sub> O <sub>3</sub>(111)/ Si (111) support systems was investigated by means of transmission electron microscopy and laboratory-based x-ray diffraction techniques. Three main types of defects were identified, namely, rotation twins, microtwins, and stacking faults, and studied as a function of Ge film thickness and after annealing at 825 ° C in ultrahigh vacuum. Rotation twins were found to be localized at the Ge(111)/cubic Pr <sub>2</sub> O <sub>3</sub>(111) interface and their amount could be lowered by the thermal treatment. Microtwins across {11 1 } were detected only in closed Ge films, after Ge island coalescence. The fraction of Ge film volume affected by microtwinning is constant within the thickness range of ∼20–260 nm . Beyond 260 nm, the density of microtwins is clearly reduced, resulting in thick layers with a top part of higher crystalline quality. Microtwins resulted insensitive to the postdeposition annealing. Instead, the density of stacking faults across {11 1 } planes decreases with the thermal treatment. In conclusion, the defect density was proved to diminish with increasing Ge thickness and after annealing. Moreover, it is noteworthy that the annealing generates a tetragonal distortion in the Ge films, which get in-plane tensely strained, probably due to thermal mismatch between Ge and Si.
    Journal of Applied Physics 11/2009; · 2.19 Impact Factor
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    Hannes Lichte, Dorin Geiger, Martin Linck
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    ABSTRACT: Electron holography allows the reconstruction of the complete electron wave, and hence offers the possibility of correcting aberrations. In fact, this was shown by means of the uncorrected CM30 Special Tübingen transmission electron microscope (TEM), revealing, after numerical aberration correction, a resolution of approximately 0.1 nm, both in amplitude and phase. However, it turned out that the results suffer from a comparably poor signal-to-noise ratio. The reason is that the limited coherent electron current, given by gun brightness, has to illuminate a width of at least four times the point-spread function given by the aberrations. As, using the hardware corrector, the point-spread function shrinks considerably, the current density increases and the signal-to-noise ratio improves correspondingly. Furthermore, the phase shift at the atomic dimensions found in the image plane also increases because the collection efficiency of the optics increases with resolution. In total, the signals of atomically fine structures are better defined for quantitative evaluation. In fact, the results achieved by electron holography in a Tecnai F20 Cs-corr TEM confirm this.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 10/2009; 367(1903):3773-93. · 2.86 Impact Factor
  • H Lichte, D Geiger, M Linck, M Lehmann
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    ABSTRACT: Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009
    Microscopy and Microanalysis 06/2009; 15:1460 - 1461. · 1.76 Impact Factor
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    ABSTRACT: Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009
    Microscopy and Microanalysis 06/2009; 15:1098 - 1099. · 1.76 Impact Factor
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    ABSTRACT: Iron nanoparticles embedded in MgO crystals were synthesized by Fe <sup>+</sup> ion implantation at an energy of 100 keV and varying fluences from 3×10<sup>16</sup> to 3×10<sup>17</sup> cm <sup>-2</sup> . Investigations of structural and magnetic properties of Fe nanoparticles have been performed using magnetometry, x-ray diffraction, transmission electron microscopy, and Mössbauer spectroscopy, as well as by theoretical Preisach modeling of bistable magnetic systems. It has been found that α - and γ -Fe nanoparticles are formed for all fluences. The content of the α -Fe phase increases at higher fluences and after annealing. The influence of postimplantation annealing at 800 ° C in vacuum and under enhanced hydrostatic pressure on the formation of nanoparticles has been analyzed.
    Journal of Applied Physics 04/2009; · 2.19 Impact Factor
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    ABSTRACT: The integration of alternative semiconductor layers on the Si material platform via oxide heterostructures is of interest to increase the performance and/or functionality of future Si-based integrated circuits. The single crystalline quality of epitaxial (epi) semiconductor–insulator–Si heterostructures is however limited by too high defect densities, mainly due to a lack of knowledge about the fundamental physics of the heteroepitaxy mechanisms at work. To shed light on the physics of stacking twin formation as one of the major defect mechanisms in (111)-oriented fcc-related heterostructures on Si(111), we report a detailed experimental and theoretical study on the structure and defect properties of epi-Si(111)/Y 2 O 3 /Pr 2 O 3 /Si(111) heterostructures. Synchrotron radiation-grazing incidence x-ray diffraction (SR-GIXRD) proves that the engineered Y 2 O 3 /Pr 2 O 3 buffer dielectric heterostructure on Si(111) allows control of the stacking sequence of the overgrowing single crystalline epi-Si(111) layers. The epitaxy relationship of the epi-Si(111)/insulator/Si(111) heterostructure is characterized by a type A/B/A stacking configuration. Theoretical ab initio calculations show that this stacking sequence control of the heterostructure is mainly achieved by electrostatic 4 Author to whom any correspondence should be addressed. 2 interaction effects across the ionic oxide/covalent Si interface (IF). Transmission electron microscopy (TEM) studies detect only a small population of misaligned type B epi-Si(111) stacking twins whose location is limited to the oxide/epi-Si IF region. Engineering the oxide/semiconductor IF physics by using tailored oxide systems opens thus a promising approach to grow heterostructures with well-controlled properties.
    New Journal of Physics 11/2008; 10(10). · 3.67 Impact Factor
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    ABSTRACT: Cs correctors have revolutionized transmission electron microscopy (TEM) in that they substantially improve point resolution and information limit. The object information is found sharply localized within 0.1 nm, and the intensity image can therefore be interpreted reliably on an atomic scale. However, for a conventional intensity image, the object exit wave can still not be detected completely in that the phase, and hence indispensable object information is missing. Therefore, for example, atomic electric-field distributions or magnetic domain structures cannot be accessed. Off-axis electron holography offers unique possibilities to recover completely the aberration-corrected object wave with uncorrected microscopes and hence we would not need a Cs-corrected microscope for improved lateral resolution. However, the performance of holography is affected by aberrations of the recording TEM in that the signal/noise properties ("phase detection limit") of the reconstructed wave are degraded. Therefore, we have realized off-axis electron holography with a Cs-corrected TEM. The phase detection limit improves by a factor of four. A further advantage is the possibility of fine-tuning the residual aberrations by a posteriori correction. Therefore, a combination of both methods, that is, Cs correction and off-axis electron holography, opens new perspectives for complete TEM analysis on an atomic scale.
    Microscopy and Microanalysis 03/2008; 14(1):68-81. · 1.76 Impact Factor
  • Microscopy and Microanalysis 09/2007; 13. · 1.76 Impact Factor
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    ABSTRACT: The stoichiometry, structure, and defects of self-assembled heteroepitaxial Ge nanodots on twin-free type B oriented cubic Pr <sub>2</sub> O <sub>3</sub>(111) layers on Si(111) substrates are studied to shed light on the fundamental physics of nanocrystal based nonvolatile memory effects. X-ray photoelectron spectroscopy studies prove the high stoichiometric purity of the Ge nanodots on the cubic Pr <sub>2</sub> O <sub>3</sub>(111)/ Si (111) support system. Synchrotron based x-ray diffraction, including anomalous scattering techniques, was applied to determine the epitaxial relationship, showing that the heteroepitaxial Ge(111) nanodots crystallize in the cubic diamond structure with an exclusive type A stacking configuration with respect to Si(111). Grazing incidence small angle x-ray scattering was used in addition to analyze the average shape, size, and distance parameters of the single crystalline Ge nanocrystal ensemble. Furthermore, transmission electron micrographs report that partial dislocations are the prevailing extended defect structure in the Ge nanodots, mainly induced by surface roughness on the atomic scale of the cubic Pr <sub>2</sub> O <sub>3</sub>(111) support.
    Journal of Applied Physics 09/2007; · 2.19 Impact Factor
  • Microscopy and Microanalysis 08/2007; 13:456 - 457. · 1.76 Impact Factor
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    ABSTRACT: Structural and magnetic properties in Mn-implanted, p-type Si were investigated. High resolution structural analysis techniques such as synchrotron x-ray diffraction revealed the formation of MnSi1.7 nanoparticles already in the as-implanted samples. Depending on the Mn fluence, the size increases from 5 nm to 20 nm upon rapid thermal annealing. No significant evidence is found for Mn substituting Si sites either in the as-implanted or annealed samples. The observed ferromagnetism yields a saturation moment of 0.21μB per implanted Mn at 10 K, which could be assigned to MnSi1.7 nanoparticles as revealed by a temperature-dependent magnetization measurement.
    Physical Review B 01/2007; 75(8). · 3.66 Impact Factor
  • M Lehmann, D Geiger, H Lichte
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    ABSTRACT: Extended abstract of a paper presented at Microscopy and Microanalysis 2005 in Honolulu, Hawaii, USA, July 31--August 4, 2005.
    Microscopy and Microanalysis 08/2005; 11(S02):2146-2147. · 1.76 Impact Factor
  • Microscopy and Microanalysis 07/2004; 10:112 - 113. · 1.76 Impact Factor
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    ABSTRACT: An electron microscope is a wave optical instrument where the object information is carried by an electron wave. However, an important information, the phase of the electron wave, is lost, because only intensities can be recorded in a conventional electron micrograph. Off-axis electron holography solves this “phase problem” by encoding amplitude and phase information in an interference pattern, the so-called hologram. After reconstruction, a rather unrestricted wave optical analysis can be performed on a computer. The possibilities as well as the current limitations of off-axis electron holography at atomic dimensions are discussed, and they are illustrated at two applications of structure characterization of ϵ-NbN and YBCO-1237. Finally, an electron microscope equipped with a Cs-corrector, a monochromator, and a Möllenstedt biprism is outlined for subangstrom holography.
    Materials Characterization 04/1999; 42(4). · 1.93 Impact Factor
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    ABSTRACT: In off-axis electron holography artefacts may arise specific to the method. In particular at high resolution they must be taken into account to obtain reliable results after correction of aberrations. In the following we describe some artefacts which we have found to be most essential so far.
    Ultramicroscopy 08/1996; · 2.75 Impact Factor
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    ABSTRACT: The exceptional progress achieved in the transmission electron microscopy (TEM) using Cs-corrector [1] allows now a true atomic resolution of 0.1 nm or better. Nevertheless, conventional TEM still suffers from the significant drawback that only a poor phase contrast can be obtained. The weaker phase structures like those produced by the meso- up to macroscopic electromagnetic fields are essential to understand solidstate properties but remain in conventional TEM virtually invisible.
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    ABSTRACT: Intrinsic functionalities of complex oxides like ferroelectricity, magnetism, superconductivity or multiferroic behaviour can be combined in electronic devices based on epitaxially grown heterostructures. Searching for new materials with special properties in nanoscience implies also the control of interfaces down to the atomic level.
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    ABSTRACT: (Multi-)ferroic materials attracted growing interest during the last decade due to their interesting (multiple)-ordering phenomena and the resulting applications (i.e. nonvolatile memories). Physical properties of boundaries are of particular importance as electronic device dimensions shrink and multiferroic bulk materials have not revealed a sufficient magneto-electric coupling so far. We will combine Density Functional calculations and microscopic techniques to examine basic properties of model boundaries, i.e. BiFeO3 71°/109°/180° domain walls (Fig. 1) and BaTiO3 90°/180° domain walls. The BaTiO3 180° domain wall is considered in the lower energetic parallel and in the higher energetic head-to-head configuration. The DFT calculations are performed within LDA+U on a plane wave basis set. PAW pseudopotentials have been incorporated to represent core states. Both, unit cell dimensions and ion positions have been relaxed to yield minimal energy structures. Transmission Electron Microscopy and in particular Electron Holography are applied to probe electric potential distributions and structure properties at the domain boundaries. Structural changes at the boundary occur due to lattice misfits and reconstruction of electronic orbitals. The thereby produced polarization change at the boundary leads to depolarization fields (Fig. 2). We particularly calculate and investigate such fields and discuss the prospects and problems of depolarization field measurements with TEM techniques. Moreover, the reconfiguration of the band structure at the boundary can lead to completely new physical properties like reduction of the band gap, magnetization change, etc [1, 2]. We calculate boundary band structures predicting ferromagnetic BiFeO3 71°/180° domain walls, band gap reduction, etc.

Publication Stats

121 Citations
35.36 Total Impact Points

Institutions

  • 2004–2010
    • Technische Universität Dresden
      • Fakultät Mathematik und Naturwissenschaften
      Dresden, Saxony, Germany