Elektronen-Holographische Tomographie zur 3D-Abbildung von elektrostatischen Potentialen in Nanostrukturen

Revealing the essential structure-property relation of materials on a (sub-)nanometer scale requires a powerful Transmission Electron Microscopy (TEM). In this context, the intrinsic electrostatic and magnetic elds, which are related to the materials nano structure, play a crucial role. Electron-holographic tomography (EHT), that is, the combination of o -axis electron holography (EH) with electron tomography (ET), provides an unique access to this information, because it allows the quantitative 3D mapping of electrostatic potentials and magnetostatic vector elds with a resolution of a few (5− 10) nanometers. The reconstruction of the 3D potential starts with the acquisition of a hologram tilt series in the electron microscope. The subsequent reconstruction of the electron object wave from each hologram yields a tilt series in both amplitude and phase images. Finally, the phase tilt series is used for the tomographic reconstruction of the 3D potential. In this work, EHT has been developed from a manual and time-consuming approach to a widely automated method. The automation includes the development of the rst software package for computer-controlled acquisition of holographic tilt series (THOMAS), a prerequisite for e cient data collection. Using THOMAS, the acquisition time for a holographic tilt series, consisting of object and reference holograms, is reduced by more than a factor of three, compared to the previous, completely manual approaches. Meanwhile, the acquisition takes 2-3 hours on average. In addition, the holographic reconstruction and corresponding methods for removal of artefacts in the phase images have been automated, now including one-step procedures for complete tilt series. Furthermore, speci c self-developed alignment routines facilitate the precise correction of displacements within the tilt series of the rod-shaped samples, which are investigated here (e.g. nanowires, FIB needles). For tomographic reconstruction, a W-SIRT algorithm based on a standard simultaneous iterative reconstruction technique (SIRT) has been developed. Within the W-SIRT, a weighted back-projection instead of a simple back-projection is used. This yields a better convergence of the W-SIRT compared to the SIRT. In most cases in EHT (likewise in other ET techniques), the reconstruction of the three-dimensional tomogram is only feasible from projections covering a limited tilt range. This leads to a so-called missing wedge in the tomogram, which causes not only a lower resolution but also artefacts. Therefore, a method is presented, how to solve the missing wedge problem for suitable objects by exploiting symmetries. The 3D potential o ers the outer (morphology) and inner structure, as well as the mean inner potential (MIP) of the nano object. This is shown by means of EHT on epitaxially grown nanowires (NWs) of GaAs and AlGaAs. The 3D morphology is studied using the corresponding iso-surfaces of the 3D potential: The facets on the nanowires surface allow conclusions about the crystalline structure. Moreover, the reconstructed 3D potential of a AlGaAs/GaAs NW clearly shows its core/shell structure due to the MIP di erence between GaAs and AlGaAs of 0.61V . In case of doped semiconductor structures with pn-junctions (e.g. transistors) the