Elektronenholographie

The phase problem of crystallography occupied the scientists for decades. It describes the fact, that the phase information of a wave gets lost in a intensity measurement. This is problematic, as in a diffraction experiment under investgation, amplitude and phase of a wave get altered, but only half of the information is obtained in a following conventional intensity measurement.

For this to overcome holographic techniques have been developed parallel to the experimental equipment. Field emission guns as a coherent wave source analogously to lasers in light optics were a prerequisite to implement holographic interference mappings. For this a undiffracted beam is deflected phase coherent toward a beam, containing the objects information. In the imageplane the coherent sum of the two beams forms the holographic image. The deflection is initiated by a a wollaston's biprism, a wire of about 100nm thickness and 3mm length under a voltage of several hundreds of volts.

But whereas in light optical holography optical properties are detected, electron holography is sensitive to the distribution of the Coulomb potential inside a specimen. Additionally the possibility of increasing the microscopes resolution arises, as the knowledge of amplitude and phase gives the opportunity to correct for the microscopes lens aberrations.

At the Laboratorium für Elektronenmikroskopieelectron holography is used to determine the mean inner potential of different industry relevant semiconductors like GaAs and AlAs. For this, a wedge of known angle is transmitted with an electron beam. Due to the mean inner potential the electrons get accelerated and their wavelength shortens a little. This leads to an increasing phase shift with respect to a vacuum wave, analogously to the transmission of light through material, with a refractive index unequal to one. The measured value of the mean inner potential can be used to determine concentration and thickness distributions by a variant of the CELFAmethod as it is shown for AlAs/GaAs-heterostructures.

Selected conference poster presentations:

Coarsening of mass-selected Au clusters on amorphous carbon at room temperature (pdf)
Mean inner Coulomb Potential of Au clusters analyzed by Transmission Electron Holography (pdf)

Selected publications:

[1] E. Müller, P. Kruse, D. Gerthsen, M. Schowalter, A. Rosenauer, D. Lamoen, R. Kling, A. Waag, Measurement of the mean inner potential of ZnO nanorods by transmission electron holography, Appl. Phys. Lett. 86, 154108 (2005)

[2] M. Wanner, D. Bach, D. Gerthsen, R. Werner, B. Tesche, Electron holography of thin amorphous carbon films: Measurement of the mean inner potential and a thickness-independent phase shift, Ultramicroscopy 106, 4, 341 (2006)

[3] E. Müller, D. Gerthsen, P. Bruckner, F. Scholz, Th. Gruber, A. Waag, Probing the electrostatic potential of charged dislocations in n-GaN and n-ZnO epilayers by transmission electron holography, Phys. Rev. B 73, 245316 (2006)

[4] Radian Popescu, Erich Müller, Matthias Wanner, Dagmar Gerthsen, Marco Schowalter, Andreas Rosenauer, Artur Bottcher, Daniel Loffler, Patrick Weis, Increase of the mean inner Coulomb potential in Au clusters induced by surface tension and its implication for electron scattering, Phys. Rev. B 76, 235411 (2007)