Seminar "Reconstruction of Nano Electric Fields and Atomic Structures Using Electron Diffractive Imaging"

SPEAKER:  Dr. Jun Yamasaki

Associate Professor, Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Japan

Abstract:

In diffractive imaging, a localized sample structure is reconstructed from its Fraunhofer diffraction pattern through iterative calculations. Although this novel imaging method has been extensively studied by coherent X-rays, electron diffractive imaging (EDI) is more suitable for analyses of nanometer-sized materials because of strong interactions between electrons and atoms.
         One of the advantages of EDI is visualization capability of phases in transmission electron waves after passing through samples. In this “Diffractive Phase Imaging”, a selected-area diffraction pattern and the corresponding bright-field TEM image are measured as amplitude information in reciprocal and real spaces, respectively. Since the complex wave fields in the both spaces are connected by Fourier transform, the amplitude information acts as constraint to estimate the phase information lost in the experimental data [1]. Recently it was clarified that the main factors limiting precision in the phase reconstructions are lens aberrations in TEM and partial spatial coherence in electron beams. To realize quantitative phase reconstructions, we developed a measurement method for those factors and also a processing method to eliminate their influences [2-5]. Using the methods, we achieved precision of 0.1-0.2 rad in the phase reconstructions [5]. So far the phase imaging of various targets such as a wedge-shaped Si crystal [1], electric potential around charged insulator particles [6] and a p-n junction in GaAs [6] have been achieved.
         Another advantage of EDI is high-resolution ability without disturbance by aberrations of the objective lens. In this approach, the shadow of the aperture is used as the real-space constraint. The atomic structure of Si view from a [110] direction was successfully reconstructed [7]. The atomic distance of 78 pm was also resolved in the result from a [112] direction, which proves the spatial resolution better than the aberration-corrected TEM [8].
 

References

[1] J. Yamasaki, et al., Appl. Phys. Lett., 101 (2012) 234105.
[2] S. Morishita, J. Yamasaki, et al, J. Electron Microsc. 60 (2011) 101.
[3] S. Morishita, J. Yamasaki, et al, Ultramicrosc. 129 (2013) 10.
[4] J. Yamasaki, et al., submitted.
[5] J. Yamasaki, et al., AMTC Lett. 5 (2016)236-237.
[6] J. Yamasaki, et al, AMTC Lett. 3 (2012) 164.
[7] S. Morishita, J. Yamasaki, et al., Appl. Phys. Lett. 93 (2008) 183103.
[8] S. Morishita, J. Yamasaki, et al, AMTC Lett. 2 (2010) 116.

Host: Prof. Tsumoru Shintake, Quantum Wave Microscopy Unit

Contact:  Dr. Ryusuke Kuwahara, Shintake Unit