Hamish Brown1 Laura Clark1 Zhen Chen2 Ryo Ishikawa3 Hirokazu Sasaki4 Naoya Shibata3 Matthew Weyland5 6 Timothy Petersen1 David Paganin1 Michael Morgan1 Les Allen7 Scott Findlay1

1, Monash University, Clayton, Victoria, Australia
2, Cornell University, Ithaca, New York, United States
3, The University of Tokyo, Tokyo, , Japan
4, Furukawa Electric Ltd., Yokohama, , Japan
5, Monash University, Melbourne, Victoria, Australia
6, Monash University, Melbourne, Victoria, Australia
7, University of Melbourne, Melbourne, Victoria, Australia

There are a variety of techniques for electromagnetic field mapping in STEM. Of particular interest are differential phase constrast (DPC) and ptychography, due to the increasing availability of segmented detector systems and fast-readout electron cameras. In differential phase contrast we relate the mean transverse deflection of the beam to the electromagnetic field at each probe position in the STEM scan. Ptychographic techniques use diffraction pattern measurements for each scan position of the probe to solve for the scattering potential and hence the electromagnetic field structure of the sample. Application of these techniques to smaller length-scales leads to some subtleties in the interpretation of experimental results.

There is a long history of STEM techniques being used to study micron-sized magnetic field domains. At these resolutions the magnetic field causes a rigid transverse diffraction shift of the electron probe. When these techniques are applied to objects less than a few tens of nm across, a more complicated diffraction plane redistribution occurs. It is possible to account for these redistributions using an analytic scattering model. We use experimental data from imaging a 17 nm wide p-n junction in [001] GaAs as a case study.

There is interest in applying STEM electromagnetic field mapping techniques at atomic resolution to directly measure charge transfer due to bonding. I will show quantitative reconstruction of the electrostatic potential of a MoS2 monolayer as a example of the atomic resolution capabilities of this technique. However application of these techniques to thicker samples is difficult: I show that quantitative reconstruction of a 7.8 nm thick sample of SrTiO3 sample fails. This is because it is necessary to assume a very thin object to apply DPC or ptychography (the projection approximation) and this assumption fails for even moderately thick samples (> 3 nm) at atomic resolution. Possible ways of circumventing this limitation are discussed.