Towards quantitative electric field measurements at hetero interfaces via precessing 4D-STEM

Abstract text (incl. figure legends and references)

Recently, four-dimensional scanning transmission electron microscopy (4D STEM) has been established to investigate internal electric fields on nanometer length scales [shibata, AB, …]. To achieve this, the shift of the electron-beam caused by the fields is measured. This becomes especially difficult at interfaces, where additional shifts, e.g., due to the different scattering potentials or tilt can occur.

In this work, we analyse whether precession electron diffraction (PED) is capable of removing dynamic effects that lead to a thickness-dependent oscillation of the detected beam-shift. Furthermore, we investigate if the shift momentum transfer of the field to the probe is determined more accurately by edge-detection or by the calculation of the centre-of-mass (COM) of the intensity. Finally, the ideal settings of a STEM for determining the displacing fields shall be found.

Using the STEMsalabim [JOE] multi-slice algorithm, we simulate electron scattering patterns assuming a precessing electron-beam. For that, we use p-n-homo- as well as hetero-interfaces, i.e., p-GaAs/n-GaAs, AlAs/GaAs and GaP/Si interfaces. From these we determine the COM of the direct beam and subsequently calculate E-fields and potentials across the junctions. In turn, we compare these with experimental results which were derived using a JEOL JEM 2200FS STEM equipped with a Nanomegas ASTAR PED system and a fast pnCCD pixelated detector.

Our investigations show that the precessing electron-beam does indeed reduce dynamic effects. However, we obtain clear deviations for the potential drop across the interfaces derived from the simulations and the experiments. In particular, thickness-dependent peculiarities are recognized, i.e. the difference of potentials at interfaces decreases with increasing sample-thickness in simulations.

In conclusion, we find that the method of precessing electron-beams still needs significant development to obtain the results that we expect.

This contribution will summarize our progress in the quantitative evaluation of internal electric fields at hetero-interfaces.