Abstract text (incl. figure legends and references)

1 Introduction

High-resolution scanning transmission electron microscopy (STEM) in principle allows to measure the atomic electric fields in a specimen as the beam electrons are deflected due to electrostatic forces. To measure the electric fields, for each scan point, information of the angular distribution of the transmitted electrons need to be obtained (4D-STEM). Two different approaches for acquisition are compared: using a pixelated camera which benefits from high resolution in momentum space and a segmented electron detector which is much faster than the camera but limited by the number of segments.

2 Objectives

Using 4D-STEM, the center of mass (COM) of the intensity distribution of the transmitted electrons was measured for each scan point in an STEM image. From the COM the effective momentum transfer of the beam electrons caused by the interaction with the atomic potentials can be calculated [1,2]. The corresponding quantities such as electric field, charge density and coulomb potential are compared for the two detector types and additionally checked by quantitative simulations. The pros and cons of these techniques are discussed as well as for which conditions a quantitative measurement is possible.

3 Materials and methods

A thin GaN crystal was analysed in a Thermo Fisher (TF) Spectra 300 TEM. The electric field seen by the scanning electron beam was measured with two different detector designs: (i) Using 4D STEM with a pixelated EMPAD electron detector, for each scan point the diffraction pattern was recorded. (ii) Using a 16-segmented TF Panther detector from the which the COM is calculated by weighting the measured intensity on each detector segment.

4 Results

Fig. 1a shows the experimental dark-field intensity for pure GaN obtained by the outer segments of the Panther. Figs. 1 b-e show the projected electric field (in units of V) for experimental and simulated Panther and EMPAD detectors in a color-wheel representation. Qualitatively and quantitatively, there is a good agreement between experiment and simulation but also between the results obtained by Panther and EMPAD detector.

5 Conclusions

We show that GaN atomic electric fields can be measured with 4D-STEM using either a pixelated EMPAD camera or a segmented Panther detector in case of thin GaN layer. The experimental results are in good agreement with quantitative simulations.

FIG1. (a) Dark-field intensity obtained from outer Panther segments. (b-e) Projected electric field in GaN calculated from the centre of mass of the transmitted-electron intensity obtained by 4D-STEM in color-wheel representation: color shows direction, brightness shows the strength with a maximum (absolute) value given in the red box in each subfigure. Data (b,c) from the segmented Panther detector (b: experiment, c: simulation) and (d,e) from the pixelated EMPAD camera (d: experiment, e: simulation).

[1] K. Müller et al., Nature Communications 5 (2014), 5653

[2] N. Shibata et al., Nature Communications 8 (2017), 15631