Filming electrons at work

Physicists at the Institute for Optics and Atomic Physics have developed a new method that allows moving images of periodically occurring processes to be recorded in transmission electron microscopy (TEM). Such processes include, for example, switching events in modern electronic components, the so-called semiconductor nanostructures. Until now, deeper "insights" into such processes were not possible.

The novelty of the method developed by Dr. Tolga Wagner under the guidance of Prof. Dr. Michael Lehmann lies in the fact that the scientists managed to invent a completely new, now patented, shutter or cover technique [1], to enable "filming" inside the TEM or, in other words, to "film electrons inside a sample during operation." This benefits the decoding of fundamental physical processes such as charge carrier dynamics in semiconductor nanostructures.

"In electron microscopy, there is always an attempt to keep the measurement conditions as stable as possible," says Dr. Tolga Wagner. High-resolution transmission electron microscopes are very sensitive to external disturbances such as vibrations, thermal instabilities, or electromagnetic field fluctuations. This is especially true for electron holography. It only provides information, for example about the potential distribution within a sample, when interference, i.e., the superposition of two coherent electron waves, occurs so that an interference pattern, the electron hologram, can be recorded. The "stability" of the electron waves relative to each other is a prerequisite for this.

However, instead of keeping the measurement conditions as stable as possible, Dr. Tolga Wagner and his colleagues intentionally disturb the measurement and allow interference only for a short period. The information generated in this way comes exclusively from the period during which interference occurred. On the one hand, this period (almost) can be chosen arbitrarily small – time resolutions in the picosecond range (one millionth of a blink) are achievable with manageable effort. On the other hand, the effort to suppress interference is low due to the high sensitivity of the setup to external disturbances. "The basic idea of our new method is to very quickly turn interference on and off through targeted disturbances. This is the principle of our shuttering technique, which we have therefore named 'interference gating'," explains Dr. Tolga Wagner. The position and width of the "gate" itself determine when and how long information is recorded. Using this method, the TU physicists succeeded in increasing the time resolution of the transmission electron microscope, which is located on the TU campus in Berlin-Charlottenburg and specifically optimized for research in electron holography, from seconds to 25 nanoseconds [2]. On these timescales, electronic processes in semiconductors, among others, occur. "With the time-resolved electron holography developed by us, it is now possible to film potential changes caused by the movement of electrons as they pass through semiconductors, which are only a few nanometers (one millionth of a millimeter) small," says Dr. Tolga Wagner.

[1] Michael Lehmann, Tore Niermann, and Tolga Wagner. "METHOD AND APPARATUS FOR CARRYING OUT A TIME-RESOLVED INTERFEROMETRIC MEASUREMENT," Publication number: EP3376522A1; TW201833521A; US2020103213A1; WO2018166786A1 (2018).
https://patentimages.storage.googleapis.com/3f/99/6a/e600762da71959/US20200103213A1.pdf
[2] Tolga Wagner, Tore Niermann, Felix Urban, and Michael Lehmann. "Nanosecond electron holography by interference gating." Ultramicroscopy 206 (2019): 112824.
https://doi.org/10.1016/j.ultramic.2019.112824

Further information is gladly provided to you:

Dr. Tolga Wagner
TU Berlin
Department of Experimental Physics / Electron and Ion Nano-Optics
Tel.: 030 314-24428
Email: tolga.wagner@tu-berlin.de