&Bullet; physics 14, 61

During the Semiconductors for Breakfast meeting, researchers woke up to conversations about light-emitting nanorods and hairy cavities.

E. Fadaly / Technical University of Eindhoven

More recent work shows that silicon-germanium nanorods, in contrast to pure silicon crystals, can emit light. The researchers grew these semiconductor materials with the required hexagonal crystal structure by sowing them on hexagonally shaped nanowires.

“I eat semiconductors for breakfast” sounds like a line from one of the Terminator Movies. However, these widespread materials were on the morning menu during a series of seminars organized by the German Physical Society (DPG) last month. The Semiconductors for Breakfast series ran for two weeks and offered a wide themed buffet, from light-emitting nanorods to gas sensor lasers. Participants were invited each morning for a colloquium-style conversation in a casual virtual environment with takeaway coffee.

“The lectures highlight current topics in the field of modern semiconductor physics and are aimed at a wide audience, including undergraduate and graduate students. Students, postdocs and leading researchers, ”says co-organizer Michael Lorke from the University of Bremen. The series was created last year as a virtual replacement for the DPG spring meeting, which was canceled due to the COVID-19 pandemic. This year, the semiconductor community was again offered the online breakfast table format, as face-to-face meetings continue to be a problem. As in the previous year, voter turnout was quite good with an average of around 120 participants per lecture. “I’m really happy that so many people came back this year,” says co-organizer Doris Reiter from the University of Münster in Germany.

The format of the meeting was open without formal registration. The only requirement was to click on the link at the appropriate morning hour. With just one lecture per day, the participants were able to take a longer coffee break without having to disrupt their entire work schedule. “A conversation is a good amount to digest,” says Reiter. “And it gives you a reason to get up in the morning!” It also offered more freedom for discussion. “We had occasions when the questions lasted an hour and a half at the end of the conversation,” says Lorke. “You just can’t do that if you pack all the conversations into a few days,” he says.

The sessions started with a presentation on hexagonal silicon-germanium crystals by Erik Bakkers from the Eindhoven University of Technology in the Netherlands. This new light emitting semiconductor was selected as the breakthrough of the year Physics world Journal in 2020. Pure silicon crystals cannot emit light because they have an indirect band gap, which means that the conduction electrons have a momentum offset from the ground state. To create a direct band gap in silicon germanium, Bakkers and his colleagues had to force the material into a hexagonal lattice structure by growing silicon germanium nanorods around a hexagonal crystal “seed”. The nanorods emit light with infrared wavelengths, making them a very promising candidate for converting electrical signals into photon signals on computer chips.

Other lectures covered the full range of semiconductor physics, from lasers to quantum dots to brain-inspired storage devices. Jesper Mørk from the Technical University of Denmark presented some recent work on photonic crystals, which are periodic structures that can manipulate light. Mørk’s group has developed photonic crystals that combine a waveguide with a small cavity. Such devices can function as optical switches, sensors, and, as more recently shown, lasers. To increase the performance of these photonic crystals, the team is working to reduce the size of the cavity, which will reduce the coupling of light with other objects, such as B. single atom switches to be reinforced. Numerical calculations by the group have uncovered volume-minimizing cavity shapes that achieve this increase in performance. Mørk shared some pictures of these topology-optimized cavities, the fly of which is surrounded by surprisingly “hairy” structures.

This computer drawing shows a cavity design that minimizes the volume in which light is confined.

Some of the speakers came from industry, such as Johannes Koeth who works for nanoplus Nanosystems and Technologies, a German company that supplies a wide range of tunable diode lasers for sensor applications. These sensors can target specific absorption lines of a desired molecule. For example, a characteristic absorption feature for water vapor occurs at 1392 nm. In a typical setup, the laser quickly sweeps a small range of wavelengths around this spectral feature while a detector extracts the gas concentration from the degree of laser light absorption. Applications include detecting leaks in pipelines, monitoring the breathing of premature babies, and detecting a drunk driver by shining a laser through a car window to detect traces of alcohol on the driver’s breath.

These and other interesting conversations made for a stimulating brew, but both Lorke and Reiter look forward to returning to face-to-face meetings. “It is important that young people have a place where they can present themselves and their work,” says Reiter. The DPG is currently planning a virtual semiconductor meeting with student contributions for September 2021. But could Semiconductors for Breakfast be continued in any form? “If there is the opportunity to do it again, I would definitely step in,” says Reiter.

–Michael Schirber

Michael Schirber is the corresponding editor for physics based in Lyon, France.

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