Trick of light: Researchers transform silicon into direct bandgap semiconductor

Development, the topic of a recent magazine cover story. ACS Nanodepends on how UC Irvine researchers converted pure silicon from an indirect bandgap semiconductor to a direct bandgap semiconductor by how it interacts with light.

The UC Irvine team, in collaboration with scientists from Kazan Federal University and Tel Aviv University, explored an innovative approach by conditioning the light rather than changing the material itself. They trapped photons on irregularities smaller than 3 nanometers near a bulk semiconductor, giving light a new property—extended momentum—that opens up new ways of interaction between light and matter. According to the researchers, by “decorating” the silicon surface, they achieved an increase in light absorption by orders of magnitude, as well as a significant increase in device performance.

“In direct bandgap semiconductor materials, electrons move from the valence band to the conduction band. This process only requires a change in energy; this is an efficient transfer,” said lead author Dmitry Fishman, an associate professor of chemistry at the University of California, Irvine. “In indirect bandgap materials such as silicon, an additional component—a phonon—is needed to provide the electron with the momentum needed to make the transition. Because the probability of a photon, phonon, and electron interacting at the same place and time is low, silicon’s optical properties are inherently weak.”

He said that as an indirect bandgap semiconductor, silicon’s poor optical properties limit the development of solar energy conversion and optoelectronics in general, which is a disadvantage given that silicon is the second most abundant element in the Earth’s crust and the foundation on which the world is built. The computer and electronics industries were built.

“Photons carry energy but almost no momentum, but if we change this story explained in textbooks and somehow give photons momentum, we can excite electrons without needing additional particles,” said co-author Eric Potma, professor in Chemistry from the University of California, Irvine. “This reduces the interaction to two particles, a photon and an electron, similar to what happens in direct bandgap semiconductors, and increases light absorption by 10,000 times, completely transforming the interaction of light and matter without changing the chemistry of the material itself. “

Co-author Ara Apkarian, professor emeritus of chemistry at the University of California, Irvine, said: “This phenomenon fundamentally changes the way light interacts with matter. Traditionally, textbooks teach us about so-called vertical optical transitions, when a material absorbs light and only the photon changes. energy state of an electron. However, photons with increased momentum can change both the energy and momentum states of electrons, opening up new transition paths that we had not previously considered. Figuratively speaking, we can “tilt the textbook” because these photons provide diagonal transitions. This significantly affects the material’s ability to absorb or emit light.”

According to the researchers, this development offers the opportunity to leverage recent advances in semiconductor manufacturing technology at the sub-1.5 nanometer scale, which could potentially impact photo-sensing and light energy conversion technologies.

“Given the growing impacts of climate change, the transition from fossil fuels to renewable energy sources is more urgent than ever. Solar energy plays a key role in this transition, but the commercial solar cells we rely on are failing,” Potma said. “Silicon’s poor ability to absorb light means these cells require thick layers—almost 200 micrometers of pure crystalline material—to effectively capture sunlight. This not only increases production costs, but also limits efficiency due to increased charge carrier recombination. Subtlety Film solar cells, which are one step closer to reality thanks to our research, are widely seen as a solution to these problems.”

Other co-authors on this study included Giovani Merham and Alexey Noskov of the University of California, Irvine; Kazan Federal University researchers Elina Battalova and Sergei Kharintsev; and Tel Aviv University investigators Liat Katrivas and Alexander Kotlyar. The project received financial support from the Chan Zuckerberg Initiative.